RHEOLOGY OF POROUS RHYOLITE

by

GENEVIEVE ROBERT

B.Sc. (Honours), McGill University, 2005

A THESIS SUBMITThD IN PARTIAL FULLFILMENT OFTHE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

in

THE FACULTY OF GRADUATE STUDIES

(Geological Sciences)

THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)

March 2008

© Genevieve Robert, 2008

ABSTRACT

I describe an experimental apparatus used to perform deformation experiments

relevant to volcanology. The apparatus supports low-load, high-temperature deformation

experiments under dry and wet conditions on natural and synthetic samples. The

experiments recover the transient rheology of complex (melt ± porosity ± solids) volcanic

materials during uniaxial deformation. The key component to this apparatus is a steel

cell designed for high-temperature deformation experiments under controlled water

pressure. Experiments are run under constant displacement rates or constant loads; the

range of accessible experimental conditions include: 25 - 1100 °C, load stresses 0 to 150

MPa, strain rates 106 to 102 i, and fluid pressures 0-150 MPa.

I present a suite of high-temperature, uniaxial deformation experiments performed

on 25 by 50 mm unjacketed cores of porous (-0.8) sintered rhyolitic ash. The

experiments were performed at, both, atmospheric (dry) and elevated water pressure

conditions (wet). Dry experiments were conducted mainly at 900 °C, but also included a

suite of lower temperature experiments at 850, 800 and 750 °C. Wet experiments were

performed at —650 °C under water pressures of 1, 2.5, 3, and 5 IVJPa, and at a fixed PH2O

of 2.5 MPa for temperatures of -385, 450, and 550 °C. During deformation, strain is

manifest by shortening of the cores, reduction of porosity, flattening of ash particles, and

radial bulging of the cores. The continuous reduction of porosity leads to a dynamic

transient strain-dependent rheology and requires strain to be partitioned between a

volume (porosity loss) and a shear (radial bulging) component. The effect of increasing

porosity is to expand the window for viscous deformation for dry melts by delaying the

onset of brittle deformation by -50 °C (875 °C to 825 °C). The effect is more

11

pronounced in hydrous melts (--0.67 — 0.78 wt. % H20) where the viscous to brittle

transition is depressed by --140 to 150 °C. Increasing water pressure also delays the onset

of strain hardening due to compaction-driven porosity reduction. These rheological data

are pertinent to volcanic processes where high-temperature porous magmas I liquids are

encountered (e.g., magma flow in conduits, welding of pyroclastic materials).

111

TABLE OF CONTENTS

ABSTRACT iiTABLE OF CONTENTS ivLIST OF TABLES viLIST OF FIGURES viiPREFACE viiiACKNOWLEDGEMENTS xCO-AUTHORSHIP STATEMENT xii

CHAPTER I: Introduction 11.1 Context 11.2 Previous studies 11.3 Goals and approach 51.4 References 7

CHAPTER II: The fluid cell 92.1 Introduction 92.2 Experimental apparatus 10

2.2.1 Fluid cell 112.2.2 Temperature calibration 13

2.3 Calibration for viscosity 142.4 Volcanological experiments 15

2.4.1 Materials 152.4.2 Experiments 192.4.3 Textural analysis of experiments 23

2.5 Discussion 24

2.6 Acknowledgements 27

2.7 Appendix 2.A: Melt viscosity of the Rattlesnake Tuff ash 28

2.8 References 30

CHAPTER III: Deformation experiments 34

3.1 Introduction 34

3.2 Experimental methods 37

3.2.1 Experimental apparatus 37

3.2.2 Fabrication of experimental cores 38

3.2.3 Pre-experimental sample characterization 41

3.3 Experimental results 44

3.3.1 Overview 44

3.3.2 Dry high-T experiments 44

3.3.3 Wet high-T experiments 46

3.3.4 Textural analysis of experimental cores 48

3.4 Post-experimental physical properties 50

3.4.1 Porosity 50

3.4.2 Water content 54

3.5 Analysis of experimental results 56

iv

3.5.1 Effect of temperature and PH2O 563.5.2 Analysis of strain 603.5.3 Effective viscosity 63

3.6 Discussion 673.7 Acknowledgements 753.8 Appendix 3.A: Correction for dwell-time effects 753.9 Appendix 3.B: Water contents of samples 783.10 References 82

CHAPTER IV: Discussion 864.1 Water 864.2 Experimental design modifications 904.3 Temperature gradient 914.4 Pore size distribution and pore shape 924.5 References 94

CHAPTER V: Summary 96

APPENDIX A: Cell design 98APPENDIX B: Data acquisition 100APPENDIX C: Experimental data 101APPENDIX D: Data processing 103

v

LIST OF TABLES

Table 2.1 Summary of calibration and deformation experiments, including conditions,properties, and composition of samples 16

Table 2.2 Measured values of viscosity for glass from melted Rattlesnake Tuff ash andVFT coefficients (A, B, C) 25

Table 3.1 Chemical composition of the Rattlesnake Tuff ash 40Table 3.2 Experimental conditions used in deformation experiments and geometry of

sample cores pre- and post-experiment 42Table 3.3 Measured values of density and porosity for pre- and post-experiment sample

cores 43Table 3.4 Analysis of strain 45Table 3.B Values of H20 and LOT (wt.%) for post-experiment cores 80

vi

LIST OF FIGURES

Figure 2.1 Experimental apparatus 12Figure 2.2 Temperature and viscosity calibration 18Figure 2.3 Pre- and post-experimental products 20Figure 2.4 Experimental results 22Figure 3.1 Overview of previous experimental studies 35Figure 3.2 Starting experimental materials 39Figure 3.3 Summary of experimental data 47Figure 3.4 Textural evolution of samples due to deformation 49Figure 3.5 Nature and distribution of porosity in sample cores 52Figure 3.6 Volume strain 55Figure 3.7 Effect of temperature and PH2O 59Figure 3.8 Analysis of strain 62Figure 3.9 Summary of apparent viscosity 64Figure 3.10 Textural comparison of samples run under dry and wet conditions 69Figure 3.11 Glass transition and relaxation timescale 73Figure 3.A Systematic corrections to experimental data 77Figure 3.B Bulk water contents of experimental samples 79Figure 4.1 Proportion of isolated porosity with deformation 87Figure A.1 Water cell design 99Figure C.1 Experiment RSO3 102

vii

PREFACE

This research comprises two complementary manuscripts prepared for publication

in peer-reviewed international scientific journals. Chapter II is published in the

American Mineralogist, under the title “High-temperature deformation of volcanic

materials in the presence of water”. I am senior author, and my co-authors are 3. K.

Russell, Daniele Giordano, and Claudia Romano. Cliff Shaw (University of New

Brunswick) and Luigi Burlini (ETH Zurich) were journal reviewers. This chapter

presents the design and calibration of a new apparatus to run uniaxial deformation

experiments on volcanic materials under temperature and water pressure conditions

relevant to volcanologic processes. The original design of the apparatus is by Daniele

Giordano with technical advice from Oliver Spieler. Ray Rodway is responsible for

machining the apparatus and helping with subsequent design changes. My experimental

work was the basis for making design modifications to improve the performance of the

apparatus.

Chapter III has been submitted for publication under the title “Rheology of porous

volcanic materials: High-temperature experimentation under controlled water pressure”

in a special volume of Chemical Geology (8th Silicate Melt Workshop, Eds. D.B.

Dingwell, R. Moretti, P. Richet), and is currently under review. I co-authored the

manuscript with 3. K. Russell and Daniele Giordano. Chapter III presents a series of

high-temperature deformation experiments run on porous aggregates of sintered volcanic

ash under both wet and dry conditions. The experiments are organized to show the

effects of (i) water pressure, and (ii) temperature on the rheological behaviour porous

volcanic materials.

viii

Pre- and post-experimental physical properties of samples, including length,

radius, mass, density, total, connected, and isolated porosity are reported in chapter III, as

well as characteristic textures of the run-products, bulk water content and whole rock

chemistry. Whole rock analyses of starting materials and run-products, including bulk

water, were performed by ALS Chemex. Karl-Fischer Titration analyses of water content

on the samples were performed by Daniele Giordano at ETH Zurich.

Chapters IV and V provide a discussion of the entire research program, including

a summary of the main results and the potential avenues for future work, respectively.

The discussion also addresses issues that were not necessarily considered prior to or

during experimentation. Four appendices are used to include detailed cell design,

complete data sets, and data processing methods.

ix

ACKNOWLEDGEMENTS

Financial support for my M.Sc. was provided by an NSERC PGS-M Scholarship.

Costs to build the experimental apparatus were met by an NSERC RTI Grant “High

temperature experiments on porosity and permeability evolution in volcanic systems”

held by J.K. Russell, G.M. Dipple, and L.A. Kennedy. Operational costs for the research

were covered by an NSERC Discovery Grant held by J.K. Russell.

Nils, I wouldn’t have made it through any of this without you, and I definitely

wouldn’t have had this much fun. I owe you a lifetime supply of Sortilège, and I shall

deliver it myself, wherever in the world you may be. Stephen, thank you for your great

listening skills, and thanks for all the fish. Krista, R-E, Curtis, Jackie, thanks for letting

me shuffle your workspace whenever i so desired. You have all contributed to my

success by keeping me in equilibrium. I am grateful I have such great people around me

making this academic experience a great life experience too. I have made friends here I

wish to keep for life.

Kelly, I want to thank you for giving me so many great opportunities and for your

guidance and unconditional support throughout this Masters, but especially for countless

scientific discussions where I felt like a colleague rather than a student. Lori, it is always

a pleasure to discuss ideas, problems and results from my experiments with someone who

understands how many hours of work one little piece of experimental data actually

represents. Mark, you brought thoughtful and unexpected arguments to our scientific

discussions, making me think and investigate further.

Thank you to Daniele, Ben and Steve for being such enthusiastic experimental lab

colleagues, and to Ben especially for bringing a different perspective to my research,

x

always being available and interested, and for being such a great friend on top of it all. I

am forever grateful to Ray Rodway for providing the technical support that made it all

possible. En terminant, je tiens a remercier tout particulièrement ma famille. Papa,

maman et Polo, tout au long de mes etudes, votre soutien et vos encouragements

m’ auront permis de réaliser mes ambitions. Je vous aime.

xi

CO-AUTHORSHIP STATEMENT

This thesis comprises two complementary manuscripts prepared for publication in

peer-reviewed international scientific journals. Chapter II is published in the American

Mineralogist, under the title “High-temperature deformation of volcanic materials in the

presence of water”. I am senior author, and my co-authors are J. K. Russell, Daniele

Giordano, and Claudia Romano. Chapter II presents the design and calibration of a new

apparatus to run uniaxial deformation experiments on volcanic materials under

temperature and water pressure conditions relevant to volcanologic processes. The

experimental cell was conceptualized by my supervisor (J.K. Russell) and was originally

designed by Daniele Giordano with technical advice from Oliver Spieler. Ray Rodway is

responsible for machining the apparatus and helping with subsequent design changes.

My experimental work was the basis for making design modifications to improve the

performance of the apparatus. I performed all calibration experiments, all experiments on

natural materials, and all experimental data reduction and analysis.

Chapter III has been submitted for publication under the title “Rheology of porous

volcanic materials: High-temperature experimentation under controlled water pressure”

in a special volume of Chemical Geology (8th Silicate Melt Workshop, Eds. D.B.

Dingwell, R. Moretti, P. Richet), and is currently under review. I am senior author, and

my co-authors are J. K. Russell and Daniele Giordano. Chapter III presents a series of

high-temperature deformation experiments that I performed on porous aggregates of

sintered volcanic ash under both wet and dry conditions.

In chapter III, I report measurements I made of pre- and post-experimental

physical properties of samples, including length, radius, mass, density, total, connected,

xii

and isolated porosity, as well as characteristic textures of the run-products, bulk water

content and whole rock chemistry. The whole rock analyses of starting materials and

run-products, including bulk water, were performed by ALS Chemex. Karl-Fischer

Titration analyses of water content on the samples were performed by Daniele Giordano

at ETH Zurich. I am responsible for data reduction and analysis of the physical and

chemical properties of the experimental samples, as well as for the reduction and analysis

of the rheological data obtained from the deformation experiments.

xiii

CHAPTER I: Introduction

1.1 Context

Experimental volcanology is an expanding field of research driven by new

methods for exploring volcanic processes through high-temperature experimentation

(Dingwell, 1998; Gardner, 1999; Tinker et al., 2004; Quane et al., 2004; Grunder et al.,

2005). Dynamic deformation of complex volcanic materials (melt ± crystals ± pores) in

the laboratory is of great interest because of the direct applications to the flow of volcanic

materials, notably in volcanic conduits, during lava transport, and during welding of

pyroclastic volcanics. Porous and hydrous volcanic materials are of special interest

because of the ubiquity of water, and consequently bubbles, in volcanic systems. There

are several sets of high-temperature experiments on natural volcanic materials that have

been performed under dry conditions (e.g., Yagi, 1966; Bierwirth, 1982; Bagdassarov and

Dingwell, 1992; Quane, 2004). However, performing similar experiments under

controlled water pressures is inhibited by the technical difficulties involved (Friedman et

al., 1963; Grunder et al., 2005). Thus, establishing the rheology of hydrous volcanic

materials remains one of the principal challenges in volcanology (Grunder and Russell,

2005).

1.2 Previous studies

Bierwirth (1982) studied the compaction and welding of the rhyolitic Bandelier

Tuff ash, New Mexico and dacitic air fall deposit from Mount St. Helens, Washington,

under dry conditions and temperatures between 650 °C and 800 °C. His experiments

were conducted on jacketed samples of loose ash, at constant load pressures between 0.72

1

and 3.62 MPa. Higher loads and higher temperatures resulted in more compaction

(greater porosity loss). Bierwirth developed an equation to describe the compaction of

Bandelier Tuff ash. The equation expresses strain, decomposed into time and strain rate,

as a function of density and material properties, which depend on a temperature-

dependent activation energy.

Bagdassarov and Dingwell (1992) performed uniaxial deformation experiments

on core samples of vesiculated natural obsidian from Little Glass Butte, Oregon. They

used constant stress (510 to i0 Pa) uniaxial deformation experiments (E = 0.01-0.015)

to determine the viscosity of the samples with low (-0-0.5), moderate (—P0.25-0.35), and

high (-4165) pore fractions at temperatures near the glass transition (—850 °C) of the melt.

They observed a decrease of apparent viscosity with increasing pore fraction and fit their

experimental data to a viscosity () vs. porosity (1) relationship of the form:

(1.1)

using a dimensionless constant C of 22.4.

Lejeune et al. (1999) performed uniaxial deformation experiments on calcium

aluminium synthetic silicate melt samples. They vesiculated the synthesized melts in air

to obtain low (t = 0-0.13) to moderately (1 = 0.32-0.47) porous samples. Deformation

experiments were conducted at temperatures ranging from 830 to 960 °C, and at a

constant stress varying from 1.1 to 67.7 MPa. The experiments of Lejeune et a!. clearly

show that the apparent viscosity of porous melt decreases with increasing porosity. The

measured decrease in viscosity due to the addition of 47% porosity in their experiments

corresponds to a viscosity change caused by an increase in temperature of 10 °C.

2

Quane (2004) used both soda lime silica glass beads as an analogue for silicate

melt and natural ash from the Rattlesnake Tuff, Oregon, to investigate the rheology of

porous volcanic materials via a series of dry, high-temperature experiments conducted at

constant displacement rate or constant load. Results from the experiments are also

reported in Quane and Russell (2005) and Quane and Russell (2006). Cores of sintered

beads or Rattlesnake Tuff ash were fabricated to produce large cores. The physical

properties of each core were fully characterized before and after each experiment. The

displacement rates used in the glass bead core deformation experiments ranged between

2.510 and ii03 cmls, and the loads between 5 and 50 kg for temperatures of 535, 550,

600, and 650 °C and starting porosities between —27 and -37%. The displacement rates

used in the Rattlesnake Tuff ash experiments ranged from 1 .25 i04 to 5.0 i04 cmls, and

the loads ranged from 22.5 to 90 kg for temperatures ranging from 800 to 900 °C and

starting porosities ranging from -.-70 to 80%. The rheology of both materials was found

to be strain dependent, and the changes in temperature to have a much greater effect on

rheology than changes in load or displacement rate. In the glass bead experiments, strain

accumulates dominantly by porosity loss at low amounts of total strain. At higher strain,

radial strain becomes more important. In contrast, deformation of natural ash cores

shows radial bulging to be dominant at lower amounts of strain with porosity loss

becoming more important at higher amounts of total strain. Strain accommodated by

porosity loss is described by the following relationship:

(1.2)1-f

where is initial total porosity and I is final total porosity. The results of Quane and

Russell (2005) suggest that, in analogue and especially in high-porosity natural materials,

3

porosity distributions control the mechanisms and extent of welding. They developed a

constitutive relationship relating porosity to melt viscosity at constant temperature by an

empirical factor (a), reflecting the starting porosity of the material, the geometry and

character of individual glass clasts and the ability of individual clasts to rearrange or

rotate during deformation to describe the rheological behaviour of the Rattlesnake Tuff

ash:

flehloe (1.3)

where 1e is the sample viscosity, rio the melt viscosity and the sample porosity.

Experiments investigating the rheology of porous hydrous volcanic materials in

which water pressure is controlled independently of the load applied to the sample or the

rate of deformation are few. Friedman et al. (1963) published the only set of deformation

experiments performed on natural volcanic ash in which water pressure was

independently controlled. They investigated the viscosity of crushed porous rhyolite

glass at temperatures between 400°C and 850 °C and at water pressures between 0 and

6.89 MPa. Most experiments were performed at temperatures above 485 °C and water

pressures below 2.07 MPa, conditions at which welded ignimbrites can form. The

deformation experiments of Friedman et al. were performed on jacketed samples of loose

ash from the Bandelier rhyolite tuff, New Mexico. They estimated the initial porosity of

the samples to be —50% on the basis of geometry, and they controlled temperature, load,

and water pressure for the duration of each experiment. The sample was brought to

temperature by a resistance furnace that surrounded the lower part of the experimental

assembly. Fluid pressure was controlled with a hand pump, and load was applied to the

sample by placing weights on a lever that was connected to the piston used to deform the

4

sample. They recorded the compaction rates of the ash and compared the results to

compaction rate curves for Pyrex glass under dry conditions to obtain viscosity values for

the Bandelier Tuff ash. For a given experimental temperature, Friedman et al. (1963)

report an increase in viscosity with increasing strain or with reduction of porosity, and

compactions rates were observed to be faster at higher water pressures. It should be

noted that the figures in Friedman et al. (1963) are mislabelled to indicate incorrectly that

compaction rate decreases with increasing water pressure (cf. Sparks et al., 1999).

The experiments reported in this thesis are the only other wet experiments that

investigate the rheology of porous volcanic materials.

1.3 Goals and approach

The objectives for this project were: (i) to build a deformation apparatus capable

of holding water pressures relevant to volcanic processes at high-temperature, (ii) to

calibrate the apparatus for viscosity and temperature, (iii) to run wet and dry experiments

on porous volcanic materials, and (iv) to use the resultant data to understand porosity in

collapsing volcanic materials. Specifically, constant displacement rate, parallel-plate

deformation experiments (Gent, 1960) were performed on the porous cores of ash from

the Rattlesnake Tuff, at high temperatures and at controlled water pressures, in a new

apparatus designed for high-temperature, uniaxial deformation experiments in the

presence of water. Strain in the experiments is expressed by a shortening and radial

increase of the sample, and a reduction in porosity from the pre-experimental values.

These goals are organized as two manuscripts. The first two objectives were met

and are published as an article in American Mineralogist (Robert et al., 2008). The latter

5

two parts of the project are in a manuscript submitted to a special volume of Chemical

Geology (Robert et al., In Review). Because of the chosen thesis format, the appendices

to this thesis are critical and more of the technical background, methods used and raw

data from the experiments are presented there. The final design of the cell is presented in

the first appendix. The second appendix explains the data acquisition process, and

contains all the experimental data files. The raw experimental data is compiled in

electronic format in the third appendix. The MATLAB code used to process the

experimental data is in the final appendix. Some repetition in the introductory sections of

Chapters II and III is unavoidable as each chapter is a separate manuscript for different

publications.

6

1.4 References

Bagdassarov, N.Sh., Dingwell, D.B., 1992. A rheological investigation of vesicular

rhyolite. Journal of Volcanology and Geothermal Research 50, 307-322.

Bierwirth, P.N., 1982. Experimental welding of volcanic ash. B.Sc. Honours Thesis,

Monash University, 74.p.

Dingwell, D.B., 1998. Recent experimental progress in the physical description of silicic

magma relevant to explosive volcanism. In: Gilbert, J.S. and Sparks, R.S.J. (eds.)

The Physics of Explosive Volcanic Eruptions, Geological Society, London,

Special Publications 145, 9-26.

Friedman, I., Long, W., Smith, R.L., 1963. Viscosity and water content of rhyolite glass.

Journal of Geophysical Research 68, 6523-6535.

Gardner, J.E., Hilton, M., Carroll, M.R., 1999. Experimental constraints on degassing

magma; isothermal bubble growth during continuous decompression from high

pressure. Earth and Planetary Science Letters 168, 201-218.

Grunder, A., Russell, J.K., 2005. Welding processes in volcanology: insights from field,

experimental, and modeling studies. Journal of Volcanology and Geothermal

Research 142, 1-9.

Grunder, A.L., Laporte, D., Druitt, T. H., 2005. Experimental and textural investigation

of welding: effects of compaction, sintering, and vapor-phase crystallization in the

rhyolitic Rattlesnake Tuff. Journal of Volcanology and Geothermal Research 142,

89-104.

Lejeune, A.M., Bottinga, Y., Trull, T.W., Richet, P., 1999. Rheology of bubble-bearing

magmas. Earth and Planetary Science Letters 166, 7 1-84.

7

Quane, S.L., 2004. Welding in pyroclastic materials, PhD Thesis, University of British

Columbia, 2O8p.

Quane, S.L., Russell, J.K., Kennedy, L.A., 2004. A low-load, high-temperature

deformation apparatus for volcanological studies. American Mineralogist 89, 873-

877.

Quane, S.L., Russell, J.K., 2005. Welding: insights from high-temperature analogue

experiments. Journal of Volcanology and Geothermal Research 142, 67-87.

Quane, S.L., Russell, J.K., 2006. Bulk and particle strain analysis in high-temperature

deformation experiments. Journal of Volcanology and Geothermal Research 154,

63-73.

Robert, G., Russell, J.K., Giordano, D., Romano, C., 2008. High-temperature

deformation of volcanic materials in the presence of water. American

Mineralogist 93, 74-80.

Robert, G., Russell, J.K., Giordano, D., In Review. Rheology of porous volcanic

materials: High-temperature experimentation under controlled water pressure.

Chemical Geology Special Issue, 8t Silicate Melt Workshop.

Sparks, R.S.J., Tait, S.R., Yanev, Y., 1999. Dense welding caused by volatile resorption.

Journal of the Geological Society, London 156, 2 17-225.

Tinker, D., Lesher, C.E., Baxter, G. M., Uchida, T., Wang, Y., 2004. High-pressure

viscometry of polymerized silicate melts and limitations of the Eyring equation.

American Mineralogist 89, 1701-1708.

Yagi, K., 1966. Experimental study on pumice and obsidian. Bulletin of Volcanology 29,

559-572.

8

CHAPTER II: The fluid cell’

2.1 Introduction

Experimental volcanology is an expanding field driven by new methods for

exploring volcanic processes through high-temperature experimentation (Dingwell, 1998;

Gardner, 1999; Tinker et al., 2004; Quane et al., 2004; Grunder et al., 2005). High-

temperature experiments are used to retrieve data on the rheological behaviour of natural

melts (e.g., Dingwell et al., 1993; Dingwell, 1998; Richet and Bottinga, 1995; Giordano

et al., 2004), the properties of pyroclastic materials (e.g., Friedman et al., 1963;

Bierwirth, 1982; Quane et al., 2004; 2005; Giordano et al., 2005), the conditions

attending explosive collapse of lava and domes (e.g., Spieler et al., 2004), and the

mechanisms of fragmentation processes in volcanic conduits (Tuffen et al., 2003;

Kennedy et al., 2005).

The explosive or effusive behaviour of volcanic systems is governed by magma

rheology, which largely reflects the abundance and nature (e.g., dissolved vs. exsolved)

of volatile components. However, the rheological properties of volcanic materials in the

presence of a fluid phase as ubiquitous as water in volcanic systems remain poorly known

(Bagdassarov and Dingwell, 1992; Lejeune et al., 1999; Stein and Spera, 1992). This gap

in knowledge results from the technical difficulties in designing and running the

appropriate experiments. Establishing the rheology of hydrous volcanic materials,

therefore, remains one of the principal challenges in volcanology (Grunder and Russell,

2005).

‘A version of this chapter has been published. Robert, G., Russell, J.K., Giordano, D.,Romano, C., 2008. High-temperature deformation of volcanic materials in the presence

of water. American Mineralogist 93, 74-80.

9

The purpose of this paper is two-fold. First, we describe a new experimental cell

for high-temperature deformation experiments of samples under controlled fluid

pressures. The “fluid cell” can be used with the Volcanology-Deformation-Rig (VDR;

Quane et al., 2004) for rheological studies of volcanic materials (e.g., pumice, ash, lava)

over T-PH2O conditions pertinent to volcanological processes. Second, we report on a

series of experiments used to: a) calibrate the apparatus; and to b) explore the properties

(e.g., viscosity) of natural pyroclastic materials at volcanic T-PH2O conditions. These data

are critical for the understanding of a variety of volcanic processes such as: welding and

compaction of ignimbrites; fragmentation and annealing of magma in volcanic conduits;

flow of volcanic domes; and amalgamation and flow of clastogenic lavas.

2.2 Experimental apparatus

The VDR (Fig. 2. la) was designed to explore the rheology of volcanic materials

by performing high-temperature, low-load (<1136 kg) deformation experiments at

constant load, or displacement rate, or at controlled load rates (Quane et al., 2004). The

apparatus comprises a GeoComp LoadTracll reinforced “T”-frame equipped with a step-

motor that moves a lower platen upwards at specified rates, or applies a prescribed load.

An S-beam type load transducer measures load; displacement is measured by a linear

variable displacement transducer (LVD transducer). A commercially purchased Zircar®

fiber-insulated heater furnace with helically-wound Fe-Cr-Al alloy resistance wire

elements allows for temperatures up to 1100 °C. The main attributes of the VDR are that

it accommodates large (D < 7.5 cm; L < 10 cm) sample cores and covers temperatures,

10

load stresses and strain rates consistent with natural volcanic processes (see Fig. 3 in

Quane et al., 2004).

The original VDR was restricted to high-temperature experiments at ambient

atmospheric conditions. We have now built a steel, sealable cell (Fig. 2. lb) that allows

high-temperature experimentation at elevated fluid pressures. The fluid cell can be used

in the VDR after only minor modifications of the original assembly. The current sample

assembly comprises, from bottom to top, a lower cooling plate, a stainless steel holder for

a ceramic spacer, the sample cell, and a stainless steel spacer (Fig. 2.1 a). The upper steel

spacer has a machined lip that aligns the water cell to the central axis of the rig.

2.2.1 Fluid cell

All parts of the fluid cell are machined out of a corrosion-resistant, high-

temperature stainless steel (grade 310) suited for experimentation involving fluids. The

cell can operate at temperatures of 25-1100°C and fluid pressures of 0-150 MPa. The

sample chamber is a 25 cm tall cylinder with a wall thickness of 1 cm and outside

diameter of 5 cm. It can accommodate sample cores up to 3 cm in diameter and 10 cm in

length. An internal piston is connected to the VDR by a 31 cm long piston shaft that

slides out of a sealed opening at the top of the fluid cell (Fig. 2.1). The VDR controls

displacement of the piston and, thus, deformation of sample.

The cell is sealed metal-on-metal at either end. The lower and upper metal seals

are fastened to the sample chamber by socket head cap screws. The top seal has a long,

narrow neck used to align the piston shaft. A high-temperature Viton® 0-ring, cooled by

a water-cooling jacket, provides a tight seal on the piston shaft. Dow Corning® high

11

piston

4—thermocouple

o-ring—— :1cIamp—

sample

Figure 2.1 (A) Volcanology-Deformation-Rig (VDR) modified from Quane et al. (2004)

for experiments using the fluid cell. (B) Cross-section of fluid cell and sample arrange

ment. Detailed line diagrams and parts list for the water cell can be found at

http://www.eos .ubc.calresearchlinfrastructure/cesl.html

0

thermocouple

valve and/ transducer

I complex

—to H20 system

loadtransducer

fiberinsulation —

ceramic __1spacer

coolingplate

to temperaturecontroller

displacement

to computer

valve andtransducercomplex

invalve

sample chamber _—

12

temperature lubricant is applied to the 0-ring to minimize friction on the piston shaft

during experiments. Normally, the piston-shaft slides into the cell under its own weight

and, for dry experiments, all applied load is used to deform the sample.

The upper part of the piston shaft is threaded so that the top of the piston can be

removed without taking apart the top of the cell. The outer diameter of the lower piston

head is slightly less (<1 mm) than the inner diameter of the cell which ensures that: (i)

there are no frictional effects between piston head and cell wall during an experiment,

and that (ii) the piston head does not create an impermeable barrier to the fluid phase;

fluid pressure is hydrostatic and equal on either side of the piston. The piston shaft is

hollow and allows a thermocouple to be placed at the lower piston head (e.g., 2 mm

above top of sample).

The valve and transducer complex comprises a safety valve, a water pressure

transducer, an air valve, and a line for introducing distilled water via a 2-way manual

fluid pressure intensifier. The intensifier serves as a fluid delivery system that allows the

operator to control fluid pressure during the experiment by adding or removing fluid (Fig.

2.1). The system can be used to compensate for slow leaks or to allow for experiments

having a variable or cyclical fluid pressure (e.g., degassing or fluid pressurization events).

2.2.2 Temperature calibration

A factory-built fiber insulated tube furnace is used to heat the sample assembly

and fluid cell. The lower cooling plate and ceramic spacer have a hole drilled in their

centers to accept a type K thermocouple (Fig. 2.1) which controls temperature at the base

of the cell. The thermocouple inside the piston reads temperature at the top of the sample

13

and helps monitor vertical temperature gradients within the sample. The top and bottom

of the furnace are stuffed and wrapped in fiber insulation to minimize temperature

gradients.

Vertical temperature profiles in the sample were measured experimentally to find

a sample position that minimizes thermal gradients. The experiments used standard cores

(2.54 cm x 5 cm) of dacite lava that had vertical (0.5 cm in diameter) holes drilled down

their centre. A special piston that allows the thermocouple to slide down the shaft, out

the piston head, and into the sample core was used for measuring the temperature

profiles. Steady-state temperatures were achieved (1 hour dwell time) and temperatures

were measured at 12.5 mm increments from the base of the sample to the piston/sample

interface. On the basis of these experiments, the minimum temperature gradient is

achieved by having the bottom of the sample positioned 65 mm above the base height of

the tube furnace (Fig. 2.2a). The maximum gradients are 4 °C over 4 cm and 8.5 °C over

5 cm with no signs of strain localization related to temperature gradients in our

experiments to date.

2.3 Calibration for viscosity

Calibration experiments were performed on solid glass cores (10 mm x 25 mm) of

NIST (NBS) standard reference material (SRM) 717a (borosilicate glass) under constant

load and dry conditions at temperatures (550-600 °C; see Table 2.1). The temperature

gradient along the length of these cores is °C. The shear viscosity of the cores is

computed for a given applied load (F; N), sample volume (V; m3), sample length at time t

14

(L; m), and rate of shortening (ãLIãt; m s’) using the no-slip (Eq. 2.1) and perfect-slip

(Eq. 2.2) models of Gent (1960) (cf. Dingwell et al., 1993):

2irL5Fc9L

(2.1)3V—(2rL3+ V)

and ?‘,(Pa s)=

(2.2)

respectively. Based on the geometry of the run-product cores (i.e. little bulging) we

chose the perfect-slip end-member model to compare viscosity values from the

deformation experiments to the temperature-dependent viscosity curve for NIST 7 17a

glass (Fig. 2.2b). The shaded field on the curve indicates the is uncertainty on the

standard glass. The uncertainty on each experimental determination of viscosity (boxes)

includes variations in temperature during the experiment (Table 2.1). Our calibration

experiments reproduce the viscosity of the standard well and suggest an experimental

accuracy of 0.2 log units.

2.4 Volcanological experiments

2.4.1 Materials

High-temperature deformation experiments were performed on fabricated cores of

ash from the Rattlesnake Tuff: a high silica rhyolite (SiO2 >75%; Table 2.1; cf. Streck

and Grunder, 1995). The ash is sieved to a 0.6-2 mm size fraction (coarse ash) and cores

are sintered by heating loose ash in a mold (2.54 cm x 8 cm) at 900 °C for 20 minutes.

Samples are trimmed to —5 cm lengths creating cores with a 2:1 aspect ratio (Fig. 2.3a).

There is little change in composition after sintering (Table 2.1).

15

*C

ompo

siti

onof

Rat

tles

nake

ash

core

sas

Si02,

Ti02,

Al

203,Fe

O(T

),M

nO,

MgO

,C

aO,

(i)

Post

-sin

teri

ng:

77.6

4;0.

17;

12.4

8;1.

17;

0.07

;0.

00;

0.31

;3.

38;

4.62

;0.

01;

0.15

.(i

i)P

ost-

expe

rim

enta

l:77

.17;

0.16

;12

.81;

1.16

;0.

07;

0.00

;0.

30;

3.44

;4.

65;

0.00

;0.

24.

aVa l

ues

ofst

rain

calc

ulat

edfr

omm

achi

nedi

spla

cem

ent

(Am

),sh

orte

ning

ofco

re(A

l).bV

alue

sof

orig

inal

and

fina

lto

talp

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ity.

eVal

ues

ofsh

ear

visc

osity

asst

ress

over

stra

inra

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)an

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13

).

Tab

le2.

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mm

ary

ofca

libr

atio

nan

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form

atio

nex

peri

men

ts,

incl

udin

gco

nditi

ons,

prop

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es,

and

com

posi

tion*

of

sam

ples

.N

o.Se

t-U

pT

P(H

20)

Loa

dA

l/At

Stra

ina

Poro

sity

bV

isco

sity

(Pa

)C

(°C

)(M

Pa)

(N)

(rn

s’)

m1p

srt

fO2

Cel

l87

8±1

Dry

—1.

25.1

060.

300.

300.

730.

6410

6.61

0b03

—

rtfo4

Cel

l65

6±10

3.3-

1.7

—1.2

5106

0.30

0.34

0.72

0.63

108.

210b

01—

rtfO

5C

ell

645±

53

—2.5

0.1

06

0.30

0.35

0.73

0.69

i07°-i0

94

—

nist

Ol

VD

R56

2±13

Dry

48.4

—0.

160.

15—

——

1010

110b02

nist

03V

DR

571±

11D

ry48

.4—

0.25

0.24

——

—10

9810

99

nist

04C

ell

575±

2D

ry48

.4—

0.30

0.30

——

—10

9610

97

nist

05C

ell

568±

1D

ry48

.4—

0.24

0.23

——

—109.8

10b00

Na

20,K20,P205,

LOT:

Figure 2.2 Calibration results for VDR and fluid cell. (A) Calibration of thermalgradient across 4 cm (grey) and 5 cm (hatch) sample cores. (B) Results of experimentsplotted against known viscosity (upper inset) of NIST glass cores (lower inset). Mainfigure shows expected values of viscosity for NIST glass over experimental range oftemperatures (shaded grey). Viscosity values derived from: (i) dry experimentsperformed in the VDR (open rectangles), and (ii) experiments performed in the fluid cellunder dry conditions (closed rectangles). See Table 2.1 for experimental conditions andresults.

17

5

4

C-)

0.

5200

1

0560 600

T(°C)620 600 580 560 540

Cl)cr3Q10

0

0)0

12.5

1 0000/T(K)

Figure 2.2 See previous page for figure caption.

570 580 590

T(°C)

11

9

11.5 12

18

The sintering process causes point annealing of shards and forms a highly-porous,

floating, shard-supported framework (Fig. 2.3b). Cores of ash comprise curvilinear and

Y-shaped bubble wall shards, complete vesicles (e.g., bubble shards), smaller proportions

of pumiceous shards, and up to 1% crystals. There are two types of bubble shards (Fig.

2.3b): (i) a population of thick-walled vesicles characteristic of the original ash, and (ii) a

subordinate population of thinner-walled vesicles produced during the sintering process.

The latter population may have resulted from nucleation and growth of new bubbles or,

more likely, represent original closed bubbles (isolated porosity) that expanded during the

heating and sintering of the cores. Total porosity of sintered cores is slightly in excess of

70% (Table 2.1). The cores have an essentially isotropic texture (e.g., little to no fabric).

The shards do not appear deformed except around bubbles that grew during sintering,

where the shards appear to conform to the shape of the thin-walled bubbles.

2.4.2 Experiments

Three unjacketed experiments were run in the fluid cell system under constant

displacement rate (_.106 m s’) and to strains of -3O% (Table 2.1). The dry experiment

(rtf2) was at 878 °C; two experiments under -3 MPa PH2O (rtf4 and rtf5) were performed

at 656 °C and 645 °C, respectively. The corresponding experimental run-products are

shown in Figure 2.3c, d. Figure 2.4a shows the relationship between applied load stress

and total strain for experiments rtf2 and rtf5. The data have been filtered to compensate

for the fact that the high sampling rate captures the oscillations of the step motor that

drives piston displacement. Smoothing the data before processing gives a more accurate

record of the changing properties (e.g., rheology) of the system with increasing strain.

19

Figure 2.3 Pre- and post-experimental products (Table 2.1). (A) Sample core of sinteredash used in deformation experiments. (B) Scanning electron micrograph of thin-sectionof sintered core of Rattlesnake Tuff ash (e.g., starting material). (C) SEM backscatteredelectron micrographs of thin section of (C) run-product rtf2 and (D) run product rtf4(load stress had a vertical orientation in these images). Ash particles are light grey andpore space is dark grey to black. White boxes highlight two populations of bubbles:thick-walled and thin-walled (see text).

20

For the dry experiment, increased load is required to maintain a constant rate of

displacement. In order to achieve 30 % shortening of the core, load stress increases from

0.1 to 0.6 MPa. This increase in load stress, at constant displacement rate, is a direct

indication of the transient properties of the core during progressive deformation.

Increasing strain causes shortening of the core by porosity reduction (volume strain),

which has the concomitant effect of increasing the effective strength (e.g., viscosity) of

the material. This trend is as described by Quane et al. (2004) and Quane and Russell

(2005) in their deformation experiments on cores of glass beads. We have also calculated

the apparent viscosity of the core sample as a function of strain (Fig. 2.4b). Once steady

deformation is achieved, viscosity rises from Pa s during the first 1-2% of strain, to

i’° Pa s at 30% strain. The increase in viscosity during deformation is broadly

consistent with constitutive relationships established for the viscosity of hot porous

aggregates (e.g., Sura and Panda, 1990; Bagdassarov and Dingwell, 1992; Quane and

Russell, 2005). The deformation path of the dry core shows a steady increase in viscosity

of -1 order of magnitude for a porosity reduction of 10%.

Experiments run at fixed PH2O (3-3.2 MPa) use similar displacement rates,

achieve similar values of strain, but are performed at -200 °C lower temperatures. At the

same temperatures and strain rates, under dry conditions, experiments on Rattlesnake ash

cores produce brittle (rather than viscous) deformation. The load in the wet experiments

is corrected for the effects of FH2O by subtracting a constant value of load from the data

set; that value is recorded at time zero before any deformation has occurred. At PH2O < 3

IVIPa piston friction is negligible. At higher water pressures a correction is needed; future

21

0. 11

0.1 0.2 0.3

Figure 2.4 Results of two experiments on cores of Rattlesnake Tuff ash: (i) rtf2 (dry at

878 °C), and (ii) rtf5 (wet at 645 °C). (A) Stress evolution vs. strain for two experimentsand corresponding water pressure for rtf5 (see text). (B) Calculated effective viscosity

vs. total strain for data in (A). (C) Results compared to independent values of melt

viscosity (solid line) and Tg for melt (e.g., i 1012 Pa s). Effect of porosity is to reduce

viscosity of the melt. Open symbols show results of two experiments (see B); arrows

indicate direction of increased strain. (D) Melt viscosity (solid line) as shown in (C).

Dashed lines are calculated effects of dissolved water (0.1, 0.25, 0.5, and 1.0 wt.% H20;

Giordano et al. 2008) and grey squares show depression of Tg for hydrous melts. Experi

mental results from two deformation experiments are plotted as in (C) (see text). The star

represents the viscosity of the Rattlesnake melt at 645 °C having 0.73 wt.% water (see

text).

0.5

0.4

0.3Cl)ci)

0.2

0.1

/1/

P(H2O)forrtf5/ —

(dry)

-D

F’)

0

2D

10

Cl)

C90

0)8C

7

0 rtf2

0.1Strain

0.2Strain

V)

CDC

0

6 8 10 121 0000/T(K)

6 81 0000/T(K)

10 12

22

work includes calibrating this effect to allow for experimentation at higher >50 MPa

PH2O.

Experiment rtf5 shows substantially different behaviour than seen in the dry

experiment (Fig. 2.4a). The load stress curve for rtf5 is essentially flat with increasing

strain, implying that a single critical load is required to maintain a constant displacement

rate throughout the entire experiment (Fig. 2.4a). The load stress shows a maximum of

120 kPa at small amounts of strain (<10%), decreases to 80 kPa after 15-20% strain and

then increases slightly to 120 kPa at 30% strain. The calculated effective viscosity is also

nearly constant (109.2 to i0 Pa s) despite the core undergoing 30% strain via porosity

loss (Table 2.1). The presence of a fluid pressure not only lowers the material strength (<

100 kPa vs. 100 - 600 kPa), but also compensates for the expected increase in viscosity

due to lower temperature (645 °C vs. 878 °C) and delays the onset of “strain hardening”

of the sample as porosity is reduced.

2.4.3 Textural analysis of experiments

Samples rtf2 (dry; Fig. 2.3c) and rtf4 (wet; Fig. 2.3d) have undergone similar

strain (-30%) and have lost identical amounts of porosity (Table 2.1). Porosity in

deformed samples occurs as intraclast voids between annealed shards, as bubble voids

(thick- and thin-walled) and as smaller (<0.01 mm) isolated pores in vitric clasts (Fig.

2.3c, 2.3d). After 30% strain, the samples develop a pronounced planar fabric caused

by rotation and flattening of shards to create a foliation. The intensity of the foliation is

virtually identical in the two experiments (cf. Fig. 2.3b vs. 2.3c and 2.3d). Thick- and

thin-walled bubble shards Fig. 2.3b) exhibit quite different behaviours during

23

deformation. The thinner-walled bubbles show much higher degrees of flattening than do

the thick-walled bubbles; this disparate behaviour is independent of bubble size although,

in general, smaller bubbles are less deformed. Another form of strain localization occurs

in curvilinear and Y-shaped shards that are near flattened bubbles. These shards show a

stronger alignment and higher degree of deformation than do shards away from large

deformed bubbles.

In summary, the SEM images of run-products from the dry and wet experiments

provide no obvious means to differentiate between them. It is somewhat enigmatic that

very similar run-products were produced even though: (i) the dry experiment was

performed at -.2OO °C higher temperature; under dry conditions the 645 °C experiment

would not support viscous deformation, and (ii) the dry experiment showed a continuous

increase in load stress and viscosity as a function of progressive strain, whereas the wet

experiment underwent the same amount of strain and porosity reduction but showed little

to no strain hardening or increase in effective viscosity.

2.5 Discussion

Our experiments address the viscosity of highly vesicular (70%) melts (Fig. 2.4c,

2.4d). The highest viscosity achieved in the dry (rtf2) experiment after 30% strain is

i00’4 Pa s, which is close to the viscosity of the Rattlesnake melt at this temperature

(1O08 Pa s; Fig. 2.4c; Table 2.2 and see Appendix A). The results of the deformation

experiment performed at -3 MPa PH2O and 645 °C (rtf5; Fig. 2.4c) indicate apparent

viscosities of 109.2- i0 Pa s. The viscosity of the Rattlesnake Tuff melt (anhydrous)

24

Table 2.2 Measured values of viscosity for glass* from melted ash from Rattlesnake Tuffash and VFT coefficients (A, B, C).T(°C) Log Ti Expta

917.80 9.92 MP975.25 9.14 MP1421.67 4.60 CC1446.28 4.43 CC1470.89 4.26 CC1495.50 4.09 CC1520.11 3.93 CC1544.72 3.78 CC1569.33 3.62 CC1593.94 3.48 CC1618.55 3.34 CC

A B CVFT -7.43 19,766 52.9

* Composition of glass by EMP as Si02 (77.42), Ti02 (0.14), Al203 (12.22), FeO(T)(1.39), MnO (0.08), MgO (0.04), CaO (0.31), Na20 (3.44), K20 (4.96), P205 (0.01).a See Appendix 2.A.

25

extrapolated to this temperature would be iO’54 Pa s (Table 2.2; Appendix 2.A), which is

substantially higher than observed. Although the wet experiment experiences the same

strain as the dry experiment, its effective viscosity remains very much lower than the

viscosity of the corresponding dry melt.

Both porosity and dissolved water serve to reduce effective viscosity. Dissolved

water causes a strong decrease in viscosity and is most pronounced in melts having the

highest values of viscosity (e.g., low temperature). There are several ways in which fluid

pressure might operate to reduce the effective viscosity of these samples during

deformation. Firstly, elevated fluid pressure will cause hydration of the shards

comprising the cores. The cores are very porous, feature high surface area to volume

ratios, and were held above Tg for several hours (Figs. 2.4c, 2.4d). The calculated effects

of H20 on melt viscosity are shown by the dashed curves in Fig. 2.4d (Giordano et al., in

Press). These elements suggest that H20 may be dissolved into the glass shards causing

a reduction in the viscosity of the framework material and, thus, a reduction in the

apparent viscosity of the deforming core. At 3 MPa the maximum (equilibrium)

dissolved water content for the shards would be 0.73 wt% (VolatileCaic 1.1; Newman

and Lowenstern, 2002). This would reduce the viscosity of the melt at 645 °C from i0’

to iO”3 Pa s (Fig. 2.4d; star symbol), which remains substantially higher than the

observed apparent viscosity of rtf5 (1092- Pa s; Fig. 2.4d). This suggests that the

low apparent viscosity recorded by experiment rtf5 reflects the combined effects of an

elevated H20 pressure and a residual porosity.

Secondly, the presence of the fluid phase itself (rather than dissolved H20) may

also cause a reduction in effective viscosity. The presence of a fluid will create a pore

26

fluid pressure (Pfljd) that can lower the (dry) strength (ai.) of the sample such that the

effective strength is: 0eff = Gdry — Pflujd (Terzaghi, 1943). Furthermore, the high porosity

ensures that virtually all the interfaces between shards are wetted by H20 vapour which

may allow for development of hydroxylated monolayers (Schlegel et al., 2002). The

hydroxylated monolayers may serve as a lubricant to the glass shards allowing shards to

glide past each other without having to deform internally. This is analogous to rock

systems in which partial wetting of crystals by a melt phase facilitates grain boundary

sliding (de Kloe et a!., 2000).

Our experiments demonstrate the importance of porosity and the fluid phase

during high-temperature deformation processes. They show that the combined effects of

porosity and a fluid (H20) phase greatly expand the window for viscous deformation of

volcanic materials. The viscosity recorded by experiments under 3 MPa PH2O (e.g., rtf5)

is too low to be ascribed solely to the effects of residual porosity or to elevated dissolved

water contents.

2.6 Acknowledgements

This research is funded by the Natural Sciences and Engineering Research

Council (NSERC) via the Research Tools and Instruments program (JKR), the Discovery

Grants program (JKR), and the PGS fellowship program (GR) and by the Italian

Dipartimiento della Protezione Civile (2004-06 Agreement, Instituto Nazionale di

Geofisica e Vulcanologia — INGV). Chemical analyses of cores of Rattlesnake Tuff ash

were generously provided by Steve Quane. We thank Don Dingwell for lab privileges to

measure viscosity at the LMU, Munich, Germany. We also thank P. Ardia at ETH

27

Zurich for microprobe analysis of silicate glasses (e.g., fused samples of Rattlesnake

Tuff). The manuscript benefited from critical reviews by Luigi Burlini and Cliff Shaw.

Finally, we would like to especially thank UBC’s Earth & Ocean Sciences machinists

Ray Rodway and JOrn Unger.

2.7 Appendix 2.A: Melt viscosity of the Rattlesnake Tuff ash

The viscosity of melted Rattlesnake Tuff ash (Streck and Grunder, 1995) was

measured independently by concentric cylinder and micropenetration techniques at the

LM(J Munich. The Vogel-Tamman-Fulcher equation (cf. Richet and Bottinga, 1995) has

been fit to the data to model the temperature dependence of viscosity for the dry and non-

vesicular melt (Table 2.2). Concentric cylinder and micropenetration techniques measure

viscosity in the ranges (10’-10 Pa s) and (1081012 Pa s), respectively, and are calibrated

against NIST SRM 717a glass. Homogeneous melts were prepared by fusing samples in

a thin-walled Pt-crucible in a MoSi2 element furnace (1 atm and 1500 — 1650 °C). The

original glass shards contained minor water, which caused vesiculation during fusion.

The sample was kept in the melting furnace for more than 1 week until all bubbles had

escaped. The sample was then transferred to the concentric cylinder viscometer furnace

and a stirring spindle was used to stir the melt. The spindle was periodically lifted out of

the melt to determine when the melt was free of crystals and bubbles. Concentric

cylinder measurements were performed once the melt was devoid of crystals and bubbles.

The crucible was removed from the furnace and allowed to cool in air to quench the

sample to a glass. The composition of the glass was determined by electron microprobe

analysis using the JEOL JXA 8200 device at ETH-Zentrum, Zurich (Table 2.2). The

28

sample was then cored to produce 3 mm thick, doubly polished disks for low-temperature

measurements of viscosity using micropenetration techniques (Giordano et al., 2005).

Measurements were performed under Argon atmosphere using a modified Bähr 802 V

vertical push-rod dilatometer (Dingwell et al., 1993; Giordano et al., 2004; 2005), and the

samples were held at temperature for 1 hour to achieve structural relaxation before each

measurement. Shear viscosity (ii) was calculated as described by Pocklington (1940) and

Toboisky and Taylor (1963).

29

2.8 References

Bagdassarov, N.Sh., Dingwell, D.B., 1992. A rheological investigation of vesicular

rhyolite. Journal of Volcanology and Geothermal Research 50, 307-322.

Bierwirth, P.N., 1982. Experimental welding of volcanic ash. B.Sc. Honours Thesis,

Monash University, 74p.

De Kloe, R., Drury, M.R., van Roermund, H.L.M., 2000. Evidence for stable grain

boundary melt films in experimentally deformed olivine-orthopyroxene rocks.

Physics and Chemistry of Minerals 27, 480-494.

Dingwell, D.B., 1998. Recent experimental progress in the physical description of silicic

magma relevant to explosive volcanism. In: Gilbert, J.S. and Sparks, R.S.J. (eds.)

The Physics of Explosive Volcanic Eruptions, Geological Society, London,

Special Publications 145, 9-26

Dingwell, D.B., Bagdassarov, N.S., Bussod, G.Y., Webb, S.L., 1993. Magma rheology.

In: Luth, R.W. (ed) Experiments at high pressure and applications to the Earth’s

mantle, Mineralogical Association of Canada, Short Course Handbook 21, 131-

196

Friedman, I., Long, W., Smith, R.L., 1963. Viscosity and water content of rhyolite glass.

Journal of Geophysical Research 68, 6523-6535.

Gardner, J.E., Hilton, M., Carroll, M.R., 1999. Experimental constraints on degassing

magma; isothermal bubble growth during continuous decompression from high

pressure. Earth and Planetary Science Letters 168, 201-218.

Gent, A.N., 1960. Theory of the parallel-plate viscometer. British Journal of Applied

Physics 11, 85-87.

30

Giordano, D., Romano, C., Papale, P., Dingwell, D.B., 2004. The viscosity of trachytes,

and comparison with basalts, phonolites, and rhyolites. Chemical Geology 213,

49-61.

Giordano, D., Nichols, A.R.L., Dingwell, D. B., 2005. Glass transition temperatures of

natural hydrous melts: a relationship with shear viscosity and implications for the

welding process. Journal of Volcanology and Geothermal Research 142, 105-118.

Giordano, D., Russell, J.K., Dingwell, D.B., In Press. Viscosity of magmatic liquids: A

model. Earth and Planetary Science Letters.

Grunder, A., Russell, J.K., 2005. Welding processes in volcanology: insights from field,

experimental, and modeling studies. Journal of Volcanology and Geothermal

Research 142, 1-9.

Grunder, A.L., Laporte, D., Druitt, T. H., 2005. Experimental and textural investigation

of welding: effects of compaction, sintering, and vapor-phase crystallization in the

rhyolitic Rattlesnake Tuff. Journal of Volcanology and Geothermal Research 142,

89-104.

Kennedy, B., Spieler, 0., Scheu, B., Kueppers, U., Taddeucci, J., Dingwell, D.B., 2005.

Conduit implosion during Vulcanian eruptions. Geology 33, 58 1-584.

Lejeune, A.M., Bottinga, Y., Trull, T., Richet, P., 1999. Rheology of bubble-bearing

magmas. Earth and Planetary Science Letters 166, 71-84.

Newman, S. and Lowenstern, B., 2002. VolatileCaic: a silicate melt-H20-C02solution

model written in Visual Basic for Excel. Computers and Geosciences 28, 597-

604.

31

Pocklington, H.C., 1940. Rough measurement of high viscosities. Proceedings of the

Cambridge Philosophical Society 36, 507—508.

Quane, S.L., Russell, J.K., 2005. Welding: insights from high-temperature analogue

experiments. Journal of Volcanology and Geothermal Research 142, 67-87.

Quane, S.L., Russell, J.K., Kennedy, L.A., 2004. A low-load, high-temperature

deformation apparatus for volcanological studies. American Mineralogist 89, 873-

877.

Richet, P., Bottinga, Y., 1995. Rheology and configurational entropy of silicate melts. In

Mineralogical Society of America, Reviews in Mineralogy 32, 67-93.

Schlegel, M.L., Nagy, K.L., Fenter, P., Sturchio, N.C., 2002. Structures of quartz (1010)-

and (1011)-water interfaces determined by X-ray reflectivity and atomic force

microscopy of natural growth surfaces. Geochimica et Cosmochimica Acta 66,

3037-3054.

Sparks, R.S.J., Tait, S.R., Yanev, Y., 1999. Dense welding caused by volatile resorption.

Journal of the Geological Society, London 156, 2 17-225.

Spieler, 0., Kennedy, B., Kueppers, U., Dingwell, D.B., Scheu, B., Taddeucci, J., 2004.

The fragmentation threshold of pyroclastic rocks. Earth and Planetary Science

Letters 226, 139-148.

Stein, D.J., Spera, F.J., 1992. Rheology and microstructure of magmatic emulsions;

theory and experiments. Journal of Volcanology and Geothermal Research 49,

157-174.

32

Streck, M.J., Grunder, A.L., 1995. Crystallization and welding variations in a widespread

ignimbrite sheet; the Rattlesnake Tuff, eastern Oregon, USA. Bulletin of

Volcanology 57, 151-169.

Sura, V.M., Panda, P.C., 1990. Viscosity of porous glasses. Journal of the American

Ceramic Society 73, 2697-2701.

Terzaghi, K., 1943. Theoretical Soil Mechanics. John Wiley and Sons, New York, NY.,

510 pages.

Tinker, D., Lesher, C.E., Baxter, G. M., Uchida, T., Wang, Y., 2004. High-pressure

viscometry of polymerized silicate melts and limitations of the Eyring equation.

American Mineralogist 89, 1701-1708.

Tobolsky, A.V., Taylor, R.B., 1963. Viscoelastic properties of a simple organic glass.

Journal of Physical Chemistry 67, 2439—2442

Tuffen, H., Dingwell, D.B., Pinkerton, H., 2003. Repeated fracture and healing of silicic

magma generate flow banding and earthquakes? Geology 31, 1089-1092.

33

CHAPTER III: Deformation experiments’

3.1 Introduction

Many volcanic processes involve the production and growth of gas-filled bubbles,

the connection of bubbles to produce permeability, and the subsequent collapse of the

bubbles. These cycles of bubble growth and collapse are important elements in processes

as diverse as magma ascent, transition from explosive to effusive volcanic eruption,

fragmentation processes in volcanic conduits, dome growth and collapse, and the

inflation, collapse, and welding of pyroclastic density currents.

Despite its obvious importance for understanding and modelling volcanic

processes, our knowledge of the rheological properties of porous magmas is incomplete.

Compaction and sintering of particulate materials in the ceramics industry has provided

insights on the effects of porosity on the viscosity of composite materials (Fig. 3.1;

Rahaman et al., 1987; Ducamp and Raj, 1989; Sura and Panda, 1990) and, more

importantly, has inspired experimentation on materials pertinent to volcanology (Fig.

3.1). For example, there are a now a number of high-temperature experimental studies on

synthetic melt systems that elucidate the rheological behaviour of porous melts (i.e., Stein

and Spera, 1992; Lejeune et al., 1999; Quane et al., 2004). There also are a smaller

number of parallel experimental studies on natural volcanic materials (i.e. Friedman et

al., 1963; Bierwirth, 1982; Bagdassarov and Dingwell, 1992; Quane, 2004; Quane and

Russell, 2005) and these studies report a wide range of rheological behaviours (see Quane

and Russell, 2005; Grunder and Russell 2005 for reviews).

‘A version of this chapter has been submitted for publication. Robert, G., Russell, J.K.,Giordano, D., In Review. Rheology of porous volcanic materials: High-temperatureexperimentation under controlled water pressure. Chemical Geology Special Issue, 8th

Silicate Melt Workshop.

34

1

Figure 3.1 Compilation of previous experimental studies on deformation of porous meltsor glasses as the relative viscosity (ii,.), taken as the ratio of apparent viscosity of theporous system (lapp) and the viscosity of the melt (1ieit)’ vs. total porosity of the system(I). Studies are grouped as deformation of (i) porous ceramic glasses or glass powders(dashed curves; (R): Rahaman et al., 1987; (D&R): Ducamp and Raj, 1989; (S&P): Suraand Panda, 1990); (ii) bubbly or porous synthetic melts (solid curves; (S&S): Stein andSpera, 1992 and (Q&R): Quane et al., 2004; and solid circles: Lejeune et al., 1999); and(iii) porous natural melts (bold, solid curves; (B&D): Bagdassarov and Dingwell, 1992;(Q): Quane et a!., 2005).

0.

35

Figure 3.1 summarizes results from some of these experimental studies, including

experiments on natural and synthetic melts. These experiments comprise two end-

member approaches: (i) deformation experiments on porous samples in which bubbles

are suspended in a coherent melt/glass phase (Bagdassarov and Dingwell, 1992; Stein

and Spera, 1992; Lejeune et al., 1999), or (ii) experiments deforming porous samples in

which the pores exist between the particles that constitute the solid framework, including

sintered ceramic particles, glass beads, or volcanic ash (Friedman et al., 1963; Bierwirth,

1982; Rahaman et al., 1987; Ducamp and Raj, 1989; Sura and Panda, 1990; Quane,

2004). In the latter case the particles, themselves, may or may not be porous.

Ultimately all hydrous melts vesiculate at or near the Earth’s surface to produce

bubble-rich melts, which commonly continue to expand to the point of fragmentation.

How the increase in porosity affects the viscosity of the magma remains unclear. Here,

we use high-temperature (T) uniaxial compression experiments on cores of volcanic

material to elucidate the rheological behaviour of high porosity magmas. Our program

uses an apparatus that allows for deformation (i.e. compaction) experiments on porous

cores of sintered volcanic ash at high-T and under controlled water pressure (PH2O).

These experiments cause a reduction in the porosity of the ash-core samples and a

concomitant change in their rheological properties. Most strain can be ascribed to

volume loss by pore destruction (volume strain); however, we also show that radial

expansion of the sample cores (shear strain) becomes increasingly important at high

values of strain. Our results also demonstrate that, at the timescales of these experiments,

the window of viscous deformation is expanded substantially by increasing porosity.

Under dry conditions, the temperature limits of viscous deformation for highly porous

36

cores of ash are reduced by --50 °C. Experiments under PH2O of 2.5 MPa also show that

increasing porosity expands the window of viscous deformation by --140-150 °C,

depending on the water content of the melt. These results have implications for the

processes governing the welding of ignimbrites (Sparks et a!., 1999), fragmentation

cycles in volcanic conduits (Tuffen et al., 2003; Kennedy et al., 2005) and the formation

and flow of clastogenic lavas (Manley, 1996; Wolff and Sumner, 2000).

3.2 Experimental methods

3.2.1 Experimental apparatus

All experiments presented are performed using the Volcanology-Deformation-Rig

(VDR) at the University of British Columbia in conjunction with a water cell specifically

designed for high-temperature experimentation at volcanic conditions in the presence of

water (Quane et al., 2004; Robert et al., 2008). The cell can operate at temperatures up to

1100 °C and fluid pressures of 0-150 IVIPa; sample sizes can be up to 30 mm in diameter

and 100 mm in length. Deformation experiments on sample cores can be performed

under a constant load (<1135 kg) or at constant displacement rate (5 10 to 2.5 102

cmls). The VDR’ s computer system records time, load, displacement; the water cell is

equipped with a transducer that records water pressure continuously.

A detailed description of the experimental apparatus and its calibration for

recovering melt viscosity can be found in Robert et a!. (2008). The original design for

the bottom of the cell has been modified slightly to prevent leaks. Specifically, the

bottom seal remains metal-on-metal, but we have developed a more efficient tightening

mechanism that provides an even pressure distribution around the entire lower seal.

37

Detailed line diagrams and a list of parts for the water cell can be found at

http://www.eos.ubc .calresearchlinfrastructure/cesl.html.

3.2.2 Fabrication of experimental cores

The deformation experiments are performed on cores created by sintering

volcanic ash collected from the Rattlesnake Tuff (Streck and Grunder, 1995). The ash is

sieved to a 0.6-2 mm size fraction and cores are sintered by heating the loose ash in a

mold (2.54 cm by 8 cm) at 900 °C for 20 minutes. Samples are trimmed to -5 cm lengths

creating cores with a 2:1 aspect ratio (Fig. 3.2a). Table 3.1 reports chemical composition

data of the Rattlesnake Tuff ash for: (i) natural (non-sieved) ash; (ii) sintered ash, pre

and post-experiment; and (iii) fused ash (glass). These measurements show that there is

little change in composition after sintering (Table 3.1).

Cores of ash comprise curvilinear and Y-shaped bubble wall shards, complete

vesicles (e.g., bubble shards), smaller proportions of pumiceous shards, and up to 1%

crystals. The sintering process causes point annealing of shards and forms a highly

porous, floating, shard-supported framework (Fig. 3.2b). The cores produced by the

sintering technique have an essentially isotropic texture and show no foliation or

preferred orientation of shards (Fig. 3 .2a, b). The sintered cores feature two types of

bubble shards (Fig. 3.2): (i) a population of thick-walled vesicle shards characteristic of

the original ash, and (ii) a subordinate population of thinner-walled vesicle shards. The

latter population appears to be produced during the sintering process and represent

originally closed vesicles (isolated porosity) that expanded during heating or new bubbles

that nucleated and grew during fabrication. The ash particles are not deformed during

38

Figure 3.2 Starting experimental materials. (A) Photograph of fabricated core (2.5 cmby 5 cm) of Rattlesnake Tuff ash used in high-T deformation experiments. (B) SEMphotomicrograph of pre-experiment core of sintered ash showing proportions of ash (lightgrey) to pore space (black) and the diversity of ash particles, including: bubble walls,glass shards, and pumiceous fragments. Large, round, thin-walled bubble shards arelikely a product of vesiculation of hydrous shards during the sintering process. Smaller,thick-walled bubble shards are a direct product of the original fragmentation.

39

Tab

le3.

1.C

hem

ical

com

posi

tion

ofth

eR

attl

esna

keT

uff

ash.

Oxi

deL

itera

ture

aS

Q0000b

SQ

00

01

cG

RR

S3O

dS

Q0821be

RS

_mel

tSi

eved

Sint

ered

Sint

ered

Post

-exp

’tX

RF

EM

P

Si02

77.1

173

.99

76.7

976

.12

76.1

176

.27

76.2

9

Ti02

0.12

0.15

0.17

0.14

0.16

0.20

0.14

A1

20311

.77

12.1

212

.34

13.1

612

.63

12.8

612

.04

FeO

(T)

1.45

1.11

1.16

1.00

1.14

1.36

1.37

MnO

0.09

0.07

0.07

0.08

0.07

0.08

0.08

MgO

0.00

0.04

0.00

0.07

0.00

0.06

0.04

CaO

0.35

0.29

0.31

0.29

0.30

0.31

0.30

Na

203.

703.

263.

343.

833.

393.

603.

39

K2O5.

234.

424.

574.

594.

594.

744.

89

P2050.

010.

020.

010.

010.

000.

010.

01H

20+

0.00

3.30

0.15

0.14

0.24

0.07

0.00

Tot

al99

.83

98.7

798

.91

98.4

398

.63

99.5

698

.54

aA

naly

sis

from

Str

eck

and

Gru

nder

(199

7).

bN

atur

alR

attl

esna

keT

uff

ash

siev

edto

coar

seas

hfr

omQ

uane

(200

4).

CR

attle

snak

eT

uff

ash

afte

rsi

nter

ing

from

Qua

ne(2

004)

.d

Rat

tles

nake

Tuf

fas

haf

ter

sint

enng

(thi

sst

udy)

.eR

attle

snak

eT

uff

ash

from

post

-exp

erim

enta

lco

re(D

RY

)fr

omQ

uane

(200

4).

XR

Fan

alys

isof

Rat

tles

nake

Tuf

fm

elt

used

inco

ncen

tric

-cyl

inde

rm

easu

rem

ents

(cf.

Rob

ert e

tal

.,20

08).

gE

MP

anal

ysis

ofR

attle

snak

eT

uff

mel

tus

edin

conc

entr

ic-c

ylin

der

mea

sure

men

ts(c

f.R

ober

t et

al.,

2008

).

sintering except around bubbles that formed or expanded during heating; there, shards are

bent around thin-walled bubbles (Fig. 3.2b). One unexpected result of the sintering

process is that the shards become extensively fractured and pitted (Fig. 3.2b); the

microfracturing may result from rapid cooling when the samples are removed from the

sintering oven.

3.2.3 Pre-experimental sample characterization

The physical properties of each sample are measured prior to running the

deformation experiments, including: geometry, density and porosity. The volume of

these highly porous cylindrical sample cores is calculated from averages of replicate

(n=10) measurements of diameter and length (Table 3.2). This volume and the sample

mass are used to compute the bulk density (Pbulk) of the core (Table 3.3: 0.37-0.43 g/cm3).

Skeletal (or framework) density (Psiceietai) is obtained by measuring sample volume via

helium pycnometry and ranges from 0.97-1.40 g/ cm3 (Table 3.3). Connected porosity

QIconnected) is calculated from skeletal and bulk density from the relationship:

connected = 1 — (3.1)Pskeletal

We obtained values of dense rock equivalent (DRE) density for sintered materials by

crushing three sintered cores and performing pycnometry on the resulting powders.

These cores for DRE measurements were also fabricated in the way described above; we

assume all experimental cores to have the same average DRE density (2.36 g/cm3).

Using this average value for powder density we compute total and isolated porosity as:

otal1Pbulk (3.2)

Ppowder

41

Tab

le3.

2.E

xper

imen

tal

cond

ition

s”us

edin

defo

rmat

ion

expe

rim

ents

and

geom

etry

bof

sam

ple

core

spr

e-an

dpo

st-e

xper

imen

t.

2.5.

106

5.26

82.

430

1.26

01.

365

2.5.

106

5.21

01.

100

1.26

71.

417

2.5.

106

5.16

52.

035

1.27

81.

377

2.5

1O

5.06

93.

609

1.27

21.

330

-5.

137

4.95

01.

278

1.27

7

2.5.

106

5.15

23.

703

1.27

11.

284

2.5

10

5.19

62.

381

1.27

21.

314

2.5.

106

5.20

03.

727

1.27

91.

301

2.5.

106

5.04

03.

578

1.28

51.

324

2.5.

106

4.42

01.

942

1.27

71.

368

2.5.

106

5.27

71.

125

1.27

81.

446

2.5

10

4.89

1-

1.29

4-

2.5.

1116

5.20

31.

155

1.28

01.

391

2.5.

106

5.26

9-

1.28

2-

2.5.

106

5.23

2-

1.28

1-

2.51

116

5.20

83.

691

1.28

51.

276

2.51

116

5.29

52.

534

1.24

11.

298

2.51

116

5.31

72.

354

1.30

61.

391

2.5

41

16

5.32

12.

612

1.32

11.

341

2.51

116

5.41

7-

1.28

2-

2.51

0-s

5.40

72.

650

1.29

2-

2.5

111

5.24

30.

817

2.59

23.

142

___V

0

V

11.3

811

.35

26.2

714

.22

9.69

9.65

26.2

86.

94

11.3

111

.24

26.4

912

.13

10.8

810

.84

25.7

620

.05

10.9

310

.71

26.3

625

.36

10.4

510

.45

26.1

519

.18

10.5

610

.58

26.4

312

.91

10.8

310

.86

26.7

019

.82

10.3

110

.09

26.1

319

.71

8.84

8.68

22.6

511

.41

10.4

610

.27

27.0

67.

39

10.1

78.

5025

.74

-

10.5

210

.51

26.7

77.

02

11.0

98.

2527

.22

-

11.1

3-

26.9

6-

11.1

411

.11

27.0

218

.88

11.7

811

.76

25.6

313

.41

11.4

711

.03

28.4

814

.32

11.6

111

.59

29.1

614

.75

12.0

711

.26

27.9

5-

12.0

210

.08

28.3

7-

11.2

811

.22

110.

6725

.34

No

Set

-Up

tT

P(H

20)

zM/A

tl

1r0

rfm0

mf

LJ

RSO

3C

ell

1072

864

0±4

1

RSO

4C

ell

1576

868

1±20

5

RSO

5C

ell

1228

065

9±9

1

RSO

7C

ell

5226

654±

41

RSO

9C

ell

-.54

00[2

5-65

0]—

2.5

RS1

OC

ell

5223

647±

75

RS1

1C

ell

1021

265

9±13

5

RS

12C

ell

5067

662±

83

RS

13C

ell

5121

654±

62.

5

RS

14C

ell

8860

650±

92.

5

RS

15C

ell

1572

366

6±18

2.5

RS

16V

DR

9650

800±

150

RS

17V

DR

1564

290

0±15

0

R51

8C

ell

1059

045

2±12

2.5

RS

19C

ell

1051

238

6±16

2.5

RS2

OV

DR

5208

900±

150

RS2

1V

DR

1060

890

0±15

0

RS

22C

ell

1071

055

0±14

2.5

RS

23V

DR

1066

885

0±15

0

RS

24V

DR

1088

475

0±15

0

RS

25V

DR

1083

850±

150

RS

29V

DR

1745

790

0±15

0a

Tim

e(t

)in

s;te

mpe

ratu

re(T

)in

°C;

wat

erpr

essu

re(P

H,o

)in

MPa

;di

spla

cem

ent

rate

(AIJ

At)

inm

is.

bD

imen

sion

sof

core

s:(1

:le

ngth

(cm

);r:

radi

us(c

m);

V:

volu

me

(cm3))

befo

re(i

.e.,

l)an

daf

ter

(i.e

.l)

expe

rim

enta

lru

ns.

Tab

le3.

3.M

easu

red

valu

esof

dens

ityan

dpo

rosi

tyfo

rpr

e-an

dpo

st-e

xper

imen

tsa

mpl

eco

res.

Den

sity

aP

oros

ityb

1T

bulk

bulk

skel

etal

skel

etal

,to

tal

tota

lco

nnec

ted

,co

nnec

ted

Tis

olat

ed,j

isol

ated

10

PoPf

PoPf

‘‘f

‘‘0

‘t’f

‘‘o

‘‘f

RSO

30.

433

0.79

41.

292

2.11

30.

816

0.66

40.

665

0.64

10.

152

0.02

3R

SO4

0.36

91.

391

1.16

42.

177

0.84

40.

411

0.68

30.

361

0.16

10.

049

RSO

50.

428

0.92

81.

109

2.18

30.

819

0.60

70.

614

0.57

50.

205

0.03

2R

SO7

0.42

20.

542

1.14

31.

753

0.82

10.

770

0.63

00.

691

0.19

10.

079

RSO

90.

415

0.42

21.

119

1.97

60.

824

0.82

00.

629

0.78

50.

195

0.03

5R

S1O

0.40

00.

545

1.10

01.

795

0.83

00.

769

0.63

60.

696

0.19

40.

073

RS1

10.

400

0.82

01.

016

1.87

80.

831

0.65

30.

607

0.56

30.

224

0.08

9R

S12

0.40

60.

548

1.12

21.

890

0.82

80.

768

0.63

90.

710

0.19

00.

058

RS

130.

395

0.51

21.

116

1.77

50.

833

0.78

30.

646

0.71

20.

187

0.07

1R

S14

0.39

00.

759

1.15

81.

991

0.83

50.

678

0.66

30.

619

0.17

20.

060

RS

150.

387

1.39

11.

074

2.20

50.

836

0.41

10.

640

0.36

90.

196

0.04

1R

S16

0.39

6-

1.12

8-

0.83

2-

0.64

9-

0.18

3-

RS

170.

393

1.49

71.

042

2.24

50.

833

0.36

60.

623

0.33

40.

211

0.03

2R

S18

0.40

8-

1.07

4-

0.82

7-

0.62

0-

0.20

7-

RS

190.

414

-1.

105

-0.

825

-0.

626

-0.

199

-

RS2

O0.

412

0.58

81.

120

1.76

90.

825

0.75

10.

632

0.66

70.

193

0.08

3R

S21

0.46

10.

877

1.39

62.

045

0.80

50.

628

0.67

00.

571

0.13

50.

057

RS

220.

403

0.77

01.

103

2.26

50.

829

0.67

40.

635

0.66

00.

195

0.01

4R

S23

0.39

70.

786

1.05

21.

812

0.83

20.

667

0.62

20.

566

0.20

90.

101

RS

240.

432

-1.

105

-0.

817

-0.

609

-0.

208

-

RS

250.

424

-1.

012

1.85

00.

820

-0.

581

-0.

239

-

RS

290.

410

1.77

10.

974

2.26

60.

826

0.24

90.

579

0.21

80.

247

0.03

1a

Bul

kan

dsk

elet

alde

nsity

ofpr

e-(P

o)an

dpo

st-

(o

f)

expe

rim

enta

lco

res

ing/

cm3

Tot

al,

conn

ecte

dan

dis

olat

edpo

rosi

tyof

pre-()

and

post

-()

expe

rim

enta

lco

res.

Pbulk — Pbulkisolated —

Pslcele:at Ppowder

(cf Michol et al., 2008). The total porosity of sintered cores varies from -0.80-0.84 and

comprises both connected (-0.58-0.67) and isolated (-0.14-0.25) fractions (Table 3.3).

3.3 Experimental results

3.3.1 Overview

A total of 21 deformation experiments were performed on sintered cores of

Rattlesnake Tuff ash, including eight at atmospheric pressure (dry) and 13 at controlled

water pressure (wet). The experimental conditions for the 21 experiments are

summarized in Table 3.2. The same constant displacement rate (2.5 i03 mmls) was used

in all deformation experiments except for sample RS25, which was deformed 1 order of

magnitude faster than the others (Table 3.2). The experiments run under dry conditions

are used to illustrate the effects of porosity on melt rheology and establish a baseline

response against which we compare results from wet experiments. The PH2O experiments

provide a closer approximation to nature in that they involve deformation of porous cores

at temperatures and fluid pressures commonly found in volcanic environments.

3.3.2 Dry high-T experiments

Four experiments were performed at atmospheric pressure conditions (dry) and at

a temperature of 900°C (Table 3.2). The four dry experiments RS2O, RS21, RS 17 and

RS29 were deformed to strains () of 0.25, 0.5, 0.75, and 0.82 respectively (Fig. 3.3a;

44

Table 3.4. Analysis of strain.Strain

a bEt St

c d

NoRSO3 0.500 0.539 0.454 0.148RSO4 0.750 0.789 0.735 0.201RSO5 0.585 0.606 0.539 0.139RSO7 0.250 0.288 0.221 0.085RSO9 0.000 0.036 0.024 -0.002RS1O 0.250 0.28 1 0.266 0.020RS11 0.500 0.542 0.513 0.062RS12 0.250 0.283 0.260 0.034RS13 0.250 0.290 0.229 0.059RS14 0.500 0.561 0.486 0.128RS15 0.750 0.787 0.722 0.219RS16 0.500 - - -

RS17 0.750 0.778 0.737 0.153RS18 0.500 - - -

RS19 0.500 - - -

RS2O 0.250 0.291 0.300 -0.014RS2I 0.500 0.521 0.474 0.085RS22 0.500 0.557 0.477 0.119RS23 0.500 0.509 0.494 0.029RS24 0.500 - - -

RS25 0.500 0.5 10 - -

RS29 0.822 0.844 0.769 0.319a Total strain from machine displacement.b Total strain from shortening of the core.C Total strain from porosity reduction.d Total strain from radial increase.

45

Tables 3.2-3.4). All four experiments show a smooth increase in load stress with

increasing strain to 0.5 followed by a much steeper increase in load stress with

additional strain. The resulting stress-strain relationships for each experiment are nearly

identical as shown by the overlapping curves in Figure 3.3a because the starting materials

were virtually identical (i.e., diameter, porosity, etc.; Table 3.3). These results indicate

the high degree of reproducibility of our experimental methods.

3.3.3 Wet high-T experiments

A total of 13 experiments were completed at elevated water pressures of: 1, 2.5

and 5 MPa (Table 3.2). All three different PH2O series were all performed at —650°C.

Experiments performed at elevated water pressure but temperatures < 650 °C are reported

in Tables 3.2-3.4, and are discussed in a later section.

The three experiments RS7, RS3, and RS5 were run at —650 °C and PH2O 1 MPa

(Table 3.2; Fig. 3.3b) and samples were deformed to Et of 0.25, 0.5 and 0.6, respectively

(Table 3.4). Overall the stress-strain relationships are similar to those observed in the

dry, high-temperature experiments. Load stress shows a smooth and continuous rise with

increasing total strain. The stresses achieved during these experiments are about an order

of magnitude lower than observed in the dry high-T experiments. However, significant

rises in stress (—20%) are achieved at lower amounts of strain (25-40%) than was

observed in the dry experiments (>60%). At this strain rate, the 1 MPa PH2O experiments

show an exponential rise in stress at —0.55.

Three experiments were conducted at —650 °C and 2.5 IVIPa (Table 3.2; Fig. 3.3c).

Samples RS13, RS14 and RS15 were deformed to = 0.25, 0.5 and 0.75, respectively

46

Figure 3.3 Summary of experimental data (cf. Table 3.2; Table 3.4) plotted as stress (a)vs. strain (8). (A) Data recorded from high-temperature (—900°C) dry series of experiments. Lower temperature (—650 °C) experiments performed under water pressures of:(B) H2O = 1 MPa, (C) H2O = 2.5 MPa, and (D) P1120 = 5 MPa. Controlled P1120 experiments (i.e. B, C, D) were terminated after —25%, 50% and 75% total strain except forsample RSO5 (B) which recorded —60% strain (Table 3.2).

Atm (Dry) at 900°C

R

A0.5

S29

. 0.25

RS211

j7

RS17

RS2O

0 0.25 0.5Et

RS2O RS21

a

30

C’,a.. 20

10

Co

C’,a

0.25 0.58

0.75

C’,0

0.58

0.58

47

(Table 3.4). The increase in water pressure from 1 MPa to 2.5 MPa allows for

continuous strain at substantially lower load stresses (<2 MPa). Moreover, there is little

to no significant rise in stress over the interval 0 to 0.3. At Et > 0.5 the load stress

required to sustain deformation increases but remains low (— 1.5 IVIPa at r = 0.75) relative

to the dry and PH2O 1 MPa experiments.

Three deformation experiments were run at —645-680 °C and a water pressure of

5 MPa (Table 3.2; Fig. 3.3d). Samples RS1O, RS11, and RS4 were deformed to Et = 0.25,

0.5 and 0.75 respectively. Experiment RS 10 ( = 0.25) shows no increase in stress over

the total deformation path, and the stress required for deformation is near the resolution

level of the apparatus. The intermediate strain experiment (RS1 1; E = 0.5) shows a saw

tooth pattern recorded during experiment that is due to sharp fluctuations in water

pressure around an average value of 5 MPa. Overall, the deformation path shows a

slight, relatively linear increase in stress from -—0.4 to 0.8 IVIPa. To a first approximation,

these experiments show that, at PH2O = 5 MPa, continuous deformation can proceed with

no increase in load stress despite the fact that porosity is being reduced from —80 to 40%.

Only at very high values of total strain (E,> 0.7) where porosity is less than 50% is there

a hint of increasing stress with progressive strain.

3.3.4 Textural analysis of experimental cores

Figure 3.4 illustrates the textural evolution of samples with progressive strain. At

values of 0.25, deformed samples are still highly porous (—0.75-0.78) and cracks that

were evident in fabricated cores are still present. Much of the porosity remains intact and

there is little annealing of shards. Ash shards show mainly point sintering. However,

48

Figure 3.4 Textural evolution of samples during high-T deformation (H2O= 2.5 MPa) of

cores represented by photographs of shortened cores and SEM images showing themicrostructures associated with flattening, folding, and annealing of ash-sized particles(e.g., bubble walls, glass shards, and pumice) and parallel loss of pore space. Images arefor experiments of: —25% (A), 50% (B), and 75 % (C) strain.

E=O.25

49

compared to the post-sintering samples, shards in the experimental run products are

clearly deformed and show more warping and re-orientation perpendicular to loading.

Deformed shards show extension cracks on the outer curvature radius. At higher values

of strain ( 0.5), samples have a porosity between -O.63-O.67. Shards are intricately

folded and alignment of shards and flattening of pumice is apparent (Fig. 3,4b). The

shards also show signs of annealing; cracks in the shards resulting from the fabrication

process are not as common and show signs of healing. Figure 3.4c illustrates the textural

evolution of the ash cores after 75% strain (Et = 0.75). Porosity is reduced to —0.37-O.41.

Most shards are highly deformed, folded and flattened; pumices are also collapsed and

flattened. Shards are collapsed and annealed into coherent masses such that the

boundaries between welded shards are hard to distinguish. Particles have a strong

preferred alignment developed perpendicular to the compression direction. Extension

cracks are absent from deformed shards in high strain samples.

3.4 Post-experimental physical properties

3.4.1 Porosity

Total, connected, and isolated porosity was measured before and after each

experiment, using the methods explained in the experimental methods section (3.2.3;

Table 3.3). As discussed above, post-sintering (pre-experimental) sample cores have a

total porosity varying between 0.81 and 0.84; connected porosity dominates but there can

be up to —0.2 isolated porosity (see Table 3.3). Values of total, connected, and isolated

porosity for the pre- and post-experimental cores are plotted in Figure 3.5.

50

Figure 3.5 Nature and distribution of porosity in pre- and post-experiment sample cores.The presence of water during the deformation experiments does not affect the distributionof the porosity; there is no difference in the trends for dry and wet experiments in termsof porosity. We therefore make no distinction between the two. (A) Measured values oftotal porosity (Fe) are plotted against values of connected (Is: circles) and isolated (Li:squares) porosity for pre-experiment (filled symbols) and post-experiment (opensymbols) cores. The overall reduction in I features an initial decrease in and parallelincrease in Ic followed by a steady loss of alone. A single “dwell-time” experiment(—120 minutes) was performed to track the porosity changes (open vs. filled triangles)prior to starting the deformation experiment (see text). (B) Isolated porosity vs.connected porosity. Pre-experiment values of porosity (grey circles) are near constantplotting between 15O-It lines 0.8 and 0.85, but comprise different proportions of isolatedand connected porosity. Porosity values of post-experiment cores are grouped by totalstrain and highlighted by labelled () grey ellipses. Triangles denote “dwell-time”experiment (as in A).

51

1A 7

0.8

ao-e-0.60

()-e- 0.4

0.2

D CQD

0.8 Z0.6

ø.::.” VN ZzN\ 7

N /‘

fl A NZNU.Lt 7N N

7 N N7 N N

7 NNN N

0.2 VV •05 •.ø.25.

0:82 O.7•5 • NIN

‘0 0.2 0.4 0.6 0.8

Figure 3.5 See previous page for caption.

52

Every experiment begins with a —2 hour equilibration period (“dwell-time”),

designed to allow the cell and sample to reach the experimental temperature and water

pressure. We ran a “dwell-time experiment” to assess the extent and nature of physical

changes occurring during the dwell time and prior to the onset of deformation (see

Appendix 3.A). In this experiment, the sample was taken to experimental conditions

(—650°C and — 2.5 MPa PH2O), given enough time to equilibrate at these conditions, and

was cooled back down to room temperature. The physical properties of the resulting core

were then measured to quantify the extent of change (Tables 3.2 and 3.3).

In addition to a systematic reduction in core length (Appendix 3.A), the major

change in the sample concerns the distribution of porosity. During the dwell time total

porosity is conserved (from 0.824 to 0.820), however, there is a shift in the proportions of

isolated and connected porosity. The data show that, during dwell time, isolated pores

are destroyed while the total abundance of connected porosity increases (triangles in Fig.

3.5). One explanation for this pattern is that, during dwell time, isolated pores become

connected either by coalescence or microfracturing.

Figure 3.5 shows the evolution in porosity during the deformation experiments.

After the dwell time, the sample retains a total porosity of 0.8 comprising both

connected (> 0.65) and a residual isolated (<0.1) porosity. As deformation proceeds,

total porosity is reduced continuously; however, the porosity reduction is mainly at the

expense of connected porosity. After the initial decrease in isolated porosity that occurs

during dwell time, deformation to very high (70-80%) values of strain produces no

further change in isolated porosity.

53

The materials we are experimenting on are extremely porous (1 0.7), and it is

reasonable to assume that most of the strain is accommodated by volume loss. Knowing

the initial and final porosity of a sample, the amount of strain due to volume loss (pore

destruction) can be calculated using the following relationship:

(3.4)l-f

where t is initial total porosity and I is final total porosity (Quane and Russell, 2005).

Figure 3.6 shows that most of the deformation in our experiments can be ascribed strictly

to volume loss (e,), but that at values of strain -0.7, the run-products are more porous

than predicted by Eq. (3.4). This suggests that deformation mechanisms other than

volume strain (strain from porosity loss) are active during the compaction. Further

evidence for more than one mechanism of deformation being active is provided by the

“onset” of significant radial increase of the samples at high amounts of total strain.

3.4.2 Water content

We measure the bulk water content of our experimental run products to verify the

amount of water incorporated into the sample during deformation. Any amount of water

dissolved into the glass will reduce the viscosity of the melt and contribute to the overall

strength reduction of the sample. We found that the timescale of our experiments (- 1.5

to 4.5 hours after initial dwell-time) is sufficient to allow water dissolution into the glass.

The bulk water content results are presented and discussed in Appendix 3.B (Table 3.B).

54

-e

t

Figure 3.6 Measured values of total porosity (J) are plotted against total machine strain

()• The black bar (top left) denotes the range of initial porosities of cores for the entiresuite of experiments (0.804 to 0.844). The grey shaded band represents the field ofmodel curves of decreasing porosity calculated for a known initial porosity as a functionof increasing strain and assuming pure volume strain. The model curves converge to zeroporosity as E approaches a value equal to the initial porosity. Volume strain is sufficientto explain these data until E -60% where the run-products may have higher porosity thanpredicted. This suggests that deformation mechanisms other than volume strain (strainfrom porosity loss) are active during compaction (see text and Fig. 3.8).

0 0.5 1

55

3.5 Analysis of experimental results

3.5.1 Effect of temperature and PH2O.

We ran six additional experiments to document the effects of temperature on the

rheological behaviour of these porous volcanic materials. Three dry experiments,

involving samples RS23, RS16 and RS24, were carried out at 850, 800 and 750 °C,

respectively (Fig. 3.7a). The three experiments deformed samples to = 0.5 and provide

data that map the temperature boundary between ductile and brittle deformation for dry

porous samples at the characteristic deformation timescale of these experiments (texp).

Sample RS23 shows a smooth increase in stress with increasing strain, but compared to

the 900 °C dry experiment (RS21), the stress increase with strain at 850 °C is more

pronounced. The lower temperature experiments (750 and 800 °C) show strikingly

different behaviours than observed in the higher temperature experiments. Instead of a

smooth increase of stress with increasing strain, they display a saw tooth pattern with

sharp (20%) stress build-ups followed by quasi-instantaneous stress drops (Fig. 3.7a).

Run-products for samples deformed at those low temperatures are extensively fractured,

often broken-up in smaller pieces, or exhibiting fractures (see picture insets in Fig. 3.7).

This suggests that, at the timescale of these experiments, the transition from ductile to

brittle deformation of the dry porous cores occurs at between 850 and 800 °C.

The effect of water pressure is summarized in Figure 3.7b by comparing results of

a 900 °C dry experiment to results from parallel experiments involving more than 50%

strain at 3 different water pressures. The dry experiment (900 °C) was performed at a

temperature -250 °C higher than any of the experiments performed at water pressures of

1, 2.5, and 5 MPa. Increasing water pressure during deformation significantly reduces

56

sample strength, and allows for large amounts of deformation to be obtained with

minimal stress imposed. The high temperature experiment plots between the 1 and 2.5

MPa PH2O experiments and illustrates the trade-off between temperature and water

pressure. At low water pressures and --650 °C, the porous cores carry more stress than

the dry core at 900 °C, while at —650 °C and water pressures greater that 2 MPa the

material is substantially weaker (Fig. 3.7b).

We also ran three high-PH2Oexperiments at reduced temperatures to document the

temperature-controlled transition from ductile to brittle deformation in the wet systems.

Samples RS22, RS18 and RS19 were deformed at 550, —450 and —385 °C, respectively,

at the same displacement rate of 2.5 i03 mrnls, to a total strain of 0.5. A PH2O of 2.5 MPa

was chosen as an intermediate response between the two end-member water pressures

used in our experiments. Sample RS22 (550°C) showed a smooth increase in stress with

increasing strain, but stress was --3 times higher than recorded in the —650 °C, 2.5 MPa

PH2O experiment (RS14) at the same values of e. Samples RS18 and RS19 showed a

pattern of brittle deformation characterized by sharp rises and drops in stress with

increasing strain, but without stress ever building up over 1 IVIPa. The pattern is very

similar to that of the dry experiments (Fig. 3.7a) but the stress drops are not as

pronounced. On the basis of these response curves, we suggest that for these

experiments, the transition from ductile to brittle behaviour occurs at between 550 and

450 °C. We expect that the brittle-ductile boundary will shift to lower temperatures with

increasing PH2O or with lower rates of displacement (e.g., longer experimental

timescales).

57

Figure 3.7 Experimental data used to illustrate the effects of temperature and PH2O onsample rheology. (A) Experimental data plotted as stress (0) vs. strain () foratmospheric dry experiments run to —50% strain and performed over a range oftemperatures (750-900 °C). Experiments at 850-900 °C show o vs. E patterns that areconsistent with ductile deformation. Dry experiments run at 750 and 800 °C show muchmore complicated patterns and are characterized by a 3-fold increase in stress andcyclical rises and drops (30-40%) in stress suggesting deformation by brittle fracturing.Run-products for samples deformed at those low temperatures are extensively fractured,often broken-up in smaller pieces, or exhibiting fractures (see picture insets). (B) Resultsof wet deformation experiments (PH2O: 1 to 5 MPa; T: —650 °C) compared to data fromdry, high-T (900 °C) experiment (RS17; Fig. 3.3). The main effects of PH2O are to reducethe strength of cores and to permit ductile deformation at temperatures well below theeffective Tg of the dry cores (see Fig. 3.7a and text).

58

A

0.1 0.2 0.3 0.4

-T

3

c0

1

00

0

Figure 3.7 See previous page for caption.

0.5

0.4

59q“

3.5.2 Analysis of strain

During these deformation experiments, total strain (es) is given by:

= L0—L,(3.5)

where L0 is the initial sample length and Lf = L0 — (total machine displacement). During

deformation samples get shorter and increase in radius (Table 3.2). We calculate volume

strain () using Eq. (3.4), and radial strain (Er) from the initial and final sample radius:

2

8r=1 (3.6)rf

Most samples show uniform radial increase along their entire length, but bulging

(greatest radial increase at the mid-point of sample length) is observed in run-products

taken to high total strain (Et > 0.6).

Figure 3.8 illustrates how the strain is progressively partitioned between volume

(E) and radial strain (Er). At low values of total strain, the two metrics are sub-equal (Eq,

- Et) and plot near the 1:1 line indicating that most of the observed strain is being

accommodated by volume loss (Fig. 3.8a). However, as total strain increases the

departure from the 1:1 line increases, as does the calculated amount of radial strain (Fig.

3. 8b). These patterns clearly show that the total strain, as manifest by shortening of the

core, cannot be fully accommodated by porosity reduction (Em: volume strain) but

requires radial bulging (Er: shear strain). The proportion of shear strain to volume strain

increases with increasing strain (Fig. 3.8a-c).

The combination of volume and shear strain (E + Er) can exceed the total strain as

represented by shortening of the core (Fig. 3.8c-d). At low values of strain the

combination of volume and shear strain are more or less equal to total strain represented

60

by shortening of the core. However at values of total strain above --0.5, the combination

of + Er is greater than the total strain (data plot above 1:1 line; Fig. 3.8d) and the

deviation increases with increasing e. These patterns indicate the nature of coupling

between the two strain mechanisms.

At low values of strain, where pore fraction>> solid fraction, total strain is

mainly accommodated by volume loss and radial strain is minimal. There is also little to

no evidence for coupling between these two strain mechanisms (i.e., volume vs. shear

strain) and they may operate independently. However, at higher amounts of strain (Et>

0.6), where porosity < 0.6, strain is accommodated by volume loss and a significant

component of radial bulging. This behaviour is also expressed in Figure 3.6 where the

measured residual porosity departs from the model vs. E curves at Et > 0.6. Figure

3.8c shows the changes in proportions of E to Er with increasing total strain. At low

values of strain E, is substantially greater than Er, however, the proportion of Er increases

steadily with increasing strain. In fact, at values of Et> 0.5, the summation of and Er

exceeds the total strain computed from shortening of the core (Fig. 3. 8d), suggesting that

there is strong coupling between the volume strain and shear strain. Moreover, at Et > 0.8

the combined values of E,+ 6r fall above the iso-strain contour for 1.0 (Fig. 3.8c) and

incremental increases in strain are dominated by radial expansion rather than by volume

loss.

The implication is that once there has been sufficient strain (Et 0.6) to reduce

porosity to a critical value ( <0.6) subsequent compaction (shortening of core) is

accommodated by porosity reduction (volume strain) and concomitant radial bulging

61

0.6C)

0.4

0.2

CO 0 1

Figure 3.8 Analysis of strain in experimentally deformed cores. (A) Total strain () asrecorded by piston displacement is plotted against the strain computed from porosity lost

(s). Values of (volume strain) increase linearly with but are always less than totalstrain. (B) Total strain () plotted against strain ascribed to increase in cross-sectionalarea (i.e., radius) of the deformed core (Er) Values of Er are always smaller than totalstrain but increase markedly with increasing total strain. (C) Values of plotted against

Er• Dashed lines are iso-EL contours (e.g., = E + (D) Values of plotted against thesum [ - Erj• Data plotting above the solid line suggest coupling of and Er (see Fig.3.8c); the extent of coupling is proportional to the distance each point is above the 1:1line and increases with total strain.

:c08 • •

%. •%%%

%. %. ‘% ‘%%. %. %..

••%

r . \p.. %.

•% %

%. ‘% ‘% ‘7•%. *%__“ •% • %..

‘Ss/% S’S s ‘S ‘S ‘S‘S ‘S ‘S ‘S ‘S ‘S

‘S ‘S ‘S

0.5E

62

(shear strain). At this point, progressive strain comprises volume strain (porosity

reduction) that is dependent on a component of shear strain; the degree of coupling

between volume and shear strain is indicated by the upwards departure from the 1:1 line

in Figure 3.8d.

In summary, the high-T deformation experiments elucidate three potential strain

regimes: (i) at low values of strain ( = < 0.5) where porosity > 0.6, most strain is

accommodated volume strain and a subordinate amount of independent (or weakly

coupled) shear strain; (ii) at intermediate values of strain ( --0.5-0.6), where porosity

50-60%, shear strain becomes increasingly important and volume and shear strain are at

least weakly coupled; and (iii) at high values of strain (> 0.7) where porosity drops to

below 40%, volume strain and shear strain are strongly coupled as evidenced by

[Ec+Er]/Et> 1.0.

3.5.3 Effective viscosity

The digital data recorded in each experiment provide load stress, total and

incremental displacement (strain) at each time step and, thus, incremental and total strain

rate. These data allow us to compute the apparent viscosity of the sample during

deformation as a function of total strain (Fig. 3.9). The apparent viscosity (Tlapp) of the

sample is the viscosity of the porous aggregate of volcanic ash at the experimental

conditions, and is calculated as:

r1app (3.7)

where a is stress and is the total strain rate. In general, these experiments show the

63

12 Atm(Dry)at900°C ‘

o RS2O l

3)o I 0

—

‘0 0.2 0.4 0.6 0.8 1

P0=5 MPa(04

- 0 (1.0)C3 RS111

D. 659°C RSO4

°10 (1.5))

9(2.0)

647°C

—

D0.8 1 0.2 0:4 0.6 0:8

Figure 3.9 Summary of apparent viscosity calculated as load stress over total strain rate,plotted as a function of total strain. (A) Calculated apparent viscosity from high-temperature (—9OO°C) dry series of experiments. Calculated apparent viscosity for lowertemperature (—65O °C) experiments performed under water pressures of: (B)

H9O= 1

MPa, (C) H2O = 2.5 MPa, and (D) H2o = 5 MPa. Open circles at = 1 represent modelmelt viscosity for water content in parentheses (wt.% H20). Grey gradient shading anddashed vertical line represent the onset of significant radial strain observed in run products.

0 0.2 0.4 0.6 0.8 1

12

0 0.2 0.4 0.6

64

porous cores of ash to have a strain-dependent behaviour where, under the constant

displacement rate constraint, stress increases with increasing strain (i.e., strain

hardening). The strain dependent rheology of these samples is a reflection of the porosity

reduction due to compaction. The strain hardening is most pronounced at high values of

strain (e.g., > 0.5; Fig. 3.3) where increases in strain cause high rates of porosity

reduction (see Fig. 3.6). In our ductile experiments, the increase in stress with increasing

strain can track the increase in apparent viscosity of the porous melt samples during

deformation due to porosity reduction.

We have demonstrated that deformation of our samples is expressed in, at least,

two different ways: (i) volume strain due to porosity loss; and (ii) shear strain manifest by

an increase in sample radius. We, therefore, recognize that the apparent viscosity values

we have plotted in Figure 3.9 are a product both of volume strain and a shear strain, and

that the contributions of these components varies as a function of total strain. Moreover,

the total experimental strain rate will also comprise varying proportions of volume strain

rate and shear strain rate. Volume strain rate dominates up to values of total strain of 0.6,

whilst at total strain > 0.6, shear strain rate is expected dominate, and Eq. (3.7) becomes a

cruder approximation of viscosity. The switchover between the two strain regimes is

illustrated schematically in Figure 3.9 by a diffuse boundary (and shading) at -0. 6 total

strain.

Values of apparent viscosity calculated for samples deformed under dry

conditions at high temperature are self-consistent and, again, demonstrate the

reproducibility of the technique used in this study (Fig. 3.9a). Overall, porosity reduces

the viscosity of the sample. The dry deformation experiments clearly show a strain

65

dependent behaviour that translates into a rise in apparent viscosity from 109.1 to 1011.9 Pa

s over the full range of porosity reduction from 0.8 to 0.25. Over the interval 0 to 0.5

viscosity rises from 109.1 to 10100 compared to melted Rattlesnake Tuff which has a

viscosity of 10102 at 900 °C. At values of strain > 0.5, Eq. (3.7) cannot be used to model

viscosity accurately because of the substantial component of shear strain (radial increase).

Post-experiment analysis of samples shows that the cores have water contents that

generally exceed values predicted by standard 1120-melt solubility models (see Appendix

3.B). The measured values may represent a combination of chemically dissolved and

mechanically trapped (e.g., nanopores) water. For the purposes of analysis we have

assumed that during deformation the melt fraction of the samples contains, at a minimum,

the H2O content predicted by the Newman and Lowenstem (2002) model. For a

temperature of 650 °C, the model predicts values of 0.42, 0.67 and 0.95 wt.% H2O for 1,

2.5 and 5 JVJPa PH2O, respectively,

The apparent viscosity of low PH2O (1 MPa) system is also clearly strain-

dependent and shows an increase in apparent viscosity from iO” to 10106 Pa s over the Et

interval 0 — 0.5. At higher values of strain the calculated apparent viscosity rises

markedly due to dominance of shear strain and the breakdown of Eq. (3.7). At 1 MPa,

the melt is expected to have 0.42 wt.% dissolved water and a melt viscosity at 650 °C of

1012 Pa s. It is apparent that the porosity reduces the effective viscosity by up to 2 orders

of magnitude. These experiments probably could not be run effectively on the bubble

free melt because these temperatures are below the glass transition temperature (Tg) of

the melt (e.g., r 1012 Pa s).

66

The same effects are not observed at higher water pressures (2.5-5 MPa). Instead,

these experiments suggest near constant values of apparent viscosity until Et exceeds 0.6.

For example, the corresponding high-strain (e 0.75) experiments for 2.5 and 5 IVIPa

PH2O show only slight rises in viscosity of 109598 and 10b0.0b02 Pa s, respectively over

0-0.5. Given the water contents predicted by Newman and Lowenstern (2002) for the

experiments RS15 (0.67 wt. %) and RSO4 (0.95 wt. %) we expect melt viscosities at 650

°C of 10112 and iO’°5 Pa s. Using these limiting values for H20 solubility, it appears that

for all water pressures the porous melts have substantially lower viscosities than their

hydrated melt (non-porous) equivalents (see white circles in Fig. 3.9). At higher values

of PH2O, the effects of strain hardening are greatly reduced (Fig. 3.3) and the apparent

viscosity of the porous melts remains approximately constant over most of the

deformation (i.e. compaction) history.

3.6 Discussion

Our experiments explore the transient rheology of particulate porous natural melt

from high (-4J.8) to moderate (—0.25) pore fractions under both atmospheric pressure

conditions and at elevated water pressure. During sample deformation strain is

accommodated by: (i) shortening of the sample core; (ii) reduction in sample porosity;

and (iii) increase in the radius of the sample core. The strain is achieved via a

combination of volume strain (reduction of pore space) and shear strain (radial

expansion). The relative contributions of these two mechanisms to vary as a function

of strain; at high strain (Et> 0.6) and relatively low pore fractions (<0.6) shear strain

begins to dominate and both mechanisms are strongly coupled. Our unjacketed

67

deformation experiments are in that way analogous to deformation occurring in an

unconfined ignimbrite sheet able to flow freely horizontally, perpendicular to the loading

direction due to gravity.

The deformation experiments clearly document the strongly strain-dependent

rheology of these cores of volcanic ash. The rate of strain hardening increases rapidly as

strain increases and probably mirrors the increasing role of shear strain as compaction

proceeds. At high values of total strain, samples develop a strong foliation from

alignment of glass and pumice shards, consistent with the major increase in shear strain.

The presence of a fluid phase (i.e. PH20) appears to reduce the extent of strain hardening.

We observe no apparent textural differences between samples deformed at dry,

higher temperature (900 °C) conditions and lower temperature (650 °C) experiments

performed at elevated PH2O. Figure 3.10 comprises thin section (A) and SEM

photomicrographs (B) of sample RS17 resulting from dry compaction at 900 °C and to

75% strain. Corresponding images are shown (Fig. 3. lOc, d) for sample RSO4 which

derives from an experiment run at -650 °C, under 5 MPa PH2O, and to 75% strain. The

run products are indistinguishable from one another. This demonstrates that different

experimental (i.e. environmental) conditions (Table 3.2) can produce distinct compaction

paths (Fig. 3.3), yet yield virtually identical products. The main difference in the run

products is their measured water contents: 0.15 wt.% for RS17 vs. 1.61 wt.% for RSO4

(see Table 3.B); the run-products have porosities of 37% and 41% respectively.

This has implications for natural systems, wherein the features of welded volcanic

deposits are used to deduce the nature of compaction and welding processes. Welding

intensity in pyroclastic deposits is a reflection of emplacement conditions of the deposit

68

Figure 3.10 Textural comparison of samples run under dry and wet conditions. Scan ofpolished thin section (A), and SEM photomicrograph (B) for sample RS 17. Scan ofpolished thin section (C), and SEM photomicrograph (D) for sample RSO4. Bothsamples were deformed to 75% strain. Sample RS17 was deformed under dry conditions,at 900 °C, and sample RSO4 was deformed at = 5 MPa, and -65O °C.

69

(e.g., emplacement temperature and accumulation rate), physical and chemical properties

of the materials (e.g., porosity, composition and water content of the melt), and dynamic

feedbacks during welding (e.g., destruction of porosity, water resorption) (Smith,

1960a,b; Guest and Rogers, 1967; Riehie et a!., 1995; Sparks et al., 1999). These

environmental parameters can combine in a multitude of ways to generate the same

overall intensity of welding. Ideally, we hope that there are features that can be observed

in the field that can be used to gauge the relative roles of these parameters (e.g., T, PH2O,

load) (Grunder and Russell, 2005; Russell and Quane, 2005). The results above cast

some doubt on this anticipation, in that material with virtually identical physical and

textural properties has resulted from two distinct end-member processes: (i) hot dry

compaction, and (ii) cool, wet compaction. Therefore, there is likely to be no unique

solution for the conditions required to develop a specific welding intensity or facies. This

insight serves to highlight the over-simplification of many early and existing models of

the welding process in pyroclastic deposits and welding facies distribution, where

temperature and load are the only conditions considered (e.g., Ross & Smith, 1961;

Ragan & Sheridan, 1972). We suggest that models of welding zonation development,

and critically, welding profiles (Reihle et al., 1995) from which porosity and permeability

information is inferred, be re-examined to account for the effects of porosity and water

pressure before, during and after welding has occurred.

Part of this study demonstrates the pronounced effect of temperature on the

rheological behaviour of these cores of volcanic ash. Under dry conditions, and a

constant displacement rate of 2.5 i0 mm/s (strain rate - i0 s1), experiments conducted

at 850 °C or higher produced stress-strain relationships consistent with viscous

70

deformation (Figs. 3.3, 3.7a). The same experiments performed at 800 °C or lower

produced stress-strain patterns indicative of brittle relaxation (Fig. 3.7a). These data

imply that, at the timescales of our experiments, the rheological glass transition

temperature (Tg) (marking the temperature boundary between ductile and brittle

behaviour) resides at between 800-850 °C (Fig. 3.1 la). The two viscous experiments

(RS 16 and RS23) were used to extract values of effective viscosity at identical values of

= 0.25 where the porosity is still very high (—0.75-0.78; Table 3.3). These values are

plotted at their experimental temperatures (filled circles; Fig. 3.11) and used to define an

Arrhenian curve having the same slope as the melt viscosity (open circles) and

representing the temperature dependence of viscosity for dry porous cores of Rattlesnake

Tuff ash. We have adopted and plotted a mid-range (e.g., 800 to 850 °C) value for the Tg

of 825 °C (Fig. 3.1 la; dashed vertical line). The same viscosity data are plotted in terms

of their characteristic relaxation timescales by scaling the melt viscosity to the bulk shear

modulus (Dingwell, 1995). The intersection of the apparent Tg and the viscosity curve

for the dry porous melt (Fig. 3.1 la) implies an average experimental timescale (texp) of

—4 s (Fig. 3.1 lb). This is illustrated by a grey horizontal dashed line on Figure 3.1 lb.

Where the porous sample has a characteristic relaxation timescale (‘tr) shorter than texp,

the experimental response will be viscous. Conversely, at lower temperatures (i.e., <825

°C) samples will have values of tr that are larger (i.e. longer) than texp; under these

conditions the rate of building stresses in the core (texp) is faster than the capacity of the

sample to relax viscously (tr). This results in brittle failure of the sample (Fig. 3.7a,

3.1 lb). The horizontal arrow marks the intersections of the experimental timescale (texp)

with viscosity curves for the Rattlesnake Tuff melt and the same melt with —75% porosity

71

Figure 3.11 Summary of variations in viscosity and relaxation time scale for theRattlesnake Tuff melt resulting from temperature, dissolved water content and porosity.(A) Viscosity of the dry, porous Rattlesnake Tuff melt (taken at Et=O.25) as a function oftemperature, and compared to the viscosity of melt alone. Solid line is based onexperimental measurement of anhydrous melt (Robert et al. 2008); dashed line is anArrhenian fit to the experimental viscosity data (this study). Low temperature, brittleexperiments are represented by filled squares, and viscous, higher temperatureexperiments by filled circles. The vertical, dashed grey line is the effective glasstransition temperature for the dry, porous system (825 °C). (B) Relaxation timescale (seetext) of the dry, porous melt as a function of temperature. Experimental data andconditions as in (A). The characteristic experimental timescale (4 s; see text) is shownas a dashed grey horizontal line. The expansion of the viscous deformation field due toviscosity is illustrated by a grey arrow (see text). (C) Viscosity of the wet (PH2O=2.5

MPa), porous Rattlesnake Tuff melt (taken at Et=O.2S) as a function of temperature, andcompared to the model viscosity (Giordano et al., In Press) of the hydrous melt (wt.%H20 in parentheses), for solubility of 0.67 wt.% at 650 °C and 0.78 wt.% at 550 °C(Newman and Lowenstern, 2002). The vertical, dashed grey line is the effective glasstransition temperature for the wet, porous system (528 °C; see text). (D) Relaxationtimescale of the wet, porous melt as a function of temperature. Experimental data andconditions as in (B). The characteristic experimental timescale (same as in C) is shownas a dashed grey horizontal line. The expansion of the viscous deformation field due toporosity is illustrated by a grey arrow (see text).

72

Figure 3.11 See previous page for caption.

T(°C)1395 975 725

12

8

4

16

U)

0

0r

0)C

U)

ct0

01

0)0

102

100

U) -2— 10

1

106

108

1

U)

1

108

1 0000/T(K)6 8 10

1 0000JT(K)

T(°C)975 725 560 440 350

P0120) = 2.5 MPa

—

;;;;;;-

C.- :/ co

— c’J, U,

._ —,-

—

8

4

8 10 12 14 16 8 10 12 14 161 0000/T(K) 1 0000/T(K)

73

(Fig. 3.1 ib); namely their respective glass transition temperatures. Under dry conditions,

the addition of porosity expands the window for viscous deformation by -5O °C.

In a similar manner, we have explored the effect of porosity on the viscosity of

hydrated melts. The viscosity of hydrated melts are calculated (Giordano et al., In Press)

for melts having fixed water contents consistent with their experimental conditions (solid

lines, Fig. 1 ic): 2.5 IVIPa PH20 and 650 (0.67 wt.%) and 550 °C (0.78 wt.%). Parallel

curves have been drawn through the experimental data points taken from the hydrous

deformation experiments that showed a viscous response. These curves represent the

temperature dependence of the effective viscosity of these hydrated porous cores. The

two lower temperature hydrous experiments (--450 and —385 °C) that gave brittle

responses are plotted as squares (Fig. 3.1 ic), and suggest that the rheological glass

transition temperature for these hydrous cores resides between 550°C and 450°C. We

have used the apparent average timescale of the experiment (Texp 4s) to constrain the Tg

of the most hydrous sample to 528 °C (Fig. 3.1 lc). Under a water pressure of 2.5 IVIPa,

the addition of porosity to these hydrated melts increases the field for viscous

deformation by 140-150 °C (grey arrow in Fig 3.1 id). Increasing the displacement rate

by 1 order of magnitude at 900 °C also pushed the material into the field of brittle

behaviour (RS25). Increasing displacement rate causes an order of magnitude decrease

in the experimental timescale (_.101 or 102 s) and the characteristic relaxation timescale

of the sample staying constant. The result is that experiments that originally featured

samples with tr <texp and deformed viscously may now find that texp <t such that

viscous relaxation of the sample cannot keep pace with the faster rate of deformation.

74

3.7 Acknowledgements

This research is funded by the Natural Sciences and Engineering Research

Council (NSERC) via the Research Tools and Instruments program (JKR), the Discovery

Grants program (JKR), and the PGS fellowship program (GR) and by the Italian

Dipartimiento della Protezione Civile (2004-06 Agreement, Instituto Nazionale di

Geofisica e Vulcanologia — INGV). Chemical analyses of cores of Rattlesnake Tuff ash

were generously provided by Steve Quane. We thank Ben Kennedy, Steve Quane and

Perrine Paquereau for countless rheology-related discussions, most of which are ongoing.

Finally, we especially thank UBC’s Earth & Ocean Sciences machinists Ray Rodway and

Jörn Unger for their support throughout this experimental program.

3.8 Appendix 3.A: Correction for dwell-time effects

Our deformation experiments are conducted at constant displacement rate and at

preset and constant values of temperature and PH20. The digital output includes load,

displacement and time, which are converted to stress, strain and strain rate. Our goal is to

invert these datasets for the rheological properties of the volcanic cores as a function of

T, PH2O and porosity. Characterization of sample cores before and after the experiments

provides additional metrics that can be used to verify or help interpret the digital output.

It is implicit that the experiments are performed on sample cores that are fully

equilibrated with the experimental conditions (i.e., T and PH20). Therefore, each core

was maintained at the experimental conditions for 1-2 hours prior to lowering the piston

and starting the deformation experiment. Whilst the pre-experiment dwell-time ensures

that a sample has had adequate opportunity to equilibrate with the experimental

75

conditions (T and PH2O), there is also the possibility that the sample will undergo physical

changes (i.e., geometry, porosity) before the deformation experiment begins.

We have made every effort to calibrate the effects of the pre-experiment

equilibration process on the sample cores by running “dwell-time” experiments. In these

experiments the core was left at experimental conditions for 1-2 hours, cooled to room

temperature in the exact same way every sample is cooled after a deformation

experiment, and then removed. Comparison of the post dwell-time core properties (i.e.,

length, radius, porosity, etc.) with its original properties demonstrated the extent of these

pre-experiment modifications (Figs. 3.5; 3.6; 3.A). The main effect is shortening of the

core as evidenced by the difference between the digitally recorded piston displacement

(i.e., no imposed displacement) and the post-experiment measured change in core length

(Fig. 3.A). A model line of unit slope was fit to the data that supports a systematic pre

experiment shortening of the core of 1.73 mm, consistent with shortening by 1.87 mm of

“dwell-time” experiment sample RSO9.

An ancillary effect of the dwell time involves the reorganization of porosity (Fig.

3.5). The dwell time experiment suggests that total porosity is conserved, but the

conservation is achieved by a parallel loss (destruction) of isolated porosity and gain

(production) of connected porosity. At the end of the dwell time, the sample cores are

left with a reduced, relatively consistent (2 to 10%), amount of isolated porosity. This

post dwell-time value of isolated porosity is maintained throughout the deformation

experiments while connected porosity is reduced continuously (Fig. 3.5).

76

c3)

Cci)

0

Cl)

50

Figure 3.A Values of piston displacement (digital output) are plotted against the mea

sured shortening (L0 — Lf) of the core for experiments listed in Table 3.1. Total shortening

of the cores is always slightly greater than piston displacement. A model line of unit

slope has been fit to the data (filled circles) and returns an intercept of 1.73 mm that

accords well with shortening (1.87 mm) during “dwell time” experiment (DTE: see text).

This suggests a systematic difference between these two metrics of strain that results

from shrinking of the core during the dwell time immediately prior to start of the experi

ment.

10 20 30displacement (mm)

77

3.9 Appendix 3.B: Water contents of samples

We analysed our post-experiment sample cores for bulk water content by

InfraRed detection of volatiles extracted at 1000 °C. All samples were crushed to < 100

tm. Replicate measurements of H20 content on sample rtf5 were made by Karl-Fischer

titration as a test of the accuracy of results from ALS-Chemex Laboratories (Fig. 3.B;

Table 3.B). The two methods agree to within 2s experimental uncertainty. Measured

water contents increase regularly with increasing experimental water pressure (Fig. 3 .B).

This pattern suggests that the cores absorb significant water during the dwell time (e.g.,

sample RSO9; black square) and that the amount taken in rises as a function of PH2O.

Indeed, our measured values of H20 content are 2-3 times higher than those predicted by

solubility models over the same P-T conditions used in the experiments (Newman and

Lowenstern, 2002).

If the measured H20 contents are true estimates of H20 solubility in this melt

under the experimental conditions (Table 3.2), then it implies that the current models for

H20 solubility in silicate melts are inadequate at these low pressures. Although unlikely

(i.e., Liu et al., 2005; Zhang, 1999), this remains at least a possibility because there are

virtually no H20 solubility data for silicate melts at our experimental pressures (i.e., <<

20 MPa; Di Matteo et al., 2006). A single exception is the older work of Friedman et al.

(1963), who also obtained higher than predicted water solubility in rhyolite over the same

pressure range as used in our experiments. We also cannot rule out the possibility that

our higher-than-expected H20 contents result from re-equilibration of the samples at

lower temperatures during the quenching process (i.e., retrograde solubility).

78

4— 0.4

dry experimentsnatural RS0.3 /

3 0.2sinte red cores

..— (pre-exp)

o__-, 0.1

RS melt0 0.1 0.2

+ 2 rtf5 (KFT)—, HO(MPa)

oC’.’

• rtf5(IR)

(MPa)2

Figure 3.B Bulk water contents measured for experimental cores by InfraRed detectionof water released at 1000°C and plotted against experimental water pressure (opencircles; black square for DTE RSO9). Water contents for atmospheric dry experimentalcores are shown in inset. Sample core rtf5 (triangles) was measured by both, InfraReddetection (filled) and Karl-Fischer titration (open) and the two methods return comparable results. Experimental cores have higher water contents than expected for meltsequilibrated at the experimental pressures and temperatures when compared to valuespredicted byH20-soiubility models for silicate melts (solid lines; Newman & Lowenstern, 2002) (Table 3.B). These results strongly suggest that the sample cores are fullysaturated with water and that the experimental time scale is long enough to reach equilibrium. Results of experimental solubility experiments of Friedman et a]. (1963; filledcircles; T: 485 (black) - 785 (light grey) °C) also plot above model solubility curves.Measured water content of natural Rattlesnake Tuff ash is plotted as open squares.

79

Table 3.B. Values of H20 and LOT (wt. %) for post-experiment coresa.

No T(°C) PH2O (MPa) H20+b LOlRSO3 640±4 1 0.76 1.22RSO4 681±20 5 1.68 2.37RSO5 659±9 1 0.67 0.95RSO7 654±4 1 0.87 1.44RSO9 [25-650] —2.5 0.98 1.23RS1O 647±7 5 2.02 2.37RS11 659±13 5 1.76 2.13RS12 662±8 3 1.63 2.13RS13 654±6 2.5 1.43 1.82RS14 650±9 2.5 1.38 1.84RS15 666±18 2.5 1.27 1.47RS16 800±15 0 0.21 1.33RS17 900±15 0 0.15 0.30RS18 452±12 2.5 1.61 2.98RS19 386±16 2.5 1.29 2.57RS2O 900±15 0 0.20 0.12RS2I 900±15 0 0.14 0.27RS22 550±14 2.5 1.50 1.39RS23 850±15 0 0.22 0.02RS24 750±15 0 0.24 1.81RS25 850±15 0 0.26 1.27rtf5 645±5 3 1.58 1.88rtf5 (KFT) 645±5 3 1.93 -

RS meltc -- 0.07 0.27

RS naturaF’ - - 3.47 3.97RS sinterede -

- 0.13 0.36a Experiments (RS#) as listed in Table 3.1.b H20+ by InfraRed detection except for Karl-Fischer titration of rtf5(KFT).C Volatile contents of melted Rattlesnake Tuff after concentric-cylinder experiments (cf.Robert et al., 2008).d Average bulk water content of natural Rattlesnake Tuff ash.e Average bulk water content of sintered cores of Rattlesnake Tuff ash (see text for details).

80

An alternative explanation is that the measured H20 contents represent a

combination of water that was chemically dissolved in the melt and water that was

mechanically trapped in pores at the interfaces of collapsed and annealed ash particles

(Fig. 3.4). For this to occur the trapped pores would have to be finer scale that the

average diameter of the crushed samples used for chemical analysis (i.e., < 100 tm)

making them nano-scale. Both water distributions would be liberated simultaneously

when the induction furnace used for the analyses brought the sample above its glass

transition temperature.

Regardless of which (if any) of the above explanations is correct, it is clear that

our samples retain water contents that are at and above the H20 solubility of silicate

melts at these P-T conditions. This supports our assertion that our pre-experiment dwell

times are sufficiently long to allow the sample to reach thermal and chemical equilibrium.

On this basis, we are able to use the deformation experiments to demonstrate the effect of

water saturation on the viscosity of bubble-rich melts.

81

3.10 References

Bagdassarov, N.Sh., Dingwell, D.B., 1992. A rheological investigation of vesicular

rhyolite. Journal of Volcanology and Geothermal Research 50, 307-322.

Bierwirth, P.N., 1982. Experimental welding of volcanic ash. B.Sc. Honours Thesis,

Monash University, ‘74.p.

Di Matteo, V., Mangiacapra, A., Dingwell, D.B., Orsi, G., 2006. Water solubility and

speciation from Campi Flegrei Caldera (Italy). Chemical Geology 229, 113-124.

Dingwell, 1995. Relaxation in silicate melts: some applications. In: Stebbins, J.F.,

McMillan, P.F., Dingwell, D.B. (eds) Reviews in Mineralogy 32. Mineralogical

Society of America, Washington, D.C. 2 1-66.

Ducamp, V.C., Raj, R., 1989. Shear and densification of glass powder compacts. Journal

of the American Ceramic Society 72, 798-804.

Friedman, I., Long, W., Smith, R.L., 1963. Viscosity and water content of rhyolite glass.

Journal of Geophysical Research 68, 6523-6535.

Giordano, D., Russell, J.K., Dingwell, D.B., In Press. Viscosity of magmatic liquids: A

model. Earth and Planetary Science Letters.

Grunder, A., Russell, J.K., 2005. Welding processes in volcanology: insights from field,

experimental, and modeling studies. Journal of Volcanology and Geothermal

Research 142, 1-9.

Guest, J.E., Rogers, P.S., 1967. The sintering of glass and its relationship to welding in

ignimbrites. Proceedings of the Geological Society. London 1641, 174-177.

Kennedy, B., Spieler, 0., Scheu, B., Kueppers, U., Taddeucci, J., Dingwell, D.B., 2005.

Conduit implosion during Vulcanian eruptions. Geology 33, 58 1-584.

82

Lejeune, A.M., Bottinga, Y., Trull, T.W., Richet, P., 1999. Rheology of bubble-bearing

magmas. Earth and Planetary Science Letters 166, 7 1-84.

Liu, Y., Zhang, Y., Behrens, H., 2005. Solubility of H20 in rhyolitic melts at low

pressures and a new empirical model for mixedH20-C02solubility in rhyolitic

melts. Journal of Volcanology and Geothermal Research 143, 2 19-235.

Manley, C.R., 1996. Physical volcanology of a voluminous rhyolite lava flow: the

Badlands lava, Owyhee plateau, SW Idaho. Journal of Volcanology and

Geothermal Research 71, 129-153.

Michol, K.A., Russell, J.K., Andrews, G.D.M., 2008. Welded block and ash flow

deposits from Mount Meager, British Columbia, Canada. Journal of Volcanology

and Geothermal Research 169, 121-144.

Newman, S., Lowenstern, J.B., 2002. VolatileCalc: a silicate melt-H20-C02solution

model written in Visual Basic for Excel. Computers and Geosciences 28, 597-

604.

Quane, S.L., 2004. Welding in pyroclastic deposits. PhD thesis, University of British

Columbia, 208 pp.

Quane, S.L., Russell, J.K., Kennedy, L.A., 2004. A low-load, high-temperature

deformation apparatus for volcanological studies. American Mineralogist 89, 873-

877.

Quane, S.L., Russell, J.K., 2005. Welding: insights from high-temperature analogue

experiments. Journal of Volcanology and Geothermal Research 142, 67-87.

Ragan, D.H., Sheridan, M.F., 1972. Compaction of the Bishop Tuff, California.

Geological Society of America Bulletin 83, 95-106.

83

Rahaman, M.N., de Jonghe, L.C., Scherer, G.W., Brook, R.J., 1987. Creep and

densification during sintering of glass powder compacts. Journal of the American

Ceramic Society 70, 166-774.

Riehie, J.R., Miller, T.F., Bailey, R.A., 1995. Cooling, degassing and compaction of

rhyolitic ash-flow tuffs: a computational model. Bulletin of Volcanology 57, 319-

336.

Robert, G., Russell, J.K., Giordano, D., Romano, C., 2008. High-temperature

deformation of volcanic materials in the presence of water. American

Mineralogist 93, 74-80.

Ross, C.S., Smith, R.L., 1961. Ash-flow tuffs; their origin, geologic relations, and

identification. U.S. Geological Survey Professional Paper 366, 81 pp.

Russell, J.K., Quane, S.L., 2005. Rheology of welding: inversion of field constraints.

Journal of Volcanology and Geothermal Research 142, 173-19 1.

Smith, R.L., 1960a. Ash-flows. Geological Society of America Bulletin 71, 795-842.

Smith, R.L., 1960b. Zones and zonal variations in welded ash-flows. U.S. Geological

Survey Professional Paper 354-F, 149-159.

Sparks, R.S.J., Tait, S.R., Yanev, Y., 1999. Dense welding caused by volatile resorption.

Journal of the Geological Society, London 156, 2 17-225.

Stein, D.J., Spera, F.J., 1992. Rheology and microstructure of magmatic emulsions;

theory and experiments. Journal of Volcanology and Geothermal Research 49,

157- 174.

84

Streck, M.J., Grunder, A.L., 1995. Crystallization and welding variations in a widespread

ignimbrite sheet; the Rattlesnake Tuff, eastern Oregon, USA. Bulletin of

Volcanology 57, 151-169.

Sura, V.M., Panda, P.C., 1990. Viscosity of porous glasses. Journal of the American

Ceramic Society 73, 2697-2701.

Tuffen, H., Dingwell, D.B., Pinkerton, H., 2003. Repeated fracture and healing of silicic

magma generate flow banding and earthquakes? Geology 31, 1089-1092.

Wolff, J.A, Sumner, J.M., 2000. Lava fountains and their products. In: Sigurdsson H.,

Houghton B.F., McNutt 5, Rymer H., Stix J. (eds) Encyclopedia of volcanoes.

Academic Press, San Diego, 321-329.

Zhang, Y., 1999. H20 in rhyolitic glasses and melts: measurement, speciation, solubility,

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85

CHAPTER IV: Discussion

In the experiments, increasing strain is manifest by a reduction in both connected

and isolated porosity. In this section I discuss the extent of water resorption and its

implications for porosity distributions and for deformation textures of the samples. I also

present a discussion of outstanding technical issues and the avenues of future work that

could answer questions that arose during this study. Finally, I present a brief discussion

of the effects of the temperature gradient and pore size distribution and shapes on

deformation.

4.1 Water

The partitioning of porosity between isolated and connected pores and the

mechanisms of pore destruction is the first order result of the experimental study. At the

densification rates used in this study, there is no evidence that welding of porous volcanic

materials produces significant amounts of isolated porosity (Fig. 3.5). Instead, connected

and isolated pores are destroyed in equal proportions; the result is that the ratios of

connected: total and isolated : total porosity remain constant throughout deformation

(Fig. 4.1). Moreover, there is no evidence for isolated porosity being created at the

expense of connected porosity.

Sparks et al. (1999) describe a welding regime in ignimbrites that would be

facilitated by water being trapped during compaction. Their idea is that, during

deformation, water pressure builds up locally to equal lithostatic loading and the increase

in pore fluid pressure forces water to be dissolved into the melt. Water resorption is

expected to reduce melt viscosity, further promoting flow and compaction (welding) of

86

1000

0

0.8

C-)”

-e-”0

0.4

0.2D

00 0.2 0:4 0.6 0.8pt

Figure 4.1 Proportions of connected (circles) and isolated porosity (squares) with deformation (decreasing total porosity).

87

the material. Sparks et al. (1999) also suggest that welding processes driven by fluid

resorption could help explain unusual welding textures such as densely welded, but

unfoliated volcanic rocks, clasts deformed into “U” or “S” shapes, or foliation parallel to

conduit wall, but with no preferred lineation orientation.

There is strong evidence in our experiments supporting the assertion that the

experimental run-products become saturated with water on the timescale of

experimentation (Fig. 3.B). However, none of the experimental run-products show the

increase in isolated porosity (Fig. 3.5) that is expected if a significant amount of water is

being trapped prior to resorption (Sparks et al., 1999). The experiments are nonetheless

examples of a gas resorption regime as described by Sparks et al. (1999) because the fluid

phase is not allowed to escape from the system (sealed cell, closed system). Water in the

system was kept in contact with the volcanic materials throughout deformation and

allowed to diffuse into the undersaturated glass, effectively reducing its viscosity by as

much as 3 orders of magnitude (Giordano et al., In Press). The magnitude of the

viscosity reduction is estimated by comparing the viscosity of the Rattlesnake Tuff melt

at 650 °C and having a post-sintering water content of 0.15 wt.% (Table 3.1) vs. the

viscosity of the melt at 650 °C and having a “model” water content of 0.95 wt.% (PH2O =

5 MPa; Newman and Lowenstern, 2002).

Textural analysis of run products offers no means of discriminating between

samples deformed under wet or dry conditions. At high degrees of total strain, the two

sets of experimental conditions (wet vs. dry): (i) produce strongly foliated samples, (ii)

show similar reduction in total porosity, and (iii) show no differences in ratios of isolated

and connected porosity. This is further evidence for extensive, uniform diffusion of

88

water into the glass phase as opposed to isolated regions of water resorption that would

produce more chaotic textures and strain localization (Sparks et al., 1999).

Measured water contents are 2-3 times higher than predicted by solubility models

(Newman and Lowenstern, 2002) and certainly require further investigation (Fig. 3.B).

Destructive chemical analysis methods were used to measure bulk water contents.

Specifically, InfraRed spectroscopy and Karl-Fischer titration of volatiles released at T

1000 °C were used to measure the bulk water content of each sample, and the two

techniques produced consistent results. These techniques provide a measurement of the

total water remaining in the samples after all moisture is removed at 110 °C; thus they

measure, and do not discriminate, between water dissolved in the glass or water that is

mechanically trapped in pores smaller than the grinding size required for analysis.

A possible explanation for the apparent excess water present in the samples,

relative to the water solubility models (Fig. 3.B; Newman and Lowenstem, 2002), is that

the excess water is trapped in nanopores or as interfacial films between annealed shards

with dimensions smaller than the crushing size fraction required for bulk water content

analysis (-100 gm). In formulating this hypothesis, it is assumed that the existing

solubility models for water accurately predict the solubility at the low temperatures and

pressures used in this study. Spot water analysis techniques such as Fourier Transform

InfraRed spectroscopy (FTIR) would provide a more accurate glass water content and

water distribution across experimental run products than the bulk water analysis methods

used in this study. Typical resolution of the FTIR technique is on the order of 30 by 30

tm, which is smaller than the size of the shards in the samples, allowing probing of

individual shards for water, and is an inexpensive, non-destructive method for water

89

analysis. The nanopore hypothesis could therefore be tested by using the FTIR water

analysis technique on doubly-polished wafers of individual shards or small intensely

welded pieces. Shard interfaces or “rims” in the case of individually polished shards

would be selectively avoided to measure water that is strictly dissolved in the glass, and

not trapped at interfaces between shards. The FTIR technique would also provide a

means of testing for chemical zoning in H20 content, or heterogeneity in water

throughout the samples.

4.2 Experimental design modifications

The design of the experimental apparatus can be improved very simply to provide

for greater water pressure stability. Switching from a one-zone resistance furnace to a

multiple, independently controlled-zone furnace would allow for more freedom in the

design and size of the water cell. The temperature gradients that may be affected by a

change in the configuration of the water cell could be minimized with the additional

control on the temperature provided by a multi-zone furnace. Thus, the design of the

water cell could be modified in the following ways: (i) the bottom of the cell could be

welded shut, and (ii) the top seal could be modified from its current metal-on-metal

configuration to a threaded and water-cooled 0-ring seal of the size of the inner diameter

of the cell. The cell diameter (outside and inside) might have to be increased slightly (-1

cm) to allow for easy sample extraction from the top of the tall (—30 cm) cell. This is the

change that is most likely to modify the temperature gradient in the assembly, and is the

reason why a two-zone furnace should be used if the water cell is to be modified from its

current configuration. A larger cell diameter would also be beneficial in that it would

90

increase the amount of total strain attainable during deformation of low to moderate

porosity samples for which bulging is a concern.

4.3 Temperature gradient

As noted earlier (cf. Chapter II; Fig. 2.2), there is a temperature gradient across

the sample assembly in the deformation experiments. That temperature gradient across a

5 cm long sample is asymmetric; the highest temperature is slightly below the middle of

the core length, and the temperature at the top of the sample is slightly lower than at the

bottom of the sample core. This asymmetry is caused by the positioning of the top of the

water cell outside of the furnace so as to not damage any of the valves and transducers

with heat. The magnitude of the temperature gradient is 8.5 °C over a sample length of 5

cm. This gradient corresponds to a 1% difference in logi between the lowest

temperature point and the highest temperature point in the sample (logi = 11.74 vs.

11.63). This is a small variation in viscosity compared to the experimental resolution of

viscosity of 0.2 log units (cf. Chapter II; Fig. 2.2). Moreover, strain localization due to

the temperature gradient was not observed at the sample scale in the experiments. This

suggests that the temperature gradient across the length of the sample is not sufficient to

localize deformation, despite the wide range of experimental conditions used (e.g., T,

PH2O). The potential impact of the temperature gradients on melt viscosity is also

mitigated by the fact that the effects of porosity on the operating deformation

mechanisms are of greater importance than temperature.

91

4.4 Pore size distribution and pore shape

Beyond the influence of the total amount of porosity present in the samples, and

its distribution as isolated vs. connected pores (cf. Fig. 3.5), pore size distribution and

pore shape are factors that may contribute to the strength of the samples during

deformation. The samples used in the experiments have extremely variable pore shapes

due to the large variations in the ash shard shapes (Figs. 3.2 and 3.4). Moreover, some of

the glass shards and all pumice shards are porous themselves, with variable pore shapes

too (e.g., tube pumice vs. bubble shard).

Most experiments on porous systems pertain to samples with either a coherent

melt fraction (e.g., Stein and Spera, 1992; Bagdassarov and Dingwell, 1992; Lejeune et

al., 1999) or a particulate melt fraction with solid (non-porous) shards (e.g., Rahaman et

al., 1987; Ducamp and Raj, 1989; Sura and Panda, 1990; Quane et al., 2004). The studies

of Friedman et al. (1963), Bierwirth (1982), and Quane (2004) are the only ones on

natural ash, i.e., deformation of samples having a particulate and porous melt fraction,

other than this study. The ability of particles to rotate during deformation may facilitate

porosity reduction. This ability is dependent on the amount of contact surface area in the

sample, itself dictated by the pore size and shape distributions, but primarily by the total

amount of porosity in the sample (Quane, 2004). The progressive compaction of the

samples during experimentation increases contact surface area between the shards and

inhibits rotation. This causes the observed strain-hardening in the viscous experiments

(Fig. 3.3). Changing the ash size fraction used to fabricate the samples would therefore

change the overall total porosity. For example, a smaller size fraction would pack more

and reduce the starting total porosity of the sample, increasing the contact surface area

92

between the shards at the onset of deformation. Strain-hardening in such a sample would

occur at lower amounts of total strain than for a sample of equal dimensions with a larger

ash size fraction.

The results presented in this study are self-consistent and map the relative

behaviour of the sintered ash materials, at the chosen size fraction, for the various

imposed temperature and water pressure conditions. Varying the size fraction in the

samples is not expected to affect this relative behaviour, but may change the temperature

at which rheological transitions are observed for the material. The mechanisms of

deformation observed in the experiments are thought to be representative of deformation

in high porosity volcanic materials, and to be primarily a function of the total porosity

followed by the pore shape and size fraction.

93

4.5 References

Bagdassarov, N.Sh., Dingwell, D.B., 1992. A rheological investigation of vesicular

rhyolite. Journal of Volcanology and Geothermal Research 50, 307-322.

Bierwirth, P.N., 1982. Experimental welding of volcanic ash. B.Sc. Honours Thesis,

Monash University, ‘74.p.

Ducamp, V.C., Raj, R., 1989. Shear and densification of glass powder compacts. Journal

of the American Ceramic Society 72, 798-804.

Friedman, I., Long, W., Smith, R.L., 1963. Viscosity and water content of rhyolite glass.

Journal of Geophysical Research 68, 6523-6535.

Giordano, D., Russell, J.K., Dingwell, D.B., In Press. Viscosity of magmatic liquids: A

model. Earth and Planetary Science Letters.

Lejeune, A.M., Bottinga, Y., Trull, T.W., Richet, P., 1999. Rheology of bubble-bearing

magmas. Earth and Planetary Science Letters 166, 71-84.

Newman, S., Lowenstern, J.B., 2002. VolatileCaic: a silicate melt-H20-C02solution

model written in Visual Basic for Excel. Computers and Geosciences 28, 597-

604.

Quane, S.L., 2004. Welding in pyroclastic deposits. PhD thesis, University of British

Columbia, 208 pp.

Quane, S.L., Russell, J.K., Kennedy, L.A., 2004. A low-load, high-temperature

deformation apparatus for volcanological studies. American Ivlineralogist 89, 873-

877.

94

Rahaman, M.N., de Jonghe, L.C., Scherer, G.W., Brook, R.J., 1987. Creep and

densification during sintering of glass powder compacts. Journal of the American

Ceramic Society 70, 166-774.

Sparks, R.S.J., Tait, S.R., Yanev, Y., 1999. Dense welding caused by volatile resorption.

Journal of the Geological Society, London 156, 217-225.

Stein, D.J., Spera, F.J., 1992. Rheology and microstructure of magmatic emulsions;

theory and experiments. Journal of Volcanology and Geothermal Research 49,

157- 174.

Sura, V.M., Panda, P.C., 1990. Viscosity of porous glasses. Journal of the American

Ceramic Society 73, 2697-2701.

95

CHAPTER V: Summary

I have successfully built and calibrated a sealable cell for high-temperature

deformation experiments. I have used the cell in support of experiments that investigate

the rheology of highly porous rhyolite under controlled water pressure. The main

experimental program was designed to explore the rheological effects of two important

variables in volcanic systems: porosity and water pressure. These effects are best studied

in experiments where other variables such as temperature and displacement rate are kept

constant. All experimental samples were created to have very similar starting porosity,

however, the total strain applied to each sample (at constant PH2O) was varied to obtain

snapshots of the final porosity along the same deformation path (Figs. 3.3-3.5). This

approach also provided a test for experimental reproducibility (Fig. 3.3a). Water pressure

was then varied within a given total strain experimental suite to elucidate the effect of

water pressure on rheological behaviour.

The data generated during the experiments include time, load, displacement, and

water pressure. Pre- and post-experimental characterization of the samples includes

measurements of radius, length, mass, volume, density, porosity, and bulk water content.

The experiments clearly show the strongly strain-dependent rheology of porous rhyolite,

and how the presence of water (and the magnitude of the water pressure) minimizes that

strain dependence. Thus, the main experimental program was ideal to explore the

dynamic rheology of porous rhyolite. A subset of the main experimental program was

designed to determine the glass transition temperature of the experimental materials at the

experimental timescales used in the study. The addition of pore space to the dry

Rattlesnake Tuff melt expanded the field of viscous deformation relative to the dry, non

96

porous melt, and the effect of porosity on the wet melt (assuming a minimum solubility

as predicted by existing solubility models) was even more dramatic.

Ultimately, the experimental data obtained in this study will be used to build a

model for the rheological behaviour of porous, wet materials during deformation, and to

extract their melt viscosity. The first step towards the comprehensive model requires

modeling the partitioning of strain during the experiments. If the relationship between

the volume and radial strain is accurately described, a relationship relating the melt

viscosity and the porosity of the sample during deformation to the recorded apparent

viscosity could be developed.

97

APPENDIX A: Cell design

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APPENDIX B: Data acquisition

For all deformation experiments on natural volcanic material, time (s), load

applied to sample (pounds), and total shortening of the sample (inches) are measured by

the VDR and recorded on the VDR computer system. For wet experiments, water

pressure is also recorded. All experimental data files are compiled in an Excel

spreadsheet on a CD at the back of the thesis.

100

APPENDIX C: Experimental data

Plots of raw, unprocessed data for all deformation experiments on natural material

are provided in a PDF document on a CD at the back of the thesis. Each experiment is

presented as a two (dry) or three (wet) part figure as (i) load vs. time, (ii) load vs.

displacement, and (iii) water pressure vs. time. An example of the figures provided

electronically is given for sample RSO3 (Fig. C. 1).

101

500 500

400 ., 400

C . V V C . V

z,nn .

0 . VV V

00 5000 10000 15000 0 0.5 1 1.5

Time (s) Displacement (in)

20

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5

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Time(s)

Figure C.1 Experiment RSO3.

102

APPENDIX D: Data processing

The following pages provide the MATLAB code used to process all the raw

experimental data files. Commented sections of the code (preceded by the percent

symbol %) provide explanations for the different data processing steps.

103

% This code uses modified VDR data files to:% convert all experimental data to SI units% correct data for accidental negatives% correct data for effect of PH2O% calculate stress from load (assumption: constant area)% calculate strain from displacement% calculate strain rate% smooth stress data% calculate viscosity as stress/strain rate

clear all

% data files are setup as such:% first row:% di(mm) li(mm) df(mm) lf(mm)% all following rows:% step t(s) load(lbs-force) displacement(inches)

% disp(’experimental data file & hit RETURN’)% raw=load(input(’experimental data file name? ‘,‘s’));load rs3.datraw=rs3;

[rows columns] = size(raw);

% sample size -- convert to SI units% initial diameter (m)dim = raw(1,1)/1000% initial length (m)lim = raw(1,2)/1000;% final diamter (m)dfm raw(1,3)/1000;% final length (m)lfm = raw(1,4)/1000;

% initial area of core (m2)Ai = pj*((dinJ2)A2);% initial volume of core (m3)Vi = Ai*lim;% final area of core (m2)Af = pj*((dfirJ2)A2);% final volume of core (m3)Vf=Af*lfm;

core_initial=[dim lim Ai Vi]core_final=[dfm lfm Af Vfj

104

% experimental datano_rdng=rows- 1;ts=raw(2:rows,2);dispin=raw(2:rows,4);

% correct load for accidental negatives & for Ph2o% correction for loadforce_lbs=raw(2:rows,3); % raw data NOT in SI units% correction for accidental negativesfor i=1:no_rdng

if force_lbs(i,1)<O;force_lbs(i, 1)=0;

endend

% correction for Ph2oforce_lbs_h2o=force_lbs-force_lbs(1,1);

% load & displacement in SIforce_N_h2o=force_lbs_h2o. *4.4482216;% dispm is the displacement in inches converted to metresdispm=dispin. *0.0254;

% load to stress (N to Pa)stress_Pa=force_N_h2o/Ai;% scale by the initial area of the core ==>

% assumption is there is no change in area

% displacement to strain (m to dimensionless)strain=dispmllim; %total strain

%%% Fit for displacement rate% AX=B% X=A\B === the solution to this is the slope with a 0 intercept% ts=A% slope=X == I will call XT1disprate_fit”% dispm=B

disprate_fit = ts\dispm % disprate_fit units are rn/sdispm_fit = ts.*disprate_fit; %this is the calculated displacement using the fit

strain_fit = dispm_fit/lim;strain_rate_fit = disprate_fit/lim;

105

plot(dispm,dispm_fit, ‘-r’)pauseplot(ts,dispm_fit, ‘-.b’)pauseplot(ts, strain_rate_fit, ‘-g’)pause

%%% Smooth stress (Pa) dataplot(ts, stress_Pa, ‘Ok’)hold on

windowSize=[ 1,2,3,5,7,15,20]

for i=1:7stress_Pa_filter=filter(ones( 1 ,windowSize(i))/windowSize(i), 1 ,stress_Pa);

plot(ts,stress_Pa_filter,color(i));disp(’size of filtering window’)windowSize(i)

xxx=input(’accept filtering window by typing desired window size (reject by hittingreturn)’,’s’)

if xxx == [1continue

elseif xxx > 0break

endendhold off

stress_Pa_final=stress_Pa_filter;

plot(strain, stress_Pa_final/1000000, ‘-k’);xlabel(’\epsilon_{ total }‘)ylabel(’\sigma (MPa)’)xlim([0 1]);disp(’Save figure?’);ANSWER=input(’hit RETURN for YES; any NTJMBER for NO ‘);if isempty(ANSWER) == 0;

‘do not save’else

saveas(gcf, input(’.fig file name? ‘,‘s’), ‘fig’)end

106

format bank

visc_nolog=stress_Pa_final./strain_rate_fit;visc_filter=log 1 O(stress_Pa_final./strain_rate_fit);plot(strain, vise_filter, ‘-k’);xlabel(’\epsilon_{ total }‘)ylabel(’log_{ 1O}\eta_{eff} (Pa s)’)pause

disp(Save figure?’);ANSWER=input(’hit RETURN for YES; any NUMBER for NO ‘);if isempty(ANSWER) == 0;

‘do not save’else

saveas(gcf, input(’.fig file name? ‘,‘s’), ‘fig’)end

% compare viscosities obtained from “raw” data% and smoothed datavisc_filter_max=log 1 0(max(stress_Pa_final)/strain_rate_fit);visc_max=log 1 0(max(stress_Pa)/strain_rate_fit);

viscosity=[visc_max; visc_filter_max]pause

107