Igneous Petrology Andri's Lecture Note

8/12/2019 Igneous Petrology Andri's Lecture Note

1/84

Created & compiled by Andri SSM Octo-2004Petrology & Economic Geology LaboratoryDept. of Geology FIKTM ITB


2/84



3/84



4/84

History of ConceptsIn the 1960s, geologists were seeking ways to prove or disprove the new idea of moving plates. Exploration

of magnetic anomalies at mid-ocean ridges provided strong support for seafloor spreading . Geologistsstudied other ocean features to see how they related to plate tectonics. While visiting Hawaii, Tuzo Wilson,

one of the founders of the theory of plate tectonics, noticed some interesting features about ocean islands.

On a map of the Pacific basin, he found three linear chains of volcanoes and submarine volcanoes

(seamounts).



5/84

An interesting pattern emerged. For each chain, the islands become progressively younger to

the southeast. The extreme southeast end of each chain is marked by active volcanoes.

Wilson proposed that the Hawaiian islands formed successively over a common source of

magma called a hot spot. The Island of Hawaii is currently located above the hot spot.

C

reated&compiledbyAnd

riSSM

Octo-2004

P

etrology&EconomicGeologyLaboratory

D

ept.ofGeologyFIKTMITB


6/84

Hot, solid rock rises to the hot spot from greater depths. Due to the lower pressure at the

shallower depth, the rock begins to melt, forming magma. The magma rises through the Pacific

Plate to supply the active volcanoes. The older islands were once located above the stationary

hot spot but were carried away as the Pacific Plate drif ted to the northwest .

Created&com

piledbyAndriSSM

Octo-2004

Petrology&Ec

onomicGeologyLaboratory

Dept.ofGeolo

gyFIKTM

ITB


7/84

ScorMafic - Dark colored

PumiceFelsic - Light coloredFrothy

ObsidianDark to black - felsic (DOES NOT

follow normal color index)Glassy

PeridotiteUltramafic

GabbroMafic - Dark colored

DioriteIntermediate

GraniteFelsic - Light colored

Coarse grained -

Generally intrusive

DiabaseMafic - Dark colored

DaciteIntermediate

- - - - - -Felsic - Light coloredMedium grained -

Dikes, sills, etc.

BasaltMafic - Dark colored

AndesiteIntermediate

RhyoliteFelsic - Light coloredFine grained -

Extrusive, volcanic

IGNEOUS

ROCKS

Interlocking

homogenous

crystalline

texture - nopreferred

orientation to the

mineral grains

Rock NameGeneral color and/or composition -Miscellaneous observations

Texture - ave.size of minerals

GeneralRock Type



8/84

Table 1: The Organization of Igneous Rocks

VolcanicBreccia

TuffF r a g m e n t a l : made ofigneous fragments

Scoria,Basalt Glass

ObsidianPumiceG l a s s y : cooled veryquickly above ground

BasaltAndesiteDaciteLatiteTrachyte,Felsite

RhyoliteF i n e - G r a i n e d : cooledquickly above ground

BasaltPorphyry

AndesitePorphyry

GranodioritePorphyry

MonzonitePorphyry

SyenitePorphyry

GranitePorphyry,RhyolitePorphyry

Po r p h y r i t ic : cooled firstbelow then aboveground

Peridotite,Dunite,Pyroxenite

GabbroDioriteGranodioriteMonzoniteSyenitePegmatite,Granite

C o u r s e - g r a i n e d : cooledslowly underground

No feldspar.Few silicates.Pyroxene,olivine.

Moreplagioclasethanorthoclase.Also biotite,amphibole,

pyroxene,augite, olivine,horn-blende,biotite

Orthoclase in similar quantities as plagioclase.Also biotite, amphibole, pyroxene, hornblende,augite

More orthoclase thanplagioclase. Also muscovite,biotite, amphibole, hornblendeOrigin

(No quartz)(No quartz)(No quartz)(No quartz)(No quartz)(Little quartz)(With quartz)Minerals

UltrabasicMaficInter-mediateInter-mediateInter-mediateFelsicFelsicSubgroup

Created&compiledbyAndriSSM

Octo-200

4

Petrology&Econ

omicGeologyLaboratory

Dept.ofGeology

FIKTM

ITB


9/84

Cre

ated&compiledbyAndri

SSM

Octo-2004

Petrology&EconomicGeologyLaboratory

De

pt.ofGeologyFIKTM

ITB


10/84

Igneous Rocks.

Crystallization of Magma

Magma is a hot liquid (made of melted rock), with a abundant gas in solut ion when it is

under pressure in the earth. It may contain crystals of high temperature minerals. It

originates in the asthenosphere. Magma is the liquid (with dissolved gases) from which all

igneous rocks solidi fy. The geothermal gradient, about 30 degrees C/km at shallow depths,would produce temperatures above 1000 degrees C at just 33 km below the surface so it

must decrease rapidly with depth. At depth pressure raises the melting point so melting

only occurs in a narrow zone where the temperature in the earth overcomes the pressure

and partial melting occurs. This zone is the asthenosphere. Click the diagram at r ight.

At high temperature in the liquid the ions have so much vibrational energy that bonds

cannot form. As the temperature drops the atoms vibrate less and bonding can occur.

Crystallization implies that the ions bond together in a regular pattern so that the exterior of

the crystal wil l have a regular geometric form (as we saw in the images of the crystals in the

chapter on mineralogy.

The rate of cool ing affects the size of the crystals There are two types of igneous rocks

Plutonic rocks (intrusive) form when the magma cools slowly beneath the surface, while

volcanic rocks (extrusive) form when magma reaches the surface and cools rapidly as lava

flows or fragmental material.



11/84



12/84

Mineral Gallery



13/84

PHYSICAL CHARACTERISTICS:

Coloris variable and tends toward pale yellows, browns, grays, and also white,

blue, black, reddish, greenish and colorless.

Lusteris adamantine to waxy.

Transparency crystals are transparent to translucent in rough crystals.

Crystal System is isometric; 4/m bar 3 2/mCrystal Habits include isometric forms such as cubes and octahedrons, twinning

is also seen.

Hardness is 10

Specific Gravity is 3.5 (above average)

Cleavage is perfect in 4 directions forming octahedrons.Fracture is conchoidal.

Streak is white.

Associated Minerals are limited to those found in kimberlite rock, an ultramafic

igneous rock composed mostly of olivine.

Other Characteristics: refractive index is 2.4 ( very high), dispersion is 0.044,fluorescent.

Notable Occurrences include South Africa and other localities throughout

Africa, India, Brazil, Russia, Australia, and Arkansas.

Best Field Indicatoris extreme hardness.



14/84

Igneous Textures

Texture is the overall size and appearance of the minerals in the rock.

The most important factor affecting the texture of the rock is the rate of

cooling.

Igneous rocks are classif ied on the basis of texture and mineral

composition(1) Aphanitic texture: This fine-grained texture indicates that the rock

crystallized rapidly at or near the surface (usually lava flows) of the

earth. Mineral composition is often difficult to identify but if the rock is

light colored it is probably made of nonferromagnesium minerals and if

dark, ferromagnesium minerals. Many aphanitic rocks (basaltic lava)have vesicles which are cavities and small openings from gas bubbles

(2) Phaneritic texture: A coarse grained texture indicates that the rock

crystallized slowly deep within the earth. These rocks are now exposed

at the surface because of uplift and erosion

Aphanitic vs phaneric texture indicates very rapid cooling at the surface

because no minerals have had a chance to grow. The rock obsidian for

example is a glassy rock. Obsidian-2, result from violent ejection from

the volcano. Some are made of f ine ejected fragments (tuff) whi le others

are large angular blocks. They are all glassy

Created&compile

dbyAndriSSM

Octo-2004

Petrology&Econo

micGeologyLaboratory

Dept.ofGeologyFIKTM

ITB


15/84

Magmatic Differentiation, Assimilation and Magma Mixing.

There is a wide variety of igneous rocks types but only a few basic types ofmagma, because the asthenosphere and upper mantle have a fairly uniform

composition.

(1) One process of developing more than one type of rock from a common

magma is called magmatic differentation. If early formed crystals sink (crystalsettling) to the bottom and the magma crystallizes or is then injected upward

into overlying rocks, the resulting rock will have a different compostion from

the original magma.

(2) When a molten body moves up through "country rock" it assimilatesrock.(melts and incorporates elements from the surrounding rock). This

changes the magma composition.

(3) Magma mixing etc. At convergent boundaries rising molten bodies, may

overtake one another and mix to form average compostions.

How do magmas move toward the surface?

(1) By Assimilation, that is, melting the surrounding rocks.

(2) By Stoping: the magma forces its way into fractures and large blocks

(inclusions) drop into the magma chamber.

(3) By Forceful Intrusion: Simply pushing up the surrounding rock.

Created&co

mpiledbyAndriSSM

Octo-

2004

Petrology&EconomicGeologyLaborat

ory

Dept.ofGeo

logyFIKTM

ITB


16/84

Naming Igneous Rocks:

This is a table of igneous rocks classified on the basis of texture vs.mineral composition.

Pumice makes lava flows and is also the main component of

volcanic tuff.--frothy

Obsidian--massive

Pyroclastic = Tuff if < about 4mm and Breccia if > 4 mm.--fragmental

Amorphous

(glass)

BasaltAndesiteRhyoliteFine-grained

PeridotiteGabbroDioriteGraniteCoarse-

grained

Olivine and

Pyroxene

Mantle rocks

High in Mg and Fe

Pyroxene and Na-

Ca feldspar

Intermediate

High in Si

and Al

K-feldsparand Qtz.

Ultramafic

Basaltic (mafic)

(high temp.

minerals)

Andesitic

Granitic

(low temp.

minerals)

Texture

It clear that the rocks on left are rich in SiO2. In contrast, Ultramafic rocks are very rich in

Mg and Fe and are "mantle rocks" . Typically granites, rhyolites and tuffs have over 70%

sil ica and basalts less than 50 %. Less sil icon makes basalts more fluid, e.g. less viscousand affects the eruptive style of the volcano.



17/84


Octo-2004

Petrology&

EconomicGeologyLaboratory

Dept.ofGeo

logyFIKTM

ITB


18/84

Basaltic Rocks

Basalt: this dark colored, fine-grained rock (often vesicular) is common

in volcanic areas such as the ocean basins and areas of continental

tension in which the magma came up from the mantle with little or no

contamination. Because of this it is often called a "primary magma".

Islands in the oceans such as Hawaii, are make almost entirely of basalt.

The ocean basins themselves are made of basalt. Basalt is composed of

pyroxene, Ca-feldspar and sometimes olivine but is very fine-grained.

When basalt cools it contracts and cracks. These fractures and often

make six-sided columns. This is called columnar jointing

Gabro is the intrusive equivalent of Basalt and is made up of same

ferromagnesium minerals as in basalt with some lighter colored Ca-

feldspar. So we should expect to find gabbro as the intrusive rocks

below the basalt in the ocean basins.



19/84

Created&compile

dbyAndriSSM

Octo-2004

Petrology&Econo

micGeologyLaboratory

Dept.ofGeologyFIKTM

ITB


20/84



21/84


22/84

Andesitic Rocks. (intermediate)

Andesite is the common kind of volcanic rock associated withe the

subduction zones in the circum-Pacific volcanic chains both on the

continents and in the island arcs.The common minerals are plagioclase

feldspars (light and rectangular) and amphiboles.(elongate and dark)Diorite is the plutonic equivalent of andesite. It looks l ike granite but

does not have quartz and has more plagioclase feldspar and dark

minerals

C


riSSM

Octo-2004

P

etrology&EconomicGeologyLaboratory

D

ept.ofGeologyFIKTMIT

B


23/84

Current Seismicity for Australia - IndonesiaUpdated as of Sun Nov 2 23:12:20 UTC 2003.

Plate boundaries in

yellow.

Open circles:

Earthquake Activi ty in

the last 30 days; not

clickable.

Creat

ed&compiledbyAndriSSM

Octo-2004

Petro

logy&EconomicGeology

Laboratory

Dept.ofGeologyFIKTM

ITB


24/84

Created&compiledbyAndriS

SM

Octo-2004


Dep

t.ofGeologyFIKTM

ITB


25/84

BASIC DEFINITIONS


26/84

BASIC DEFINITIONSMinerals

Minerals are:

Naturally occurringInorganic

Solids

Minerals have a definite chemical composit ion

Minerals have an orderly internal crystal structure

Minerals are the building blocks of rocks. Each mineral has dif ferent physical and

chemical properties which allow it to be identified. Physical properties you wil l use toidentify the minerals include color, hardness, luster, cleavage, magnetism, reaction to

acid, etc.

Rocks

An aggregate of one or more minerals. Rocks are the building blocks of the Earth's

crust. The Earth's cont inental crust is dominated by granite, and the oceanic crust is

dominated by basalt. Both of these are igneous rocks.

There are three basic categories of rocks:

Igneous (or crystallized from hot lava or magma) - ex. granite, basalt

Sedimentary (or fragments laid down by water or wind) - ex. sandstone, shale,

limestone

Metamorphic (or rocks changed by heat and or pressure) - ex. gneiss, schist, slalte,marble

Physical Properties of Minerals

Color - The color of the mineral as it appears in reflected light to the naked eye.

Luster - The character of the light reflected from the mineral. A mineral may have a

metallic luster (in other words, you would call i t a metal), or a non-metallic luster.

Non-metallic lusters may be described in more detail as:



27/84

..7Quartz4Fluorite

..6.5-7Peridot6-7Epidote

..2.5-4.5Pearl7.5-8Emerald

6-7Zoisite5-6.5Opal5Dioptase

7.5Zircon (med.)5-5.5Obsidian5-6Diopside

6.5Zircon (low)6-6.5Nephrite10Diamond

4-5Variscite5.5Moldavite8.75Cubic Zirconia

5-6Turquoise6-6.5Marcasite9Corundum

(Sapphire / Ruby)

7-7.5Tourmaline3.5-4Malachite3.5-4Coral

8Topaz5-6Lapis-Lazuli8.5Chrysoberyl

8Spinel6Labradorite6.5-7Chalcedony

5-5.5Sphene5-6Lazulite3Calcite

5-6Sodalite2.5-4Jet5.5Brazilianite

1-1.5Soapstone6.5-7Jadeite3.5-4Azurite

2-4Serpentine7-7.5Iolite5Apatite

6-6.5Rutile5.5-6.5Hematite7-7.5Andalusite

5.5-6.5Rhodonite5-6Glass2-2.5Amber

1.5-4.5Rhodochrosite7-7.5Garnet2Alabaster

ALPHABETICAL MOHS TABLE OF HARDNESS



28/84

IGNEOUS COMPOSITIONAL NAMES AND MAGMA TYPES

Ca/Na or Ca/K

Mg/Fe

Water Content

Mafic Mineral

Content

LiquidusTemperature

granitegranodioritediorite or

quartz dioritedioritegabbroperidotite

Intrusive Rock

Name

rhyolitedaciteandesitebasalticandesitebasaltkomatiiteExtrusive RockName

felsicintermediate to

felsicintermediate

mafic to

intermediatemaficultramaficMagma Type

acidicor

silicic

intermediate to

acidic or silicicintermediate

basic to

intermediatebasicUltrabasic

Compositionalor Chemical

Equivalent

>6863 - 6857 - 6352 - 5745 -52


29/84


30/84

TextureIgneous textures are classif ied by the presence or absence of crystals,the size of the crystals, and the size and density of vesicles (holes).

Check out this page for a nice summary of IGNEOUS TEXTURES

Extrusive rocks:Pyroclastic rocks are classif ied by grain size from BOMBS (>64mm) toash (


31/84


Octo-2004

Petrology&E

conomicGeologyLaborato

ry

Dept.ofGeologyFIKTM

ITB


32/84

Igneous Rocks:

Rocks that form from magma:

mixture of liquid, mineral crystals and gas

(mostly water vapor); 95% of earth's crust is igneous or metamorphosed

igneous rock

usually divided into two broad genetic groups:

Intrusive (plutonic)rocks that form from magma cooling beneath the earth's surface

Extrusive (volcanic): rocks that form from magma cooling at the earth'ssurface; but how can you tell where an igneous rock or iginally formed?Texture :is a proxy for cooling rate; coarse-grained (phaneritic) cooled slowly

all coarse-grained igneous rocks are intrusive

Fine-grained (aphanitic) cooled quickly includes all volcanic rocks AND someintrusive rocks (for example: those that formed when magma squeezed into

other rocks along fracture planes) a mixture of coarse and fine crystals can

reflect two stages of cooling and sometimes cooling can be so rapid that gas

bubbles are trapped (vesicles, pumice) or that mineral crystals don't even

have time to form (volcanic glass)



33/84

Igneous Rock Types

- formed by crystallization of molten rocks called magma- Classified based on:

A. Texture- primarily crystal size.

Intrusive - coarse-grained, slow cooling at depth; "plutonic rocks" (plutons,

batholiths, stocks, dikes, sills).

Extrusive - fine-grained, rapid cooling at or near surface; "volcanic rocks"

(lavas, pyroclastic rocks)

B. Chemical and mineral composition (SiO2 varies from 45 to 70 wt%)

Felsic- mostly quartz, K- and Na-feldspars, muscovite; high SiO2; light-

colored; low-T (~700-800C), high-viscosity. Granite, Rhyolite.

Mafic- mostly Fe-Mg rich ol ivine and pyroxene, Ca-rich feldspars; lowSiO2; dark-colored; high-T (~1100-1200C), low-viscosity. Basalt, Gabbro.

Intermediate- Na-rich feldspars, amphibole, biotite, minor

quartz. Granodiorite, Dacite, Diorite, Andesite.

Ultramafic - Mg-rich olivine and pyroxenes. Peridotite, Komatiite

Created

&compiledbyAndriSSMOcto-2004

Petrology&EconomicGeologyLab

oratory

Dept.ofGeologyFIKTM

ITB


34/84

2. Magma formation and differentiation

Magma forms when rocks melting T is exceeded; melting T depends on rockcomposition and conditions of T and P. Rocks are multicomponent systems,

thus they melt over a range of T's. Minerals that melt at the lowest T melt f irst

and produce a partial melt. Partial melts can have very dif ferent composition

than a completely melting rock (ie., basalt from the mantle). Partial melts r ise

and coalesce to form magma chambers. Increasing P causes melting T toincrease. Explains why most of crust and mantle are solid. Partial melting may

be induced by lowering P rapidly (decompression melting).

Composition:

Rocks with minerals that crystallize at low-T also melt at low T. Water lowersthe melting T significantly.

Diversity of igneous rocks was first explained by Magmatic Differentiation

(Bowen, 1928). Process by which a uniform parent magma evolves into

daughter magmas of varied composition. Occurs via fractional Crystallization.(gravity settling or by compaction/deformation). Bowen's Reaction

Series. Modern theories are more complex but incorporate Bowen's

theories. Important concepts are: (1) partial melting of various source rock

compositions (2) dynamic magma chamber processesCreated & compiled by Andri SSM Octo-2004

Petrology & Economic Geology LaboratoryDept. of Geology FIKTM ITB


35/84

Volcanism

1. VOLCANIC ROCKS- Lavas - magma that extrudes relatively quietly onto surface.

A. Basaltic- high T (~1100C), dark, low viscosity, far traveling (10's of km).

Flood basalts- immense plateaus, fluid-lava, on flat terrain, 100's m thick.

Pahoehoe - smooth, ropey flows, dissolved gases, forms tubes.

Aa - blocky, jagged flows, degassed lava.

Pillow lavas - underwater cooling, spheriodal blocks, like toothpaste.

B. rhyolitic - low T (800-1000C). light, high viscosity, local, forms domes.-

Pyroclastic Deposits - vapor P release explosively ejects magma into air.

Pyroclasts- material ejected, classed by size. Ash to house-size; tuffs; breccias.

Pyroclastic Flow - hot mix of ash, gas, and dust that travels near surface, flows downflanks of volcanoes up to 200 km/hr; most dangerous volcanic hazard.

Created & compiled by Andri SSM Octo-2004



36/84

VOLCANIC LANDFORMS and eruptive styles- Shield Volcanoes - very large, convex, broad, "shield-shaped" cone, thousands

of very fluid-lavas (mostly basalt), typical at hotspots; Mauna Loa, Kilauea.

- Cinder Cones - small, concave cones, made of layers of cinders, commonly

basaltic

- Composite Volcanoes - large, concave, steep-sided, alternating lavas flows and

pyroclastic deposits, andesitic, erupt explosively, subduction zones; Mt. St.

Helens.

- Volcanic Domes - small, steep-sided domes, viscous rhyolitic magma, usually

plugs the vent of composite cones after explosive eruption, periodically collapse or

explode.

- Calderas - large collapse structures; emptying of large, shallow magma chamber

during violent eruptions. Occur on shield and composite volcanoes. Very

dangerous. Yellowstone; Long Valley

- Phreatic Eruption - magma in contact with water (ground, sea, lake, ice); can be

very explosive; Krakatoa 1883.

- Fissure Eruptions - large volumes of lava from linear cracks; shield volcanoes;mid-ocean ridges; Iceland; flood basalts.

- Lahars (mudflows) - warm mix of wet volcanic debris; moves rapidly in stream

valleys; melting glacial ice or rain on recent pyroclastic deposits. Dangerous



Origin Created & compiled by Andri SSM Octo 2004


37/84

Origin

"fire-formed rocks"Crystallize from molten material:

Magma - below the Earth's surface

Lava - erupts onto the Earth's surface through a volcano or crack (fissure)

Lava cools more quickly because it is on the surface.

Cooling Rates

Cooling rates influence the texture if the igneous rock :

Quick cooling = fine grains

Slow cooling = coarse grains

Rhyolite Granite

Aphanitic - fine grain size (< 1 mm);

result of quick coolingPhaneritic - coarse grain size; visible

grains (1-10 mm); result of slow cooling


Created & compiled by Andri SSM Octo 2004


38/84

Porphyritic-

Mixture of grain sizes caused by mixed cooling history; slow cooling first, followedby a period of somewhat faster cooling.

Terms for the textural components: Phenocrysts - the large crystals;

Groundmass ormatrix - the finer crystals surrounding the large crystals. The

groundmass may be either aphanitic or phaneritic. Types of porphyritic textures:

Porphyritic-aphanitic & Porphyritic-phaneritic

Origin: mixed grain sizes and hence cooling rates, imply upward movement ofmagma from a deeper (hotter) location of extremely slow cooling, to either:

a much shallower (cooler) location with fast cooling (porphyritic- aphanitic), ora somewhat shallower (slightly cooler) location with continued fairly slow

cooling (porphyritic-phaneritic).



39/84




40/84




41/84





42/84

p yPetrology & Economic Geology LaboratoryDept. of Geology FIKTM ITB

Hydrothermal cycles at spreading zone that

mainly associated with Mid Oceanic Ridge


43/84

Primary magmas

Primary magma-any chemically unchanged melt derivedfrom a partial melting of its (mantle) source rock

Primitive magma-ambiguous but means unmodified

Parental-magmas that give rise to other derivativemagmas.

Interested in knowing if magmas are co-genetic

Primary Melt reflect process-fractional vs equilibrium,depths (init, cease), source, degree of melting

Modified by -fractional crystallization-magma mixing-assimilation (crust, lithoshere)

-liquid immiscibility



44/84

Vessiculation: depressurization leads to gas becoming lesssoluble and formation of bubbles-depends on amount of gas,composition of melt, and pressure Once bubbles form, magma is less

dense, rises more, loses more gas...

If viscosity is low, bubbles coalesce and escapeif slow flow, bubbles leave non-violently.if fast flow, magma gets thrown out with a burp-bombs or scoria.

If viscosityis

high, bubbles cant coalesce

they get bigger and eventually cause disruption of magma, explosive

eruption

Other possible driving forces

tectonic forces-squeezing l ike toothpaste?diapirs-blobs rising



H d R k M lt?


45/84

How do Rocks Melt?

Magma, Lava and their products

Created&comp

iledbyAndriSSM

Octo-2004


Dept.ofGeologyFIKTM

ITB


46/84

The Geotherm-Why is the Interior Hot?

Sources of Heat

Adiabatic CompressionPrimordial Heat

Radioactivity

Sunlight

How does Earth get rid of Heat?

Radiation

Conduction

ConvectionPlate Tectonics

Created & compiled by Andri SSM Octo-2004Petrology & Economic Geology Laboratory

Dept. of Geology FIKTM ITB


47/84

Temperature of The Earth

The Geotherm

Created&comp

iledbyAndriSSM

Octo-2004

Petrology&Eco

nomicGeologyLaboratory

Dept.ofGeologyFIKTM

ITB

Wh i th E th' I t i H t?


48/84

Why is the Earth's Interior Hot?Sources of Heat

Pressure-Adiabatic Compression

Primordial Heat-Left over from the formation

Decay of Radioactive Isotopes- (K, Th, U)

Sunlight-Only penetrates a few inches



H t t id f H t?


49/84

How to get rid of Heat?Convection-(mass)

Radiation-(light)

Conduction-(atomic vibrations)

Most efficient for Earth's Interior-leads to Plate Tectonics

Created&compiledbyAndri

SSM

Octo-2004

Pe

trology&EconomicGeologyLaboratory

De

pt.ofGeologyFIKTM

ITB

When do Rocks Melt?


50/84

When Geotherm Crosses Melting Curve

All Materials have a Melt ing Point

The Melting Point changes with Pressure & Composition

(Melting Curve)



Causes of Melting


51/84

Causes of Melting

Adiabatic Decompression or Pressure Release Melting


Octo-2004

Petrology&EconomicGeolo

gyLaboratory

Dept.ofGeologyFIKTMITB


52/84

Addition of Water



What Happens after Melting?


53/84

pp gWhere does melting happen?Transport

Eruption

Ponding

Solidification


Octo-2004

Petrology&EconomicGeolo

gyLaboratory

Dept.ofGeologyFIKTMITB

What Solidifies First?


54/84

Bowen's Reaction SeriesOne Magma-All sorts of rocks

Discontinuous Series-Fe, Mg rich Silicates

Continuous Series-Ca, Na, K, rich Silicates



PLUTONIC=COOL BELOW THE SURFACE; COOL SLOWLY -> LARGE CRYSTALS

VOLCANIC=COOL ABOVE THE SURFACE; COOL FAST > SMALL CRYSTALS & GLASS


55/84

VOLCANIC=COOL ABOVE THE SURFACE; COOL FAST -> SMALL CRYSTALS & GLASS

RHYOLITEANDESITEBASALT

VOLCANIC

GRANITEDIORITEGABBROPLUTONIC

LIGHT/FELSICCOLORDARK/MAFIC

HIGHLOW

as high as 72%~50%

Created&compiledb

yAndriSSM

Octo-2004

Petrology&Economic

GeologyLaboratory

Dept.ofGeologyFIK

TM

ITB

Igneous textures: 2004

ory


56/84

Igneous textures:Glassy - instantaneous cooling

Obsidian = volcanic glass

Aphanitic - fine grain size (< 1 mm); result of quick cooling Basalt

Rhyolite

Andesite

ObsidianRhyolite


Octo-2

Petro


Laborato

Dept

.ofGeologyFIKTM

ITB

PHANERIC


57/84

Granite

DioriteGabbro

Pegmatitic - very large crystals (many over 2 cm)

Phaneritic - coarse grain size; visible grains (1-10 mm); result of slow coolingGranite pegmatite or pegmatitic granite


Octo-2004

Petrology&EconomicG

eologyLaboratory

Dept.ofGeologyFIKTM

ITB

Porphyritic- Mixture of grain sizes caused by mixed


58/84

cooling history; slow cooling first, followed by a period of

somewhat faster cooling.Terms for the textural components:

Phenocrysts - the large crystals

Groundmass ormatrix - the finer crystals

surrounding the large crystals. The groundmass

may be either aphanitic or phaneritic.

Types of porphyritic textures:Porphyritic-aphanitic

Porphyritic-phaneritic

Origin: mixed grain sizes and hence cooling rates, imply

upward movement of magma from a deeper (hotter)

location of extremely slow cooling, to either:

a much shallower (cooler) location with fast cooling

(porphyritic- aphanitic), or

a somewhat shallower (slightly cooler) location with

continued fairly slow cooling (porphyritic-phaneritic).

Rock name = porphyry

Granite porphyry or porphyritic granite (porphyritic-phaneritic) - phenocrysts usually potassium feldspar

Andesite porphyry or porphyritic andesite

(porphyritic-aphanitic) - phenocrysts usually

hornblende (amphibole)Created & compiled by Andri SSM Octo-2004Petrology & Economic Geology Laboratory



59/84

Low viscosity basaltic lava flow from an active volcano on one of the

Hawaiian Islands.

Created&co

mpiledbyAndriSSM

Octo-

2004

Petrology&EconomicGeologyLaborat

ory

Dept.ofGeo

logyFIKTM

ITB


60/84

A cinder cone is a small volcano (high viscosity magma), between 100 and 400

meters tall, made up of exploded rock blasted out of a central vent at a high

velocity

Created&co

mpiledbyAndriSSM

Octo-2004

Petrology&E

conomicGeologyLaboratory

Dept.ofGeologyFIKTM

ITB

Mount St. HelensThe above image is a post-eruption

computer rendering of Mount St. Helens


61/84

eruption on May 18, 1980 from a U.S.Geological Survey digital

elevation model (DEM). The lateraleruption removed 2.8 cubic kilometers of

rock and sediment from from the volcano

and lowered its height by 400 meters.

Detectable amounts of ash were spread

over 50,000 square kilometers of area

surrounding the volcano. The large cratercreated by the explosive erupt ion is about

600 meters deep and can be seen in the

center of the image above.



The most explosive type of volcano is the CALDERA The


62/84

The most explosive type of volcano is the CALDERA. The

cataclysmic explosion of these volcanoes leaves a huge circulardepression at the Earth's surface. This depression is usually less

than 40 kilometers in diameter.

These volcanoes form when "wet" GRANITIC MAGMA quickly rises

to the surface of the Earth. When it gets to within a few kilometers

of the surface the top of the magma cools to form a dome. Beneath

this dome the gaseous water in the magma creates extreme

pressures because of expansion.

When the pressure becomes too great the dome and magma are

sent into the Earth's atmosphere in a tremendous explosion. On

the island ofKRAKATOA , a caldera type volcano exploded in 1883

ejecting 75 cubic kilometers of material in the air and left a

depression in the ground some 7 kilometers in diameter.




63/84



Diamonds ascend to Earth's

surface in rare molten rock, or-

2004

tory


64/84

magma, that originates at great

depths. Carrying diamonds andother samples from Earth's

mantle, this magma rises and

erupts in small but violent

volcanoes. Just beneath such

volcanoes is a carrot-shaped

"pipe" filled with volcanic rock,

mantle fragments, and some

embedded diamonds. The rock

is called kimberlite after the city

of Kimberley, South Afr ica,

where the pipes were firstdiscovered in the 1870s.

Another rock that provides

diamonds is lamproite.

The volcano that carries diamond to the surface emanates from deep cracks and

fissures called dikes. It develops its carrot shape near the surface, when gasesseparate from the magma, perhaps accompanied by the boiling of ground water, and a

violent supersonic eruption follows. The volcanic cone formed above the kimberlite

pipe is very small in comparison with volcanoes like Mount St. Helens, but the magma

originates at depths at least 3 times as great. These deep roots enable kimberlite to tap

the source of diamonds. Magmas are the elevators that bring diamonds to Earth'ssurface.

Create

d&compiledbyAndriSSM

Octo-

Petrology&EconomicGeologyL

aborat

Dept.ofGeologyFIKTM

ITB

-2004

atory


65/84


Octo

Petrology&EconomicGeologyLabora

Dept.o

fGeologyFIKTM

ITB

While minor diamond discoveries were

made among alluvial gold in New South

W l i i 1851 di i



66/84

Wales starting in 1851, a discovery in

1979 on the Kimberley Plateau ofWestern Australia enabled the country

to be the world's most prolific diamond

producer. Based on ancient bedrock,

diamond exploration began in 1972, with

a kimberlite pipe discovery coming in1976 in the Ellendale area. In 1979, a

large lamproite pipe was found and

named the Argyle mine; by 1992 over

200 mill ion carats had been mined there.

Only 5% of the production is gemquality. A unique feature of the Argyle

mine, though, is a small but consistent

supply of valuable pink to red or purple

diamonds

The Argyle mine on the Kimberley

plateau of Western Australia.

Australian Production

Total: 428 million carats Annual: 35-

40 million carats

Most diamonds consist of pr imeval carbon from Earth's mantle, but those

from eclogites probably contain carbon recycled from the ocean crust by


67/84

from eclogites probably contain carbon recycled from the ocean crust by

plate tectonics -- the carbon of microorganisms.

How do we know? Carbon atoms occur in three different masses, or

isotopes. Unlike high-temperature processes in deep Earth, low-

temperature, biological processes, such as photosynthesis, are sensitive to

the differences in mass, and actively sort different carbon isotopes.Thus, the ratios of carbon isotopes in organic materials -- plants, animals,and shells -- vary, and also differ from those in the carbon dioxide of the

atmosphere and the oceans. Geochemists " read" the carbon isotopes in

samples to interpret nature's record.

Virtually all carbon atoms, the ones in a diamond or a tree or you, came from

the stars. Particularly at Earth's surface the proportions of 12C and 13C (the

carbon isotopes of mass 12 and 13) get redistributed. Expressed as simple

numbers in 13C notation -- in which larger numbers mean more 13C --organic carbon has large negative values, average Earth has a mildly

negative value, and the carbon in shells is near zero


The narrow range of 13C values

for harzburgitic diamonds in the


68/84

for harzburgitic diamonds in the

histogram on the top resemblesthe range of average Earth,

indicating that the mantle is the

likely carbon source.

The large range for eclogites

suggests mixing of organic

carbon (the strongly negative

numbers), mantle carbon

(mildly negative numbers), andshell-like carbon (values near

zero).

These data support recycling ofonce-living carbon from Earth's

surface deep into the mantle to

form diamond.Created & compiled by Andri SSM Octo-2004Petrology & Economic Geology LaboratoryDept. of Geology FIKTM ITB


69/84

When ocean floor slides into the mantle, the basaltic rock becomes

eclogite, and organic carbon in sediments may become diamond



70/84


Octo-200

4

Petrology&Econ

omicGeologyLaboratory

Dept.ofGeology

FIKTM

ITB


71/84

Created&c

ompiledbyAndriSSM

Octo

-2004

Petrology&

EconomicGeologyLabora

tory

Dept.ofGe

ologyFIKTM

ITB

Forms of Intrusive Plutons


72/84

Forms of Intrusive Plutons

Differentiated Sills

Layered Complexes

Differentiated SillsAlways sill-like in shape and generally hypabyassal

Very sharp contacts with host rocks marked by a thin chill zone

Systematic varaitions in chemical and mineralogic composition

Modal qtz+kspar increase upward. Plag and pyroxene decreaseFe/Mg increase upward in the pyroxene

Plag systematically becomes more albitic

Elements Si, Fe, Na, K increase upward; Ca, Mg decrease

Grain size of the final differentiate is often pegmatitic

Basal rocks are often ultramaficUpper chilled margin closely resembles lower chilled margin in

composition (i.e. mafic)



73/84

Created&com

piledbyAndriSSM

Octo-2004

Petrology&Ec

onomicGeologyLaboratory

Dept.ofGeolo

gyFIKTM

ITB

Origin


74/84

Origin

Differentiation (Crystal- Liquid Fractionation) Early formeddifferentiates sink gravitationally giving rise to lower layer ofultramafics. Chilled zone of mafics is due to attachment of crystals to the

roof of the magma chamber. It should be smaller than the basal zone and

generally it is. Sill then changes compoistion upward due to the

differentiation process.

Assimilation of Country Rock - Not thought to be importantbecause the contacts between host and pluton are too sharp andinclusions are lacking.

Mixing -Also not important as this would invalidate the systematicchemical variations

Created & compiled by Andri SSM Octo-2004Petrology & Economic Geology Laboratory


Layered Intrusive Complexes


75/84

CharacteristicsExtremely large - Bushveld (65,000 km2, Great Dyke 5000 km2Usually have the shape of an inverted funnel

Layering is very promiment and always discordant with the walls of the

funnelOften linear to elliptical in shape

Possess both rhythmic (cyclic) layering

Also prosses cryptic mineral and chemical variations

Graded bedding is common

Locally slump structures and cross bedding has been noted

OriginProblem has always been explaining rhythmic and cryptic layering and

graded bedding. Rhythmic layering thought to be the result of repeated

reinjection of new batches of magma, but cryptic layering invalidates this.

Graded bedding implies gravity settling. Cryptic layering suggests

fractionation is the dominant process. Local cross beds and slump

structures indicate the magma chamber was not static, but rather

undergoing convective motion.

Octo-2004

boratory


76/84

Chemical analyses can be obtained, and a chemical classification, such as

the LeBas et al., IUGS chemical classif ication of volcanic rocks (based on

total alkalies [Na2O + K2O] vs. SiO2 diagram shown above)


O

Petro


Lab

Dept.ofGeologyFIKTM

ITB

SiO2 (Silica) Content

> 66 wt. % - Acid



77/84

66 wt. % Acid

52-66 wt% - Intermediate45-52 wt% - Basic

< 45 wt % - Ultrabasic

This terminology is based on the onetime idea that rocks with a high % SiO2were precipitated from waters with a high concentration of hyrdosil icic acid

H4SiO4. Although we now know this is not true, the acid/base terminology is

well entrenched in the literature.

Silica SaturationIf a magma is oversaturated with respect to Silica then a silica mineral, suchas quartz, cristobalite, tridymite, or coesite, should precipitate from the

magma, and be present in the rock. On the other hand, if a magma is

undersaturated with respect to si lica, then a silica mineral should not

precipi tate from the magma, and thus should not be present in the rock. Thesilica saturation concept can thus be used to divide rocks in silica

undersaturated, silica saturated, and silica oversaturated rocks. The first

and last of these terms are most easily seen.

Silica Undersaturated Rocks - In these rocks we should find minerals that, in

general, do not occur with quartz. Such minerals are:

Melanite - Ca Fe+3Si OPerovskite - CaTiO

Hayne - 6NaAlSiO4.(Na2,Ca)SO4Nosean - 6NaAlSiO4

.Na2SO4

Sodalite - 3NaAlSiO4.NaClForsteritic Olivine - Mg2SiO4

SSM

Octo-2004

gyLaboratory


78/84

Thus, if we find any of these minerals in a rock, with an exception thatwe'll see in a moment, then we can expect the rock to be silica

undersaturated.

If we calculate a CIPW Norm (we'll see how to do this in lab) thenormative minerals that occur in silica undersaturated rocks arenepheline and/or leucite.Silica Oversaturated Rocks. These rocks can be identified as possibly any

rock that does not contain one of the minerals in the above list.

If we calculate a CIPW Norm, silica oversaturated rocks will containnormative quartz.

Silica Saturated Rocks. These are rocks that contain just enough silicathat quartz does not appear, and just enough silica that one of the silicaundersaturated minerals does not appear. In the CIPW norm, these rockscontain olivine, or hypersthene + olivine, but no quartz, no nepheline, andno leucite.To get an idea about what silica saturation means, let's look at a simple

silicate system - the system Mg2SiO4 - SiO2

Melilite - (Ca,Na)2(Mg,Fe+2,Al,Si)3O7

Melanite - Ca2

Fe+3Si3

O12

Perovskite - CaTiO3

C


ri

P

etrology&EconomicGeolog

D

ept.ofGeologyFIKTMITB

Note how composit ions between Fo

and En wil l end their crystallization


79/84

with only Fo olivine andenstatite. These are SiO2-undersa-

turated. compositions. All compo-

sitions between En and SiO2 will end

their crystallization with quartz and

enstatite. These are SiO2 over-saturated compositions.

Note also that this can cause some

confusion in volcanic rocks that do

not complete their crystallization due

to rapid cooling on the surface. Let'simagine first a composition in the

silica-undersaturated field. Cooling

to anywhere on the liquidus will

result in the crystallization of Fo-rich

olivine. .

If this liquid containing olivine is erupted and the rest of the liquid quenches

to a glass, then this will produce a rock with phenocrysts of olivine in a

glassy groundmass. Created & compiled by Andri SSM Octo-2004Petrology & Economic Geology Laboratory


// //


80/84

1mm

Olivine-Leucite Basalt (Basanite) of Komba Volcano Flores Sea

X X

Created&com

piledbyAndriSSM

Octo-2

004

Petrology&EconomicGeologyLaborato

ry

Dept.ofGeolo

gyFIKTM

ITB

// //


81/84

1mm

Olivine-Leucite Basalt (Basanite) of Komba Volcano Flores Sea

X X


Octo-2004

Petrology&EconomicGe

ologyLaboratory

Dept.ofGeologyFIKTM

ITB


82/84


83/84


84/84

Documents

Igneous Petrology Andri's Lecture Note