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IX. Metamorphic Rocks
A. Evidence of metamorphism
B. The recipe for metamorphism
C. Prograde metamorphism of shale
D. Classification of Metamorphic Rocks
E. Metamorphism and Plate Tectonics
The Rock Cycle
Rock
Metamorphism (Increased T & P)
Meta- morphic rock
Partial Melting
Magma
Geological Materials
Transformation Processes
MetamorphismMetamorphic Rocks
Definition: Re-crystallization of
minerals While still solid Into minerals that are
stable at different temperatures and
Different pressures
(Usually progressively higher temperatures and pressures)
A. Evidence of Metamorphism
E.g., Sedimentary Rocks Made of minerals derived
from weathering of a parent rock
Stable at atmospheric temperatures and pressures (low T & P)
Originally horizontal, continuous and uniform layers
1. Bent (deformed) layers
Fig. 8-1, p. 168
Evidence of Metamorphism
Differential pressure “squashes” rock and included features
2. Flattened Pebbles
3. Crystalline Texture Minerals tightly interlocking
due to recrystallization under pressure
Evidence of Metamorphism
Evidence of Metamorphism
4. New mineral
assemblages E.g., Shale: Clay minerals
(some quartz)
Metamorphism
Forms Schist: Mica, Feldspar and other silicate minerals
B. Recipe of Metamorphism
1. Parent Rock Even though
minerals will change Most elements are
provided by parent rock
Except water and some dissolved ions
Shale
Schist
B. Ingredients of Metamorphism
2. Increased Temperature (high geothermal gradients) Minerals stable at lower temperatures converted to minerals stable at higher temperatures Solid state chemical reactions are accelerated
0 500 1000Temp. (ºC)
A B C
Ingredients of Metamorphism
3. Increased Pressure (and stresses) Increased Lithostatic Pressure as rocks are buried Differential Pressure at convergent plate boundary or
Fig. 8-3, p. 169
Results of Stress (Pressure)
Differential Pressure
Parallel alignment of platy minerals (e.g., micas) and needle-like minerals (e.g., amphiboles)
Forms Foliation
Ingredients of Metamorphism
4. Addition or removal of fluids (and elements) Water (and other fluids) within rocks and minerals Moving during metamorphism Accelerates solid-state chemical reactions and May change rock composition
5. Time Millions of years!
C. Prograde Metamorphism of Shale (and the classification of metamorphic rocks)
Shale: Fine grained Clay (and quartz)
Minerals stable under low T&P (atmospheric)
Compaction due to accumulation of sediment
Fissility along laminations Water bound in crystalline
structure of clay
Prograde Metamorphism of Shale
Metamorphic grade 1: Low Grade Rock: Slate Low grade metamorphic T&P
(Water is expelled from crystalline structure of clay)
Minerals stable under low grade metamorphic conditions
Mineral composition: Fine grained Mica (and Quartz)
Foliation: Slaty Cleavage due to realignment of platy minerals
Prograde Metamorphism of Shale
2: Medium-Grade Metamorphism Phillite
Courser grained Mica and quartz
Medium-grade metamorphic T&P Causes minerals to grow
Slaty cleavage becomes rippled and rock has a sheen
Prograde Metamorphism of Shale
3: Medium to High-Grade Metamorphism Schist
Course grained Mica and Quartz
Medium-grade metamorphic T&P Causes minerals to grow
Minerals stable under medium grade metamorphic conditions appear: Garnet, amphibole and biotite
Schistosity due to alignment of platy and needle likeminerals
Prograde Metamorphism of Shale
4: High-Grade Metamorphism Gneiss
Course grained Feldspar, Quartz, Amphibole, Biotite
High-grade metamorphic T&P Causes minerals to separate into bands
Minerals stable under high grade metamorphic conditions appear: feldspar
Gneissic banding bands of dark and light minerals
Fig 7.12
Prograde Metamorphism of Shale
5: Very High-Grade Metamorphism and partial melting Migmatite
Silica rich minerals melt first (quartz and feldspar)
Forming silicic magma Injected into fractures
resulting in silicic veins if intrusive igneous rock
Fig 7.12
Metamorphic Rocks of other Parent Rocks
Limestone Marble
Bioclastic calcite Crystalline calcite
Metamorphic Rocks of other Parent Rocks
Quartz Sandstone Quartzite
Granular quartz Crystalline quartz
Metamorphic Rocks of other Parent Rocks
Basalt Amphibole Schist Granite Gneiss
Uniform texture Foliation (schistosity and
gneissic banding)
Where does Prograde Metamorphism occur?
Where temperatures and pressures are progressively increased: Subduction zones and Mountain belts
Regional Metomorphism at Convergent plate boundaries
0 600 1200Temp. (ºC)
A B C
Prograde Metamorphism in Mountain Belts
MetamorphicGrade
High
Low
NortheasternNorth America
NorthwesternAfrica
Review for Exam
For each rock type discussed know: Grain size (rounding and sorting if
sedimentary) Texture (clastic or crystalline) Other characteristics
Various types of metamorphic foliation Various sedimentary structures
Environment of formation History of formation
Roc
kC
lass
ifica
tion
Reading the Grand CanyonReading the Grand Canyon
2. Grand Canyon Pink Granite
The Grand Canyon
Intrusive IgneousSedimentary Rocks
Metamorphic
1. Vishnu Schist (and gneiss)
3. Bright Angel Shale
4. Redwall Limestone
5. Coconino SandstoneThe Grand Staircase
Fig. 8.16
The Grand Staircase
Fig. 8.16
Eastern Zion National Park
Checkerboard Butte Zion National Park
Vishnu Schist and Pink Granite