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