Metamorphic Rocks

A. Evidence of metamorphism

B. The ingredients of metamorphism

C. Classifying metamorphic rocks

D. Metamorphism of shale (& other parent rocks!)

E. Metamorphic minerals

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

MetamorphismMetamorphic Rocks

WHY??? do the minerals recrystallize? To reach chemical

equilibrium!!!

Animation of metamorphic rock formation

A. Evidence of Metamorphism

For example… 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

For example… 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

Shale

Gneiss

Evidence of Metamorphism

Differential pressure “squashes” rock and included features

2. Flattened Pebbles

Conglomerate

Metaconglomerate

3. Crystalline Texture Minerals tightly interlocking

due to recrystallization under pressure

Evidence of Metamorphism

Quartz Sandstone

Quartzite

Evidence of Metamorphism

4. New mineral assemblages Example: Shale: Clay minerals (some quartz)

Metamorphism(Mid-grade)

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. Recipe of Metamorphism

2. Heat Geothermal gradient: more pressure = more heat!! Minerals stable at lower temperatures converted to minerals stable at higher temperatures Solid state chemical reactions are accelerated

0 600 1200Temp. (ºC)

A B C

A C B

Recipe of metamorphism: Heat Geotherm: Geotherm: Temperature increases

with depth beneath Earth’s surface Typical: 30° C/km

Igneous intrusions: Igneous intrusions: Localized increases in temperature

Recipe of Metamorphism

3. Increased Pressure & Stress Increased Confining Pressure as rocks are buried Compression at convergent plate boundary or Sheared as plates slide past each other

Results of Stress

Compressive Stress

Shear Stress

Causes 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!

Metamorphic Grade High grade

(High P & T)

Low grade (Low P & T)

Intensity of Metamorphism:Intensity of Metamorphism:

Metamorphic Facies

The minerals in a rock are clues to the (pressure and temperature) history of the rock

Facies = a set of metamorphic conditions

Each facies is characteristic of its Tectonic environment

Source rocks in the environment (like shale)

Metamorphism of Shale

Fig. Story 9.7b

Metamorphic Rocks of other Parent Rocks

Limestone Marble

Bioclastic calcite Crystalline calcite

Nonfoliated

Metamorphic Rocks of other Parent Rocks

Quartz Sandstone Quartzite

Granular quartz Crystalline quartz

Nonfoliated

Metamorphic Rocks of other Parent Rocks

Basalt Amphibole Schist Granite Gneiss

Uniform texture Foliation (schistosity and

gneissic banding)

Characteristic Metamorphic Minerals

For more images: http://skywalker.cochise.edu/wellerr/mineral/

kyanite

andalusite

sillimanite

staurolite garnet

Distinct Mineral Assemblages

GradeGrade

GradeGrade

Distinct Mineral Assemblages

Index Minerals

Define the metamorphic conditions or zones

Give information about the “metamorphic grade”

Fig. Story 9.7a

SummaryA. Evidence of metamorphism

A. Deformed layers, flattened pebbles, crystalline texture, & new mineral assemblages

B. The ingredients of metamorphismA. Parent rock, heat, pressure, fluids, & time

C. Classifying metamorphic rocksA. Low grade --> high grade & facies

D. Metamorphism of shale (& other parent rocks!)A. Shale --> Gneiss, Sandstone --> Quartzite, Limestone -->

Marble

E. Metamorphic mineralsA. Muscovite and Garnet = Index minerals

To think about… Metamorphic Rocks & Plate Tectonics Tracing changes in pressure and

temperature conditions that a rock experienced …

… gives insight into plate tectonic settings!!