Metamorphic Rocks

Chapter 7

Metamorphic Rx

“meta” = change

“morphos” = shape/form

Parent rock = pre-existing rock that is subject to metamorphism

Metamorphic Rx

Form by alteration of parent rock due to:

- increase in pressure

- increase in temperature

- exposure to chemically active fluid

Metamorphism

Occurs within Earth’s crust

- minerals change until they are ________ under the new conditions

Metamorphic Conditions

1) Increasing pressure (stress)

- occurs for two reasons:

a) deep burial

b) mountain building

Rx reaction to pressure

Behave differently depending upon temperature

Shallow depths = cooler temp

- rx are brittle and tend to break

Ex: fault breccia

Called “cataclastic metamorphism”

Rx reaction to pressure

Deep burial = higher temp’s

- rx behave plastically and fold under pressure (Fig. 7.1)

Types of pressure

a) Differential stress

- applied unequally from different directions

- produces foliated texture

Foliated texture

Foliation = parallel alignment of minerals w/in a rock

- creates a layered appearance

- associated w/mountain building

Types of pressure

b) Confining pressure

- applied equally from all sides

- associated with deep burial

- nonfoliated texture

Metamorphic Conditions

2) Increasing temperature

NOTE: Temperature cannot exceed the melting point of all minerals in rock

Why?

Increasing temperature

- occurs for two reasons:

a) deep burial

- geothermal gradient

- increases 30o C per km

b) magma intrusion

Increasing temperature

Changes to parent rock:

a) Compositional change

- unstable minerals either melt or rearrange into new minerals

- partial melting: remaining rock is metamorphic rock

Compositional Change

Ex: Shale Slate

(clay minerals) (microscopic

micas)

Increasing temperature

b) Textural Change

- partial melting allows minerals to recrystallize

- produces nonfoliated texture

Textural Change

Ex: Sandstone Quartzite

(quartz) (quartz)

Note: The composition is still the same

Metamorphic Conditions

3) Chemically active fluid

- superheated water with ions in solution

- “hydrothermal solution”

- occurs near magma chambers

“Microscopic Water”

Water in crystalline structure

- necessary for atoms to rearrange themselves

Sources of Water

a) Groundwater

b) Magma chambers

c) Crystalline structure of hydrous minerals

Ex: Gypsum = CaSO4 2H2O

Metamorphic Rock Classification

Foliated textures

- degrees of increasing metamorphic intensity

(temperature, pressure)

Foliated textures

Parent rock = shaleLow degree Slate

(microscopic micas)Low degree Phyllite

(light reflects from larger micas)

Foliated textures

Low degree Phyllite

Medium degree Schist

(visible micas)

Foliated textures

Low degree Slate

Medium degree Schist

High degree Gneiss

(black & white layers)

Transitional change

Migmatite

- rock w/both igneous & metamorphic characteristics

- represents high degree of metamorphism (Fig. 7.23)

Foliated textures

Most schists have muscovite & biotite = mica schists

Accessory minerals w/micas indicate degree of metamorphism w/in schist range

Accessory minerals

Referred to as “index minerals”

Ex: chlorite mica schist

Low grade mm (~200oC)

Ex: garnet mica schist

Intermediate grade mm

Nonfoliated textures

Composition controlled by parent rock

Parent Rock Meta. Rock

Sandstone Quartzite

Limestone Marble

Regional metamorphism

Index minerals

Regional Metamorphism

Foliated rocks are often folded

One rock is the raw material for another