Chapter 9 (part I): Metamorphic Rocks

Study Help for Chapter 9

Definition of metamorphism, its causes, and the agents of metamorphism.

Textures of metamorphic rocks, rock cleavage, foliation. The most common metamorphic rocks and what they are formed from (parent rock).

Understand the main types/environments of metamorphism, which are the most/least common, where do they occur? How are they related to plate tectonics?

Know the slate gneiss progression of metamorphism

Progressive regional metamorphism and index minerals (Figure Story 9.7)

Chapter 9 (part I): Metamorphic Rocks

1) Introduction

2)Metamorphic environments

3)Agents of metamorphism

Recall: 3 fundamental rock types

Igneous

Sedimentary

Metamorphic

Example: granite

crystallization

1) Introduction

Igneous

Sedimentary

Metamorphic

crystallization

Example: Grand Canyon

deposition

Recall: 3 fundamental rock types

1) Introduction

Igneous

Sedimentary

Metamorphic

crystallization

deposition

today

Recall: 3 fundamental rock types

1) Introduction

“metamorphism” Lit. “change form”

Transform:

Igneous

Sedimentary

Metamorphic

Metamorphic

into:

“process”

occurs deep

Recall: 3 fundamental rock types

1) Introduction

2) Metamorphic environments

Contact metamorphism

Hydrothermal metamorphism

Regional metamorphism

rock near or touching magma

high temperaturesof magma “bake” rock

Chemical reactions with ion-richwater

Associated with igneousActivity, seafloor spreading centers

associated w/ mountain building

Large scale deformations,directed forces,high temperatures,(most common),Mostly associated withConvergent plate boundariesFig. 9.3

3) Agents of metamorphism

Heat

Pressure (stress)

Chemical active fluids

HeatThe most important agent

Recrystallization results in new, stable minerals

Two sources of heatContact metamorphism – rocks are baked by heat from magma

An increase in temperature with depth due to the

geothermal gradient (20-30 °C per km in upper crust)

3) Agents of metamorphism

See Fig. 9.1

Pressure (stress)

Increases with depth (300-400 bar/km)

Confining pressure applies forces equally in all directions

Rocks may also be subjected to differential stress which is unequal in different directions

3) Agents of metamorphism

See Fig. 9.1

Origin of pressure in metamorphism

3) Agents of metamorphism

3) Agents of metamorphism

Pressure/Stress

3) Agents of metamorphism

Pressure/Stress

3) Agents of metamorphism

Pressure/Stress

3) Agents of metamorphism

Pressure/Stress

near surface deformation:brittle

fractures

shearingforce

3) Agents of metamorphism

Pressure/Stress

Deep crust/mantle deformation:ductile

shearingforce

(no fractures)

Chemically active fluids = hydrothermal fluids

Mainly water with other volatile components

Enhances migration of ions

Aids in recrystallization of existing minerals

Where is the water from from?– Squeezed out of pore spaces of sedimentary rocks

– Squeezed out of fractures in igneous rocks

– Hydrated minerals such as clays and micas

3) Agents of metamorphism

4) How metamorphism alters rocks

Hydrothermal fluids cause mineralogical changes

3) Agents of metamorphism

The importance of parent rock

Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed

Mineral makeup determines, to a large extent, the degree to which each metamorphic agent will cause change

e.g., in shale, clays recrystallize to micas, but quartz and feldspars remain largely unaltered

3) Agents of metamorphism

Important: parent rock mineralogy

Example: magma intrudes rock

host rock alterationquartz sandstone not much

limestone lots

3) Agents of metamorphism