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