IX. Metamorphic Rocks
A.
B.
C.
D.
E.
Evidence of metamorphism
The recipe for metamorphism
Prograde metamorphism of shale
Classification of Metamorphic Rocks
Metamorphism and Plate Tectonics
The
Rock Cycle

Geological Materials
Transformation Processes

Rock
Metamorphism
(Increased T & P)

Metamorphic
rock
Partial Melting

Magma
Metamorphism
Metamorphic 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
Evidence of Metamorphism

3. Crystalline Texture
Minerals tightly interlocking
due to recrystallization under
pressure
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
Temp. (ºC)
0 500 1000
A B C

2. Increased Temperature (high geothermal gradients)



Minerals stable at lower temperatures converted to
minerals stable at higher temperatures
Solid state chemical reactions are accelerated
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 needlelike 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
Fig 7.12
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
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
Granite

Uniform texture




Amphibole Schist
Gneiss
Foliation (schistosity and
gneissic banding)
Where does Prograde
Metamorphism occur?
Temp. (ºC)
0 600 1200
A B C



Where temperatures and pressures are progressively
increased: Subduction zones and Mountain belts
Regional Metomorphism at
Convergent plate boundaries
Prograde Metamorphism in
Mountain Belts
Northeastern
North America
Metamorphic
Grade
High
Northwestern
Africa
Low
Review for Exam



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
Rock
Classification
For each rock type discussed know:
Reading the Grand Canyon
The Grand Canyon
Fig. 8.16
The Grand Staircase
5. Coconino Sandstone
4. Redwall Limestone
Sedimentary Rocks
Intrusive Igneous
Metamorphic
3. Bright Angel Shale
2. Grand Canyon
Pink Granite
1. Vishnu Schist (and gneiss)
The Grand Staircase
Fig. 8.16
Eastern Zion National Park
Checkerboard Butte
Zion National Park
Vishnu Schist and Pink Granite