Metamorphic Rocks
A.
B.
C.
D.
Evidence of metamorphism
The ingredients of metamorphism
Classifying metamorphic rocks
Metamorphism of shale (& other
parent rocks!)
E. Metamorphic minerals
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
Metamorphism
Metamorphic Rocks
WHY??? do the minerals
recrystallize?
 To reach chemical
equilibrium!!!
Animation of metamorphic rock formation
A. Evidence of Metamorphism
Shale
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
Gneiss
Evidence of Metamorphism

2. Flattened Pebbles
Conglomerate
Differential
pressure
“squashes” rock
and included
features
Metaconglomerate
Evidence of Metamorphism
Quartz Sandstone
Quartzite

3. Crystalline Texture
Minerals tightly interlocking
due to recrystallization under
pressure
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
A
C
B
Temp. (ºC)
0 600 1200
A B C

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


Recipe of metamorphism: Heat

Geotherm: Temperature increases
with depth beneath Earth’s surface


Typical: 30° C/km
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

Intensity of Metamorphism:

High grade
(High P & T)

Low grade
(Low P & T)
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
Nonfoliated

Bioclastic calcite

Crystalline calcite
Metamorphic Rocks of other
Parent Rocks

Quartz Sandstone  Quartzite
Nonfoliated

Granular quartz  Crystalline quartz
Metamorphic Rocks of other
Parent Rocks

Basalt
Granite

Uniform texture




Amphibole Schist
Gneiss
Foliation (schistosity and
gneissic banding)
Characteristic Metamorphic Mineral
sillimanite
kyanite
staurolite
garnet
andalusite
For more images: http://skywalker.cochise.edu/wellerr/mineral/
Distinct Mineral Assemblages
Grade
Distinct Mineral Assemblages
Grade
Index
Minerals

Define the
metamorphic
conditions or
zones

Give information
about the
“metamorphic
grade”
Fig. Story 9.7a
Summary
A. Evidence of metamorphism
A. Deformed layers, flattened pebbles, crystalline
texture, & new mineral assemblages
B. The ingredients of metamorphism
A. Parent rock, heat, pressure, fluids, & time
C. Classifying metamorphic rocks
A. Low grade --> high grade & facies
D. Metamorphism of shale (& other parent rocks!)
A. Shale --> Gneiss, Sandstone --> Quartzite,
Limestone --> Marble
E. Metamorphic minerals
A. 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!!