Transcript No Slide Title

Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 8
Metamorphic Rocks
Gregory G. Dimijian/Photo Researchers
Metamorphic rocks
Metamorphism: to change form
Metamorphic rock: any rock
(sedimentary, volcanic, plutonic,
metamorphic) that has undergone
changes in texture or
mineralogical composition in the
solid state.
No single mode of origin
• Metamorphic rocks result from the
partial or complete recrystallization
of minerals in rocks over long
periods of time.
• Rocks remain essentially solid
during metamorphism.
Metamorphic processes
HEAT: stability region of mineral sensitive to
T. With increased T, pore fluid decreases.
PRESSURE: greater pressure tends to
decrease space available; metamorphic
mineral tend to be dense. Increased P can
come from any directed stress (burial).
Stress will bring about a preferred
orientation of minerals.
FLUIDS (H2O): acts as a catalyst during
metamorphism; aids the exchange of ions
between growing crystals.
Metamorphic grade
Refers to the intensity of
metamorphism.
High grade: high T, P
Low grade: low T, P
Pressure and Temperature vs. Depth
Fig. 8.1
Metamorphic
Grade
Fig. 8.2
Types of metamorphism
Regional: Widespread changes in temperature
and pressure bring about changes in rocks due
to tectonic forces.
Contact: Intrusion of magma against colder
rocks (affected area is proportional to the size
and temperature of the intrusion but always
only a local phenomenon).
Deformational: Changes in rocks associated with
faulting and folding (regional or local).
Types of metamorphism (cont.)
Burial: Changes in a rock due to the gradual changes in T
and P due to successive burial (regional).
Impact: Changes due to rapid increase in pressure (only
localized).
Augen Mylonitic Gneiss
Plate Tectonics and Metamorphism
Fig. 8.3
Metamorphic reactions
Mineralogical changes (e.g., clay to mica): Many complicated
reactions — depend on pressure, temperature,
composition.
Common metamorphic minerals include amphiboles,
garnet, mica, staurolite, and kyanite.
Textural changes: recrystallization (grain
boundaries more compact) and foliation (preferred
orientation of minerals).
How much can a rock change?
The amount of change during
metamorphism depends on:
 grade
of metamorphism
 duration
of metamorphism
 composition
of the rock
Changes in texture
Grain size
• Recrystallization
• Mineral size can either decrease or increase.
Orientation of minerals
• Recrystallization
• Directed stress will orient minerals:
– Lineation
– Foliation
Slate with Foliation and Relict Bedding
Fig. 8.4
Metamorphic Foliation
Fig. 8.5a
Metamorphic Foliation
S. Dobos
Fig. 8.5b
Direction of
Compressive
Forces
S. Dobos
Fig. 8.5b
Slaty Cleavage
Martin Miller
Fig. 8.6
Classification of Metamorphic Rocks
Based on Texture: Table 8.1
Classification of Foliated Rocks
Fig. 8.7
Slate
Andrew J. Martinez/Photo Resrachers
Fig. 8.8a
Schist
Biophoto Associates/Photo Researchers
Fig. 8.8b
Schist in Thin Section
Gneiss
Breck P. Kent
Fig. 8.8c
Quartzite
Breck P. Kent
Fig. 8.9a
Quartzite in Thin Section
Marble
Breck P. Kent
Fig. 8.9b
Marble in Thin Section
Garnet
Porphyroblasts
Schist Matrix
Chip Clark
Fig. 8.10
Stability of minerals
• Most minerals are stable over a
relatively narrow range of P and T
(e.g., ice unstable above 0°C).
• The stability range of different
minerals overlap and provide
constraints on the metamorphic
history of rocks.
Metamorphic facies
• A given set of metamorphic
conditions
• Each facies is characteristic of
particular tectonic environments and
will have certain minerals that are
diagnostic of those conditions.
• Therefore, the minerals in a rock can
be clues to the (P,T) history of the
rock.
Metamorphic reactions
Prograde: Mineral changes that take
place during an increase in
temperature.
Retrograde: Mineral changes that
take place during an decrease in
temperature.
Mineralogic Changes in Metamorphosed Shales
Fig. 8.11a
Mineralogic Changes in Metamorphosed Shales
Fig. 8.11b
Mineralogic Changes in Metamorphosed
Mafic Rocks
Fig. 8.12
Metamorphic Facies
Fig. 8.13
Classification of metamorphic rocks
Based on the texture and composition of
the rock:
Low grade
Slate
Phyllite
Schist
Gneiss
High grade
Migmatite
Major Minerals of Metamorphic Facies from
Parent Rocks of Different Composition: Table
8.2
Contact
Metamorphism
of Limestones
Fig. 8.14
Contact
Metamorphism
of Sandstones
and Shales
Fig. 8.15
Fig. 8.16