1. Factors in Metamorphism
Metamorphism is effected by four main factors:
Fluids
Temperature
Pressure
Deviatoric stress

2. Factors in Metamorphism
Pressure
A uniform compressive stress created by
Load (weight of overlying rocks)
Fluid pressure
Increases greatly deeper in the crust.
With temperature, controls what minerals are stable.

3. Factors in Metamorphism
Deviatoric / Differential Stress
Regional Metamorphism:
Stress imbalance causes compression in some directions and expansion in others. Controls:
Foliation (orientation of minerals into layers)
Need to have either platey (micas) or elongate (hornblende) minerals
Non-Foliated rocks, with no platey or elongated minerals
(quartz, feldspar, calcite, dolomite, etc.)

4. Hydrostatic Pressure vs Deviatoric Stress

5. Deviatoric Stress
Deviatoric stress is the primary control on foliation
Foliation - alignment of minerals (platey or elongated minerals)
Slate – Alignment of fine grained minerals (dull)
Phyllite – Alignment of fine grained minerals (sheen, wavy texture, still too small to see individual grains)
Schist – Alignment of fine to coarse grained minerals
Gneiss – Alignment of medium to coarse grained minerals, and segregation of different minerals into light and dark bands

6. Foliation

7. Foliation - Slaty Cleavage
Slate
slaty cleavage.
Protolith– fine grained rock like shale, mudstone, or siltstone

8. Foliation - Gneissic Banding
shows gneissic banding
Arizona State University

9. Low Grade High Grade Foliated Metamorphic Rocks
Metamorphic rock with aligned flat minerals
Slate
Schist
Gneiss
Low Grade
High Grade
Foliation perpendicular to direction of maximum differential stress
Foliation and crystallization perpendicular to direction of maximum differential stress
Foliation, crystallization and differentiation perpendicular to direction of maximum differential stress

10. Metamorphic Grade
Index Mineral – a mineral in a metamorphic rock which indicates a certain metamorphic grade
Isograd – line marking the first appearance of an index mineral on a map or cross-section

11. Marble Quartzite Anthracite Amphibolite Marble Quartzite Anthracite
Metamorphic rock without minerals that can be foliated (or from contact metamorphism)
Non- Foliated Metamorphic Rocks
Marble
Quartzite
Anthracite
Amphibolite
quartz
sandstone
Protolith
limestone
coal
mafic rocks
Mineralogy
carbonates
quartz
none
Amphiboles
(i.e. hornblende)
Marble
Quartzite
Anthracite
Amphibolite
Can be either