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Factors in Metamorphism

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Presentation on theme: "Factors in Metamorphism"— Presentation transcript:

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

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