Factors in Metamorphism

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

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Factors in Metamorphism. Metamorphism is effected by four main factors: Fluids Temperature Pressure Deviatoric stress. Factors in Metamorphism. Pressure. A uniform compressive stress created by Load (weight of overlying rocks) Fluid pressure. Increases greatly deeper in the crust. - PowerPoint PPT Presentation

Transcript of Factors in Metamorphism

Page 1: Factors in Metamorphism

Factors in Metamorphism

Metamorphism is effected by four main factors:

Fluids

Temperature

Pressure

Deviatoric stress

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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.

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Factors in Metamorphism

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.)

Deviatoric / Differential Stress

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Deviatoric Stress Hydrostatic Pressure

Hydrostatic Pressure vs Deviatoric Stress

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Deviatoric Stress

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

Deviatoric stress is the primary control on foliation

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Foliation

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Slate slaty cleavage.

Protolith– fine grained rock like shale, mudstone, or siltstone

Foliation - Slaty Cleavage

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Arizona State University

http://www.slu.edu

Gneiss shows gneissic banding

Foliation - Gneissic Banding

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Foliated Metamorphic Rocks Metamorphic rock with aligned flat minerals

http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

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

Low Grade High Grade

SchistSlate Gneiss

http://hyperphysics.phy-astr.gsu.edu/hbase/geophys

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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

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Non- Foliated Metamorphic Rocks Metamorphic rock without minerals that can be foliated (or from contact metamorphism)

Marble Quartzite Anthracite Amphibolite

Protolith

Mineralogy

limestone

carbonates

quartzsandstone

quartz

coal

none

mafic rocks

Amphiboles(i.e. hornblende)

Marble

http://www.mii.org/ http://www.mii.org/

Quartzite

http://www.smccd.edu/

AmphiboliteCan be either

http://www.mii.org/

Anthracite