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. With temperature, controls what minerals are stable.

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.)

Hydrostatic Pressure vs Deviatoric Stress

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

Foliation

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

Foliation - Gneissic Banding shows gneissic banding Arizona State University http://www.slu.edu

Low Grade High Grade Foliated Metamorphic Rocks Metamorphic rock with aligned flat minerals http://hyperphysics.phy-astr.gsu.edu/hbase/geophys 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 http://www.tulane.edu/~sanelson/geol212/contactmeta.htm

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

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) http://www.mii.org/ http://www.mii.org/ http://www.mii.org/ http://www.smccd.edu/ Marble Quartzite Anthracite Amphibolite Can be either