factors in metamorphism
DESCRIPTION
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 PresentationTRANSCRIPT
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
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
Deviatoric Stress Hydrostatic Pressure
Hydrostatic Pressure vs Deviatoric Stress
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
Foliation
Slate slaty cleavage.
Protolith– fine grained rock like shale, mudstone, or siltstone
Foliation - Slaty Cleavage
Arizona State University
http://www.slu.edu
Gneiss shows gneissic banding
Foliation - Gneissic Banding
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
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
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