Metamorphic rocks

Metamorphic rocks

Form by changes in composition of preexisting rocks due to pressure and temperature

Sources of heat

Conduction (mantle), advection (magma), radioactive decay

Temperature range for metamorphism

Occurs 250 - 1000 degrees C

Sources of pressure

Overlying rock mass (lithostatic pressure)

Lithostatic pressure formula

density x gravity x height

Pressure range for metamorphism

1 kbar - 30 kbar (3km - 30km)

Metamorphic grade

Pressure and temperature conditions experienced by the rock

Protolith

Original rock that metamorphic rock formed from

Metamorphosed basalt

Mafic igneous rocks

Metamorphosed shale

Clay-rich sediments

Regional metamorphism

Occurs over large areas due to burial

Contact metamorphism

Occurs over small areas due to localized heat sources around igneous intrusions

Hydrothermal metamorphism

Caused by hot fluids, e.g. ocean floor basalts interacting with hot fluids

Impact metamorphism

Occurs when a meteorite hits Earth, causing large pressure and temperature changes

Fault related metamorphism

Brittle or ductile deformation in faults or shear zones; intense deformation allows new minerals to grow

Classification of metamorphic rocks

Based on appearance, composition, pressure and temperature, and deformation

Inherited features of metamorphic rocks

Bedding, large igneous crystals

Features formed during metamorphism

Minerals of different sizes, distinct layers, aligned grains, folds

Layering in metamorphic rocks

Alternating layers of different compositions

Foliation

Planar feature defined by the preferential orientation of mineral grains

Lineation

Linear feature defined by the preferential orientation of mineral grains

Crenulations

Small scale folds

Porphyroblasts

Grains much bigger than the average grain size

Matrix

Finer grained minerals that host the porphyroblast

Metamorphic assemblages

A list of minerals that appear to co-exist stably within a rock

Metamorphic facies

Rocks with the same chemical conditions that form under the same pressure and temperature conditions will have the same minerals in the same proportion

Greenschist facies

Low pressure and temperature; includes chlorite, epidote, actinolite

Amphibolite facies

Medium pressure, medium to high temperature; includes plagioclase-hornblende-quartz + epidote + clinopyroxene + garnet

Granulite facies

Medium pressure, high temperature; includes orthopyroxene-clinopyroxene-plagioclase + quartz + hornblende + garnet

Metamorphic zones

Chlorite -> biotite -> garnet -> staurolite -> kyanite -> sillimanite