MetamorphismTemperature, fluids, pressure, deviatoric stressMetamorphic gradeMetamorphic faciesContact metamorphismRegional metamorphism

The Rock Cycle

MetamorphismTemperature, fluids, pressure, deviatoric stressMetamorphic gradeMetamorphic faciesContact metamorphismRegional metamorphism

The Rock Cycle

Rocks are classified by their mode of formation. There are three major rock forming processes on Earth, producing three kinds of rocks.

Igneous Rocks

Formed when magma (molten rocks) solidifies

Environment: Hot enough to melt rock, pressure varies

Sedimentary Rocks

All non-igneous rocks formed by processes acting on the surface of the Earth

Environment: normal for Earth’s surface

Metamorphic Rocks

Formed by chemically and physically altering rocks under heat and pressure deep within the Earth’s crust.

Environment: High pressure, not hot enough to melt rock

Metamorphic Rocks Metamorphic rocks form deep within Earth’s crust under high pressure and temperature. The buried rock (protolith) undergoes solid state transformation, altering the mineral composition and texture of the rock. The rock can not melt, or it would become an igneous rock after cooling.

Metamorphism that occurs around igneous intrusions is called contact metmorphism.

Metamorphism occurring due to stresses caused by mountain building is called regional metamorphism. These complex metamorphic environments may include zones of contact metamorphism if igneous intrusions occur.

The type of metamorphic rock produced is controlled by the composition of the protolith and the temperature, pressure and duration of the metamorphism. Metamorphic rocks come in two flavors – foliated and non-foliated.

Factors in Metamorphism

Metamorphism is effected by four main factors:

Fluids

Temperature

Pressure

Deviatoric stress

Factors in Metamorphism

Temperature

Controlled by:

Geothermal gradient (increasing temperature with greater depth in Earth)

Tectonic setting (higher in areas of igneous activity)

Most rocks melt at less than 2000oC, the upper limit of metamorphism (rocks formed from melt are igneous!)

Factors in Metamorphism

Fluids

Moving fluids within the rock body can effect:

Pressure (increases with temperature)

Heat (moves it around)

Dissolved ions and gases (moves them around)

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

Stress imbalance causes compression in some directions and expansion in others. Controls:

Foliation (orientation of minerals into layers)

Lineation (linear orientation of minerals)

Deviatoric Stress

Deviatoric Stress Hydrostatic Pressure

Hydrostatic Pressure vs Deviatoric Stress

Foliated Metamorphic Rocks

Foliation occurs when a protolith with flat minerals like mica is subjected to deviatoric stress. The flat minerals align perpendicular to the direction of maximum stress.

Increasing metamorphism causes more severe reordering and alteration of the rock’s metamorphic texture.

Metamorphic rock with aligned flat minerals

Non- Foliated Metamorphic Rocks Metamorphic rock without minerals that can be foliated

Marble Quartzite Anthracite Amphibolite

Protolith

Mineralogy

limestone

carbonates

quartzsandstone

quartz

coal

none

mafic rocks

amphiboles

Marble

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

Quartzite

http://www.smccd.edu/

Amphibolite

http://www.mii.org/

Anthracite

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

Deviatoric Stress

Maximum crystal growth occurs perpendicular to direction of

maximum stress

Deviatoric Stress

Foliation - alignment of flat minerals

Slaty Cleavage – Alignment of small mica flakes

Schistosity – Alignment of large mica flakes

Gneissic Banding – segregation of different minerals into light and dark bands

Lineation - alignment of pointy minerals

Deviatoric stress is the primary control on foliation and lineation

Deviatoric Stress

http://www.aqd.nps.gov/grd/usgsnps/deform/gfoliation.html

Foliation and lineation

Foliation

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

Foliation - Slaty Cleavage

Orientation of existing minerals

Slate slaty cleavage.

Foliation - Slaty Cleavage

Slate

slaty cleavage.

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

Foliation - Slaty Cleavage

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

SchistFoliation - Schistosity

Orientation of existing minerals and crystallization of new

minerals

Foliation - Schistosity

Schist exhibits schistosity

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

Foliation - Schistosity

Schist

New minerals crystallize - in this

case garnets

New minerals crystallize - in this

case garnets

Foliation - Gneissic Banding

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

Orientation and crystallization continue, minerals become

segregated into bands

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

SchistSchistSlateSlate GneissGneiss

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

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

Metamorphic Grade

Metamorphic grade increases with both increasing temperature and pressure, both of which increase with depth in the Earth

Metamorphic Grade

Materials with the same chemical formula can have different crystalline structures, and thus different physical properties, depending on the

conditions of formation

Rocks containing diamonds formed under different P/T conditions than rocks containing graphite

GraphiteGraphite

Temperature (K)1000 3000

Pre

ssur

e

DiamondDiamond

Metamorphic GradePolymorphs - minerals with the same chemical formula but different crystallize

structures (=different chemical and physical properties)

Finding one polymorph in a rock narrows down the P/T

conditions at metamorphism

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

Metamorphic Grade

sillimanitekyanite

andalusite

Metamorphic Grade

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

Contact Metamorphism

Contact Metamorphism

Shale Rock

Igneous Intrusion

Metamorphism adjacent to an igneous intrusion

Map hig

h g

rad

e

hig

h g

rad

e

low

gra

de

ign

eou

s

low

gra

de

Contact Metamorphism

Finds first specimens of “green” metamorphic

index mineral

Finds first specimens of “yellow” metamorphic

index mineral

Finds igneous intrusion

Metamorphic Grade

A B C

Index Mineral – a mineral in a metamorphic terrain which indicates a certain metamorphic grade

Isograd – line marking the first appearance of an index mineral on a map.

ABC

Map Contact Metamorphism

Metamorphic Grade

high grade

low grade

igneous

unmetamorphosed

Metamorphic Grade

http://www.sun.ac.za/geology/METCOURSE/2nd%20year/metamorphic%20zones.htm

300oC 700oC

Contact Metamorphism

Isograds of index minerals

Regional Metamorphism

Large-scale metamorphism is usually related to tectonic events (rifting, mountain building, etc.)

A single regional event can produce many different kinds of metamorphism, including smaller zones of contact metamorphism.

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

Metamorphic Facies – Rocks that have been exposed to the same degree of metamorphism (P/T regime), and thus are the same metamorphic grade

Metamorphic Facies

Metamorphic Facies

http://www.dc.peachnet.edu/~janderso/images/metafaci.jpg

All rocks that have experienced this much metamorphism belong

to Greenschist facies.

Metamorphic Facies

However, all the rocks have undergone the same amount of

metamorphism, and all belong to the Greenschist facies.

QuartzK-feldsparPlagioclaseMuscoviteBiotiteChloriteEpidoteCalciteMagnetiteActinolite

Mudstone Limestone Granite Gabbro

Min

eral

s in

Met

am

orp

hic

Roc

kProtoliths

Metamorphic RocksSchist Schist SchistMarble

Metamorphic Facies

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

Carolina Slate Belt Greenschist facies

Carolina Slate Belt Greenschist facies

The Rock Cycle

Igneo

us R

ocks

Sedi

men

tary

Roc

ks

Metamorphic Rocks

The Rock Cycle

1. Melt (form magma) 2. Cool (crystallize)

Igneous Process making igneous rocks

geothermal gradient, Bowen’s Reaction Series, felsic, intermediate, mafic, magma, lava, texture, basalt, gabbro, andesite, diorite, rhyolite, granite

1. Chemical and physical weathering (breaks down rock)

Sedimentary Processesmaking sedimentary rocks

(air/water)

(ice)

2. Transport clasts and/or ions

3. Deposition/ precipitation in basin

4. Lithification

weathering, transportation, deposition, lithification, clasts, clastic rocks, chemical rocks, depositional environment

Sedimentary Processesmaking sedimentary rocks

Add heat and pressure to pre-existing rock (protolith)

Metamorphic Processesmaking metamorphic rocks

metamorphism, temperature, fluids, pressure, deviatoric stress, metamorphic grade, metamorphic facies, index minerals, isograds, contact metamorphism,

regional metamorphism

Igneous Process

1. Heat (melt)2. Cool (crystallize)

Metamorphic Process

Add heat and pressure

Sedimentary Process1. Breakdown (weather)2. Transport3. Deposit/precipitate4. Lithify

original rock mass

sedimentary rock

metamorphic rock

igneous rockTh

e R

ock

Cycl

e