METAMORPHIC ROCKS

We Should Be As Water, Which Is Lower Than All

Things, Yet Stronger Even Than The

Rocks.Oglala Sioux

• Metamorphism (meta – change; morphe – form).• Minerals are most stable in the environment they formed.• Minerals change when rocks are moved deeper into the earth.• Heat, pressure, & chemical reactions alter mineral content, texture (size, shape, & distribution), & structure within rocks without melting them. • Bonds of unstable minerals break down w/ freed atoms/ions of elements recrystallizing, or recombining to form new, stable minerals.• A new environment of stability is achieved for the altered minerals.• Not all bonds are broken. Why?• Elemental composition of metamorphic rocks is the same as the original rock.• Metamorphic rocks form the basement foundation for many mountain belts.• Metamorphic rocks appear at the surface due to erosion of overlying rocks, tectonic & uplifting processes, or faulting.

MetamorphismDanny M. Vaughn, Ph.D., CMS

Crystallization of Magma: Bowen’s Reaction Series

• Minerals melt & solidify at ~ the same temperature.

• Minerals that melt first crystallize last.

• Minerals that melt last crystallize first.

• Process relates silicate mineral crystallization tomagma composition.

(Anorthite)

(Albite)

(Orthoclase)

(Hornblende)

Driving MechanismsHeat

• Heat accelerates reactions & the most important factor driving metamorphism.

• Geothermal gradient ~72°F per mile in the crust.

• Metamorphism occurs when rocks created in a cooler environment are relocated to a hotter environment.

• Heat sources: tectonics, convection w/inmantle.

Metamorphism Results in:

1. Solid-state chemical reactions

2. Recrystallization

Metamorphism

Metamorphic Rocks

of North America

Driving Mechanisms

Pressure• Lithostatic pressure. Litho – rock, static – stand in place.

• Also known as, confining pressure.

• Volume changes, not shape.

• Compressional forces.

• Pressure in bars (1.02 km/sq.m,14.7 lbs/sq.in).

• Metamorphism generally occurs at>1 kilobar (1,00 bars) & temperatures below 6 miles.

Driving MechanismsPressure

• Directed pressure. Forces are in one direction.

• Shape changes, typically flattened in one direction.

• Lengthened perpendicular to the direction of force.

• Mineral alignment (foliation) also results.

• Occurs in platy (thin, elongated, & flaky) minerals.

Driving Mechanisms

Directed PressureUndeformed

Stretched

Driving MechanismsPressure

Photomicrograph of mica mineralsaligned in a preferred direction (foliated).

Driving MechanismsFoliation

• Fluids speed up chemical reactions.• Weak, unstable ions are broken from a mineral. • Water (particularly under pressure) aids in moving ions.• Water promotes an exchange of ions with adjacent grains.• Basalts formed by mantle melting contain no water & are stable.• Adding water changes anhydrous olivene, pyroxene, plagioclase into hydrous minerals (serpentine & chlorite).

Driving Mechanisms

Circulating Fluids

• Minerals w/in the parent rock determine the elements to work with.

• Parent rocks w/ only one mineral will contain only one mineral when metamorphosed.

Driving MechanismsWhat Controls the

Mineralogy?

Types of Metamorphism

Contact Metamorphism

• Magma in contact w/ preexisting rocks.• Variation in temperature w/ surrounding rocks.


Regional Metamorphism

• Also termed dynamothermalmetamorphism.

• Alters rocks over 1,000’s km2.

• Majority of the continental crust.


Regional MetamorphismWhy do metamorphosed

rocks not become unstable at the surface?

• Rocks exposed at the surface are generally weathered & eroded before the rock becomes unstable from a change in environment of creation.

• Removal of CO2 and H2O during heating degases & dehydratesmetamorphosed rocks. • Slows a reverse reaction since water is the catalyst.


Burial metamorphism

• Occurs when sediment is buried >6 miles.

• Lithostatic pressure & geothermal heatingrecrystallize sediments.

• Foliation is weak, if developed at all.

• Where would you find this process developing?

Types of MetamorphismHydrothermal metamorphism• Rocks are chemically altered by water.

divergent plate boundaries.• Olivene, pyroxene, & plagioclase (anhydrous) to hydrous serpentine, chlorite,epidote, & talc.• Rising soluble magma is precipitated out Yielding copper, nickel, iron, & lead.


Shock metamorphism

• Meteorite impact.• Intense heat & pressure.• Minerals shattered & recrystallized.• Stishovite (SiO2) is formed.

Other forms of metamorphism. • Fault-zone. • Pyrometamorphism.

Summary of Metamorphic Types

Metamorphic Rock Classification

TextureSize and shape of the mineral crystals – in particular, the presence and type of foliation.

and

CompositionType of minerals present in the

rock.

Based on:

Metamorphic Rock ClassificationMetamorphism of mud (shale to slate

to phyllite)• Slate• Phyllite• Schist• Gneiss

Increa sing P & T

• Clay minerals to micas.• Changes in texture & minerals.• Highly unstable at high temperatures.• Red – hematite; Green – chlorite; purple – manganese; black – carbon.• As temperatures increase, mica, chlorite, & graphite flakes increase.• Ions re-bond forming phyllite.• Wavy foliation.• Some minerals visible.


Metamorphism of mud (Schist)

• Slate• Phyllite

• Schist• Gneiss

Increa sing P & T

• Foliated w/ preferred alignment of micas. • Cleaves (fractures) between planes of oriented minerals (Rock cleavage).• Temperature & pressure increase.• Mica becomes clearly visible.


Metamorphism of mud (Gneiss)• Slate• Phyllite• Schist

• Gneiss

Increa sing P & T

• At still higher temperatures, quartz & feldspar mobilize.• Migrate to regions of lower stress.• Denser, higher temperature minerals(biotite, garnet, amphibole, & pyroxene) are separated from quartz/feldspar.• A banded appearance results.• Termed metamorphic differentiation.• Gneiss forms from continental/continental & alongsubduction zones.

Slate Phyllite SchistGneiss

• Some melting can occur forming migmatites.• Highly contorted, swirling patterns.• Light colored region, granitic (felsic) minerals.• Darker region contains biotite.• From along convergent boundaries.• Large portion of continental crust.


Metamorphism of igneous rocks(Migmatites)


Metamorphism of Limestone

• From limestone/dolostone to marble.• Calcite/dolomite minerals.• Equant grains are interlocked when metamorphosed.• Foliation is rare.


Metamorphism of Sandstone

• From sandstone to quartzite.• Foliated when quartz grains are stretched or flattened.• Highly resistant to weathering.

Metaconglomerate w/ stretched quartz pebbles


Metamorphism of Igneous Rocks• Mineral dependent.

• Basalt crystallizes at high temperatures.• Major minerals are anhydrous (w/out water).• When basalt is hydrothermally metamorphosed, minerals hydrate.• Calcium plagioclase is converted to lower temperature sodium plagioclase.• Anhydrous pyroxene & olivene are converted to chlorite, epidote, & amphibole.• The low temperature green shist is created as a result.• At higher pressures & relatively low temperatures, the green minerals react to form glaucopane (blue amphibole) which yields a blue schist.• Blue schists are found only in subduction zones.• Granite & diorite form gneisses.• Mineralogy is basically in tact as the original minerals remain stable at high temperatures.• Foliation is common.

Metamorphic Grade

Degree of temperature and pressure a rock is exposed to

during metamorphism

Low Grade

Intermediate Grade

High Grade(~1,800°F)

Incr easi ng P & T

Index Minerals & Metamorphic Zones

Metamorphic Grade

• Proximity of intrusion.• Intensity of metamorphism.• Temperature dependent.

Metamorphic Index Minerals

Aluminum silicate minerals that form

at a specific temperature and

pressure

Metamorphic Rocks & Plate

Tectonics

Metamorphic Facies

Metamorphic Rock

& Daily Life

Potential Hazards from Metamorphic Rocks

The Rock Cycle Revisited

METAMORPHIC ROCKS

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