metamorphism and metamorphic rocks factors influencing metamorphism pressure more pressure is...

METAMORPHISM AND METAMORPHIC ROCKS

FACTORS INFLUENCING METAMORPHISM

PRESSURE

More pressure is exerted on rocks at depth than at Earth’s surface.

LITHOSTATIC or CONFINING PRESSURE

pushes on rocks in all directions.

Grains become smaller anddenser at depth.



PRESSURE

Pressure is greatest at thecontact point betweengrains.

Bonds can break.Ions migrate and re-bond.Rock becomes

compressed.



PRESSURE

Pressure may be greater insome directions.

DIRECTED PRESSUREFlattens rock in the

direction at which thegreatest pressure isapplied.

Lengthens rock in the direction to the greatest pressure.



PRESSURECauses existing minerals and

features to deform.Doesn’t affect minerals in

the rocks.

Induces FOLIATION, in which the tabular minerals are aligned to the direction ofgreatest directed pressure.



FOLIATION



PRESSURE

For metamorphism to occur, 1 kilobar of pressure isrequired.1 Bar = pressure applied to the Earth’s surface

by the atmosphere.1 Bar = 1.02 kg/cm2

1 kilobar = 1000 Bar (or 1020 kg/ cm2)

These pressures occur at ~ 3 km depth.



CIRCULATING FLUIDS

Water greatly influences metamorphism.Aids in migration of unbonded atoms and ions.Allows for exchange of ions between adjacent grains.

Water comes from a variety of sourcesGround waterSubducted oceanic crustCrystallization of magmaDecomposition of water-rich minerals like

amphiboles and clays



CIRCULATING FLUIDS

Influence production of new minerals by ionic exchange.

Speed up reactions and change overall composition of the rock.



PARENT ROCK

Composition of parent rock influences the type of metamorphic rock produced.


TYPES OF METAMORPHISMCONTACT METAMORPHISM

Solid rocks in close proximity to magma will changein response to heating and circulation of hotfluids, i.e., INTRUSIONS.

Produces a “baked zone” in the adjacent country rock.Pressure is not significant in reaction.Impact of the intrusion is local.Only rocks in immediate contact with the intrusion

will be highly metamorphosed.


CONTACT METAMORPHISM

Minerals stable at high temperatures will be formed closest to the intrusion.

Coarser grained metamorphic minerals will also beformed closest to the intrusion.

Size of the intrusion also influences the amount ofmetamorphism.

Differences in temperature is also significant.


CONTACT METAMORPHISM


REGIONAL METAMORPHISMAlters rocks over a large geographic region.

Appalachians of New EnglandNorth Cascades of Washington-British Columbia


REGIONAL METAMORPHISM

BURIAL METAMORPHISM

Occurs deep in sedimentary basins.Requires depths > 10 km

At these depths and greater, lithostatic pressure andgeothermal heat drive metamorphic reactions andrecrystallization.

Does not require a tectonic process.Does not create mountain belts.Generally no foliation because no directed pressure.



BURIAL METAMORPHISM



DYNAMOTHERMAL METAMORPHISM

Affected by pressure (squeezing) and heat (magma).Occurs when converging plates squeeze rock caught

between the plates.Crust generally thickens.Surface rocks get taken deep into the Earth.Rocks are subjected to high temperatures, lithostatic

pressure and directed pressure.Rocks become foliated.



DYNAMOTHERMAL METAMORPHISM

Produced metamorphic rocks found in the Alps,Himalaya, and Appalachian Mountains.

Also produced the greatly eroded metamorphic rocksin the Great Lakes region.

These rocks are exposed by uplift (faulting) and rapiderosion.

The process doesn’t reverse itself.


OTHER TYPES OF METAMORPHISM

HYDROTHERMAL METAMORPISM

Chemical alteration of pre-existing rocks by hot water.Occurs primarily beneath ocean floor near divergent

plate boundaries.Creates water-rich minerals.

Includes serpentine, talc, and chloriteIn addition, when steam hits ocean water and cools

rapidly, valuable metals such as Cu, Ni, Fe and Pb are precipitated.



FAULT ZONE METAMORPISM

Produced by rocks grinding past each other at a fault.

Produced by directed pressure and frictional heat.



SHOCK METAMORPISM

Produced by meteorite impact.Forms shock varieties of quartz.

StishoviteCoesite



PRYOMETAMORPHISM

Results from ultra-high temperatures that occur in lowpressure environments.

Usually produced by lightning hitting surface rocks orsediments.

Fulgurite



PRYOMETAMORPHISMFulgurite

metamorphism and metamorphic rocks factors influencing metamorphism pressure more pressure is...

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