Geo - Metamorphic rocks

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Metamorphic rocks

• rocks made from material that has been changed by temperature, pressure, and/or fluids

• both igneous and sedimentary rocks can become metamorphic rocks, as well as other metamorphic rocks

• metamorphism is caused by plate tectonic motion

• provides information on past metamorphic processes

Protoliths

preexisting rocks

metamorphic rocks are solid state alterations of protoliths

uses of metamorphic rocks

building and construction (quartzite, marble, slate, etc.)

monuments

roofing/paving

decorative stone

jewlery

industry

phase change

new minerals form with:

• same chemical formula

• different crystal structure

changes texture

main types of metamorphism

1. Heat (T)

2. Pressure (P)

3. Compression and shear

4. Hot fluids

heat driven metamorphism

heat energy causes solid state diffusion (atomic bonds breaking and rearranging)

occurs at temperatures from 200C up to 700C - 1100C

where does metamorphism take place?

in the lithosphere & upper mantle

metamorphism due to Pressure (P)

• creates denser material

• involves phase change or recrystallization

• formation of many minerals depends on both P & T

normal stress

acts at right angles

shear stress

acts parallel to surface

elastic deformation

strain that is reversable after stress is released

ex. rubber band returns to normal when not stretched

ductile deformation

changes in shape due to stress are permanent

ex. bending a metal bar keeps it in that shape

brittle deformation

rocks fraction under increasing stress

confining pressure

pressure exerted from rocks above is matched by equal pressure from all sides

causes chemical reactions just like heat

directed stress

unequal balance of stress from multiple directions

generated by movement of lithospheric plates

occurs at much lower pressures and doesn’t result in chemical changes

recrystallization

minerals dissolve from an area of rock experiencing high stress and regrow in a lower stress location

minerals change size and shape but not identity

chemically reactive fluids

• expelled by crystallizing magma

• react with minerals in protolith changing chemical and mineral composition

  • dominant near mid ocean ridges

metasomatism

hydrothermal alteration

foliation

parallel surfaces or layers in metamorphic rocks

foliated metamorphic rocks

• slate

• phyllite

• schist

• Gneiss

mineralogy temp

Light colored (felsic) melts at lower T

Dark colored (mafic) melts at higher T

non-foliated

lack a planar fabric

low grade

low temp and pressure

high grade

high temp and pressure

arrange schist, migmatite and slate from lowest to highest grade

slate, schist, migmatite

as grade increases

new and larger minerals form

index minerals

form at certain temps and pressures

at a temp of 800C and a pressure of 6 km, what mineral is present?

sillimanite

thermal metamorphism

heat by plutonic intrustion

burial metamorphism

increases in P and T by burial in a basin

dynamic metamorphism

shearing in a fault zone

regional metamorphism

P and T alteration due to mountain building

produces the most metamorphic rock

hydrothermal metamorphism

alteration by hot water leaching

subduction metamorphism

high P low T alteration in subduction zones

shock metamorphism

extreme high P resulting from meteorite impact

shields

ancient high grade rocks

remanent of eroded mountain belts

why they are important

• provide insight on physical and chemical changes deep within Earth

• metamorphic minerals have economic value