metamorphism cheat sheet

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Metamorphism
A change in form of preexisting rocks of all types.
Sedimentary
Igneous
Metamorphic
Contact By the action of Heat alone
Dynamic By the action of Pressure alone
Regional By the action of Heat and Pressure in combination
Metamorphism Excludes:
Weathering, diagenesis and lithification
Environments where temperatures are below 200 ­ 300 degrees centigrade
Melting Of Rocks environments where temperatures are above 650 degrees centigrade
Environments less than 2km depth and at pressures below 1000 bars
The extent of change to existing rocks
Low Grade ­ slight alteration (further away in the metamorphic aureole)
Medium Grade ­ significant alteration
High Grade ­ extensive/total alteration (close to the edge of the metamorphic aureole)
Metamorphic changes are assumed to be isochemical ­ so the chemical composition will
remain the same e.g % Si , O, Al
The only loss from the system is water mostly in clay because as temperature is increased
then the water is boiled off.
Contact Metamorphism
Changes due to the action of heat alone. In Batholiths, Plutons, Sills and Dykes you will find
contact metamorphism. Found anywhere with large scale igneous bodies. Metamorphic
aureole refers to the volume of rock affected by heat from the intrusion
Mafic intrusion is hotter than Silicic one.
Argillaceous rocks have the most clay they undergo the most change.
Low Grade ­ Spotted Rock
Medium Grade ­ Chiastolite Rock
High Grade ­ Hornfels
Spotted Rock

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Increased temperature to 300 ­ 400 degrees centigrade.
Partial recrystallization occurs
New minerals occur as oval spots 2 ­ 5mm in diameter.…read more

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Usually at a mountain building site. Because of the pressure they often have aligned crystals.
Occurs due to a progressive increase in pressure and temperature conditions. Argillaceous
rocks undergo most change as they are composed of chemically complex clays
Low grade Slate
Medium grade Schist
High grade ­ Gneiss
Slate
Occurs at shallow depths (515km, relatively high pressures
and low temperatures less than 500 degrees centigrade. Forms
Platey minerals at 90 degrees to stress, grain size is too small
to see with the naked eye.…read more

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Metamorphism caused by pressure alone. Associated with major fault planes, especially
reverse and thrust faults. Eg Lizard Thrust, Moine Thrust, Glarus Nappe. Very localised,
restricted to 1 or 2 metres immediately adjacent to the fault plane.
Fault Metamorphism
Fault Breccia
At shallow depth depths and moderate fault movement rocks simply
break up into angular fragments. Low to moderate pressures at
shallow depths < 5km below the surface. Long axis of the clast may
show parallel orientation to the fault plane. Easily eroded.…read more

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