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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…

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Increased temperature to 300 ­ 400 degrees centigrade.
Partial recrystallization occurs
New minerals occur as oval spots 2 ­ 5mm in diameter.
Spots show sieve or poikiloblastic texture Spots have overgrown and included grains of the
original argillaceous rock
Relic structures such as bedding/lamination and fossils may be evident

Hornfels…

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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…

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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…

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