These are dynamic,
regional and contact metamorphic rocks.
Dynamic
Metamorphics - Pressure
Mylonite. By Dave
Schumaker via Flickr.com
Dynamic metamorphics are the rocks that have been experiencing dynamic pressure.
They have been stretched or flattened, maybe twisted when large scale
movements happen in the Earth's crust and different rock masses are
forced over each other and crush each other as they come into
contact. These are the rocks with some impressive foliations.
Contact
Metamorphics - Temperatures
Skarn. By Maggie via Flickr.com
Contacts
are mainly affected by hot
temperatures and very little by pressures.
They have therefore got no foliation or "schistosity". These rocks
include hornfels,
skarn
and fulgurite. There
are also some rocks that can be
both regional
and contact metamorphics. They include marble,
quartzite,
serpentinite, greenstone,
eclogite and amphibolite.
Regional
Metamorphics - both Pressures and
Temperatures
Regionals
are
the rocks that have
the "schisty"
texture which break along the layers (except gneiss). They are formed by both
increasing pressuresand temperatures.
These
rocks include slate, phyllite, schist, gneiss, granulate and
migmatite. At low pressures and temperatures, the sedimentary
rock shale forms slate.
With increasing pressures and temperatures,
phyllite, schist
and gneiss
are formed. Each
stage also have their own
mineral composition. Shale contains clay minerals, while chlorites are
common in slate; quartz and muscovite are common in phyllite; garnet
and biotite are common in schist; and gneiss is known for minerals like
sillimanite
and K-feldspar.
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