Intense Heat and/or Pressure, Yo!
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Metamorphic rocks are formed from existing
bedrock by the action of heat, pressure, and
chemical action within the Earth’s crust.
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Pressure: (Regional/Dynamic Metamorphism)
 At 6 miles beneath the surface, The downward pressure ~
20 tons/in2
 Horizontal pressure is present during mountain building
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College Definition: Metamorphic rock is created
when heat and pressure break the bonds
between some of the ions in a mineral, allowing
them to migrate to other sites in the rock and rebond. Such migration of ions, usually through
fluids circulating through the rock, results in
recrystallization of the mineral. Metamorphism
occurs when heat and pressure exceed certain
threshold levels, destabilizing the minerals in
rocks, but do not become high enough to cause
melting.
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Heat and Chemical Action (Thermal or Contact
Metamorphism)
 High temps may cause partial melting and recrystallization
 A “baked” zone is present along the edge of an igneous
intrusion. This will contain contact metamorphosed rocks
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Rock Transformations: (Original  Morphed)
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Limestone Marble (acid test reveals carbonate)
Sandstone Quartzite (harder and denser)
ShaleSlatePhylliteSchistGneiss
ConglomerateMeta-conglomerate (pebbles stretched and
squished)
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Quartzite and Marble may result from regional / dynamic
metamorphism but they do not show foliation (see ESRT)
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Foliation – under intense pressure and heat, some
minerals may partially melt and re-crystallize as
new minerals or larger crystals. They may also align
themselves in sheets or plates (distorted layers)of
mineral crystals (Schist)
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or they may arrange into bands of alternating
light and dark minerals (Gneiss)
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High-grade metamorphism will also
show distorted layering and banding
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Foliation