Geology, Geologic Processes and Geologic History
Metamorphic rocks are pre-existing rocks that have been changed by heat and pressure and are often deformed and infiltrated by fluids.
Where it is obvious, some low grade metamorphic rocks are named by their precursors: metasandstone, metasiltstone, metachert, metagranite
Foliated metamorphic rocks contain aligned needle-like or platy minerals, whereas non-foliated metamorphic rocks have no preferred alignment of minerals.
Common foliated metamorphic rocks are:
has a slaty cleavage (thin parallel splits in rock). Usually a
metamorphosed mudstone or shale
•schist: has a schistosity (parallel alignment of coarse-grained crystals)
•gneiss: has a gneissic texture consisting of parallel bands of light and dark colored minerals.
A gneiss that was a granite is often called a granitic gneiss
Common non-foliated metamorphic rocks are:
•quartzite: composed of quartz. Usually a metamorphosed sandstone.
•marble: composed of calcite or dolomite. Usually a metamorphosed limestone or dolomite
•ampbibolite: dark green to black rock composed of plagioclase and ampbibole minerals and may have a foliation. Usually a metamorphosed basalt