Alabama Landscapes

Geology, Geologic Processes and Geologic History

Table of Contents
 

Metamorphic Rocks

 

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: 

•slate: has a slaty cleavage (thin parallel splits in rock).  Usually a metamorphosed mudstone or shale

•phyllite: fine-grained with a slaty cleavage and has a luster due to micrscopic mica crystals.

•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

  • greenstone: a dark-green fine-grained rock

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