Earth Science Laboratory

METAMORPHIC ROCKS


    The word metamorphic is derived from the Greek Words meta ("change") and morph("form") When applied to rocks, it therefore refers to rocks that have changed their form - rocks that may have originally been sedimentary, igneous, or another metamorphic rock; then subjected to new conditions of temperature and pressure, causing "recrystallization" and a completely new form. This occurs when limestone, for example, is subjected to heat and pressure and turns into a more coarsely-crystalline and sometimes banded rock called marble. The soft, clay-rich rock known as shale, when subjected to pressure becomes a harder rock called slate. Often, the changes are even more spectacular, with new minerals formed, such as garnet, mica, and tourmaline (as seen in schist). Even coal can be metamorphosed; bituminous (soft) coal, a sedimentary rock, can be changed into anthracite, or hard coal. Anthracite can be found in the folded belts of rocks in the Appalachian Mountains, where temperatures and pressures of mountain building have caused these changes to occur. There are two major types of metamorphism...

Regional (pressure-dominated) metamorphism occurs over large areas during periods of mountain building. Regional metamorphism causes a layered or platy structure in rocks, called foliation. Foliated rocks are the result of intense pressure (and sometimes, to a lesser degree, are also the result of heat). Different grades of metamorphism are demonstrated when a shale is subjected to increasingly greater pressure and heat - first it becomes slate, then phyllite, then schist, and finally, gneiss.

Contact (heat-dominated) metamorphism occurs when molten rock (magma) comes into contact with other surrounding rocks (called the country rock). The heat causes a localized "baked zone" of metamorphosed rock. Metamorphic rocks produced by contact metamorphism are not foliated, as the major factor here involves heat rather than pressure.
 
Summary Chart of Common Metamorphic Rocks
Original Rocks Metamorphic Equivalent Foliated? Metamorphism
sandstone quartzite no regional & contact
shale slate >> phyllite >> schist >> gneiss yes regional
limestone marble no contact
clay-rich rock hornfels no contact
granite gneiss yes regional
basalt schist yes regional
bituminous coal anthracite coal no regional

Some characteristics of metamorphic rocks to look for:

1. They are invariably bright and lustrous.

2. Foliated metamorphic rocks appear layered.

3. Coarse-grained metamorphic rocks may show crystals that grew larger during metamorphism.


Copyright © 1983 by William K. Tong