Metamorphic Grade

Metamorphic rocks form below the ground surface, beyond the reach of near-surface sedimentary processes of disintegration and consolidation. Pressure and temperature increase as depth below the ground surface increases. For each 1 mi (1.6 km) increase in depth below the Earth's surface, pressure typically increases by 0.56 kilobars, while temperature increases an average of 70°F (40°C). We know that minerals and rocks form under specific conditions, including unique pressure and temperature ranges. Once formed, if changes to these conditions occur, the rock becomes unstable and must undergo change to again reach stability. If these changes include the long-term application of increased temperature and pressure to any type of rock, whether igneous, sedimentary or metamorphic, the end result is a metamorphic rock. These processes alter the mineral composition and texture of the parent rock, thereby creating a different type of rock. This is metamorphism.

In the order of increasing pressure and temperature, the metamorphic rocks formed from the sedimentary rocks shale and/or mudstones are slate, phyllite, schist and gneiss; from volcanic tuff (ash turned to rock), various types of schist and amphibolite, a dark rock containing hornblende and feldspar; from sandy limestone or dolomite, marble, tremolite marble, and diopside marble; the latter two being coarse-grained, impure forms of marble.

Although metamorphism produces particular types of rocks, when interpreting metamorphic grade, geologists often focus on metamorphic facies, as opposed to a specific type of metamorphic rock. This is because the environment in which metamorphic rocks formed is not easily identified based on a single type of rock.

A metamorphic facies consists of metamorphic rocks that form within a similar environment with respect to pressure and temperature, and is identified by the presence of specific mineral groups.