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Metamorphic Grades Describe “How Much”

Sometimes rocks are metamorphosed just a little bit. Other times, rocks are altered intensely. The degree that rocks are changed by metamorphism depends on how much heat and pressure they are put under, and for how long they are subject to these conditions. The amount of metamorphism is called the metamorphic grade. Millions of years after a rock has been metamorphosed, geologists can figure out how much heat and pressure it was subject to by looking at its minerals. That’s because different minerals will form during different amounts of metamorphism. The minerals listed below are clues to the metamorphic grade.

  • Low grade metamorphic rocks form in temperatures of about 200 degrees Celsius. They contain minerals such as chlorite, muscovite, and biotite.
  • Intermediate grade metamorphic rocks contain minerals such as garnet and staurolite.
  • High grade metamorphic rocks form in temperatures of about 800 degrees Celsius. In that environment the rocks are on verge of melting. One mineral that forms in this intense heat is sillimanite, which is a common mineral in the rock gneiss.

Last modified June 20, 2003 by Lisa Gardiner.

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