Metamorphic Grades Describe “How Much”

Just like baking cookies, metamorphic rocks need to be "cooked" just right! Click on image for full size (24K GIF) Windows Original

Sometimes rocks are metamorphosed just a little bit. Other times, rocks are changed extremely. The degree that rocks are changed by metamorphism depends on how much heat and pressure they are put under, and for how long. 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 exposed to. They look at its minerals 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 at temperatures of about 200 degrees Celsius. That’s a little hotter than the oven temperature needed to cook chocolate chip cookies! At this temperature, mica minerals grow as the rock changes.
• Intermediate grade metamorphic rocks contain mica as well as minerals such as garnet.
• High grade metamorphic rocks form in places that are so hot the rocks almost melt, about 800 degrees Celsius. One mineral that forms in this intense heat is sillimanite, which is often found in the rock gneiss.