13.2: What Is a Metamorphic Rock?
What Is a Metamorphic Rock?
The word metamorphism Links to an external site. means to change form; for rocks, this means a recrystallization of minerals (crystals) under I conditions (temperatures too low for melt production). All rocks form at certain temperatures and pressures at, or more commonly, beneath the Earth’s surface. These rocks are most stable at the conditions under which they form. Therefore, changing the temperature and/or pressure conditions may lead to a different rock, one that changed in order to be stable under the new external conditions. This new rock that forms in response to changes in its physical and chemical environment is called a metamorphic rock. A metamorphic change can also occur if the rock’s composition is altered by hot, chemically reactive fluids, causing a change in the mineral content of the rock.
Metamorphic rocks form by the physical and sometimes chemical alteration of a pre-existing rock. To distinguish between the pre-existing rock and the new metamorphic one, the term protolith Links to an external site. (proto- meaning “original” and -lith meaning “rock”), or parent rock, is used to describe the pre-existing rock (Figure 13.2). All metamorphic rocks have at least one protolith that was altered during metamorphism. Igneous, sedimentary, or in some cases, even metamorphic rocks can be altered into a completely new or different metamorphic rock.
Figure 13.2: Heat and pressure alter shale, the protolith, to slate. (Left: CC-BY 2.0; James St. John via Flickr Links to an external site.; Right: CC-BY 4.0; Chloe Branciforte, own work)
A person who studies metamorphic rocks is typically referred to as a metamorphic petrologist. A petrologist Links to an external site. is a geoscientist that studies rocks and the conditions under which rocks form. A metamorphic petrologist specializes in metamorphic rocks and their unique conditions of formation. Like many other geoscientists, working with other disciplines is common, with a heavy influence from both math and technology. Many are employed by universities where they teach and/or do research, and at state and federal agencies, including geological surveys, like the California Geological Survey Links to an external site. or United State Geological Survey (USGS) Links to an external site.. Additional career pathways are available in the private sector including mining and natural resource extraction. Many of these career options require a college degree and postgraduate work. If you are interested, talk to your geology instructor for advice. We recommend completing as many math and science courses as possible (chemistry is incredibly important for mineralogy). Also, visit National Parks Links to an external site., CA State Parks Links to an external site., museums, gem & mineral shows, or join a local rock and mineral club. Typically, natural history museums will have wonderful displays of rocks, including those from your local region. Here in California, there are a number of large collections, including the San Diego Natural History Museum Links to an external site., Natural History Museum of Los Angeles County Links to an external site., Santa Barbara Museum of Natural History Links to an external site., and Kimball Natural History Museum Links to an external site.. Many colleges and universities also have their own collections/museums.