Metamorphic Rocks

Our final stop in looking at the basic rock types defined by the rock cycle are the metamorphic rocks. A metamorphic rock results from extensive changes to texture and mineralogy of the parent rock. This usually occurs with changes in pressure and temperature that occur above the point of diagenesis and below the point of actually melting the rock. One thing to remember, metamorphism occurs while the rock remains solid. If it reaches a point where the rock melts, it then becomes an igneous rock. (See my article Igneous Rocks for more infor on them.) Metamorphic rocks are the result of the dynamic nature of our Earth. Unlike sedimentary and igneous rocks, metamorphic rocks are very complex. Given different conditions of heat and pressure, the same parent rock can form any number of different metamorphic rocks. Because the same parent rock can result in different metamorphic rocks, geologists classify metamorphic rocks in groupings called facies. The different facies reflect the range of pressures and temperatures that the parent rock may be subjected to.

Knowing the facies of the metamorphic rock is useful when looking at them in the lab, but in the field, a geologist doesn’t always know the specifics of temperature and pressure that might have affected the parent rock. Therefore, in the field, geologists identify the origin of metamorphic rocks based on the extent of the metamorphism. There are two main types of metamorphism described this way. Regional metamorphism occurs over a large regional area and usually results from mountain building events. Contact metamorphism results when molten rock makes contact with existing rock. This contact creates a zone of altered parent rock around the intruding igneous rock.

Knowing the origin of the metamorphic rock is but the first step. After identifying how the rock originated, a geologist must name the rock. This can be the difficult part as the naming of metamorphic rocks is not as systematic as the naming of sedimentary or igneous rocks. Metamorphic rocks are named based on the texture of the rock.

We will use a generalized mudrock to help name the basic metamorphic rocks. Initial changes in pressure and temperature will result in recrystallization of the minerals present. Generally, early during the metamorphism, the changed minerals remain microscopic, but their orientation may alter to a direction of preferred orientation. Essentially, all the minerals line up in the same direction. The resulting rock has excellent cleavage (the splitting along the planes of the minerals in the rock) and is called slate. As metamorphism continues, we generally see an increase in the grain size of the minerals. This rock is known as a phyllite. Phyllites are very similar to slate, but have a silky sheen on their surfaces. As more heat or pressure builds up metamorphism continues and crystals become visible without the aid of a magnifying lens. At this point, the rock has distinct platy or elongate minerals in a strongly preferred orientation. The rock is now called schist. It can still be spilt into thin flakes or slabs, but the minerals that make up the rock are visible to the eye. As metamorphism continues, the rock will begin to develope bands of medium to course grained textures and different minerals. This rock is called gneiss (pronounced ‘nice’).

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