Metamorphic rocks, along with sedimentary and igneous rocks, are one of the three types of rock studied by geologists. The name "metamorphic" comes from the word metamorphism – "meta" meaning "change" and "morph" meaning "form" – and describes that these rocks are the result of metamorphism of other rocks, referred to as parent rocks.
Major Factors in Rock Metamorphism
Six major factors are involved in the metamorphism of a rock. These are:
- Chemical composition of the parent rock, which is a major control over the new mineral assemblage of the metamorphic rock
- Change in temperature
- Change in pressure
- The presence or absence of fluids, which also influences factors (2) and (3)
- The length of time the parent rock is subjected to high pressure and temperature
- Whether or not the parent rock is compressed, twisted or broken during metamorphism
These factors all play a role in what type of metamorphic rock will be created, but the chemical composition of the parent rock is the greatest factor in determining the mineral assemblage of the metamorphic rock. Only the elements that existed in the form of minerals in the parent rock will be present in the metamorphic rock, although the mineral's crystal shapes may be different or there may be new minerals all together.
For example, a sandstone rock made entirely of quartz grains will have the elements Silicon (Si) and Oxygen (O), the only two elements in quartz crystals. If this parent rock is subjected to metamorphism, the resulting metamorphic rock will still only contain the elements Si and O, although the crystal forms of these elements may be different.
The combining effects of pressure and temperature control how the texture and mineral assemblage in the parent rock will change in the metamorphic rock. One way that geologists describe how much pressure and temperature have changed a rock during metamorphism is to describe the process as low-grade metamorphism or high-grade metamorphism. Low-grade metamorphism occurs at relatively low pressures and temperatures of 100 to 500 degrees Celsius. High-grade metamorphism occurs at relatively high pressures and temperatures above 500 degrees Celsius.
The Principal Metamorphic Rocks
There are eight principal metamorphic rocks. These rocks and their parent rocks are:
- Cataclasite, which can form from the metamorphism of any rock and is characterized as a rock that is created from mechanical deformation, such as breaking and fragmenting.
- Slate, which is a low-grade metamorphic product of the sedimentary rocks shale and mudstone.
- Phyllite, which is a medium-grade metamorphic product of shale and mudstone.
- Schist and gneiss, which are a high-grade metamorphic product of shale and mudstone.
- Greenschist is a low-grade metamorphic product of the igneous rock basalt.
- Amphibolite and granulite are high-grade metamorphic products of basalt.
- Marble is a metamorphic product of the sedimentary rock limestone.
- Quarzite is a metamorphic product of the sedimentary rock sandstone.
Metamorphic rocks are named for their texture and mineral assemblage. Coarse-grained rocks such as schist and gneiss are named for their texture, but the mineral assemblages may be added to the name. For example, a metamorphic rock containing visible grains of quartz, plagioclase, biotite and garnet will be named a quartz-plagioclase-biotite-garnet gneiss. Finer grained rocks such as phyllite and slate are also named for their texture, but since the individual minerals can not be seen with the naked eye, no mineral assemblages are added to the name.
One very unique feature of some metamorphic rocks is a feature called foliation. Foliation occurs when the platy minerals in a rock are squeezed flat under heat and pressure. These minerals then become aligned and form bands within the rock that reflect the direction that pressure was applied. Foliation is common in metamorphic rocks such as phyllite, gneiss, and greenschist.
Source:
Dietrich, Richard V. and Skinner, Brian J., Rocks and Rock Minerals. NY: John Wiley and Sons, 1979.
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