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What Is a Metamorphic Rock?

Some people may not realize that rocks come in various types. Although most of the time, people refer to rocks simply as rocks, rocks are categorized by their formation. Igneous rocks result when lava cools and sedimentary rocks are rocks that have been loosened or eroded from mountains over time. Metamorphic rocks occur when any type of rock is subjected to pressure and high temperatures that alter its structure.

Slate
- Slate is formed from shale, which is a soft sedimentary rock. When shale is subjected to light pressure and warm temperatures, it undergoes a metamorphosis and forms slate. Formed mainly in mountainous areas, slate is typically gray or black, but is found in other colors as well. Slate is used in roofing and flooring, and crushed slate is added to paints.
Schist
- A type of metamorphic rock, schist is typically formed from sedimentary rocks like shale. When these sedimentary rocks are subjected to extreme heats and pressures, their mineral structure is realigned and schist is formed. Schist has a large amount of crystallization of mica minerals, such as biotite, chlorite and muscovite, which gives this type of rock a sparkly appearance. Schist is used for decorative purposes.
Gneiss
- Gneiss rocks have visible mineral streaks throughout them. These streaks are usually feldspar, quartz or mica, or a combination of the three. Gneiss rocks develop under high pressure and temperature, which cause the minerals to form bands within the rocks. The structure of gneiss rocks cause them to split in a specific way that yields the characteristic parallel banding. Gneiss is used as a building material.
Marble
- Marble forms when the calcite in limestone melts due to having pressure and hot temperatures exerted on it. Pure limestone that undergoes a metamorphosis yields white marble, which does not occur very often. Normally mineral impurities are present in the limestone. This creates marble's characteristic marbling. The color of the marble itself and the veins are determined by the type of mineral impurities the limestone contains.
Quartzite
- Quartzite is formed from quartz grains in sandstone. As sandstone undergoes a gradual heat and pressure buildup, these grains recrystallize to form larger interlocking crystals. The result is a much harder metamorphic rock called quartzite. Quartzite is used for flooring and as a surfacing material, as well as in the glass and ceramic industries. It is similar in appearance to marble, but can be distinguished from marble because it cannot be scratched with a knife and does not react with weak acids.

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