Metamorphic rock

Metamorphic rock is rock that has changed from one type of rock into another. The word metamorphic comes from Greek and means "of changing form." Metamorphic rock is produced from igneous, sedimentary, or even other metamorphic rocks. Most of Earth's crust is made up of metamorphic rock. Igneous and sedimentary rocks become metamorphic rock as a result of intense heat from magma and pressure from burial within Earth. Although the rock undergoes extreme heat and a great deal of pressure, it does not melt. If the rock melted, it would become igneous and not metamorphic rock. Instead, the heat and pressure combine to change the mineral makeup of the rock. Essentially, metamorphic rocks are made of the same minerals as the original rock, but the various minerals have been rearranged to make a new rock.

There are two basic types of metamorphic rock: regional and thermal. Regional metamorphic rock, found mainly in mountainous regions, is formed by pressure. Different amounts of pressure produce different types of rock. The greater the pressure, the more drastic the change (also, the deeper the rock the higher the temperature, which adds to the potential for diverse changes). For example, a pile of mud can turn into shale (a fine-grained sedimentary rock) with relatively low pressure, about 3 miles (5 kilometers) down into Earth. With more pressure and some heat, shale can transform into slate and mica. Carried even deeper, slate transforms into schist (pronounced shist) and then gneiss (pronounced nice).

Thermal metamorphic rock, also called contact metamorphic rock, is formed by considerable pressure and, more important, intense heat. When molten rock pushes up into Earth's crust, the incredible pressure behind it forces the molten rock into any empty space. The accompanying intense heat causes the surrounding rock to completely recrystallize, forming a new rock. An example of this type of thermal metamorphic rock is marble, which is actually limestone whose calcite has recrystallized. Sandstone made mostly of quartz fragments recrystallizes into quartzite. Thermal metamorphic rocks are not as common or plentiful as regional metamorphic rocks.

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