Metamorphic Rock

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Metamorphic rock is rock that has changed from one type of rock into another. The word metamorphic (from Greek) means "of changing form." Metamorphic rock is produced from either igneous rock (rock formed from the cooling and hardening of magma) or sedimentary rock (rock formed from compressed and solidified layers of organic or inorganic matter). 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 tectonic shifting. Although the rock becomes extremely hot and under a great deal of pressure it does not melt. If the rock melted, the process would result in igneous, not metamorphic rock. Metamorphic alteration of rock causes the texture and/or mineral composition to change. New textures are formed from a process called recrystallization. New minerals (which are simply various combinations of elements) are created when elements recombine.

There are two basic types of metamorphic rock: regional and thermal. Regional metamorphic rock, found mainly in mountainous regions, is formed mainly by pressure, as opposed to heat. 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 mi (5 km) down into the earth. With more pressure and some heat, shale can transform into slate and mica. Metamorphic rock found closer to Earth's surface, or produced by low pressure, characteristically splits or flakes into layers of varying thickness. This is called foliation. Slate is often used as roofing tiles and paving stones. With lots of pressure and increasing heat, rock called schist forms. Schist, which is a medium-grained regional metamorphic rock also has a tendency to split in layers, is subjected to high temperatures, and often contains crystals, such as garnets. Gneiss (pronounced "nice") is formed by a higher pressure and temperature than schist. These rocks are coarse grained and, although layered as schist is, do not split easily. Essentially, metamorphic rocks are made of the same minerals as the original rock or parent rock but the various minerals have been rearranged to make a new rock.

Thermal metamorphic rock, also called contact metamorphic rock, is formed not only by considerable pressure but, more importantly, by intense heat. Imagine molten rock pushing up into Earth's crust. The incredible pressure fills any empty space, every nook and cranny, with molten rock. This intense heat causes the surrounding rock to completely recrystallize. During recrystallization, the chemical composition regroups to form 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. Sometimes a metamorphic rock can become metamorphosed. This is known as polymetamorphism.

Metamorphism