Metamorphic

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 fromeither 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). Mostof 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 simplyvarious combinations of elements) are created when elements recombine.
Thermal metamorphic rock, also called contact metamorphic rock, is formed not only by considerablepressure 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 moltenrock. This intense heat causes the surrounding rock to completely recrystallize. During recrystallization, the chemical composition regroups to form a new rock. An example of thistype 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.