Metamorphic rocks are one of three broad types or categories of rocks, the other two being igneous rocks and sedimentary rocks. The word 'metamorphic' comes from the Greek for 'change' and 'form'. Metamorphic rocks refer to any rock that has been changed due to the action of heat, pressure and/or solutions into a new appearing rock at or below the surface of the Earth.
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Some metamorphic rocks exhibit what is called schistosity. Metamorphic rocks like granite gneiss and biotite schist are have high schistosity because they are heavily banded or foliated. This quality gives the metamorphic rock a striped appearance. Schistosity occurs when new grains of mica and/or chlorists become larger and bend or crinkle to create a new foliation. The pressure from the weight of overlying rock and the forces of converging plate boundaries is primarily responsible for creating schistosity in metamorphic rocks.
Metamorphic rocks such as marble and quartzite are formed by only one mineral. Marble comes from the mineral calcite, and quartzite comes from the mineral quartz sand. This differs from rocks like sedimentary rocks, which form from the accumulation of many different types of sediment.
Some metamorphic rocks will react with acid. Metamorphic rocks like marble contain limestone, which is formed by calcite. Calcite always reacts with acid and thus dissolves some of the rock. Remember that not all metamorphic rocks will react with acid because not all of them have calcite. Metamorphic rocks like slate will not react with acid because they are formed from shale, which comes from clay.
Metamorphic rocks exhibit one or more of four typical results of metamorphism. The extreme heat and pressure of the Earth's core will create new minerals in metamorphic rocks. Mica, for instance, needs extreme heat and pressure applied to clay in order to form. Second, the pore space in metamorphic rocks is generally smaller than in igneous rocks, for instance, because of densification. Third, metamorphic rocks will exhibit new structure called rock cleavage. Finally, rocks like quartzite will exhibit recrystallization of their original mineral.
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