Explain how metamorphic rocks are formed, and explain the differences between regional and contact metamorphism.
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Metamorphic rocks are formed when pre-existing rocks undergo changes in mineralogy, texture, and sometimes chemical composition due to intense heat, pressure, or a combination of both. There are two main types of metamorphism: regional metamorphism and contact metamorphism.
Regional metamorphism occurs over large areas and is usually associated with mountain-building processes. It is caused by the tectonic forces responsible for the formation of mountain ranges, such as the collision of continental plates. These forces subject rocks to high temperatures and pressures over long periods of time. Regional metamorphism typically occurs at depths greater than 6 to 10 kilometers below the Earth's surface. The rocks involved in regional metamorphism are commonly exposed to directed pressure, resulting in the development of foliation, which is a parallel alignment of minerals or mineral bands. Examples of regional metamorphic rocks include gneiss, schist, and slate.
Contact metamorphism, on the other hand, occurs when rocks come into direct contact with magma or hot fluids. This type of metamorphism usually happens on a smaller scale and is localized around areas of igneous activity, such as volcanic intrusions or magma chambers. When the heat from the magma or fluids is transferred to the surrounding rocks, it causes them to undergo changes. Contact metamorphism is characterized by the formation of a distinct rock zone, known as an aureole, which surrounds the intruding igneous body. The rocks within the aureole can exhibit a wide variety of textures and mineral assemblages depending on the temperature and duration of exposure to the heat source. Examples of contact metamorphic rocks include hornfels, marble, and skarn.
In summary, metamorphic rocks are formed through the process of metamorphism, which involves changes in mineralogy, texture, and sometimes chemical composition of pre-existing rocks due to heat and pressure. Regional metamorphism occurs over large areas due to tectonic forces, resulting in the development of foliation, while contact metamorphism occurs in localized areas where rocks come into direct contact with magma or hot fluids, resulting in the formation of an aureole around the heat source.