Metamorphic rocks are formed when existing rocks are changed due to high heat and pressure. After metamorphism, the parent or original rock does not resemble the newly formed rock. The three factors that can lead to metamorphism are heat, pressure, and hydrothermal solutions.

• Heat
  • Heat provides the energy that is needed for metamorphism to occur.
  • The source of heat comes from two sources: magma, and depth (geothermal heat).
• Pressure
  • As depth increases, so does pressure.
  • Pressures present on rocks cause them to be more compacted and have more density.
• Hydrothermal solutions
  • These are hot solutions that escape from magma.
  • They promote a faster rate of metamorphism as well as recrystallization.

Metamorphic rocks can be classified by their mineral composition and texture. The two types of texture are foliated and nonfoliated. Foliated rocks have a layer-like appearance. The rocks have alternating layers of dark and light bands that are parallel to each other. The common foliated metamorphic rocks are schist, slate, and gneiss. Gneiss consists of bands of light and dark minerals from quartz or feldspar.

Nonfoliated metamorphic rocks do not have layers or alternating bands of different colors. These rocks are usually made from one type of mineral.  Limestone is metamorphosed into marble. Marble is used to build monuments and statues. Sandstone is metamorphosed into quartzite. Quartzite rocks may form hills and mountains because of its ability to resist weathering.

 

There are two major types of metamorphism.  Contact metamorphism occurs when hot magma moves into the rocks.  The magma can change the mineral composition of the rocks that are in direct contact with the hot magma. Therefore the effect of contact metamorphism is strongest near the magma. Contact metamorphism is associated with low pressure and high temperatures.

Regional metamorphism occurs when rocks are exposed and changed due to high temperatures and pressures. Geologic structures created by regional metamorphism are The Rocky Mountains and the Cascades. Regional metamorphism takes place over large areas of Earth’s crust.

• Types of regional metamorphism:
  • Burial metamorphism: happens when rocks are covered by greater than 10 kilometers (6 miles) of rock or sediment. Pressure and geothermal heat at these depths begins and causes the minerals in the rocks to recrystallize which, in turn, changes them.
  • Dynamothermal metamorphism: happens during the process of mountain building, when rocks are at a very high temperature and pressure – between converging plates and deep beneath rising mountains.
  • Hydrothermal Metamorphism: caused by hydrothermal solutions that can come directly from magma, can be pulled from the molecular structure of nearby rocks during metamorphosis, or it can be superheated groundwater.
    • Yellowstone National Park is located above a hydrothermal hot spot.
  • Fault Metamorphism: near an active fault the pressure and temperature can be high enough to produce metamorphic recrystallization. This has been noted near the San Andreas Fault in California during large earthquakes.

  Types of Rock
  Metamorphic Rocks		Foliated and Nonfoliated Marble Formed when existing rocks are changed due to high heat and pressure Burial metamorphism
  Igneous Rocks		Extrusive Gabbro Intrusive Magma cools and solidifies
  Sedimentary Rocks		Evaporites Clastic Compaction and Cementation Gypsum

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