Metamorphic Rocks

The word metamorphic means "of changed form." Metamorphic rocks are rocks that have been changed from one form, such as igneous or sedimentary, to another. This can occur when a given rock is exposed to intense heat, pressure, or hot fluids.

Recalling igneous rocks, the specific rock that forms from a given magma depends on the components of the magma, the temperature at which the magma cools, how quickly the magma cools and also the pressure at which the rock forms. As materials crystallize, the atoms form a structure that is most stable at the temperature and pressure of the environment.

Imagine a group of people filling a small room. If only a few people are in the room there is plenty of space for people to spread out and move around while they talk. However, as more and more people enter the room, those same people no longer have the same space and may have to adjust their bodies to become more comfortable.

Although, rocks differ significantly from people, pressure and temperature will have the same effect on the atoms that make up the rock. As pressure or temperature increases, the atoms that make up the original rock must rearrange themselves in order to form a more stable or "comfortable" structure, just as people in a room. However, since rocks are made of minerals and each mineral has a specific chemical structure and form, changing the structure and or form will then change the mineral.

The type of metamorphic rock that can form depends on the rock that is being metamorphosed as well as the amount of pressure and heat to which the rock is exposed. Since varied temperature and pressures will produce different minerals, the exact type of metamorphic rock produced depends on the grade (intensity) of metamorphism. The grade of metamorphism can most often be determined by the minerals present within the metamorphic rock and the type of rock formed. 
Some of the more common metamorphic minerals include quartz, feldspar, biotite, muscovite, chlorite, garnet, tourmaline, calcite, and amphibole. The rock that has undergone change is called the parent rock. Parent rocks can be any of the three types of rocks: igneous, sedimentary, or even metamorphic rocks which can be metamorphosed again.

Metamorphic Processes

Metamorphism often occurs on two scales: contact and regional. Contact metamorphism occurs relatively locally, in areas adjacent to igneous intrusions or in areas that are in contact with hot water known as hydrothermal fluids.
Typically this kind of metamorphism is caused by moderate pressure and extreme heat that may last for a period of days to thousands of years. In cases of contact metamorphism, the most extreme metamorphism occurs at the contact point between the parent rock and the intrusive magma or hydrothermal fluid. The degree of metamorphism then decreases rapidly as distance from the heat source increases. Depending on the source, zones of contact metamorphism may range from a few millimeters to tens of meters.

In contrast to contact metamorphism, regional metamorphism occurs over much larger areas. In areas such as rising mountain ranges, large igneous intrusions can form and warp overlying strata increasing the pressures above the intrusion. Also the extreme heat and pressures caused by tectonic forces or deep burial will cause widespread metamorphism.

In order to identify a metamorphic rock and understand the history of the rock, it is necessary to describe the mineralogical composition of the rock. Mineral composition of metamorphic rocks is often described in terms of relative abundance. The most abundant mineral will most often be used in naming the rock and, more importantly, will be helpful in determining the environment from which the rock formed.

Recrystallization, Neometamorphism, and Metasomatism

The most common process in the formation of metamorphic rocks is recrystallization. In this process, small crystals of one mineral in contact with one another slowly come together to form fewer larger crystals. For example, the sedimentary rock, limestone. Limestone is composed of microscopic crystals of calcite that once formed the shells of marine creatures. When metamorphosed, those tiny crystals are slowly forced together under high pressure and begin to form larger, more visible crystals of calcite found in the metamorphic rock marble.

In other situations, mineral crystals may completely rearrange to form completely new minerals that are more stable under increased pressure and/or temperatures. This process is known as neometamorphism. This is most often observed with the sedimentary rock shale, which is composed of fine grained quartz, feldspars, and other clay minerals. When metamorphosed, these minerals grains will change to muscovite mica and garnet.

The process of metasomatism results in the loss or addition of chemicals. This can best be seen in the production of anthracite coal, which is almost pure carbon. The parent rock bituminous coal is produced from the aggregation of dead plant material, and anthracite is produced by the loss of the more volatile materials such as nitrogen, oxygen, and methane.

Classification of Metamorphic rocks 

In order to identify any metamorphic rock, it is first necessary to determine its texture. The texture of a metamorphic rock is a description of its constituent minerals along with their arrangement and shape. Typically this will be initially described as foliated or non-foliated. Non foliated textures have no obvious features and are classified mainly on the basis of crystal size.

Foliation is the existence or appearance of layers. Foliated textures result from a parallel arrangement of flat, platy minerals such as muscovite. This is usually a result of mineral recrystallization in the presence of a directed pressure. Under this condition, the mineral grains will grow in an orientation that best distributes the force of the pressure. Typically, mineral grains will grow perpendicular to the force being enacted on them.