How does contact metamorphism change rocks

Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other.

Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during regional metamorphism. As the rocks become heated at depth in the Earth during regional metamorphism they become ductile, which means they are relatively soft even though they are still solid.

The folding and deformation of the rock while it is ductile may greatly distort the original shapes and orientations of the rock, producing folded layers and mineral veins that have highly deformed or even convoluted shapes.

The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress. The photograph below shows high-grade metamorphic rock that has undergone several stages of foliation development and folding during regional metamorphism, and may even have reached such a high temperature that it began to melt. Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma.

Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated.

Where intrusions of magma occur at shallow levels of the crust, the zone of contact metamorphism around the intrusion is relatively narrow, sometimes only a few m a few feet thick, ranging up to contact metamorphic zones over m over feet across around larger intrusions that released more heat into the adjacent crust. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole.

The rocks closest to the contact with the intrusion are heated to the highest temperatures, so the metamorphic grade is highest there and diminishes with increasing distance away from the contact.

Because contact metamorphism occurs at shallow to moderate depths in the crust and subjects the rocks to temperatures up to the verge of igneous conditions, it is sometimes referred to as high-temperature, low-pressure metamorphism. Hornfels, which is a hard metamorphic rock formed from fine-grained clastic sedimentary rocks, is a common product of contact metamorphism.

Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. The difference in composition between the existing rock and the invading fluid drives the chemical reactions. The hydrothermal fluid may originate from a magma that intruded nearby and caused fluid to circulate in the nearby crust, from circulating hot groundwater, or from ocean water.

If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rock—changed its chemical composition. Ocean water that penetrates hot, cracked oceanic crust and circulates as hydrothermal fluid in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to mid-ocean spreading ridges and other ocean-floor volcanic zones.

Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. Greenschist contains a set of minerals, some of them green, which may include chlorite, epidote, talc, Na-plagioclase, or actinolite. The fluids eventually escape through vents in the ocean floor known as black smokers, producing thick deposits of minerals on the ocean floor around the vents.

Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. Burial metamorphism is the lowest grade of metamorphism.

The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals. It usually requires a strong microscope see the small grains of zeolite minerals that form during burial metamorphism. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle.

In most subduction zones the subducting plate is relatively cold compared with the high temperature it had when first formed at a mid-ocean spreading ridge. Subduction takes the rocks to great depth in the Earth relatively quickly. This produces a characteristic type of metamorphism, sometimes called high-pressure, low-temperature high-P, low-T metamorphism, which only occurs deep in a subduction zone.

In oceanic basalts that are part of a subducting plate, the high-P, low-T conditions create a distinctive set of metamorphic minerals including a type of amphibole, called glaucophane, that has a blue color. Blueschist is the name given to this type of metamorphic rock. Blueschist is generally interpreted as having been produced within a subduction zone, even if the plate boundaries have subsequently shifted and that location is no longer at a subduction zone. The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combination labratory experiments, physics-based theoretical calculations, along with evidence in the textures of the rocks and their field relations as recorded on geologic maps.

The knowledge of temperatures and pressures at which particular types of metamorphic rocks form led to the concept of metamorphic facies. Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions. Even though the name of the each metamorphic facies is taken from a type of rock that forms under those conditions, that is not the only type of rock that will form in those conditions. For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for.

However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. The diagram below shows metamorphic facies in terms of pressure and temperature condiditons inside the Earth. Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. One MPa equals nearly 10 atmospheres. A pressure of MPa corresponds to a depth of about 35 km inside the Earth. Although pressure inside the Earth is determined by the depth, temperature depends on more than depth.

Temperature depends on the heat flow, which varies from location to location. The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. Or more properly steam. Minerals are carried by the steam. When this hot fluid escapes from the magma it is called Hydrothermal Solution. These hot fluids can change the crystallization in rock by dissolving the minerals and then depositing new ones. Rocks that come in contact with this hydrothermal solution can have their composition altered as a result of this recrystalization.

The Classification of Metamorphic Rocks Metamorphic rocks are classified as foliated or nonfoliated. Foliated metamorphic rocks appeared banded or layered. Nonfoliated metamorphic rock usually contains one mineral. It is uniform in texture. Rock and Mineral Collections. Fossil and Geologic Time Book Sets. Fossil Collections. Rock Gallery. Gem and Mineral Clubs. Mineral Identification. Privacy Policy. Home Minerals What is a Mineral? Facebook Twitter.

Enjoy this page? Please pay it forward. Here's how Download Google Earth For Free. Remote Sensing Downloader. Thunder Egg. How it formed? Share on Facebook. Contact Metamorphism Contact Metamorphism Contact metamorphism is a type of metamorphism that occurs adjacent to intrusive igneous rocks due to temperature increases resulting from hot magma intrusion into the rock. Sea levels influence eruptions on volcanic island. Discovery of new geologic process calls for changes to plate tectonic cycle.

New research uncovers continental crust emerged million years earlier than thought. Among these factors are: The size and temperature of the intrusion. Albite - Epidote Hornfels Facies. Pelitic rocks will be characterized by an assemblage of quartz, albite, epidote, muscovite or andalusite, chlorite, biotite Quartzo-feldspathic rocks will be characterized by an assemblage of microcline, quartz, muscovite, albite, and biotite.

Pelitic rocks will be characterized by an assemblage of quartz, plagioclase, muscovite or andalusite, cordierite, or quartz, plagioclase cordierite, muscovite, and biotite Note the absence of epidote and chlorite in these assemblages. Pelitic rocks will be characterized by an assemblage of quartz, plagioclase, K-spar, andalusite or sillimanite, and cordierite Note the absence of muscovite. Sanidinite Facies The sanidinite facies is relatively rare in contact metamorphic aureoles, although it is somewhat more common in rocks found as xenoliths in igneous rocks.

It represents the highest conditions of temperature. The facies is characterized by the absence of hydrous minerals, particularly micas. Pelitic and quartzo-feldspathic rocks contain unusual phases like mullite 3Al 2 O 3. Sometimes tridymite is present in place of quartz. Basic rocks of the sanidinite facies are more common, and are often found along the conduit walls of dikes.

Several assemblages have been reported. Skarns Sometimes when a siliceous magma intrudes carbonate rocks like limestone and dolostone, significant chemical exchange metasomatism takes place between the magma and the carbonate rock. Here, quartz monzonite intruded an Mg-rich limestone. Metamorphism and metasomatism produced four zones near the contact three ranging in size from 3 cm to 15 m in width. The outer zone consists of calcite marble or calcite - brucite [MgOH 2 ] marble, showing little metasomatism.

Closer to the contact is the montecellite zone. A thin zone along the contact shows evidence of assimilation of the limestone by the magma. Examples of questions on this material that could be asked on an exam What is contact metamorphism, why does it occur and why is it generally restricted to relatively shallow depths in the earth's crust?

What are the characteristics of a contact metamorphic aureole? What factors control the size of a contact metamorphic aureole? What are the contact metamorphic facies in order from lowest grade to highest grade? What is a skarn and how do skarns form?