Were and how is metamorphic rock formed?

Answer 1

Metamorphic comes from "metamorphism", which means "to change in form" (not to destroy). Metamorphosis can happen with any rock that isn't completely melted. (Completely melting the rock turns it into magma/lava, and when it cools, it is considered "igneous"). However, changing limestone to marble may be the easiest to understand since you will be more likely to know what these are.

Limestone is that rock that is often grey, can contain fossils and is frequently where caves are created from. If you had a layer of this rock, and a volcano started to grow from under it, the high heat and pressure created from the growing volcano changes the parts of the limestone that come into contact with the magma into marble - you know: the stuff floors, counters, and statues can be made from.

So to recap...
Where:
Near volcanically active areas where pre-existing rock comes into contact with the heat and pressure from the volcano/magma chamber.

How:
From temperatures hot enough to partially melt, but not too high as to totally melt it; and pressures high enough to change the internal structure. Usually, at the edges of the magma.

Answer 2

Metamorphic rocks are formed in many different environments. Contact metamorphism is high temp/low pressure and usually takes place less than 10km beneath Earth's surface. This happens when a body of magma intrudes into cool country rock and "bakes" the rock it touches. These tend to be small area metamorphic process ~ 1-100m wide.
Regional metamorphism is large scale metamorphism at high temp/ high pressure (800 degrees C and greater than 5 km depth) and as the name implies - over a region.
There is also hydrothermal metamorphism involving the circulation of hot fluids.
The types of rocks formed give clues about the type of metamorphism, especially whether they are foliated (regional) or unfoliated crystalline (contact).

Answer 3

They form all over the place and do so in different ways. To qualify as metamorphic rock has to be changed from one form into another. That's rather what metamorphic means. Heat and pressure are usual suspects.

Answer 4

Metamorphic rock is formed when existing rock is subjected to extreme heat and pressure transforming it's physical and chemical identity.

Answer 5

A metamorphic rock is a rock thats physical appearance can be changed by; heat or preasure.

Answer 6

Metamorphism is defined as follows:

The mineralogical and structural adjustment of solid rocks to physical and chemical conditions that have been imposed at depths below the near surface zones of weathering and diagenesis and which differ from conditions under which the rocks in question originated.

The single most distinctive feature of regional metamorphic rocks that separates them from both igneous and sedimentary rocks is the presence of a preferred orientation of the individual minerals that comprise the rock. The term used to describe a preferred orientation is foliation.

Metamorphic rocks are sedimentary or igneous rocks that have been altered by heat , pressure and fluid alteration. They are derived from previously existing igneous, sedimentary or even metamorphic rock. They are identified by the types of minerals they contain and their texture. Thermally metamorphosed rocks occur bordering igneous intrusions, which altered the surrounding rock originally because of their intense hear, resulting also in the formation of new minerals such as andalusite and garnet. Regionally metamorphosed rocks occur in the roots of mountain ranges, where intense pressures and high temperatures formed platy minerals (e.g. mica) and high-pressure minerals (e.g. staurolite).

* Metamorphism, therefore occurs at temperatures and pressures higher than 200oC and 300 MPa. Rocks can be subjected to these higher temperatures and pressures as they are buried deeper in the Earth. Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.

* The upper limit of metamorphism occurs at the pressure and temperature where melting of the rock in question begins. Once melting begins, the process changes to an igneous process rather than a metamorphic process.

Grade of Metamorphism

As the temperature and/or pressure increases on a body of rock we say the rock undergoes prograde metamorphism or that the grade of metamorphism increases. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.

* Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals. With increasing grade of metamorphism, the hydrous minerals begin to react with other minerals and/or break down to less hydrous minerals.

* High-grade metamorphism takes place at temperatures greater than 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O, and non-hydrous minerals become more common

Types of Metamorphism

Contact Metamorphism
Contact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone surrounding the intrusion, called a metamorphic or contact aureole. Outside of the contact aureole, the rocks are not affected by the intrusive event. The grade of metamorphism increases in all directions toward the intrusion. Because the temperature contrast between the surrounding rock and the intruded magma is larger at shallow levels in the crust where pressure is low, contact metamorphism is often referred to as high temperature, low pressure metamorphism. The rock produced is often a fine-grained rock that shows no foliation, called a hornfels.

Regional Metamorphism
Regional metamorphism occurs over large areas and generally does not show any relationship to igneous bodies. Most regional metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions. Thus, regional metamorphism usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gniesses. The differential stress usually results from tectonic forces that produce compressional stresses in the rocks, such as when two continental masses collide. Thus, regionally metamorphosed rocks occur in the cores of fold/thrust mountain belts or in eroded mountain ranges. Compressive stresses result in folding of rock and thickening of the crust, which tends to push rocks to deeper levels where they are subjected to higher temperatures and pressures.

Cataclastic Metamorphism
Cataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing. Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the shearing occurred.

Hydrothermal Metamorphism
Rocks that are altered at high temperatures and moderate pressures by hydrothermal fluids are hydrothermally metamorphosed. This is common in basaltic rocks that generally lack hydrous minerals. The hydrothermal metamorphism results in alteration to such Mg-Fe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. Rich ore deposits are often formed as a result of hydrothermal metamorphism.

Burial Metamorphism
When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 300oC may develop in the absence of differential stress. New minerals grow, but the rock does not appear to be metamorphosed. The main minerals produced are often the Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.

Shock Metamorphism (Impact Metamorphism)
When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion, ultrahigh pressures can be generated in the impacted rock. These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite. In addition they can produce textures known as shock lamellae in mineral grains, and such textures as shatter cones in the impacted rock.