Metamorphic rock

File:Quartzite.jpg
Quartzite, a form of metamorphic sandstone
File:Migma ss 2006.jpg
Folded strata in a metamorphic rock from near [Geirangerfjord], Norway

A metamorphic rock is a type of rock which has been changed by extreme heat and pressure. Its name is from 'morph' (meaning form), and 'meta' (meaning change).[1]

The original rock gets heated (temperatures greater than 150 to 200 °C) and pressured (1500 bars).[2] This causes profound physical and/or chemical change. The original rock may be sedimentary rock, igneous rock or another older metamorphic rock.

There is always more pressure and higher temperature under the Earth's surface. In the root of a mountain chain or a volcano these forces will be enough to change shape of the strata and the minerals it is made of. Sedimentary rock which has been near such forces often looks as if a giant had twisted it and heated it over a fire. Examples of metamorphic rock:

The recrystallisation of minerals after heating generally causes the destruction of any fossils the rocks might have contained. These rocks are formed when igneous or sedimantory rocks are subjected to extreme heat and pressure as a result they undergo a complete change in their form and characterstics. Such form rocks are called metamorphic rocks.

Types of metamorphism

Regional metamorphism

Regional metamorphism, or dynamic metamorphism, occurs in great masses of rock. Rocks can be metamorphosed just by being at great depths below the Earth's surface. There they get high temperatures and the great weight of the rock layers above.

Much of the lower continental crust is metamorphic, except for recent igneous intrusions. Horizontal tectonic movements such as the collision of continents create orogenic belts. High temperature, pressures and deformation occurs along these belts. If the metamorphosed rocks are later uplifted and exposed by erosion, they are seen as long belts or other large areas at the surface.

Contact metamorphism

File:Eozoon01.jpg
A contact metamorphic rock made of interlayered calcite and serpentine from the Precambrian of Canada. Once thought to be a fossil called Eozoön canadense. Scale in mm.

Contact metamorphism occurs when magma is injected into the surrounding solid rock (country rock). The changes that occur are greatest where the magma comes into contact with the rock. The temperatures were highest there and decreased with distance from it.

Metamorphic minerals

Metamorphic minerals are those that form only at the high temperatures and pressures of metamorphism. These minerals, known as index minerals, include sillimanite, kyanite, staurolite, andalusite, and some garnet.

Other minerals, such as olivines, pyroxenes, amphiboles, micas, feldspars, and quartz may be found in metamorphic rocks. They are not necessarily the result of the process of metamorphism. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process.

Related pages

References

  1. This originally Greek prefix has several meanings, but here it means 'change'.
  2. Blatt, Harvey and Robert J. Tracy, 1996. Petrology. 2nd ed, W.H.Freeman, p.355. Template:Catalog lookup linkScript error: No such module "check isxn".

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