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Metamorphic rock

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Quartzite, a form of metamorphic rock, from the Museum of Geology at
   University of Tartu collection.
   Enlarge
   Quartzite, a form of metamorphic rock, from the Museum of Geology at
   University of Tartu collection.

   Metamorphic rock is the result of the transformation of a pre-existing
   rock type, the protolith, in a process called metamorphism, which means
   "change in form". The protolith is subjected to heat (greater than 150
   degrees Celsius) and extreme pressure causing profound physical and/or
   chemical change. The protolith may be sedimentary rock, igneous rock or
   another older metamorphic rock.

   Metamorphic rocks make up a large part of the Earth's crust and are
   classified by texture and by chemical and mineral assemblage (
   metamorphic facies). They are formed deep beneath the Earth's surface
   by great stresses from rocks above and high pressures and temperatures.
   They are also formed by the intrusion of molten rock, called magma,
   into solid rock and form particularly at the place of contact between
   the magma and solid rock where the temperatures are high.

   The study of metamorphic rocks (now exposed at the Earth's surface
   following erosion and uplift) provides us with very valuable
   information about the temperatures and pressures that occur at great
   depths within the Earth's crust.

   Some examples of metamorphic rocks are gneiss, slate, marble, and
   schist.

Metamorphic minerals

   Metamorphic minerals are those that form only at the high temperatures
   and pressures associated with the process 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, but 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. However, all minerals are stable only
   within certain limits, and the presence of some minerals in metamorphic
   rocks indicates the approximate temperatures and pressures at which
   they were formed.

   The change in the particle size of the rock during the process of
   metamorphism is called recrystallization. For instance, the small
   calcite crystals in the sedimentary rock limestone change into larger
   crystals in the metamorphic rock marble, or in metamorphosed sandstone,
   recrystallisation of the original quartz sand grains results in very
   compact quartzite, in which the often larger quartz crystals are
   interlocked. Both high temperatures and pressures contribute to
   recrystallization. High temperatures allow the atoms and ions in solid
   crystals to migrate, thus reorganizing the crystals, while high
   pressures cause solution of the crystals within the rock at their point
   of contact.

Foliation

   The layering within metamorphic rocks is called foliation (derived from
   the Latin word folia, meaning "leaves"), and it occurs when a strong
   compressive force is applied from one direction to a recrystallizing
   rock. This causes the platy or elongated crystals of minerals, such as
   mica and chlorite, to grow with their long axes perpendicular to the
   direction of the force. This results in a banded, or foliated, rock,
   with the bands showing the colours of the minerals that formed them.

   Textures are separated into foliated and non-foliated categories.
   Foliated rock is a product of differential stress that deforms the rock
   in one plane, sometimes creating a plane of cleavage: for example,
   slate is a foliated metamorphic rock, originating from shale.
   Non-foliated rock does not have planar patterns of stress.

   Rocks that were subjected to uniform pressure from all sides, or those
   which lack minerals with distinctive growth habits, will not be
   foliated. Slate is an example of a very fine-grained, foliated
   metamorphic rock, while phyllite is coarse, schist coarser, and gneiss
   very coarse-grained. Marble is generally not foliated, which allows its
   use as a material for sculpture and architecture.

   Another important mechanism of metamorphism is that of chemical
   reactions that occur between minerals without them melting. In the
   process atoms are exchanged between the minerals, and thus new minerals
   are formed. Many complex high-temperature reactions may take place, and
   each mineral assemblage produced provides us with a clue as to the
   temperatures and pressures at the time of metamorphism.

   Metasomatism is the drastic change in the bulk chemical composition of
   a rock that often occurs during the process of metamorphism. It is due
   to the introduction of chemicals from other surrounding rocks. Water
   may transport these chemicals rapidly over great distances. Because of
   the role played by water, metamorphic rocks generally contain many
   elements that were absent from the original rock, and lack some which
   were originally present. Still, the introduction of new chemicals is
   not necessary for recrystallization to occur.

Types of metamorphism

   Contact metamorphism is the name given to the changes that take place
   when magma is injected into the surrounding solid rock (country rock).
   The changes that occur are greatest wherever the magma comes into
   contact with the rock because the temperatures are highest at this
   boundary and decrease with distance from it. Around the igneous rock
   that forms from the cooling magma is a metamorphosed zone called a
   contact metamorphism aureole. Aureoles may show all degrees of
   metamorphism from the contact area to unmetamorphosed (unchanged)
   country rock some distance away. The formation of important ore
   minerals may occur by the process of metasomatism at or near the
   contact zone.

   Regional metamorphism is the name given to changes in great masses of
   rock over a wide area, often within orogenic belts. The high
   temperatures and pressures in the depths of the Earth are the cause of
   the changes, and if the metamorphosed rocks are uplifted and exposed by
   erosion, they may occur over vast areas at the surface. The process of
   metamorphism may have destroyed the original features that could have
   revealed the rock's previous history. Recrystallization of the rock
   will destroy the textures and fossils present in sedimentary rocks.
   Metasomatism will change the original composition.

Metamorphic rock textures

   The five basic metamorphic textures with typical rock types are:
     * Slaty: slate and phyllite; the foliation is called 'slaty cleavage'
     * Schistose: schist; the foliation is called 'schistocity'
     * Gneissose: gneiss; the foliation is called 'gneisocity'
     * Granoblastic: granulite, some marbles and quartzite
     * Hornfelsic: hornfels and skarn

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