Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The protoliths are exposed to heat (temperatures greater than 150 to 200 ° C) and the (150 megapascal (1,500 bar)) pressure, causing massive physical or chemical changes. Protolit may be a sedimentary, frozen, or existing metamorphic rock.
Metamorphic rocks form a large part of the Earth's crust and make up 12% of Earth's land surface. They are classified by texture and with chemical and mineral aggregations (metamorphic facies). They can be formed only by being deep beneath the Earth's surface, experiencing the high temperatures and the enormous pressure of the rock layers above it. They can be formed from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when the rock is heated by a hot liquid rock intrusion called magma from Earth's interior. The study of metamorphic rocks (now exposed to the earth's surface after erosion and removal) provides information about the temperature and pressure that occur at very deep depths within the Earth's crust. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.
Video Metamorphic rock
Metamorfik mineral
Metamorphic minerals are minerals that only form at the high temperatures and pressures associated with metamorphism processes. This mineral, known as the mineral index, includes sillimanite, kyanite, staurolite, andalusite, and some garnets.
Other minerals, such as olivine, pyroxene, amphiboles, mycas, feldspar, and quartz, can be found in metamorphic rocks, but not necessarily the result of the metamorphism process. These minerals formed during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain unchanged chemically during the metamorphic process. However, all minerals are only stable within certain limits, and the presence of some minerals in metamorphic rocks shows the approximate temperature and pressure at which they are formed.
The change in particle size of rocks during the process of metamorphism is called recrystallization. For example, small calcite crystals in limestone sedimentary rocks and lime changes into larger crystals in metamorphic marble rocks; in metamorphic gemstones, the recrystallization of the original quartz sand grains produces a very densely packed quartzite, also known as metaquartzite, in which often larger quartz crystals are interlocked. Both temperature and high pressure contribute to recrystallization. High temperatures allow atoms and ions in solid crystals to migrate, thus reorganizing crystals, while high pressure causes the crystalline solution in the rock at its point of contact.
Maps Metamorphic rock
Foliation
The layer in metamorphic rock is called foliation (derived from the Latin folia , meaning "leaf"), and it occurs when the stone is being shortened along one axis during recrystallization. This causes platy or elongated mineral crystals, such as mica and chlorite, to be rotated in such a way that the long axis is perpendicular to the orientation of shortening. This produces rock banded, or foliated, with bands showing the mineral colors that make up them.
Textures are separated into foliated and non-foliated categories. Foliated rock is a product of differential voltage that destroys rocks in one plane, sometimes creating a cleavage field. For example, slate is a foliated metamorphic rock, derived from shale. Non-foliated rocks do not have a planar strain pattern.
Rocks that are subjected to uniform pressure from all sides, or those lacking minerals with typical growth habits, will not peel off. Where the stone has been subjected to differential pressure, the type of foliation that develops depends on the metamorphic level. For example, starting with the mudstone, the following sequence develops with increasing temperatures: slate is a very fine, characteristic metamorphic rock, characteristic of very low class metamorphism, while phyllite is a fine grain and is found in a low-level metamorphic region, the schist is a medium for coarse and found in the middle-class metamorphism region, and coarse rough to very coarse, found in high-grade metamorphic areas. Marble is generally not foliated, which allows its use as a material for sculpture and architecture.
Another important mechanism of metamorphism is the chemical reaction that occurs between minerals without melting. In atomic processes are exchanged between minerals, and thus new minerals are formed. Many complex high-temperature reactions can occur, and each set of minerals generated gives us a clue about the temperature and pressure at the time of metamorphism.
Metasomatism is a drastic change in the bulk chemical composition of the stones that often occurs during the process of metamorphism. This is due to the introduction of chemicals from surrounding rocks. Water can transport these chemicals quickly at great distances. Due to the role played by water, metamorphic rocks generally contain many absent elements from the original rocks, and some lack thereof that were originally present. However, the introduction of new chemicals is not necessary for recrystallization to occur.
Type metamorphism
Contact metamorfisme
Metamorphism of contact is the name given to the changes that occur when the magma is injected into the surrounding solid rock (rock state). The change that occurs is the largest wherever the magma comes in contact with the stone because the highest temperature is at this limit and decreases with distance from it. Around the igneous rocks formed from the cooling magma is a metamorphosis zone called metamorphism aureole contact . Aureoles can show all degrees of metamorphism from the contact area to the unabsorbed (unchanged) state rocks, somewhat far away. The formation of important ore minerals can occur by metasomatism processes at or near the contact zone.
When a contact stone is altered by a frozen intrusion it very often becomes more indurated, and the crystal is more rough. Many rocks of this type were once called horns, and the term hornfels are often used by geologists to signify smooth, dense, and uncontaminated contact metamorphosis products. Flakes can be dark argillaceous hornfels, full of small plates of brownish biotite; marble or impure limestone may turn gray, yellow or greenish-silicate or silicate, hard and flaky marble or silver, with abundant augits, garnets, wollastonite and other minerals where calcite is an essential component. Diabas or andesite can be hornfels diabas or hornet andesite with new hornblende and biotite development and partial recrystallization of the original feldspar. Chert or flint can be a fine quartz crystal stone; the sandstone loses its clastic structure and is converted to a small quartz grain mosaic fitting in a metamorphic rock called quartzite.
If the stone is initially woven or coated (such as, for example, laminated sandstone or calcified sekis) this character may not be obliterated, and hornfels banded is the product; fossils may even have a preserved form, though fully recrystallized, and in many contact lavas, vesicles are still visible, although their contents are usually incorporated into new combinations to form minerals that did not originally exist. The minute structure, however, disappears, very often, if the thermal changes are very deep. Thus the tiny quartz granules in the shale disappear or merge with the surrounding clay particles, and the subtle soils of the lava are completely reconstructed.
With recrystallization in this way, strange rocks of a very different kind are often produced. Thus flakes may enter cordierite rocks, or may exhibit large crystals of andalusite (and chiastolite), staurolite, garnet, kyanite and sillimanite, all from the alumina content of the original flakes. A large number of mica (both muscovite and biotite) often form together, and the resulting product has close similarities to many types of schists. Limestone, if pure, often turns into rough crystal marbles; but if there is a mixture of clay or sand in the original rocks, minerals such as garnet, epidote, idocrase, wollastonite, will be present. Sandstones when heated can turn into rough quartzite consisting of large quartz grains. This more intense phase of change is less common in igneous rocks, because their minerals, which are formed at high temperatures, are not so easily altered or recrystallized.
In some cases, unified rocks and in small dark crystals of spinel, sillimanite and cordierite may be separate. Shales are sometimes altered by basal dikes, and the feldspathic sandstones may be fully vitrified. Similar changes can be induced in flakes by burning coal seams or even by ordinary stoves.
There is also a tendency to metasomatism between frozen magma and state sedimentary rocks, in which chemicals in each are exchanged or introduced to another. Granite can absorb flake fragments or bits of basalt. In this case, a hybrid rock called a skarn appears, which does not have the characteristics of igneous rocks or normal sediments. Occasionally granite magma attacks penetrate rocks around, filling joints and bed planes, etc., with quartz and feldspar yarns. This is extraordinary but the examples are known and may occur on a large scale.
Regional metamorphism
Regional metamorphism , also known as dynamic metamorphosis , is the name given for large rock mass changes in large areas. The rocks can morpheme only by being at a very deep depth beneath the surface of the Earth, experiencing high temperatures and large pressures caused by the massive layers of rocks above. Most of the lower continental crust is metamorphic, except for recent frozen rock intrusions. Horizontal tectonic movements such as continental collisions create an orogenic belt, and cause high temperatures, pressures and deformations in the rocks along this belt. If the metamorphosed rocks are then lifted and exposed by erosion, they can occur in long belts or other large areas on the surface. The process of metamorphism may have destroyed the original features that could unravel previous rock history. Rock recrystallization will destroy the texture and fossils present in sedimentary rocks. Metacomatism will change its original composition.
Regional metamorphoses tend to make rocks sharper and at the same time provide a foliated, shistose or gneissic texture, which consists of a planar mineral arrangement, so that clay or prismatic minerals such as mica and hornblende have the longest axis arranged in parallel. one another. For that reason, many of these rocks are split in one direction along the mica-bearing zone (sekis). In gneisses, minerals also tend to be separated into bands; thus there are quartz and mica stitches in mica schist, very thin, but essentially composed of one mineral. Along layers of minerals composed of soft minerals or fissiles, the rocks will split easily, and newly detached specimens will appear to be confronted or coated with this mineral; for example, a mica piece visible on the surface may be entirely composed of shining mica scales. On the edge of the specimen, however, the granular white quartz folia will be visible. In these alternating folia gneisses it is sometimes thicker and less orderly than in schist, but most importantly less bloom; they may be lenticular, die quickly. Gneisses also, as a rule, contain more feldspar than schists, and are harder and less fissile. The controversy or the crumbling of foliation is not uncommon; the cleaved face is undulose or shrink. Schistosity and gneissic appeal (two main types of foliation) are formed by pressure directed at high temperatures, and for interstitial movements, or internal streams that regulate mineral particles as they crystallize in the directed pressure plane.
The rocks that were originally sediments and rocks are undoubtedly igneous rocks can morpheme into sekis and gneisses. If initially the same composition they may be very difficult to distinguish from each other if metamorphism is great. A quartz porphyry, for example, and a fine feldspathic sandstone, may both metamorphose into gray or pink mica-gray.
Texture of metamorphic rocks
Five basic metamorphic textures with typical rock types are slaty (including slate and phyllite; foliation is called "slaty cleavage"), schistose (including schist; foliation is called "schistosity") , gneissoses (gneissos foliation), granoblastic (including granulite, some marbles and quartzite), and hornfelsic (including hornfels and skarn).
See also
- Blueschist
- List of rock types
- List of rock textures
- Metavolcanic stones
- Migmatite
- Neomorphism
References
External links
- Metamorphic textures - Middle East Technical University
- Examples of contact metamorfism
- Metamorphic Rock Database (MetPetDB) - Department of Earth Sciences and Environment, Rensselaer Polytechnic Institute
- Metamorphic Rocks Tour, Metamorphic Stone Introduction
- Atlas Metamorphic Rocks - Detailed specimens of field and hand photographs of metamorphic rocks grouped by arrangement and composition (Department of Earth Sciences, University of Oxford)
Source of the article : Wikipedia
0 Comments