Geology Science

Metamorphic rocks are formed within the Earth’s crust. Changing temperature and pressure conditions may result in changes to the mineral assemblage of the protolith. Metamorphic rocks are eventually exposed at the surface by uplift and erosion of the overlying rock.

There are fundamental types of metamorphism: regional metamorphism and phone, or thermal, metamorphism. Metamorphic rocks are categorised by way of texture and mineralogy. Specific zones of temperature and stress outline distinctive metamorphic facies. Rocks containing minerals diagnostic of each facies can be related to the temperature and stress that characterise that facies. For example, blueschist and eclogite facies rocks form below low to medium temperatures and high pressures. Such conditions as commonly associated with subduction zones. Hornfels facies rocks generally arise beneath the medium to high temperature and occasional stress conditions related to touch aureoles.

Each metamorphic facies is associated with a collection of index minerals from which it is able to be recognized. The actual set of indicator minerals from every organization that is found in a specific example relies upon at the chemical composition of the protolith. Information approximately the mineralogy of each metamorphic facies may be found in Grotzinger and Jordan.

Metamorphic facies. Greenschist, amphibolite and granulite conditions are typically associated with regional metamorphism. Blueschist and eclogite are associated with the type of regional metamorphism that occurs at subduction zones. Hornfels and sanidinite are typically associated with contact metamorphism. (Image from Wikipedia Commons.)

Types of metamorphism

1.Contact Metamorphism

Contact metamorphism normally happens round intrusive igneous rocks, and is the end result of temperature growth associated with the introduction of warm magma into the colder u . S . Rocks. The region surrounding an intrusion, wherein metamorphism is gift, is referred to as a metamorphic aureole. Metamorphism is best closest to the intrusion and dissipates as distance from the intrusion increases. The size of the aureole depends on the temperature of the igneous intrusion, the size of the intrusion and the temperature of the united states rock. Dykes can also have a small band of adjoining rock which has been regionally metamorphosed, but the outcomes of this metamorphism are generally minimum. Large intrusions, along with granite batholiths, may additionally have metamorphic aureoles covering a sizeable area. Where the united states rocks are pelitic in composition (i.E. They’re shaped from small silt debris), the rock closest to the intrusion is “baked”, forming a hard, splintery kind of rock known as a hornfels. Hornfels may additionally have a spotted look due to the boom of minerals such as cordierite and andalusite. In rocks of different compositions other minerals will shape. For instance olivine may form in an impure limestone.

The metamorphic grade of an aureole quarter is quantified via which minerals arise within that zone. Shale closest to an intrusion can be altered to hornfels and comprise minerals such as sillimanite or cordierite. Further from the intrusion, the same shale might also appear reasonably unaltered other than the development of some crystals of andalusite.

Contact metamorphism and associated minerals for special types of u . S . Rock. Aluminium-rich shale will form indicator minerals consisting of cordierite and sillimanite. Impure limestone may also host indicator minerals including olivine. Where there are giant volumes of fluid gift, skarns might also shape. (Image: Wolf 2010.)

2.Regional Metamorphism

Regional metamorphism typically consequences in the formation of rocks which might be strongly foliated, including slates, schists, and gneisses (see underneath). The differential pressure needed to produce such foliated rocks typically consequences from tectonic forces that produce compressional stresses in the rocks consisting of arise while two continental masses collide. Thus, locally metamorphosed rocks are determined within the cores of fold/thrust mountain belts or in eroded mountain degrees. Compressive stresses result in folding of rock and thickening of the crust, which results in a number of the rocks being driven to deeper stages wherein they’re subjected to better temperatures and pressures.

The pressure and heat which drive metamorphism come from 3 assets: the internal warmness of the Earth; the load of the overlying rock; and the horizontal pressures advanced as rocks become deformed.

Temperature in the crust increases with growing intensity from the surface by using 20 to 60°C in keeping with kilometre of intensity; 30°C in line with km is considered to be the average. For instance, at a intensity of 15km the temperature will be about 450°C. The stress at 15km intensity is derived from the weight of the overlying rock, and is ready 4000 times the pressure on the Earth’s surface.

Grades of metamorphic rock. A metamorphic rock which has experienced a temperature of 600°C and a pressure of 600MPa is termed a “high-grade metamorphic rock”. (Image from Tulane. edu.)

Metamorphic Minerals

All rocks are composed of minerals, and each mineral is simplest solid over a specific range of temperatures and pressures. Outwith that range it will generally tend to break down or combine with a neighbouring mineral to shape a brand new mineral.

The system of metamorphism is associated with the formation of particular metamorphic minerals. Some silicate minerals are so diagnostic of metamorphism that their presence suggests that the rock is metamorphic. These mineral are referred to as index minerals and include andalusite, kyanite, staurolite and sillimanite. Some other minerals, including garnet, muscovite, feldspar, quartz and micas, also are found in metamorphic rocks, but those will also be discovered in igneous and sedimentary rocks.

Metamorphic zones

The idea of metamorphic zones can be illustrated by means of thinking about what occurs to mudrock as it becomes subjected to growing stages of metamorphism. Mudrock is a quality-grained sedimentary rock which frequently carries aluminium-rich minerals, and it produces the following minerals as it is steadily metamorphosed, from a low-grade to a high-grade rock:

Chlorite zone: quartz, chlorite, muscovite, albite

Biotite area: quartz, muscovite, biotite, chlorite, albite

Garnet region: quartz, muscovite, biotite, garnet, Na plagioclase

Staurolite sector: quartz, muscovite, biotite, garnet, staurolite, plagioclase

Kyanite region: quartz, muscovite, biotite, garnet, kyanite, plagioclase, +/- staurolite

Sillimanite sector: quartz, muscovite, biotite, garnet, sillimanite, plagioclase

Metamorphic rock textures

The 5 simple metamorphic textures with usual rock types are slaty (consists of slate and phyllite; the foliation is known as “slaty cleavage”), schistose (consists of schist; the foliation is referred to as “schistosity”), gneissose (gneiss; the foliation is known as “gneissosity”), granoblastic (consists of granulite, a few marbles and quartzite), and hornfelsic (consists of hornfels and skarn).

Foliation

The layering within metamorphic rocks is called foliation (derived from the Latin phrase folia, that means “leaves”), and it takes place when a rock is being shortened alongside one axis for the duration of recrystallization. This causes the platy or elongated crystals of minerals, including mica and chlorite, to emerge as circled such that their lengthy axes are perpendicular to the orientation of shortening. This results in a banded, or foliated rock, with the bands displaying the colours of the minerals that shaped them.

Foliated Metamorphic Rocks:

Some varieties of metamorphic rocks — granite gneiss and biotite schist are examples — are strongly banded or foliated. (Foliated approach the parallel association of certain mineral grains that gives the rock a striped appearance.) Foliation paperwork while pressure squeezes the flat or elongate minerals within a rock so that they grow to be aligned. These rocks expand a platy or sheet-like structure that reflects the route that pressure become applied.

• Slaty cleavage – closely spaced planar surfaces along which rocks are easily split; seen in slate, which is a very fine-grained foliated rock derived from an original shale protolith.
• Schistosity – medium sized platy minerals in a preferred orientation, often interlayered with quartz and feldspar; characteristic of the rock type schist.
• Gneissic texture – seen in high-grade metamorphic rocks and characterised by medium- to coarse- grained crystals arranged in alternating bands of dark and light minerals (gneissic banding)

Non-Foliated Metamorphic Rocks:

Non-foliated metamorphic rocks do no longer have a platy or sheet-like shape. There are numerous ways that non-foliated rocks may be produced. Some rocks, including limestone are product of minerals that aren’t flat or elongate. No remember how a whole lot strain you practice, the grains will no longer align! Another type of metamorphism, touch metamorphism, happens whilst hot igneous rock intrudes into a few pre-present rock. The pre-existing rock is basically baked by way of the heat, converting the mineral shape of the rock with out addition of pressure.

• Granoblastic – a mosaic of equidimensional grains; hornfels, marble and quartzite have a granoblastic texture.
• Porphyroblastic – characterised by porphyroblasts, which are large crystals surrounded by a finer-grained groundmass in a metamorphic rock. This texture is often seen in garnet-mica schists.