Magma

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Magma

Magma (Plurals include: magmas and magmata) is molten rock

Rock (geology)

In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock....
that is found beneath the surface of the Earth

Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber

Magma chamber

A magma chamber is a large underground pool of molten Rock lying under the surface of the earth's crust. The molten rock in such a chamber is under great pressure, and given enough time pressure can gradually fracture the rock around it creating outlets for the magma....
inside a volcano

Volcano

A volcano is an opening, or rupture, in a planet's surface or Crust , which allows hot, molten rock, ash, and gases to escape from below the surface....
.

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Magma (Plurals include: magmas and magmata) is molten rock

Rock (geology)

Earth

Magma chamber

Volcano

A volcano is an opening, or rupture, in a planet's surface or Crust , which allows hot, molten rock, ash, and gases to escape from below the surface....
. Magma is capable of intrusion into adjacent rocks, extrusion onto the surface as lava

Lava

Lava is molten Rock expelled by a volcano during an eruption. When first expelled from a volcanic vent, it is a liquid at temperatures from 700 ?C to 1,200 ?C ....
, and explosive ejection as tephra

Tephra

Tephra is air-fall material produced by a Volcano regardless of composition or fragment size. Tephra is typically Rhyolite in composition, as most explosive volcanoes are the product of the more viscosity felsic or high silica magmas....
to form pyroclastic rock

Pyroclastic rock

Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic....
.

Magma is a complex high-temperature fluid substance. Temperatures of most magmas are in the range 700°C to 1300°C (or 1292°F to 2372°F), but very rare carbonatite

Carbonatite

Carbonatites are intrusive or extrusive igneous Rock defined by mineralogic composition consisting of greater than 70 volume percent carbonate minerals....
melts may be as cool as 600°C, and komatiite

Komatiite

Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO....
melts may have been as hot as 1600°C. Most are silicate

Silicate

A silicate is a compound containing an anion in which one or more central silicon atoms are surrounded by electronegative ligands. This definition is broad enough to include species such as hexafluorosilicate , [SiF6]2-, but the silicate species that are encountered most often consist of silicon with oxygen as the ligand...
solution

Solution

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent....
s.

Environments of magma formation and compositions are commonly correlated. Environments include subduction zones, continental rift zones, mid-oceanic ridges, and hotspot

Hotspot (geology)

In geology, a hotspot is a location on the Earth's surface that has experienced active volcano for a long period of time. J. Tuzo Wilson came up with the idea in 1963 that volcanic chains like the Hawaiian Islands result from the slow movement of a tectonic plate across a "fixed" hot spot deep beneath the surface of the planet....
s, some of which are interpreted as mantle plume

Mantle plume

A mantle plume is an upwelling of abnormally hot rock within the Earth's mantle . As the heads of mantle plumes can partly melt when they reach shallow depths, they are thought to be the cause of volcano centers known as Hotspot and probably also to have caused flood basalts....
s. Despite being found in such widespread locales, the bulk of the Earth's crust and mantle

Mantle (geology)

The mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers....
is not molten. Rather, most of the Earth takes the form of a rheid

Rheid

In geology, a rheid is a solid material that deforms by viscosity flow. To be considered a rheid, deformation by flow should exceed Elasticity deformation by at least a factor of three....
, a form of solid that can move or deform under pressure. Magma, as liquid, preferentally forms in high temperature, low pressure environments within several kilometers of the Earth's surface.

Magma compositions may evolve after formation by fractional crystallization

Fractional crystallization (geology)

Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and Earth's mantle....
, contamination, and magma mixing. By definition, all igneous rock

Igneous rock

Igneous rock is one of the three main Rock types . Igneous rock is formed by magma being cooled and becoming solid . They may form with or without crystallization, either below the surface as Intrusion rocks or on the surface as extrusive rocks....
is formed from magma.

While the study of magma has historically relied on observing magma in the form of lava outflows, magma was discovered in situ

In situ

In situ is a Latin phrase meaning in the place. It is used in many different contexts....
for the first time in 2008.

Melting of solid rock

Melting of solid rock to form magma is controlled by three physical parameters: its temperature, pressure, and composition. Mechanisms are discussed in the entry for igneous rock

Igneous rock

Temperature

At any given pressure and for any given composition of rock, a rise in temperature past the solidus

Solidus (chemistry)

In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid . The solidus is applied, among else, to metal alloys, ceramics, and natural rocks and minerals....
will cause melting. Within the solid earth, the temperature of a rock is controlled by the geothermal gradient

Geothermal gradient

The geothermal gradient is the rate of increase in temperature per unit depth in the Earth. It varies with location and is typically measured by determining the bottom open-hole temperature after borehole drilling....
and the radioactive decay

Radioactive decay

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide....
within the rock. The geothermal gradient averages about 25°C/km with a wide range from a low of 5-10°C/km within oceanic trenches and subduction zones to 30-80°C/km under mid-ocean ridges and volcanic arc environments.

Pressure

As magma buoyantly rises it will cross the solidus-liquidus and its temperature will reduce by adiabatic cooling. At this point it will liquify and if erupted onto the surface will form lava. Melting can also occur due to a reduction in pressure by a process known as decompression melting.

Composition

It is usually very difficult to change the bulk composition of a large mass of rock, so composition is the basic control on whether a rock will melt at any given temperature and pressure. The composition of a rock may also be considered to include volatile phases such as water

Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
and carbon dioxide

Carbon dioxide

Carbon dioxide is a chemical compound composed of two oxygen atoms covalent bond to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state....
.

The presence of volatile phases in a rock under pressure can stabilize a melt fraction. The presence of even 0.8% water may reduce the temperature of melting by as much as 100°C. Conversely, the loss of water and volatiles from a magma may cause it to essentially freeze or solidify.

Partial melting

When rocks melt they do so incrementally and gradually; most rocks are made of several minerals, all of which have different melting points, and the phase diagram

Phase diagram

A phase diagram in physical chemistry, mineralogy, and materials science is a type of Graph of a function used to show conditions at which thermodynamically-distinct phase can occur at thermodynamic equilibrium....
s that control melting commonly are complex. As a rock melts, its volume changes. When enough rock is melted, the small globules of melt (generally occurring in between mineral grains) link up and soften the rock. Under pressure within the earth, as little as a fraction of a percent partial melting may be sufficient to cause melt to be squeezed from its source.

Melts can stay in place long enough to melt to 20% or even 35%, but rocks are rarely melted in excess of 50%, because eventually the melted rock mass becomes a crystal and melt mush that can then ascend en masse as a diapir

Diapir

A diapir is a type of intrusion in which a more mobile and ductily-deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dike...
, which may then cause further decompression melting.

Primary melts

When a rock melts, the liquid is known as a primary melt. Primary melts have not undergone any differentiation and represent the starting composition of a magma. In nature it is rare to find primary melts. The leucosomes of migmatite

Migmatite

Migmatite is a rock at the frontier between igneous rock and metamorphic rocks. They can also be known as diatexite.Migmatites form under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks....
s are examples of primary melts. Primary melts derived from the mantle are especially important, and are known as primitive melts or primitive magmas. By finding the primitive magma composition of a magma series it is possible to model the composition of the mantle from which a melt was formed, which is important in understanding evolution of the mantle.

Parental melts

Where it is impossible to find the primitive or primary magma composition, it is often useful to attempt to identify a parental melt. A parental melt is a magma composition from which the observed range of magma chemistries has been derived by the processes of igneous differentiation. It need not be a primitive melt.

For instance, a series of basalt flows are assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a parental melt. Fractional crystallization models would be produced to test the hypothesis that they share a common parental melt.

Geochemical implications of partial melting

The degree of partial melting is critical for determining what type of magma is produced. The degree of partial melting required to form a melt can be estimated by considering the relative enrichment of incompatible elements versus compatible elements. Incompatible elements

Compatibility (geochemistry)

In geochemistry, compatibility is a measure of how readily a particular trace element will substitute for a major element within a mineral.Compatibility of an ion is controlled by two things: its valence and its ionic radius....
commonly include potassium

Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
, barium

Barium

Barium is a chemical element. It has the symbol Ba, and atomic number 56. Barium is a soft silvery metallic alkaline earth metal. It is never found in nature in its pure form due to its reactivity with Earth's atmosphere....
, caesium

Caesium

Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of , which makes it one of only liquid metal that are liquid at or near room temperature....
, rubidium

Rubidium

Rubidium is a chemical element with the symbol Rb and atomic number 37. Rb is a soft, silvery-white metallic element of the alkali metal group....
.

Rock types produced by small degrees of partial melting in the Earth's mantle are typically alkaline (Ca

Calcium

Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the earth's Crust ....
, Na

Sodium

Sodium is an element which has the symbol Na , atomic number 11, atomic mass 23 amu , and a common oxidation number +1. Sodium is a soft, silvery white, highly reactive element and is a member of the alkali metals within "group 1" ....
), potassic (K

Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
) and/or peralkaline (high aluminium to silica ratio). Typically, primitive melts of this composition form lamprophyre

Lamprophyre

Lamprophyres are uncommon, small volume ultrapotassic igneous rocks igneous rocks primarily occurring as Dike , lopoliths, laccoliths, stocks and small intrusions....
, lamproite

Lamproite

Lamproites are ultrapotassic igneous rocks earth's mantle-derived igneous rock. They have low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements....
, kimberlite

Kimberlite

Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley, Northern Cape in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
and sometimes nepheline

Nepheline

Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Sodium3PotassiumAluminium4silicon4Oxygen16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites....
-bearing mafic rocks such as alkali

Alkali

In chemistry, an alkali is a Base , Ionic compound salt of an alkali metal or alkaline earth metal Chemical element. Alkalis are best known for being Base s that dissolve in water....
basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
s and essexite

Essexite

Essexite , also called nepheline monzogabbro is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, MA....
gabbros or even carbonatite

Carbonatite

Carbonatites are intrusive or extrusive igneous Rock defined by mineralogic composition consisting of greater than 70 volume percent carbonate minerals....
.

Pegmatite

Pegmatite

Pegmatite is a very coarse-grained igneous rock that has a grain size of 20 mm or more; such rocks are referred to as pegmatitic.Most pegmatites are composed of quartz, feldspar and mica; in essence a "granite"....
may be produced by low degrees of partial melting of the crust. Some granite

Granite

Granite is a common and widely occurring type of Intrusion , felsic, igneous rock rock . Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as Porphyry ....
-composition magmas are eutectic (or cotectic) melts, and they may be produced by low to high degrees of partial melting of the crust, as well as by fractional crystallization. At high degrees of partial melting of the crust, granitoids such as tonalite

Tonalite

Tonalite is an igneous rock, plutonic rock , of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar....
, granodiorite

Granodiorite

Granodiorite is an intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar. It usually contains abundant biotite mica and hornblende, giving it a darker appearance than true granite....
and monzonite

Monzonite

Monzonite is an intermediate igneous intrusive rock composed of approximately equal amounts of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende, biotite and other minerals....
can be produced, but other mechanisms are typically important in producing them.

At high degrees of partial melting of the mantle, komatiite

Komatiite

Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO....
and picrite are produced.

Composition and melt structure and properties

Silicate melts are composed mainly of silicon

Silicon

Silicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. The atomic mass is 28.0855....
, oxygen

Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
, aluminium

Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
, alkalis (sodium

Sodium

Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
, calcium

Calcium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, atomic weight 24.3050 and common oxidation number +2.Magnesium, an alkaline earth metal, is the ninth most abundance of the chemical elements in the universe by mass....
and iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
. Silicon atoms are in tetrahedral coordination with oxygen, as in almost all silicate minerals

Silicate minerals

The silicate minerals make up the largest and most important class of rock-forming minerals, comprising approximately 90 percent of the crust of the Earth....
, but in melts atomic order is preserved only over short distances. The physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition.

Viscosity

Viscosity

Viscosity is a measure of the Drag of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness"....
is a key melt property in understanding the behaviour of magmas. More silica-rich melts are typically more polymerized, with more linkage of silica tetrahedra, and so are more viscous. Dissolution of water drastically reduces melt viscosity. Higher-temperature melts are less viscous.

Generally speaking, more mafic magmas, such as those that form basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite

Rhyolite

This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous rock, volcanic rock , of felsic composition ....
. Low viscosity leads to gentler, less explosive eruptions.

Characteristics of several different magma types are as follows:

Ultramafic (picritic)

SiO₂ < 45% Fe-Mg >8% up to 32%MgO Temperature: up to 1500°C Viscosity: Very Low Eruptive behavior: gentle or very explosive (kimberilites) Distribution: divergent plate boundaries, hot spots, convergent plate boundaries; komatiite

Komatiite

Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO....
and other ultramafic lavas are mostly Archean

Archean

The Archean is a geology eon before the Proterozoic and Paleoproterozoic, before 2.5 Ga . Instead of being based on stratigraphy, this date is defined chronometrically....
and were formed from a higher geothermal gradient

Geothermal gradient

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric"....
(basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually gray to black and fine-grained due to rapid cooling of lava at the surface of a planet....
ic)

SiO₂ < 50% FeO and MgO typically < 10 wt% Temperature: up to ~1300°C Viscosity: Low Eruptive behavior: gentle Distribution: divergent plate boundaries, hot spots, convergent plate boundaries

Intermediate (andesitic

Andesite

Andesite is an igneous rock, volcanic rock, of Igneous rock#Chemical classification, with aphanitic to porphyritic texture. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende....
)

SiO₂ ~ 60% Fe-Mg: ~ 3% Temperature: ~1000°C Viscosity: Intermediate Eruptive behavior: explosive Distribution: convergent plate boundaries, island arcs

Felsic

Felsic is a term used in geology to refer to silicate minerals, magma, and rock which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
(rhyolitic)

SiO₂ >70% Fe-Mg: ~ 2% Temp: < 900°C Viscosity: High Eruptive behavior: explosive Distribution: hot spots in continental crust (Yellowstone National Park

Yellowstone National Park

Yellowstone National Park, established by the U.S. Congress as a national park on March 1, 1872, is located primarily in the U.S. state of Wyoming, though it also extends into Montana and Idaho....
), continental rifts

Magma

Melting of solid rock

Temperature

Pressure

Composition

Partial melting

Primary melts

Parental melts

Geochemical implications of partial melting

Composition and melt structure and properties

See also