Magma

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Lava flow on Hawaii. Lava is the extrusive equivalent of magma. Pahoehoe toe.jpg
Lava flow on Hawaii. Lava is the extrusive equivalent of magma.

Magma (from Ancient Greek μάγμα (mágma) meaning "thick unguent" [1] ) is the molten or semi-molten natural material from which all igneous rocks are formed. [2] Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. [3] Besides molten rock, magma may also contain suspended crystals and gas bubbles. [4] Magma is produced by melting of the mantle and/or the crust at various tectonic settings, including subduction zones, continental rift zones, [5] mid-ocean ridges and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chambers [6] or trans-crustal crystal-rich mush zones. [7] During their storage in the crust, magma compositions may be modified by fractional crystallization, contamination with crustal melts, magma mixing, and degassing. Following their ascent through the crust, magmas may feed a volcano or solidify underground to form an intrusion [8] (e.g., an igneous dike or a sill). While the study of magma has historically relied on observing magma in the form of lava flows, magma has been encountered in situ three times during geothermal drilling projects twice in Iceland (see Magma usage for energy production), and once in Hawaii. [9] [10] [11]

Ancient Greek Version of the Greek language used from roughly the 9th century BCE to the 6th century CE

The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is often roughly divided into the Archaic period, Classical period, and Hellenistic period. It is antedated in the second millennium BCE by Mycenaean Greek and succeeded by medieval Greek.

An unguent is a soothing preparation spread on wounds, burns, rashes, abrasions or other topical injuries. It is similar to an ointment, though typically an unguent is less viscous and more oily. It is usually delivered as a semi-solid paste spread on the skin and is often oily to suspend the medication or other active ingredients.

Igneous rock Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.

Contents

Physical and chemical properties of magma

Most magmatic liquids are rich in silica. [8] Silicate melts are composed mainly of silicon, oxygen, aluminium, iron, magnesium, calcium, sodium, and potassium. The physical behaviours of melts depend upon their atomic structures as well as upon temperature and pressure and composition. [12]

Silicon dioxide chemical compound

Silicon dioxide, also known as silica, silicic acid or silicic acid anhydride is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics (as an electrical insulator), and as components in the food and pharmaceutical industries.

Oxygen Chemical element with atomic number 8

Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O
2
. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up almost half of the Earth's crust.

Aluminium Chemical element with atomic number 13

Aluminium or aluminum is a chemical element with symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth's crust; it is the third most abundant element after oxygen and silicon and the most abundant metal in the crust, though it is less common in the mantle below. The chief ore of aluminium is bauxite. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals.

Viscosity 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.

Viscosity physical property of a fluid

The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.

Generally speaking, more mafic magmas, such as those that form basalt, are hotter and less viscous than more silica-rich magmas, such as those that form rhyolite. Low viscosity leads to gentler, less explosive eruptions.

Mafic Silicate mineral or igneous rock that is rich in magnesium and iron

Mafic is an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron, and is thus a portmanteau of magnesium and ferric. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar.

Basalt A magnesium- and iron-rich extrusive igneous rock

Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.

Rhyolite An igneous, volcanic rock, of felsic (silica-rich) composition

Rhyolite is an igneous, volcanic rock, of felsic (silica-rich) composition (typically > 69% SiO2 – see the TAS classification). It may have any texture from glassy to aphanitic to porphyritic. The mineral assemblage is usually quartz, sanidine and plagioclase (in a ratio > 2:1 – see the QAPF diagram). Biotite and hornblende are common accessory minerals. It is the extrusive equivalent to granite.

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 and other ultramafic lavas are mostly Archean and were formed from a higher geothermal gradient and are unknown in the present
Mafic (basaltic)
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)
SiO2 ~ 60%
Fe–Mg: ~ 3%th
Temperature: ~1000°C
Viscosity: Intermediate
Eruptive behavior: explosive or effusive
Distribution: convergent plate boundaries, island arcs
Felsic (rhyolitic)
SiO2 > 70%
Fe–Mg: ~ 2%
Temperature: < 900°C
Viscosity: High
Eruptive behavior: explosive or effusive
Distribution: common in hot spots in continental crust (Yellowstone National Park) and in continental rifts

Temperature

Temperatures of most magmas are in the range 700 °C to 1300 °C (or 1300 °F to 2400 °F), but very rare carbonatite magmas may be as cool as 490 °C, [13] and komatiite magmas may have been as hot as 1600 °C. [14] At any given pressure and for any given composition of rock, a rise in temperature past the solidus will cause melting. Within the solid earth, the temperature of a rock is controlled by the geothermal gradient and the radioactive decay 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.

Carbonatite Igneous rock with more than 50% carbonate minerals

Carbonatite is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geochemical verification.

Komatiite An ultramafic mantle-derived volcanic rock

Komatiite is a type of ultramafic mantle-derived volcanic rock defined as having crystallised from a lava with ≥ 18 wt% MgO. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content. Komatiite was named for its type locality along the Komati River in South Africa, and frequently displays spinifex texture composed of large dendritic plates of olivine and pyroxene.

In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid (crystallized). The solidus is applied, among other materials, to metal alloys, ceramics, and natural rocks and minerals.

Density

TypeDensity (kg/m3)
Basalt magma2650–2800 [15]
Andesite magma2450–2500 [15]
Rhyolite magma2180–2250 [15]

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 and carbon dioxide.

Water chemical compound

Water is a transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's streams, lakes, and oceans, and the fluids of most living organisms. It is vital for all known forms of life, even though it provides no calories or organic nutrients. Its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. Water is the name of the liquid state of H2O at standard ambient temperature and pressure. It forms precipitation in the form of rain and aerosols in the form of fog. Clouds are formed from suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor. Water moves continually through the water cycle of evaporation, transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea.

Carbon dioxide chemical compound

Carbon dioxide is a colorless gas with a density about 60% higher than that of dry air. Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms. It occurs naturally in Earth's atmosphere as a trace gas. The current concentration is about 0.04% (410 ppm) by volume, having risen from pre-industrial levels of 280 ppm. Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, ice caps, glaciers and seawater. It is present in deposits of petroleum and natural gas. Carbon dioxide is odorless at normally encountered concentrations. However, at high concentrations, it has a sharp and acidic odor.

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.

Also a major portion of almost all magma is silica, which is a compound of silicon and oxygen. Magma also contains gases, which expand as the magma rises. Magma that is high in silica resists flowing, so expanding gases are trapped in it. Pressure builds up until the gases blast out in a violent, dangerous explosion. Magma that is relatively poor in silica flows easily, so gas bubbles move up through it and escape fairly gently.

Origins of magma by partial melting

Partial melting

Melting of solid rocks to form magma is controlled by three physical parameters: temperature, pressure, and composition. The most common mechanisms of magma generation in the mantle are decompression melting [16] , heating (e.g., by interaction with a hot mantle plume [17] ), and lowering of the solidus (e.g., by compositional changes such as the addition of water [18] ). Mechanisms are discussed further in the entry for igneous rock.

When rocks melt, they do so slowly and gradually because most rocks are made of several minerals, which all have different melting points; moreover, the physical and chemical relationships controlling the melting are complex. As a rock melts, for example, its volume changes. When enough rock is melted, the small globules of melt (generally occurring between mineral grains) link up and soften the rock. Under pressure within the earth, as little as a fraction of a percent of partial melting may be sufficient to cause melt to be squeezed from its source. [19] 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, which may then cause further decompression melting.

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 commonly include potassium, barium, caesium, and rubidium.

Rock types produced by small degrees of partial melting in the Earth's mantle are typically alkaline (Ca, Na), potassic (K) and/or peralkaline (high aluminium to silica ratio). Typically, primitive melts of this composition form lamprophyre, lamproite, kimberlite and sometimes nepheline-bearing mafic rocks such as alkali basalts and essexite gabbros or even carbonatite.

Pegmatite may be produced by low degrees of partial melting of the crust. Some granite-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, granodiorite and monzonite can be produced, but other mechanisms are typically important in producing them.

Evolution of magmas

Primary melts

When a rock melts, the liquid is 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 migmatites 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.[ clarification needed ]

Parental melts

When it is impossible to find the primitive or primary magma composition, it is often useful[ according to whom? ] 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.

At high degrees of partial melting of the mantle, komatiite and picrite are produced.

Migration and solidification of magmas

Magma develops within the mantle or crust where the temperature and pressure conditions favor the molten state. After its formation, magma buoyantly rises toward the Earth's surface. As it migrates through the crust, magma may collect and reside in magma chambers (though recent work suggests that magma may be stored in trans-crustal crystal-rich mush zones rather than dominantly liquid magma chambers [20] ). Magma can remain in a chamber until it cools and crystallizes forming igneous rock, it erupts as a volcano, or moves into another magma chamber.There are two known processes by which magma changes: by crystallization within the crust or mantle to form a pluton, or by volcanic eruption to become lava or tephra.

Plutonism

When magma cools it begins to form solid mineral phases. Some of these settle at the bottom of the magma chamber forming cumulates that might form mafic layered intrusions. Magma that cools slowly within a magma chamber usually ends up forming bodies of plutonic rocks such as gabbro, diorite and granite, depending upon the composition of the magma. Alternatively, if the magma is erupted it forms volcanic rocks such as basalt, andesite and rhyolite (the extrusive equivalents of gabbro, diorite and granite, respectively).

Volcanism

During a volcanic eruption the magma that leaves the underground is called lava. Lava cools and solidifies relatively quickly compared to underground bodies of magma. This fast cooling does not allow crystals to grow large, and a part of the melt does not crystallize at all, becoming glass. Rocks largely composed of volcanic glass include obsidian, scoria and pumice.

Before and during volcanic eruptions, volatiles such as CO2 and H2O partially leave the melt through a process known as exsolution. Magma with low water content becomes increasingly viscous. If massive exsolution occurs when magma heads upwards during a volcanic eruption, the resulting eruption is usually explosive.

Magma usage for energy production

The Iceland Deep Drilling Project, while drilling several 5,000m holes in an attempt to harness the heat in the volcanic bedrock below the surface of Iceland, struck a pocket of magma at 2,100m in 2009. Because this was only the third time in recorded history that magma had been reached, IDDP decided to invest in the hole, naming it IDDP-1.

A cemented steel case was constructed in the hole with a perforation at the bottom close to the magma. The high temperatures and pressure of the magma steam were used to generate 36MW of power, making IDDP-1 the world’s first magma-enhanced geothermal system. [21]

Related Research Articles

Volcanism phenomena and processes associated with the action of volcanos, geysers and fumaroles

Volcanism is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics and volcanic gases erupt through a break in the surface called a vent. It includes all phenomena resulting from and causing magma within the crust or mantle of the body, to rise through the crust and form volcanic rocks on the surface.

Stratovolcano Tall, conical volcano built up by many layers of hardened lava and other ejecta

A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice and ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica, with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).

Dacite Volcanic rock intermediate in composition between andesite and rhyolite

Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains where the rock was first described.

Andesite An intermediate volcanic rock

Andesite ( or ) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and rhyolite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Volcanic rock rocks composing or associated with volcanoes, volcanic activity or volcanism

Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.

Peridotite A coarse-grained ultramafic igneous rock

Peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica (SiO4−
4
). It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Eclogite A dense, mafic metamorphic rock

Eclogite is a mafic metamorphic rock. Eclogite forms at pressures greater than those typical of the crust of the Earth. An unusually dense rock, eclogite can play an important role in driving convection within the solid Earth.

Ultramafic rock igneous rock type

Ultramafic rocks are igneous and meta-igneous rocks with a very low silica content, generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals. The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks.

Rock cycle Transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous

The rock cycle is a basic concept in geology that describes the transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. As the adjacent diagram illustrates, each of the types of rocks is altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption.

Fractional crystallization (geology) One of the main processes of magmatic differentiation

Fractional crystallization, or crystal fractionation, is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. It is important in the formation of igneous rocks because it is one of the main processes of magmatic differentiation. Fractional crystallization is also important in the formation of sedimentary evaporite rocks.

The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it fractionally crystallizes to become a felsic magma, which is low in magnesium and iron and produces rhyolite or granite. Calc-alkaline rocks are rich in alkaline earths and alkali metals and make up a major part of the crust of the continents.

Subduction zone metamorphism Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets created by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

Tonalite-trondhjemite-granodiorite

Tonalite-trondhjemite-granodiorite rocks or TTG rocks are intrusive rocks with typical granitic composition but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. Post Archean TTG rocks are present in arc-related batholiths, as well as in ophiolites, while Archean TTG rocks are major components of Archean cratons.

Earths crustal evolution

Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface.

Crystal mush

A crystal mush is a magmatic body which contains a significant amount of crystals suspended in the liquid phase (melt). As the crystal fraction makes up less than half of the volume, there is no rigid large-scale three-dimensional network as in solids. As such, their rheological behavior mirrors that of absolute liquids. Within a single crystal mush, there is grading to a higher solid fraction towards the margins of the pluton while the liquid fraction increases towards the uppermost portions, forming a liquid lens at the top. Furthermore, depending on depth of placement crystal mushes are likely to contain a larger portion of crystals at greater depth in the crust than at shallower depth, as melting occurs from the adiabatic decompression of the magma as it rises, this is particularly the case for mid-oceanic ridges.

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