Hawaii
Hawaii ( ; haw, Hawaii or ) is a U.S. state, state in the Western United States, located in the Pacific Ocean about 2,000 miles from the U.S. mainland. It is the only state outside North America, the only state that is an archipelago, a ...
. Lava is the extrusive equivalent of magma.
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth
Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...
, and evidence of magmatism
Magmatism is the emplacement of magma
300px, Lava flow on Hawaii (island), Hawaii. Lava is the extrusive equivalent of magma.
Magma (from Ancient Greek μάγμα (''mágma'') meaning "thick unguent") is the molten or semi-molten natural materia ...
has also been discovered on other terrestrial planets
A terrestrial planet, telluric planet, or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, th ...
and some natural satellite
A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit
In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...
s. Besides molten rock, magma may also contain suspended crystals and gas bubbles.
Magma is produced by melting of the mantle or the crust in various 
tectonic
Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building
A mountain is an elevated portion of the Earth's crust, gene ...
settings, which on Earth include subduction zone
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate
This is a list of tectonic plates on Earth's surface. Tec ...
s, continental rift zone
upright=1.2, East Rift Zone on Kīlauea, Hawaii
A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined re ...
s, mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics
File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale)
Plate ...
s and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chamber
A magma chamber is a large pool of liquid rock
Rock most often refers to:
* Rock (geology)
A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical ...
s or trans-crustal zones. During magma's storage in the crust, its composition may be modified by fractional crystallization, contamination with crustal melts, magma mixing, and degassing. Following its ascent through the crust, magma may feed a 
volcano
A volcano is a rupture in the crust of a planetary-mass object
A planet is an astronomical body orbit
In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ar ...
and be extruded as lava
of pāhoehoe lava, Hawaii, United States
, Iceland in 1984
Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon. Magma is generated by the inte ...
, or it may solidify underground to form an , such as a 
dike
Dyke or dike may refer to:
General uses
* Dyke (slang), a slang word meaning "lesbian"
* Dike (geology), a subvertical sheet-like intrusion of magma or sediment
* Dike (mythology), the Greek goddess of moral justice
* Dikes, diagonal pliers, diag ...
, a sill, a laccolith
A laccolith is a sheet-like intrusion (or concordant pluton) that has been injected within or between layers of sedimentary rock
Sedimentary rocks are types of Rock (geology), rock that are formed by the accumulation or deposition of mineral o ...
, a pluton#REDIRECT Intrusive rock
, an igneous ''intrusion'' exposed when the surrounding softer rock eroded away
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as bath ...
, or a batholith
Image:Yosemite 20 bg 090404.jpg, upright=1.3, Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada Batholith
A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of Intrusive rock, intrusiv ...
.
While the study of magma has relied on observing magma after its transition into a lava flow
of pāhoehoe lava, Hawaii, United States
, Iceland in 1984
Lava is molten Rock (geology), rock ( magma) that has been expelled from the interior of a terrestrial planet (such as Earth
Earth is the third planet from the Sun and the onl ...
, magma has been encountered in situ
''In situ'' (; often not italicized in English) is a Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as ...
three times during geothermal drilling projects, twice in Iceland (see Use in energy production
Use may refer to:
* Use (law), an obligation on a person to whom property has been conveyed
* Use (liturgy), a special form of Roman Catholic ritual adopted for use in a particular diocese
* Use–mention distinction
The use–mention distincti ...
) and once in Hawaii.
Physical and chemical properties
Magma consists of liquid rock that usually contains suspended solid crystals. As magma approaches the surface and theoverburden pressure
Pressure
Pressure (symbol: ''p'' or ''P'') is the force
In physics
Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that ...
drops, dissolved gases bubble out of the liquid, so that magma near the surface consists of materials in solid, liquid, and gas phases.
Composition
Most magma is rich insilica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , 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 ...
. Rare nonsilicate magma can form by local melting of nonsilicate mineral deposits or by separation of a magma into separate immiscible
Miscibility () is the property of two Chemical substance, substances to mix in all Mixing ratio, proportions (that is, to fully dissolution (chemistry), dissolve in each other at any concentration), forming a Homogeneity and heterogeneity, homogen ...
silicate and nonsilicate liquid phases.
Silicate magmas are molten mixtures dominated by oxygen
Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
and silicon
Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetravalence, tetravalent metalloid and semiconductor. It is a member ...
, the most abundant chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nu ...
s in the Earth’s crust, with smaller quantities of aluminium
Aluminium (aluminum in American
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the United States
The United States of America (USA), commonly known as the Unit ...
, calcium
Calcium is a chemical element with the Symbol (chemistry), symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties a ...
, magnesium
Magnesium is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a pure substance consisting only of atoms that all have the same ...
, iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
, sodium
Sodium is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a pure substance consisting only of atoms that all have the same num ...
, and potassium
Potassium is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a pure substance consisting only of atoms that all have the same ...
, and minor amounts of many other elements. Petrologist
A volcanic sand grain seen under the microscope, with plane-polarized light in the upper picture, and cross polarized light in the lower picture. Scale box is 0.25 mm.
Petrology (from the grc, πέτρος, translit=pétros, lit=rock and grc, ...
s routinely express the composition of a silicate magma in terms of the weight or molar mass
In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance in that sample, measured in mole (unit), moles. The molar mass is a bulk, not molecular, property of a subst ...
fraction of the oxides of the major elements (other than oxygen) present in the magma.
Because many of the properties of a magma (such as its viscosity and temperature) are observed to correlate with silica content, silicate magmas are divided into four chemical types based on silica content: ''felsic'', ''intermediate'', ''mafic'', and ''ultramafic''.
Felsic magma
''Felsic'' orsilicicSilicic is an adjective to describe magma
300px, Lava flow on Hawaii (island), Hawaii. Lava is the extrusive equivalent of magma.
Magma (from Ancient Greek μάγμα (''mágma'') meaning "thick unguent") is the molten or semi-molten natural mater ...
magmas have a silica content greater than 63%. They include rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matrix ...
and dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. I ...
magmas. With such a high silica content, these magmas are extremely viscous, ranging from 108 cP for hot rhyolite magma at to 1011 cP for cool rhyolite magma at . For comparison, water has a viscosity of about 1 cP. Because of this very high viscosity, felsic lavas usually erupt explosively to produce pyroclastic
300px, USGS scientist examines Mount_St._Helens.html"_;"title="pumice_blocks_at_the_edge_of_a_pyroclastic_flow_from_Mount_St._Helens">pumice_blocks_at_the_edge_of_a_pyroclastic_flow_from_Mount_St._Helens_
File:Volcanic_Stone_3D.ogg.html" ;"titl ...
(fragmental) deposits. However, rhyolite lavas occasionally erupt effusively to form lava spine
250px, A lava spine at the summit of Mount Pelée in 1902
A lava spine (or lava spire) is a vertical growth of solid lava
of pāhoehoe lava, Hawaii, United States
, Iceland in 1984
Lava is molten Rock (geology), rock (magma) that has been exp ...
s, lava dome
In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow Extrusive rock, extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6 ...
s or "coulees" (which are thick, short lava flows). The lavas typically fragment as they extrude, producing block lava flows. These often contain obsidian
Obsidian (; ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
Obsidian is produced from felsic lava, rich in the lighter elements ...
.
Felsic lavas can erupt at temperatures as low as . Unusually hot (>950 °C; >1,740 °F) rhyolite lavas, however, may flow for distances of many tens of kilometres, such as in the Snake River Plain
The Snake River cutting through the plain leaves many canyons and Canyon#List of gorges">gorges, such as this one near Twin Falls, Idaho
, 2008
The Snake River Plain is a geology, geologic feature located primarily within the U.S. state of Id ...
of the northwestern United States.
Intermediate magma
''Intermediate'' orandesitic
Andesite ( or ) is an extrusive volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being o ...
magmas contain 52% to 63% silica, and are lower in aluminium and usually somewhat richer in magnesium
Magnesium is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a pure substance consisting only of atoms that all have the same ...
and iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
than felsic magmas. Intermediate lavas form andesite domes and block lavas, and may occur on steep composite volcano
A stratovolcano, also known as a composite volcano, is a conical volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and Volcanic gas, gases to escape from a magma cham ...
es, such as in the Andes
The Andes, Andes Mountains or Andean Mountains ( es, Cordillera de los Andes) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of Sout ...
. They are also commonly hotter, in the range of ). Because of their lower silica content and higher eruptive temperatures, they tend to be much less viscous, with a typical viscosity of 3.5 × 106 cP at . This is slightly greater than the viscosity of smooth peanut butter
Peanut butter is a food Paste (food), paste or Spread (food), spread made from Grinding (abrasive cutting), ground, dry roasting, dry-roasted peanuts. It commonly contains additional ingredients that modify the taste or texture, such as salt, sw ...
. Intermediate magmas show a greater tendency to form phenocrysts
image:montblanc granite phenocrysts.JPG, 300px, Granites often have large feldspar, feldspathic phenocrysts. This granite, from the Switzerland, Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that gi ...
, Higher iron and magnesium tends to manifest as a darker groundmass
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ord ...
, including amphibole or pyroxene phenocrysts.
Mafic magmas
''Mafic'' orbasalt
Basalt (, ) is a fine-grained extrusive
A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope
Extrusi ...
ic magmas have a silica content of 52% to 45%. They are typified by their high ferromagnesian content, and generally erupt at temperatures of . Viscosities can be relatively low, around 104 to 105 cP, although this is still many orders of magnitude higher than water. This viscosity is similar to that of ketchup
Ketchup is a table condiment. The unmodified term ("ketchup") now typically refers to tomato ketchup, although original recipes used egg whites, mushroom ketchup, mushrooms, oysters, grapes, mussels, or walnuts, among other ingredients.
Tomat ...
. Basalt lavas tend to produce low-profile shield volcano
A shield volcano is a type of volcano
A volcano is a rupture in the crust of a planetary-mass object
A planet is an astronomical body orbit
In physics, an orbit is the gravitationally curved trajectory of an physical body, objec ...
es or flood basalt
A flood basalt is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot (geology), hotspot reaching ...
s, because the fluidal lava flows for long distances from the vent. The thickness of a basalt lava, particularly on a low slope, may be much greater than the thickness of the moving lava flow at any one time, because basalt lavas may "inflate" by supply of lava beneath a solidified crust. Most basalt lavas are of ''ʻAʻā
of pāhoehoe lava, Hawaii, United States
Hawaii ( ; haw, Hawaii or ) is a U.S. state in the Western United States, in the Pacific Ocean about 2,000 miles (3,200 km) from the U.S. mainland. It is the only state outside North America, ...
'' or ''pāhoehoe
of pāhoehoe lava, Hawaii, United States
Hawaii ( ; haw, Hawaii or ) is a U.S. state in the Western United States, in the Pacific Ocean about 2,000 miles (3,200 km) from the U.S. mainland. It is the only state outside North America, ...
'' types, rather than block lavas. Underwater, they can form pillow lavas
A pillow is a support of the body at rest for comfort, therapy, or decoration. Pillows are used by many species, including humans. Some types of pillows include throw pillows, body pillows, decorative pillows and many more. Pillows that aid sl ...
, which are rather similar to entrail-type pahoehoe lavas on land.
Ultramafic magmas
''Ultramafic'' magmas, such as picritic basalt,komatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igne ...
, and highly magnesian magmas that form boninite
Boninite is a mafic extrusive rock high in both magnesium
Magnesium is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a ...
, take the composition and temperatures to the extreme. All have a silica content under 45%. Komatiites contain over 18% magnesium oxide, and are thought to have erupted at temperatures of . At this temperature there is practically no polymerization of the mineral compounds, creating a highly mobile liquid. Viscosities of komatiite magmas are thought to have been as low as 100 to 1000 cP, similar to that of light motor oil. Most ultramafic lavas are no younger than the Proterozoic
The Proterozoic () is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two form ...
, with a few ultramafic magmas known from the Phanerozoic
The Phanerozoic Eon is the current geologic eon in the geologic time scale
The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontolo ...
in Central America that are attributed to a hot mantle plume
A mantle plume is a proposed mechanism of convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid
In physics, a fl ...
. No modern komatiite lavas are known, as the Earth's mantle has cooled too much to produce highly magnesian magmas.
Alkaline magmas
Some silicic magmas have an elevated content ofalkali metal oxide
The alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origi ...
s (sodium and potassium), particularly in regions of continental rifting
200px, Gulf of Suez Rift showing main extensional faults">extensional_fault.html" ;"title="Gulf of Suez Rift showing main extensional fault">Gulf of Suez Rift showing main extensional faults
In geology, a rift is a linear zone where the lithos ...
, areas overlying deeply subducted
Subduction is a geological process in which the oceanic lithosphere
A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On ...
plates, or at intraplate hotspots. Their silica content can range from ultramafic (nephelinite of peridotite (yellow), Kaiserstuhl, Germany.
Nephelinite is a fine-grained or aphanitic
is aphanitic.
Image:LvMS-Lvv.jpg, An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope
Aphanite, or aph ...
s, basanite
Basanite () is an igneous, volcanic ( extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structur ...
s and tephrite
Leucite tephrite from Mayen, Eifel, Germany
Tephrite is an igneous rock, igneous, volcanic (extrusive) rock (geology), rock, with aphanitic to porphyritic Texture (crystalline), texture. Mineral content is usually abundant feldspathoids (leucite ...
s) to felsic (trachyte
Trachyte is an extrusive igneous rock
Igneous rock (derived from the Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area ar ...
s). They are more likely to be generated at greater depths in the mantle than subalkaline magmas. Olivine nephelinite of peridotite (yellow), Kaiserstuhl, Germany.
Nephelinite is a fine-grained or aphanitic
is aphanitic.
Image:LvMS-Lvv.jpg, An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope
Aphanite, or aph ...
magmas are both ultramafic and highly alkaline, and are thought to have come from much deeper in the mantle of the Earth
Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...
than other magmas.
Nonsilicic magmas
Some lavas of unusual composition have erupted onto the surface of the Earth. These include: *Carbonatite
Carbonatite lava at Ol Doinyo Lengai volcano, Tanzania
Carbonatite () is a type of Intrusive rock, intrusive or extrusive igneous Rock (geology), rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbon ...
and natrocarbonatite lavas are known from Ol Doinyo Lengai
Ol Doinyo Lengai (Oldoinyo Lengai), "Mountain of God" in the Maasai language, is an active volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and Volcanic gas, gases ...
volcano in Tanzania
Tanzania (;This approximates the Kiswahili pronunciation. However, is also heard in English. ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes regi ...
, which is the sole example of an active carbonatite volcano. Carbonatites in the geologic record are typically 75% carbonate minerals, with lesser amounts of silica-undersaturated silicate minerals (such as mica
Micas ( ) are a group of minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect Cleavage (crystal), basal cleavage ...
s and olivine), apatite
Apatite is a group of phosphate minerals, usually referring to hydroxyapatite
Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaki ...
, magnetite
Magnetite is a mineral and one of the main iron ore
Iron ores are rocks and minerals from which metal
A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, po ...
, and pyrochlore
Pyrochlore (sodium, Na,calcium, Ca)2niobium, Nb2oxygen, O6(hydroxide, OH,fluorine, F) is a mineral group of the niobium end member of the pyrochlore supergroup.
The general formula, A2B2O7 (where A and B are metals), represent a family of phases i ...
. This may not reflect the original composition of the lava, which may have included sodium carbonate
Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, water-soluble salts that yield moderately alkaline solutions in water. ...
that was subsequently removed by hydrothermal activity, though laboratory experiments show that a calcite-rich magma is possible. Carbonatite lavas show stable isotope ratio
The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
s indicating they are derived from the highly alkaline silicic lavas with which they are always associated, probably by separation of an immiscible phase. Natrocarbonatite lavas of Ol Doinyo Lengai are composed mostly of sodium carbonate, with about half as much calcium carbonate and half again as much potassium carbonate, and minor amounts of halides, fluorides, and sulphates. The lavas are extremely fluid, with viscosities only slightly greater than water, and are very cool, with measured temperatures of .
* Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen
Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, ...
magmas are thought to be the source of the iron ore
Iron ores are rocks
A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in which it is formed. Rock ...
at Kiruna
(; se, Giron; fi, Kiiruna) is the northernmost town
A town is a human settlement. Towns are generally larger than villages and smaller than city, cities, though the criteria to distinguish between them vary considerably in differen ...
, Sweden
Sweden ( sv, Sverige ), officially the Kingdom of Sweden ( sv, links=no, Konungariket Sverige ), is a Nordic countries, Nordic country in Northern Europe.The United Nations Group of Experts on Geographical Names states that the country's for ...
which formed during the Proterozoic
The Proterozoic () is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two form ...
. Iron oxide lavas of Pliocene
The Pliocene ( ; also Pleiocene) epoch (geology), Epoch is the epoch in the geologic timescale that extends from 5.333 million to 2.58El Laco volcanic complex on the Chile-Argentina border. Iron oxide lavas are thought to be the result of
File:Single tet.png, A single silica tetrahedron
File:Double tet.png, Two silica tetrahedra joined by a bridging oxygen ion (tinted pink)
The tendency towards polymerization is expressed as NBO/T, where NBO is the number of non-bridging oxygen ions and T is the number of network-forming ions. Silicon is the main network-forming ion, but in magmas high in sodium, aluminium also acts as a network former, and ferric iron can act as a network former when other network formers are lacking. Most other metallic ions reduce the tendency to polymerize and are described as network modifiers. In a hypothetical magma formed entirely from melted silica, NBO/T would be 0, while in a hypothetical magma so low in network formers that no polymerization takes place, NBO/T would be 4. Neither extreme is common in nature, but basalt magmas typically have NBO/T between 0.6 and 0.9, andesitic magmas have NBO/T of 0.3 to 0.5, and rhyolitic magmas have NBO/T of 0.02 to 0.2. Water acts as a network modifier, and dissolved water drastically reduces melt viscosity. Carbon dioxide neutralizes network modifiers, so dissolved carbon dioxide increases the viscosity. Higher-temperature melts are less viscous, since more thermal energy is available to break bonds between oxygen and network formers.
Most magmas contain solid crystals of various minerals, fragments of exotic rocks known as xenoliths and fragments of previously solidified magma. The crystal content of most magmas gives them thixotropy, thixotropic and shear thinning properties. In other words, most magmas do not behave like Newtonian fluids, in which the rate of flow is proportional to the shear stress. Instead, a typical magma is a Bingham fluid, which shows considerable resistance to flow until a stress threshold, called the yield stress, is crossed. This results in plug flow of partially crystalline magma. A familiar example of plug flow is toothpaste squeezed out of a toothpaste tube. The toothpaste comes out as a semisolid plug, because shear is concentrated in a thin layer in the toothpaste next to the tube, and only here does the toothpaste behave as a fluid. Thixotropic behavior also hinders crystals from settling out of the magma. Once the crystal content reaches about 60%, the magma ceases to behave like a fluid and begins to behave like a solid. Such a mixture of crystals with melted rock is sometimes described as ''crystal mush''.
Magma is typically also Viscoelasticity, viscoelastic, meaning it flows like a liquid under low stresses, but once the applied stress exceeds a critical value, the melt cannot dissipate the stress fast enough through relaxation alone, resulting in transient fracture propagation. Once stresses are reduced below the critical threshold, the melt viscously relaxes once more and heals the fracture.
2005-02-11 Magma expands slightly at lower pressure or higher temperature. When magma approaches the surface, its dissolved gases begin to bubble out of the liquid. These bubbles had significantly reduced the density of the magma at depth and helped drive it toward the surface in the first place.
When rocks melt, they do so over a range of temperature, because most rocks are made of several minerals, which all have different melting points. The temperature at which the first melt appears (the solidus) is lower than the melting temperature of any one of the pure minerals. This is similar to the lowering of the melting point of ice when it is mixed with salt. The first melt is called the ''eutectic'' and has a composition that depends on the combination of minerals present.
For example, a mixture of anorthite and diopside, which are two of the predominant minerals in
Most magmas are fully melted only for small parts of their histories. More typically, they are mixes of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have different densities, and so they can separate as magmas evolve.
As magma cools, minerals typically crystallize from the melt at different temperatures. This resembles the original melting process in reverse. However, because the melt has usually separated from its original source rock and moved to a shallower depth, the reverse process of crystallization is not precisely identical. For example, if a melt was 50% each of diopside and anorthite, then anorthite would begin crystallizing from the melt at a temperature somewhat higher than the eutectic temperature of 1274 °C. This shifts the remaining melt towards its eutectic composition of 43% diopside. The eutectic is reached at 1274 °C, the temperature at which diopside and anorthite begin crystallizing together. If the melt was 90% diopside, the diopside would begin crystallizing first until the eutectic was reached.
If the crystals remained suspended in the melt, the crystallization process would not change the overall composition of the melt plus solid minerals. This situation is described as ''equillibrium crystallization''. However, in a series of experiments culminating in his 1915 paper, ''Crystallization-differentiation in silicate liquids'', Norman L. Bowen demonstrated that crystals of olivine and diopside that crystallized out of a cooling melt of forsterite, diopside, and silica would sink through the melt on geologically relevant time scales. Geologists subsequently found considerable field evidence of such '' fractional crystallization''.
When crystals separate from a magma, then the residual magma will differ in composition from the parent magma. For instance, a magma of gabbroic composition can produce a residual melt of granite, granitic composition if early formed crystals are separated from the magma. Gabbro may have a liquidus temperature near 1,200 °C, and the derivative granite-composition melt may have a liquidus temperature as low as about 700 °C. Incompatible elements are concentrated in the last residues of magma during fractional crystallization and in the first melts produced during partial melting: either process can form the magma that crystallizes to pegmatite, a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma.
Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Assimilation near the roof of a magma chamber and fractional crystallization near its base can even take place simultaneously. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions.
immiscible
Miscibility () is the property of two Chemical substance, substances to mix in all Mixing ratio, proportions (that is, to fully dissolution (chemistry), dissolve in each other at any concentration), forming a Homogeneity and heterogeneity, homogen ...
separation of iron oxide magma from a parental magma of calc-alkaline
The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic
...
or alkaline composition.
* Sulfur
Sulfur (in nontechnical British English: sulphur) is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a pure substance consis ...
lava flows up to long and wide occur at volcano, Chile. They were formed by the melting of sulfur deposits at temperatures as low as .
Magmatic gases
The concentrations of differentgas
Gas is one of the four fundamental states of matter (the others being solid, liquid
A liquid is a nearly incompressible fluid
In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an appl ...
es can vary considerably. Water vapor
(99.9839 °C)
, -
, Boiling point
The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
The boiling point of a liquid v ...
is typically the most abundant magmatic gas, followed by carbon dioxide
Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...
and sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a Toxicity, toxic gas responsible for the smell of burnt matches. It is released naturally by vol ...
. Other principal magmatic gases include hydrogen sulfide
Hydrogen sulfide is a chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by ...
, hydrogen chloride
The compound
Compound may refer to:
Architecture and built environments
* Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall
** Compound (fortification), a version of the above fortified with d ...
, and hydrogen fluoride
Hydrogen fluoride is a chemical compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock in the preparation ...
.
The solubility of magmatic gases in magma depends on pressure, magma composition, and temperature. Magma that is extruded as lava is extremely dry, but magma at depth and under great pressure can contain a dissolved water content in excess of 10%. Water is somewhat less soluble in low-silica magma than high-silica magma, so that at 1,100 °C and 0.5 GPa
Grading in education is the attempt to apply standardized measurements of varying levels of achievement in a course.
Grades can be assigned as letters (for example, A through F), as a range (for example, 1 to 6), as a percentage, or as a numbe ...
, a basaltic magma can dissolve 8% while a granite pegmatite magma can dissolve 11% . However, magmas are not necessarily saturated under typical conditions.
Carbon dioxide is much less soluble in magmas than water, and frequently separates into a distinct fluid phase even at great depth. This explains the presence of carbon dioxide fluid inclusions in crystals formed in magmas at great depth.
Rheology
Viscosity
The viscosity of a fluid is a measure of its drag (physics), 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.
Viscosity can be ...
is a key melt property in understanding the behaviour of magmas. Whereas temperatures in common silicate lavas range from about for felsic lavas to for mafic lavas, the viscosity of the same lavas ranges over seven orders of magnitude, from 104 cP for mafic lava to 1011 cP for felsic magmas. The viscosity is mostly determined by composition but is also dependent on temperature. The tendency of felsic lava to be cooler than mafic lava increases the viscosity difference.
The silicon ion is small and highly charged, and so it has a strong tendency to coordinate
In geometry
Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space t ...
with four oxygen ions, which form a tetrahedral arrangement around the much smaller silicon ion. This is called a ''silica tetrahedron''. In a magma that is low in silicon, these silica tetrahedra are isolated, but as the silicon content increases, silica tetrahedra begin to partially polymerize, forming chains, sheets, and clumps of silica tetrahedra linked by bridging oxygen ions. These greatly increase the viscosity of the magma.
Temperature
Temperatures of lava, which is magma extruded onto the surface, are in the range , but very rare carbonatite magmas may be as cool as , andkomatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igne ...
magmas may have been as hot as . Magma has occasionally been encountered during drilling in geothermal fields, including drilling in Hawaii that penetrated a dacitic magma body at a depth of . The temperature of this magma was estimated at . Temperatures of deeper magmas must be inferred from theoretical computations and the geothermal gradient.
Most magmas contain some solid crystals suspended in the liquid phase. This indicates that the temperature of the magma lies between the solidus (chemistry), solidus, which is defined as the temperature at which the magma completely solidifies, and the liquidus, defined as the temperature at which the magma is completely liquid. Calculations of solidus temperatures at likely depths suggests that magma generated beneath areas of rifting starts at a temperature of about . Magma generated from mantle plumes may be as hot as . The temperature of magma generated in subduction zones, where water vapor lowers the melting temperature, may be as low as .
Density
Magma densities depend mostly on composition, iron content being the most important parameter.usu.edu - ''Geology'' 326, "Properties of Magmas"2005-02-11 Magma expands slightly at lower pressure or higher temperature. When magma approaches the surface, its dissolved gases begin to bubble out of the liquid. These bubbles had significantly reduced the density of the magma at depth and helped drive it toward the surface in the first place.
Origins
The temperature within the interior of the earth is described by the geothermal gradient, which is the rate of temperature change with depth. The geothermal gradient is established by the balance between heating through radioactive decay in the Earth's interior and heat loss from the surface of the earth. The geothermal gradient averages about 25 °C/km in the Earth's upper crust, but this varies widely by region, from a low of 5–10 °C/km within oceanic trenches and subduction zones to 30–80 °C/km alongmid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics
File:Earth cutaway schematic-en.svg, upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale)
Plate ...
s or near mantle plume
A mantle plume is a proposed mechanism of convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid
In physics, a fl ...
s. The gradient becomes less steep with depth, dropping to just 0.25 to 0.3 °C/km in the mantle, where slow convection efficiently transports heat. The average geothermal gradient is not normally steep enough to bring rocks to their melting point anywhere in the crust or upper mantle, so magma is produced only where the geothermal gradient is unusually steep or the melting point of the rock is unusually low. However, the ascent of magma towards the surface in such settings is the most important process for transporting heat through the crust of the Earth.
Rocks may melt in response to a decrease in pressure, to a change in composition (such as an addition of water), to an increase in temperature, or to a combination of these processes. Other mechanisms, such as melting from a Impact event, meteorite impact, are less important today, but impacts during the accretion (geology), accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our early Earth was probably an ocean of magma. Impacts of large meteorites in the last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces.
Decompression
Decompression melting occurs because of a decrease in pressure. It is the most important mechanism for producing magma from the upper mantle. The solidus (chemistry), solidus temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during the mantle convection, convection of solid mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3 °C per kilometer. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3 °C to 4 °C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from the upward movement of solid mantle is critical in the evolution of the Earth. Decompression melting creates the ocean crust at mid-ocean ridges, making it by far the most important source of magma on Earth. It also causes volcanism in intraplate regions, such as Europe, Africa and the Pacific sea floor. Intraplate volcanism is attributed to the rise ofmantle plume
A mantle plume is a proposed mechanism of convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid
In physics, a fl ...
s or to intraplate extension, with the importance of each mechanism being a topic of continuing research.
Effects of water and carbon dioxide
The change of rock composition most responsible for the creation of magma is the addition of water. Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometers, peridotite begins to melt near 800 °C in the presence of excess water, but near 1,500 °C in the absence of water. Water is driven out of the oceanic lithosphere insubduction zone
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate
This is a list of tectonic plates on Earth's surface. Tec ...
s, and it causes melting in the overlying mantle. Hydrous magmas composed of basalt and andesite are produced directly and indirectly as results of dehydration during the subduction process. Such magmas, and those derived from them, build up island arcs such as those in the Pacific Ring of Fire. These magmas form rocks of the calc-alkaline series, an important part of the continental crust.
The addition of carbon dioxide
Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules consist of a carbon atom covalent bond, covalently double bonded to two oxygen atoms. It occurs naturally in At ...
is relatively a much less important cause of magma formation than the addition of water, but genesis of some normative mineralogy, silica-undersaturated magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experiments document that the peridotite solidus temperature decreases by about 200 °C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. At greater depths, carbon dioxide can have more effect: at depths to about 200 km, the temperatures of initial melting of a carbonated peridotite composition were determined to be 450 °C to 600 °C lower than for the same composition with no carbon dioxide. Magmas of rock types such as nephelinite of peridotite (yellow), Kaiserstuhl, Germany.
Nephelinite is a fine-grained or aphanitic
is aphanitic.
Image:LvMS-Lvv.jpg, An aphanitic volcanic sand grain, with fine-grained groundmass, as seen under a petrographic microscope
Aphanite, or aph ...
, carbonatite, and kimberlite are among those that may be generated following an influx of carbon dioxide into mantle at depths greater than about 70 km.
Temperature increase
Increase in temperature is the most typical mechanism for formation of magma within continental crust. Such temperature increases can occur because of the upward intrusion of magma from the mantle. Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression at a plate boundary. The plate boundary between the Indian and Asian continental masses provides a well-studied example, as the Tibetan Plateau just north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal continental crust. Studies of electrical resistivity deduced from Magnetotellurics, magnetotelluric data have detected a layer that appears to contain silicate melt and that stretches for at least 1,000 kilometers within the middle crust along the southern margin of the Tibetan Plateau. Granite andrhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matrix ...
are types of igneous rock commonly interpreted as products of the melting of continental crust because of increases in temperature. Temperature increases also may contribute to the melting of lithosphere dragged down in a subduction zone.
The melting process
When rocks melt, they do so over a range of temperature, because most rocks are made of several minerals, which all have different melting points. The temperature at which the first melt appears (the solidus) is lower than the melting temperature of any one of the pure minerals. This is similar to the lowering of the melting point of ice when it is mixed with salt. The first melt is called the ''eutectic'' and has a composition that depends on the combination of minerals present.
For example, a mixture of anorthite and diopside, which are two of the predominant minerals in basalt
Basalt (, ) is a fine-grained extrusive
A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope
Extrusi ...
, begins to melt at about 1274 °C. This is well below the melting temperatures of 1392 °C for pure diopside and 1553 °C for pure anorthite. The resulting melt is composed of about 43 wt% anorthite. As additional heat is added to the rock, the temperature remains at 1274 °C until either the anorthite or diopside is fully melted. The temperature then rises as the remaining mineral continues to melt, which shifts the melt composition away from the eutectic. For example, if the content of anorthite is greater than 43%, the entire supply of diopside will melt at 1274 °C., along with enough of the anorthite to keep the melt at the eutectic composition. Further heating causes the temperature to slowly rise as the remaining anorthite gradually melts and the melt becomes increasingly rich in anorthite liquid. If the mixture has only a slight excess of anorthite, this will melt before the temperature rises much above 1274 °C. If the mixture is almost all anorthite, the temperature will reach nearly the melting point of pure anorthite before all the anorthite is melted. If the anorthite content of the mixture is less than 43%, then all the anorthite will melt at the eutectic temperature, along with part of the diopside, and the remaining diopside will then gradually melt as the temperature continues to rise.
Because of eutectic melting, the composition of the melt can be quite different from the source rock. For example, a mixture of 10% anorthite with diopside could experience about 23% partial melting before the melt deviated from the eutectic, which has the composition of about 43% anorthite. This effect of partial melting is reflected in the compositions of different magmas. A low degree of partial melting of the upper mantle (2% to 4%) can produce highly alkaline magmas such as melilitites, while a greater degree of partial melting (8% to 11%) can produce alkali olivine basalt. Oceanic magmas likely result from partial melting of 3% to 15% of the source rock. Some Calc-alkaline magma series, calk-alkaline granitoids may be produced by a high degree of partial melting, as much as 15% to 30%.
High-magnesium magmas, such as komatiite
Komatiite () is a type of ultramafic mantle-derived volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igne ...
and picrite, may also be the products of a high degree of partial melting of mantle rock.
Certain chemical elements, called incompatible elements, have a combination of ionic radius and ionic charge that is unlike that of the more abundant elements in the source rock. The ions of these elements fit rather poorly in the structure of the minerals making up the source rock, and readily leave the solid minerals to become highly concentrated in melts produced by a low degree of partial melting. Incompatible elements commonly include potassium
Potassium is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a pure substance consisting only of atoms that all have the same ...
, barium, caesium, and rubidium, which are large and weakly charged (the large-ion lithophile elements, or LILEs), as well as elements whose ions carry a high charge (the high-field-strength elements, or HSFEs), which include such elements as zirconium, niobium, hafnium, tantalum, the rare-earth elements, and the actinides. Potassium can become so enriched in melt produced by a very low degree of partial melting that, when the magma subsequently cools and solidifies, it forms unusual potassic rock such as lamprophyre, lamproite, or kimberlite.
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. Melt rapidly separates from its source rock once the degree of partial melting exceeds 30%. However, usually much less than 30% of a magma source rock is melted before the heat supply is exhausted.
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 (geology), fractional crystallization.
Evolution of magmas
Most magmas are fully melted only for small parts of their histories. More typically, they are mixes of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have different densities, and so they can separate as magmas evolve.
As magma cools, minerals typically crystallize from the melt at different temperatures. This resembles the original melting process in reverse. However, because the melt has usually separated from its original source rock and moved to a shallower depth, the reverse process of crystallization is not precisely identical. For example, if a melt was 50% each of diopside and anorthite, then anorthite would begin crystallizing from the melt at a temperature somewhat higher than the eutectic temperature of 1274 °C. This shifts the remaining melt towards its eutectic composition of 43% diopside. The eutectic is reached at 1274 °C, the temperature at which diopside and anorthite begin crystallizing together. If the melt was 90% diopside, the diopside would begin crystallizing first until the eutectic was reached.
If the crystals remained suspended in the melt, the crystallization process would not change the overall composition of the melt plus solid minerals. This situation is described as ''equillibrium crystallization''. However, in a series of experiments culminating in his 1915 paper, ''Crystallization-differentiation in silicate liquids'', Norman L. Bowen demonstrated that crystals of olivine and diopside that crystallized out of a cooling melt of forsterite, diopside, and silica would sink through the melt on geologically relevant time scales. Geologists subsequently found considerable field evidence of such '' fractional crystallization''.
When crystals separate from a magma, then the residual magma will differ in composition from the parent magma. For instance, a magma of gabbroic composition can produce a residual melt of granite, granitic composition if early formed crystals are separated from the magma. Gabbro may have a liquidus temperature near 1,200 °C, and the derivative granite-composition melt may have a liquidus temperature as low as about 700 °C. Incompatible elements are concentrated in the last residues of magma during fractional crystallization and in the first melts produced during partial melting: either process can form the magma that crystallizes to pegmatite, a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma.
Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Assimilation near the roof of a magma chamber and fractional crystallization near its base can even take place simultaneously. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions.
Primary magmas
When rock melts, the liquid is a ''primary magma''. Primary magmas have not undergone any differentiation and represent the starting composition of a magma. In practice, it is difficult to unambiguously identify primary magmas, though it has been suggested thatboninite
Boninite is a mafic extrusive rock high in both magnesium
Magnesium is a chemical element
Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements
In chemistry, an element is a ...
is a variety of andesite crystallized from a primary magma. The Great Dyke of Zimbabwe has also been interpreted as rock crystallized from a primary magma. The interpretation of leucosomes of migmatites as primary magmas is contradicted by zircon data, which suggests leucosomes are a residue (a cumulate rock) left by extraction of a primary magma.
Parental magma
When it is impossible to find the primitive or primary magma composition, it is often useful to attempt to identify a parental magma. A parental magma 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 magma''. Fractional crystallization models would be produced to test the hypothesis that they share a common parental magma.Migration and solidification
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, due to its lower density than the source rock. As it migrates through the crust, magma may collect and reside inmagma chamber
A magma chamber is a large pool of liquid rock
Rock most often refers to:
* Rock (geology)
A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical ...
s (though recent work suggests that magma may be stored in trans-crustal crystal-rich mush zones rather than dominantly liquid magma chambers ). Magma can remain in a chamber until it either cools and crystallizes to form intrusive rock, it erupts as a volcano
A volcano is a rupture in the crust of a planetary-mass object
A planet is an astronomical body orbit
In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ar ...
, or it moves into another magma chamber.
Plutonism
When magma cools it begins to form solid mineral phases. Some of these settle at the bottom of the magma chamber forming Cumulate rock, 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 asbasalt
Basalt (, ) is a fine-grained extrusive
A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope
Extrusi ...
, andesite and rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matrix ...
(the extrusive equivalents of gabbro, diorite and granite, respectively).
Volcanism
Magma that is extruded onto the surface during a volcanic eruption is calledlava
of pāhoehoe lava, Hawaii, United States
, Iceland in 1984
Lava is molten Rock (geology), rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon. Magma is generated by the inte ...
. 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
Obsidian (; ) is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
Obsidian is produced from felsic lava, rich in the lighter elements ...
, scoria and pumice.
Before and during volcanic eruptions, Volatility (chemistry), volatiles such as CO2 and H2O partially leave the melt through a process known as exsolution. Magma with low water content becomes increasingly Viscosity, viscous. If massive exsolution occurs when magma heads upwards during a volcanic eruption, the resulting eruption is usually explosive.
Use in energy production
The Iceland Deep Drilling Project, while drilling several 5,000 m holes in an attempt to harness the heat in the volcanic bedrock below the surface of Iceland, struck a pocket of magma at 2,100 m 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 36 MW of power, making IDDP-1 the world's first magma-enhanced geothermal system.References
{{Authority control Igneous petrology Igneous rocks Volcanism Earth's crust