Magma 2
Levi Satterley
Magma (from Ancient Greek μάγμα (mgma) 'thick unguent')[1] is the molten or semi-molten natural material from which all igneous rocks are formed.[2] Magma (sometimes colloquially but incorrectly referred to as lava by laypeople) 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 or the crust in various tectonic settings, which on Earth include 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 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 and be extruded as lava, or it may solidify underground to form an intrusion,[8] such as a dike, a sill, a laccolith, a pluton, or a batholith.[9]
While the study of magma has relied on observing magma after its transition into a lava flow, magma has been encountered in situ three times during geothermal drilling projects, twice in Iceland (see Use in energy production) and once in Hawaii.[10][11][12][13]
Magma consists of liquid rock that usually contains suspended solid crystals.[14] As magma approaches the surface and the overburden pressure drops, dissolved gases bubble out of the liquid, so that magma near the surface consists of materials in solid, liquid, and gas phases.[15]
Most magma is rich in silica.[8] Rare nonsilicate magma can form by local melting of nonsilicate mineral deposits[16] or by separation of a magma into separate immiscible silicate and nonsilicate liquid phases.[17]
Silicate magmas are molten mixtures dominated by oxygen and silicon, the most abundant chemical elements in the Earth's crust, with smaller quantities of aluminium, calcium, magnesium, iron, sodium, and potassium, and minor amounts of many other elements.[18] Petrologists routinely express the composition of a silicate magma in terms of the weight or molar mass fraction of the oxides of the major elements (other than oxygen) present in the magma.[19]
Felsic lavas can erupt at temperatures as low as 800 C (1,470 F).[24] 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 of the northwestern United States.[25]
Some silicic magmas have an elevated content of alkali metal oxides (sodium and potassium), particularly in regions of continental rifting, areas overlying deeply subducted plates, or at intraplate hotspots.[35] Their silica content can range from ultramafic (nephelinites, basanites and tephrites) to felsic (trachytes). They are more likely to be generated at greater depths in the mantle than subalkaline magmas.[36] Olivine nephelinite magmas are both ultramafic and highly alkaline, and are thought to have come from much deeper in the mantle of the Earth than other magmas.[37]
The concentrations of different gases can vary considerably. Water vapor is typically the most abundant magmatic gas, followed by carbon dioxide[43] and sulfur dioxide. Other principal magmatic gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride.[44]
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.[46]
The silicon ion is small and highly charged, and so it has a strong tendency to coordinate 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.[47]
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.[15]
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 thixotropic and shear thinning properties.[48] 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.[49] 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.[50] 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.[51]
Magma is typically also 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.[52]
Temperatures of molten lava, which is magma extruded onto the surface, are almost all in the range 700 to 1,400 C (1,300 to 2,600 F), but very rare carbonatite magmas may be as cool as 490 C (910 F),[53] and komatiite magmas may have been as hot as 1,600 C (2,900 F).[54] Magma has occasionally been encountered during drilling in geothermal fields, including drilling in Hawaii that penetrated a dacitic magma body at a depth of 2,488 m (8,163 ft). The temperature of this magma was estimated at 1,050 C (1,920 F). Temperatures of deeper magmas must be inferred from theoretical computations and the geothermal gradient.[13]
Most magmas contain some solid crystals suspended in the liquid phase. This indicates that the temperature of the magma lies between the 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.[14] Calculations of solidus temperatures at likely depths suggests that magma generated beneath areas of rifting starts at a temperature of about 1,300 to 1,500 C (2,400 to 2,700 F). Magma generated from mantle plumes may be as hot as 1,600 C (2,900 F). The temperature of magma generated in subduction zones, where water vapor lowers the melting temperature, may be as low as 1,060 C (1,940 F).[55]
Magma expands slightly at lower pressure or higher temperature.[56] 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.[57]
Rocks may melt in response to a decrease in pressure,[60] to a change in composition (such as an addition of water),[61] to an increase in temperature,[62] or to a combination of these processes.[63] Other mechanisms, such as melting from a meteorite impact, are less important today, but impacts during the accretion of the Earth led to extensive melting, and the outer several hundred kilometers of the early Earth was probably a magma ocean.[64] 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.[65]
The 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 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.[63]
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