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magma(măg`mə): see lavalava
, molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure. It solidifies into igneous rock that is also called lava.
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a molten mass, primarily silicate in composition, which forms in zones deep within the earth. Magma is usually a complex mutual solution of compounds of many chemical elements, predominant among which are oxygen, Si, Al, Fe, Mg, Ca, Na, and K. Sometimes magma contains up to several percent dissolved volatile components, chiefly water, and to a lesser extent, oxides of carbon, hydrogen sulfide, hydrogen, fluorine, and chlorine. When the magma crystallizes at depth the volatiles partially become part of various minerals (amphiboles and micas, for example). In rare cases magmatic melts of nonsilicate composition are observed; examples include alkali-carbonate (the volcanoes of East Africa) or sulfide magma.
In volcanic areas magma that reaches the earth’s surface issues forth in the form of lava, forms extrusive bodies in volcanic vents, or is erupted with gases as fragmented material. The fragmented material, mixed with fragments of country rock and sedimentary material, is deposited in the form of various tuffs.
Magmatic masses that solidify at depth form intrusive bodies of various shape and size; they range from small masses that fill fissures to enormous masses with areas up to many thousands of square kilometers in horizontal section. When magma is intruded into the earth’s crust or when it is extruded onto the earth’s surface, igneous rocks form. These rocks are what gives us an idea of the composition of magma.
Types. After studying the distribution of various igneous rocks on the earth’s surface and demonstrating that basalts and granites predominate, the Soviet geologist F. Iu. Levinson-Lessing hypothesized that all known igneous rocks formed from two types of parent magmas: basic (basaltic), which is rich in Mg, Fe, and Ca with an SiO2 content of 40-55 percent of the mass, and acid (granitic), which is rich in alkali metals and contains 65-78 percent Si(>2. The English geologist A. Holmes advanced the hypothesis that in addition to the basic and acid magmas there is ultrabasic (peridotitic) magma, which is erupted directly from subcrustal chambers, contains less than 40 percent SiO2, and is rich in Mg and Fe. Then, in the late 1920’s when it was established that volcanoes extrude primarily basic magma (lava) and acid rocks are encountered only as intrusive formations, the American petrologist N. Bowen proposed the hypothesis that there is just one parent magma, basaltic, and suggested that granites form as the result of crystallization differentiation of basaltic magma during its solidification process. In the late 1950’s N. Bowen proved the possibility of the existence of granitic magma under conditions of high pressures, the presence of water (2-4 percent), and a temperature of about 600°C.
It was initially supposed that magma forms continuous shells in the earth’s interior. Then geophysical research proved that there are no such permanent layers of molten magma, but that magma forms periodically within separate chambers of the earth that differ in composition and depth.
In the early 1970’s, on the basis of results from numerous experiments, it was assumed that granitic magma forms in the earth’s crust and upper mantle, while basic magma probably forms in the region of the asthenosphere as a result of the segregation of relatively easily fusible material. In addition to granitic and basaltic magma the existence of rarer, local magmas is postulated, but their nature is as yet unclear. It is thought that a local rise in temperature (heating up of the earth’s interior) promotes the appearance of magma; the introduction of fluxes (water, alkalies, and the like) and a drop in pressure are also postulated.
Intensive experimental research is under way in the USSR, the United States, Japan, and Australia to study the formation conditions of melts that resemble magma. Data from geophysical investigations on the condition of the crust and upper mantle (in particular, data on temperatures in the earth’s interior) are very important for clarifying the nature of magma.
Igneous rocks of approximately the same age and chemical composition that formed from a common initial magmatic melt (called comagmatic rocks) are often found in zones measuring thousands of kilometers. Moreover, the igneous rocks of each such zone (or province) are distinguished by a higher or lower content of some oxide (for example, Na or K) and characteristic metallogenesis. On this basis it was hypothesized that enormous magma reservoirs existed for entire geological epochs lasting tens of millions of years. According to other theories, the reason for this homogeneity is the closeness in the compositions of the initial rocks and in the temperatures and pressures under which the magma formed.
Magmas of differing compositions have different physical properties that also depend on temperature and volatile content. Basaltic magma is distinguished by lower viscosity and the lava flows formed by it are very mobile. The speed at which such flows move may sometimes be as much as 30 km/hr. Acid (granitic) magma is usually more viscous, especially after losing volatile components. This magma forms extrusive domes in volcanic vents; more rarely it forms flows. Acid magma, which is rich in volatiles, is usually erupted and forms thick layers of ignimbrite. Under intrusive conditions where volatile components are still present, acid magma is more mobile and may form thin dikes. The temperature of magma varies widely. Measurements of the temperature of lava in present-day volcanoes have shown that it ranges from 900° to 1200°C. According to experimental figures, granitic (eutectic) magma remains liquid to about 600°C.
Evolution. When it meets with conditions that differ from those under which it formed, magma can evolve, changing its composition. Differentiation of magma occurs, during which several different magmas are derived from a single parent magma. Differentiation of magma may occur before crystallization (magmatic differentiation) or in the process of crystallization (crystallization differentiation). Magmatic differentiation may be the result of liquation of the magma—that is, its separation into two immiscible liquids—or it may result from a difference in temperatures or some other physical parameter in the magma chamber.
Crystallization differentiation is related to the fact that the minerals which separate in the initial stages of the solidification of magma have a different specific gravity than does the melt. This causes some of them to rise (for example, the plagioclase crystals in the diabases of the Kola Peninsula), while others sink (for example, olivine and augite in the basalts of Nova Scotia). As a result, rocks of different composition form in the vertical section of a magmatic rock body. It is possible for the composition of the magma to change when residual liquid is expelled from the separated crystals and as the result of interaction between the magma and the enclosing rock.
At first it was thought that magmatic differentiation and interaction with enclosing rock (assimilation, contamination) cause the variety of magmas. Today these processes are more often used to explain structural details of particular masses of igneous rock, the banded structure of intrusive bodies, differences in the composition of lavas issuing from a volcano at the same time at different hypsometric levels, and change in the composition of lavas issuing from a volcano.
The order of separation of minerals during the crystallization of magma is very important for determining the course of the evolution of magma. The German petrologist K. H. Rosenbusch and the American petrologist N. Bowen worked out a sequence according to which the first to separate during the crystallization of magma are always rare (accessory) minerals, followed by ferromagnesian silicates and basic plagioclases, then amphibole and intermediate plagioclases, and finally biotite, alkali feldspars, and quartz. In basic magma the same rule determines the precipitation of olivine first, then pyroxenes, and amphiboles and micas last. However, there is no universal sequence for the crystallization of magma. This conforms to ideas of magma as a complex solution in which the precipitation of solid phases is determined by the law of mass action and the solubility of the components. In magmas that are rich in aluminosilicate and alkali components, therefore, feldspars separate out earlier than dark-colored minerals (in granites). In rocks that are strongly supersaturated with silica, quartz (quartz porphyries) is often the first to separate. Even in magmas of homogeneous composition the order of crystallization changes depending on the content of volatile substances.
Associated minerals. Magma is a carrier of many useful mineral components, which are concentrated in particular sectors during magmatic crystallization, creating endogenous deposits. Some ore minerals (minerals of Cr, Ti, Ni, Pt) and apatite become separated during the process of the crystallization of magma and form magmatic deposits in stratiform complexes. It is believed that hydrothermal deposits, greisen, skarn, and other deposits of nonferrous, rare, and precious metals as well as some iron deposits are formed in the last stages of the formation of intrusives (the postmagmatic stage) by volatile components contained in the magma.
There are also links between the main concentrations of ores of rare alkalimetals, boron, beryllium, rare earths, tungsten, and other rare elements, on the one hand, and the derivatives of granitic magma on the other. Other associations include ores of chalcophile elements with basaltic magma and deposits of chromium and diamonds with ultrabasic magma.
REFERENCESZavaritskii, A. N. Izverzhennye gornye porody. Moscow, 1955.
Levinson-Lessing, F. Iu. Petrografiia, 5th ed. Moscow-Leningrad, 1940.
Rittmann, A. Vulkany i ikh deiatel’nost’. Moscow, 1964. (Translated from German.)
Joder, H. S., and C. E. Tilley. Proiskhozhdenie bazaVtovykh magm. Moscow, 1965. (Translated from English.)
Mehnert, K. Migmatity i proiskhozhdenie granitov [part 1]. Moscow, 1971. (Translated from English.)
Bayly, B. Vvedenie v petrologiiu. Moscow, 1972. (Translated from English.)
F. K. SHIPULIN
Magma(symbolic mathematics, tool)
[W. Bosma, J. Cannon and C. Playoust, The Magma algebra system I: The user language, J. Symb. Comp., 24, 3/4, 1997, 235-265].