—magmatic /mag mat"ik/, adj. —magmatism, n./mag"meuh/, n., pl. magmas, magmata /-meuh teuh/.1. Geol. molten material beneath or within the earth's crust, from which igneous rock is formed.2. any crude mixture of finely divided mineral or organic matter.3. Chem., Pharm. a paste composed of solid and liquid matter.[1400-50; late ME < L: dregs, leavings < Gk mágma kneaded mass, salve, equiv. to mag- (base of mássein to knead, press; see MASS) + -ma n. suffix of result]
* * *Molten or partially molten rock from which igneous rocks form, usually consisting of silicate liquid.Magma migrates either at depth or to the Earth's surface, where it is ejected as lava. The interactions of several physical properties, including chemical composition, viscosity, content of dissolved gases, and temperature, determine the characteristics of magma. Numerous events that can occur during crystallization influence the resulting rock: separation of early crystals from liquid prevents reaction between them; magma can cool too rapidly for reaction to occur; and loss of volatiles may remove some components from the magma.
* * *▪ rockmolten or partially molten rock from which igneous rocks form. It usually consists of silicate liquid, although carbonate and sulfide melts occur as well. Magma migrates either at depth or to the Earth's surface and is ejected as lava. Suspended crystals and fragments of unmelted rock may be transported in the magma; dissolved volatiles may separate as bubbles and some liquid may crystallize during movement. Several interrelated physical properties determine the characteristics of magma, including chemical composition, viscosity, dissolved gases, and temperature. As magma cools, crystals form in a systematic manner, which is most simply expressed in the form of Bowen's reaction series; early high-temperature crystals will tend to react with the liquid to form other minerals at lower temperatures. Two series are recognized: (1) a discontinuous reaction series, which from high to low temperatures is composed of olivine, orthopyroxene, clinopyroxene, amphibole, and biotite; and (2) a continuous reaction series, represented by high-temperature calcium-rich plagioclase to low-temperature sodium-rich plagioclase. Numerous variations can occur during crystallization to influence the resulting rock. Such variations include separation of early crystals from liquid, preventing a reaction; cooling of magma too rapidly for reactions to occur; and loss of volatiles, which may remove some components from the magma. Transport and emplacement of magma is strongly affected by its viscosity and by the fracture characteristics of rocks through which it moves. Viscosity is reduced by water and a lower silica content.
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