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Composition
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Magma, then, is most likely neither uniform nor homogenous. Parts may be largely crystalline, while other parts are largely liquid. The chemical composition in different parts of a melt may also vary.
Glassy volcanic rocks originate from magma that was probably a mixture of liquid and gas alone (see VOLCANO). Volcanologists analyzing gases escaping from volcanic vents can infer what gases might have escaped from magmas but not their amounts. Volcanic rocks with various proportions of crystals and glass suggest magmas containing many solids. The various lava types indicate a range of fluid to viscous magmas. Liquid and gas may contribute as little as 5 percent to the volume of some magmas.
Chemically most magmas are rich in silicon dioxide and aluminum oxide and also contain iron, magnesium, calcium, sodium, potassium, titanium, manganese, phosphorus, and water. Carbonate magmas (carbonatites) and other unusual compositions also exist. Molten material in the Earth\'s core may be largely nickel and iron.
William D. Romey
Overview
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Magma is a complex, mobile mixture of crystals, rock fragments, liquid, and gases deep within the Earth. The source of IGNEOUS ROCKS, it can intrude into adjacent rocks or extrude at the surface. Never directly observed, magmas are inferred from volcanic material, such as LAVA and natural volcanic glass, and from solid rocks that presumably crystallized from a melt at depth. Through experimental studies with artificial melts geologists seek to determine the physical conditions and appearance, chemical compositions, paths of evolution, loci of origin, and causes of formation of magmas.
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