These are planar or linear features that result from flowage of magma with or without contained crystals.Various forms of faintly to sharply defined layering and lining typically reflect compositional or textural inhomogeneities, and they often are accentuated by concentrations or preferred orientation of crystals, inclusions, vesicles, spherulites, and other features.As the magma cools and begins to crystallize, silica is taken from the magma to be combined with the other cationic oxides to form the silicate minerals.For example, one mole of Si O), along with the pyroxene, since the olivine requires only one-half as much silica for every mole of magnesium oxide.Their emanations have provided the water for the oceans, the gases for the primordial oxygen-free atmosphere, and many valuable mineral deposits.Clarke estimated that 95 percent of crustal rocks are of igneous origin (formed from molten silicate masses, or magmas).The major ) are depleted in the rocks that have a silica content of less than about 45 weight percent, but, above 45 percent, calcium oxide can be as high as 10 percent; this amount decreases gradually as the silica increases.
(Denudation is the wearing away of the terrestrial surface by processes including weathering and erosion.) Generally, the intrusive rocks have cross-cutting contacts with the country rocks that they have invaded, and in many cases the country rocks show evidence of having been baked and thermally metamorphosed at these contacts.
Sedimentary rocks occur as a thin veneer on an igneous or metamorphic basement, except where locally thickened in mountain belts. ) was erupted from Ol Doinyo Lengai, a volcano in northern Tanzania, Africa.
Because of the limited occurrence of such carbonate-rich igneous rocks, however, the following discussion will consider the chemistry of silicate rocks only.
Igneous rocks constitute one of the three principal classes of rocks, the others being metamorphic and sedimentary.
Igneous rocks are formed from the solidification of Earth is composed predominantly of a large mass of igneous rock with a very thin veneer of weathered material—namely, sedimentary rock.
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