The elements of the crust are combined in inorganic chemical compounds called minerals . These minerals are mixed together in various proportions to form many kinds of rock . Igneous rocks form when molten material from the earth's interior cools and solidifies in the crust. Magma cooled slowly below the surface forms coarse-textured intrusive igneous rocks. Lava cooled rapidly at the surface forms fine-textured extrusive igneous rocks.
Igneous rocks consist mainly of silicate minerals containing silicon, oxygen and metallic elements.
The kind of metallic elements present determines the mineral density. Less dense felsic minerals dominate the igneous rocks of the upper crust while more dense mafic and ultramafic minerals dominate those of the lower crust. Silicate minerals undergo chemical changes called mineral alteration when exposed to air and water at the earth's surface. Most clay minerals are produced by mineral alteration. Weathering , or the breakdown of rocks into smaller particles known collectively as sediment , occurs through both mineral alteration and physical disintegration.