Strahler Companion Ch. This is the first chapter of Part II which deals with the systems and cycles of the solid earth. These are linked to the cycle of rock change which describes how different rock types develop as earth materials are cycled and recycled through geologic time. 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.