The elements silicon and oxygen make up 75 percent of all the elements in the outer layer, or crust, of the earth. The elements combine to make crystalline chemical compounds called minerals. A mineral is defined as a naturally occurring, inorganic solid element or compound with a crystalline structure.

About 92 percent of the minerals of the earth's crust are composed of silicon and oxygen, the silicate minerals. The basic unit of the silicates is a tetrahedral structure that combines with positive metallic ions or with other tetrahedral units to form chains, sheets, or an interlocking framework. The ferromagnesian silicates are tetrahedral structures combined with ions of iron, magnesium, calcium, and other elements. The ferromagnesian silicates are darker in color and more dense than other silicates. The nonferromagnesian silicates do not have irons or magnesium ions and they are lighter in color and less dense than the ferromagnesians. The nonsilicate minerals do not contain silicon and are carbonates, sulfates, oxides, halides, sulfides, and native elements.

A rock is defined as an aggregation of one or more minerals that have been brought together into a cohesive solid. Igneous rocks formed as hot, molten magma cooled and crystallized to firm, hard rocks. Sedimentaryrocks are formed from sediments, accumulations of weathered rock materials that settle out of the atmosphere or out of water. Sediments become sedimentary rocks through a rock-forming process that involves both the compaction and cementation of the sediments. Metamorphicrocks are previously existing rocks that have been changed by heat, pressure, or hot solution into a different kind of rock without melting. The rock cycle is a concept that an igneous, a sedimentary, or a metamorphic rock is a temporary stage in the ongoing transformation of rocks to new types.

The earth has a layered interior that formed as the earth's materials underwent differentiation, the separation of materials while in the molten state. The center part, or core, is predominantly iron with a solid inner part and a liquid outer part. The core makes up about 15 percent of the earth's total volume and about a third of its total mass. The mantle is the middle part of the earth's interior that accounts for about twothirds of the earth's total mass and about 80 percent of its total volume. The Mohorovicic discontinuity separates the outer layer, or crust, of the earth from the mantle. The crust of the continents is composed mostly of less dense granite-type rock. The crust of the ocean basins is composed mostly of the more dense basaltic rocks.

Another way to consider the earth's interior structure is to consider the weak layer in the upper mantle, the asthenosphere that extends around the entire earth. The rigid, solid, and brittle layer above the asthenosphere is called the lithosphere. The lithosphere includes the entire crust, the Moho, and the upper part of the mantle.

Evidence from the ocean floor revived interest in the idea that continents could move. The evidence for seafloor spreading came from related observations concerning oceanic ridge systems, sediment and fossil dating of materials outward from the ridge, and from magnetic patterns of seafloor rocks. Confirmation of seafloor spreading led to the plate tectonic theory. According to plate tectonics, new basaltic crust is added at diverging boundaries of plates, and old crust is subducted at converging boundaries.Mountain building, volcanoes, and earthquakes are seen as related geologic features that are caused by plate movements. The force behind the movement of plates is uncertain, but it may involve convection in the deep mantle.