The elements silicon and oxygen make up 75 percent of all the elements
in the outer layer, or crust, of 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 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. Sedimentary
rocks 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. Metamorphic
rocks 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.
Earth has a layered interior that formed as 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 Earth's total
volume and about a third of its total mass. The mantle is the middle part
of Earth's interior that accounts for about two-thirds of Earth's total mass
and about 80 percent of its total volume. The Mohorovicic discontinuity
separates the outer layer, or crust, of 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 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 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.
