There are three major classes of rocks, grouped according to the way in which they are formed: igneous, sedimentary, and metamorphic.

are formed by the cooling of molten mixtures of minerals, called magmas, which are found far below the earth's crust. When magmas cool below the surface, the rocks formed are called intrusive. When they cool above the surface, the rocks formed are called extrusive.

Granite is the best-known intrusive igneous rock. It is composed mainly of feldspar and quartz, mixed with mica or hornblende. This firm, enduring rock is widely used for buildings and monuments. Other intrusive rocks include pegmatite, granite porphyry, diorite, gabbro, syenite, and peridotite.

Large masses of intrusive rock below the surface of the earth are called batholiths. Some cooled magma rises vertically to form projections called dikes. Some dikes have been uncovered by erosion and extend above the earth's surface. Another form of cooled magma, sills, consists of horizontal layers of igneous rock between layers of other kinds of rock.

he most common form of this rock is lava from volcanoes. Basalt, a heavy, dark-colored rock, is a form of cooled lava. Other rocks formed by cooled lava include obsidian (sometimes called natural glass), pumice, rhyolite, and andesite. Metal ores are often found near masses of igneous rock. These ores were originally produced by the cooling of magma that had a metallic content, or by the chemical interaction of other rocks with cooling magma.

are formed of loose materials—mud, sand, pebbles, and bits of organic matter—moved from one place to another. These materials are called sediment. Streams deposit vast amounts of sediment in the oceans. It is then moved about by waves and currents and left in smooth layers known as strata. Much sediment is also deposited on land by winds and ice. After millions of years, the sediment solidifies into rock because of chemical action and the pressure caused by overlying layers of material. About three-fourths of the earth's surface is covered with sedimentary rock.

Sandstone, a common sedimentary rock, is formed mainly of quartz particles cemented together by silica, calcium carbonate, or iron oxide. Shales are clays that have hardened into rock. Limestone is formed from calcium carbonate deposits made up largely of the microscopic remains of various sea organisms that produce shells. Conglomerate is sedimentary rock that is composed of gravel and other small, usually rounded rock fragments, often of quartz or limestone. These are held together by much finer-grained material that fills the spaces between the fragments. Breccia is similar to conglomerate, but its fragments are less rounded.

are rocks formed underground by the altering of sedimentary or igneous rocks. Such altering can be caused by heat, pressure, chemical action, or movement of the earth's crust, either separately or in combination. Slate, marble, quartzite, gneiss, and schist are all classed as metamorphic rocks.

Many metamorphic rocks display foliation, a banding or layering similar to that of sedimentary rocks. Foliation is particularly characteristic of gneiss and schist, and can also be seen in many examples of slate.