Rocks form in three main ways. These processes result in three kinds of rocks, igneous, sedimentary, and metamorphic. Igneous rocks are born in fire, or at least in great heat. Sedimentary rocks are built up from sediment. Metamorphic rocks have been drastically changed by geologic processes.
Igneous rocks form when molten rock solidifies. Liquid rock, magma, forms deep underground when solid rock melts. Rock can also be melted by a volcano, and be extruded out onto the surface. Liquid rock that is extruded into the air or an ocean is called lava.
Pumice and obsidian are two kinds of rock formed in volcanoes. Pumice is extremely light, because it is full of gas bubbles. Obsidian is volcanic glass, usually tinted dark by minerals. Basalt is an igneous rock that often forms when rock liquefies as it rises to zones where the pressures keeping it solid decrease. It forms a layer beneath the bottom of the ocean, and also builds volcanic islands.
Wherever volcanic rock is found, there has been geologic activity, but not necessarily a volcano. Sometimes igneous rock forms below ground when molten rock forces its way between existing layers of rock, and solidifies before it ever reaches the surface. Granite can form this way.
Igneous rocks probably make up 90% of the earth's outer crust, but they are mostly hidden beneath layers of sedimentary and metamorphic rock.
Sedimentary rocks form from small particles that are deposited by wind, water, or sometimes glaciers. These deposits build up in layers on a lake or ocean bed, in a valley, or at the foot of a cliff. Because of the pressure of new layers, or due to chemical reactions, they solidify to rock.
Rock that is eroded elsewhere and becomes new rock is called clastic. Material that drops out of solution to form rock is a chemical precipitate. Sediment from plants or animals may become organic sedimentary rock.
Limestone is an example of organic sedimentary rock; it is formed from the shells and skeletons of marine creatures, along with coral and algal waste. Coal is another organic rock. Sandstone is created from rock fragments, while the minerals in shale are mostly clays. Both rocks are clastic. Gypsum and halite (rock salt) are two rocks that precipitate out of solution in a chemical reaction.
Sedimentary rock probably only makes up 5% of the earth's upper crust, but it is an important part. It carries the history of the earth in fossils and ruins, and shows where rivers ran and waters stood in ancient seas.
Metamorphic rocks have been altered. The very minerals that make up the rock can be changed into something new that shows the effects of the pressures and temperatures at great depths. Some metamorphic rocks are easily recognizable as forms of igneous or sedimentary rocks, but others are utterly changed.
Shale, hardened clay, becomes slate when it is first transformed. With increased pressure it becomes phyllite, and then schist, a hard, foliated (easily split) rock often dotted with flakes of mica and sometimes a sprinkling of red garnets. Schist can also form from igneous rocks such as basalt and tufa. Sandstone becomes sturdy quartzite. Limestone becomes marble.
Particular metamorphic rocks indicate particular conditions. For example, the blueschist of California marks a subduction zone. Geologists can illuminate the composition of an area by mapping and tracing the limits of areas of metamorphic rocks.
Some rocks are formed from material deposited in layers, which creates sedimentary rocks. Liquid magma or lava solidifies into igneous rock. Other rocks are metamorphic, having been changed from their original state by temperature and pressure.
The classifications of sedimentary, igneous, and metamorphic are only the beginning of understanding rocks. There is much more to know. Each individual rock tells a unique story of its origin, composition, and history.