Rock is the hard mineral substance that forms the solid part of Earth's crust. Mountains and canyons expose many different types of rock on their surfaces. Great cliffs of rock line the seashore in such places as Maine, California, and southern England. In many desert regions, rock formations rise above sandy plains. In most fairly flat areas, a layer of soil covers the underlying rock. Soil consists of tiny fragments of rock and grains of minerals mixed with decaying remains of plants and animals. Stones and pebbles are pieces of rock, and gravel consists of loose, rounded rock fragments.
Most rocks are aggregates. Aggregates contain crystals or grains of two or more different minerals. Much granite, for example, contains grains of clear quartz, pinkish potassium feldspar, white plagioclase, and black biotite or hornblende. Some rocks have grains so small that they can be seen only when a thin slice of the rock is examined under a microscope. A few kinds of rock consist almost entirely of only one mineral. Quartzite, for example, is rock composed of the mineral quartz, and limestone is rock composed of the mineral calcite.
People use rocks in many ways. Builders use granite, marble, and other rocks as construction materials. Cement is made from finely crushed and heated limestone. Sand and gravel or crushed stone mixed with wet cement makes strong, durable concrete, an artificial rock, for use in buildings, highways, and dams.
Such metals as aluminum, copper, iron, lead, tin, and zinc come from rocks called ores. Ores also supply radioactive metals, such as uranium, and nonmetallic minerals, such as borax, graphite, and trona. In Minnesota and western Australia, deposits of iron ore make up entire mountains. In tropical climates, weathering (the wearing away of rock) creates thick soils rich in the aluminum-bearing ore bauxite.
Some rocks hold valuable crystals. In Africa and Australia, workers mine diamonds from a rock called kimberlite. Beautiful green emeralds come from rocks in Colombia, India, Russia, and South Africa. Most blue aquamarine comes from Brazil and Madagascar. Emerald and aquamarine are gem forms of the mineral beryl.
Geologists study rocks to trace Earth's history (see Geology). Petroleum geologists analyze the age, structure, and composition of rock layers to find petroleum deposits. Paleontologists study fossils found in rock to learn about living things that existed millions of years ago (see Fossil).
Thousands of young people and adults collect rocks and minerals as a hobby. They trade rocks just as stamp collectors trade stamps. There are thousands of rock and mineral clubs throughout the world. These clubs hold regular meetings, sponsor study groups and museum exhibits, and organize field trips to collecting areas.
Geologists divide rock into three different classes: (1) igneous rock, (2) sedimentary rock, and (3) metamorphic rock. Igneous rock forms when molten rock cools. Sedimentary rock forms from loose deposits of rocky material. Metamorphic rock is created when other rocks are changed by heat and pressure.
