Rock

Rock is a hard natural substance composed of one mineral or several different minerals held together. A mineral is a natural solid with a defined chemical composition and crystalline structure. Rocks make up the Earth's crust and the layers beneath it, and are also found on other celestial bodies and the moon.

Rocks are classified according to how they form. Three major types are recognized: igneous rock, which forms from the cooling of molten material; sedimentary rock, which forms from the compression and cementation of broken fragments; and metamorphic rock, which forms when heat and pressure alter a rock's previous structure.

Igneous rock

Igneous rock forms when magma—molten rock material—cools and solidifies. If cooling occurs beneath the earth's surface, the crystals are large, as in granite. If it cools on the surface after a volcanic eruption, the crystals are very small, as in basalt.

Sedimentary rock

Water, wind, and ice break down fragments from older rocks. These fragments accumulate in lakes or on the sea floor, and pressure from overlying layers and the weight of material above transforms them into hard rock. Examples include sandstone and limestone. These types commonly contain fossils, which help establish a record of the history of life.

Metamorphic rock

When older rock encounters high heat and pressure without melting, its minerals alter and form new structures. Examples: hard clay becomes slate, and limestone becomes marble. This process typically occurs in areas where mountains form and the Earth's layers collide.

The rock cycle and uses

None of the three types is permanent: sedimentary rock can transform, metamorphic rock can melt, and magma can solidify again. This continuous transformation is called the rock cycle. Humans use rock for construction, roads, cement, and mineral extraction including iron, gold, and oil.