Rock

Rock, hard, solid matter of the earth's crust, sometimes a combination of one or more minerals. Rock may occur close to the earth's surface or deep underground. Rocks are classified according to their origin. Igneous rock forms when magma (molten material deep within the earth) rises toward the surface and cools. Intrusive igneous rock, such as granite, results if the magma solidifies before it reaches the earth's surface. Extrusive igneous rock, such as obsidian, occurs when magma reaches the surface, as it does in a volcano eruption. Sedimentary rock, such as sandstone, gypsum, and chalky limestone, forms when sediments, or parts of other rocks, mix together and harden. These sediments can be formed by erosion, chemical action, or an accumulation of plant and animal parts. Metamorphic rock, such as marble and slate, occurs when igneous or sedimentary rocks are exposed to intensive pressure or heat and subsequently change their form. Rock is used primarily as a building material and is an important component of concrete.