Rock

Rocks are naturally occurring substance composed primarily of crystals of different kinds of minerals, or broken pieces of crystals, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants. Rocks are all around us. They make up the backbones of hills and mountains and the foundations of plains and valleys. Beneath the soil you walk on and the deep layers of soft mud that cover the ocean basins is a basement of hard rock.

We can learn something about the way a rock formed from by looking carefully at the evidence preserved inside. What a rock is made of, the shapes of the grains or crystals within the rock, and how the grains or crystals fit together all provide valuable clues to help us unlock the rock's history hidden within.

Types
They are divided into three basic types, igneous, sedimentary and metamorphic, depending upon how they were formed. Plate tectonics provides an explanation for how rocks are recycled from igneous to sedimentary to metamorphic and back to igneous again.


 * Igneous rock (from the Greek word for fire) form from when hot, molten rock (magma) crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.


 * Sedimentary rock are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock.


 * Metamorphic rock started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot, mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet. by the U.S. Geological Survey, last updated on 01/13/04.