Rocks are separate into sedimentary, igneous and metamorphic. They are further classified according to particle size. The change of one rock type to another is described by the geological model called the rock cycle.
SEDIMENTARY ROCKS are formed by the accumulation of sediments from weathering and erosion and deposition of existing rocks. They may consist of minerals, rock debris or organic matter. They have a layered appearance, from the accumulation and compression of the sediments from which they are formed. They may be soft, like clays, or hard like limestones. The layers may be thick and massive, as in some sandstones or close together as in shales. Texture may be fine, as in chalk, grainy as in sandstones, or pebbly as in conglomerates.
IGNEOUS ROCKS are formed when magma solidifies. When ít solidifies above the surface the rocks are known as extrusive igneous rocks. When the magma solidifies beneath the surface the rocks formed are known as intrusive igneous rocks; they are often squeezed into cracks and intruded between layers of older rocks. In appearance, igneous rocks are crystalline (although the crystals may be too small to see with the naked eye;) they may be uniformly grainy in texture, especially when the crystals are tiny, as in basalts, or granular, as in granites. The crystals may be arranged in zones but the rocks never present a layered appearance.
METAMORPHIC ROCKS are rocks that have been altered from their original form by changes in heat and pressure at depth inside the earth. These changes may occur during folding and Fault movement or baked by contact with magma. .Metamorphic rocks are especially common in mountain-building areas; they may be formed from igneous, sedimentary or even existing metamorphic rocks. Contact metamorphic rocks form a zone around an intrusion of magma, where the molten rock has come into contact with the surrounding rocks. The appearance of a metamorphic rock depends on its origin.
The three classes of rocks — the sedimentary, the igneous and the metamorphic — are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks.
