The Rock Cycle is a convenient method of illustrating the relationship of the families of igneous, sedimentary, and metamorphic rocks. Igneous rocks are aggregates of minerals that crystallize from a molten mass of material referred to as magma. Magma is a complex high-temperature liquid generated either within or beneath the crust, contained within a magma chamber, and composed of water, various silicate compounds, and a variety of gases. Magma may either be extruded onto the surface of earth in the form of lava and various other forms of extrusive igneous rocks or intruded into the crust to form various types of intrusive igneous rocks. Igneous and sedimentary rocks are converted to metamorphic rocks when subjected to pressures and temperatures of a magnitude sufficient to transform the existing rock types to other rock types, but less than the pressures and temperatures required to melt rock.
Metamorphic rocks can be themselves metamorphosed or remetamorphosed.
Both sedimentary and metamorphic rock may also be weathered and eroded to form sediment and then sedimentary rock, or melted to form magma when buried and subjected to sufficiently high temperature and pressure. Rocks can be composed of only one mineral such as quartzose sandstone which is composed of essentially one mineral, quartz. However, most rocks consist of more than one mineral such as common sandstone which contains quartz as the dominant mineral, but also includes accessory minerals such as feldspar, mica, and others. It is the relative abundance of different minerals in a rock that provide much of the information about its history of formation. The common minerals found in each of the three classes of rocks are listed in Table 3-1.