Hence the definitions adopted in rock names simply correspond to selected points in a continuously graduated series. By increase or decrease in the proportions of their minerals, they pass through gradations from one to the other the distinctive structures of one kind of rock may thus be traced, gradually merging into those of another. There are, however, no hard-and-fast boundaries between allied rocks. Those three classes are subdivided into many groups. This transformation produces three general classes of rock: igneous, sedimentary and metamorphic. Over the course of time, rocks can be transformed from one type into another, as described by a geological model called the rock cycle. These physical properties are the result of the processes that formed the rocks. Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size. Rock outcrop along a mountain creek near OrosÃ, Costa Rica. The scientific study of the character and origin of rocks is called petrology, which is an essential subdiscipline of geology. This occurs, for example, when continental plates collide. Metamorphic rocks are formed when existing rocks are subjected to such high pressures and temperatures that they are transformed without significant melting. Sedimentary rocks are formed by diagenesis and lithification of sediments, which in turn are formed by the weathering, transport, and deposition of existing rocks. Igneous rocks are formed when magma cools in the Earth's crust, or lava cools on the ground surface or the seabed. Rocks are usually grouped into three main groups: igneous rocks, sedimentary rocks and metamorphic rocks. Rocks form the Earth's outer solid layer, the crust, and most of its interior, except for the liquid outer core and pockets of magma in the asthenosphere. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter.
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