A rock is a natural mass of minerals that forms a considerable part of the earth's crust, and is originated by some geological process.
When rocks are made up of a single mineral, they are called simple, like marble, which is calcite; Most rocks are composed, in other words, aggregates of various minerals, such as granite that is a mixture of quartz, feldspar, and mica.
Most of the known simple elements or bodies make up rocks, but only a small number are involved in such a preponderant way that they constitute 98% of the earth's crust (oxygen 47%, silicon 28% and sodium 23.5%).
Rocks are classified in relation to their composition, texture, and origin. According to the nature of the processes that gave rise to it, three divisions are established: igneous rocks (or endogenous or eruptive), whose formation takes place inside the earth due to high temperatures and pressures.
The sedimentary rocks, caused by the effects of erosion on other rocks and sediment consolidation of animal or vegetable organic remains. And the metamorphic rocks, which are pre-existing of the two previous types, later transformed in depth.
Among these rocks there is an evolutionary process known as the " Rock Cycle". Igneous rocks are formed by cooling and crystallization of magma. Exposed on the earth's surface to the action of weathering agents, they decompose, the originating products are transported and form deposits or sediments, thus originating sedimentary rocks.
These buried at great depths, subjected to high temperatures and penetrated by gases and chemically active solutions, are transformed into metamorphic rocks. In metamorphism, the rock does not melt but the minerals that compose it change their shape and often their nature. If the rock melts, magma originates, and a new cycle would start again.
The term rock is also referred to a very hard, solid, firm and constant person, animal or thing; for example, Paul is a rock because it is difficult to move his heart.
