Rock cycle is defined as the changes involved in the different types of rocks, however the rock cycle illustrates and explains how the three types of rocks are related to each other, and how processes change from one type of rock to another over time. The rock cycle takes place due to two forces:

1. Earths internal heat engine: this internal heat moves material around in the core and the mantle and leads to slow but significant changes within the crust.
2. The hydrological cycle- this is the movement of water, ice, and air at the surface and it is made possible by the sun.

The rock cycle steps

When describing the rock cycle, one can start anywhere since the three types of rocks- igneous rocks, sedimentary rocks and metamorphic rocks are directly related to each other by the rock cycle. However, it is preferable to start with molten magma.

Magma can cool on the surface of the Earth to form an igneous rock, in this case, it produces avolcanicorextrusive igneous rock, or it can cool beneath the surface of the Earthas a result, it produces aplutonicorintrusive igneous rock. Plutonic rocks cool and crystallize slowly below the Earths surface giving crystals more time to develop and turn into a rock with large mineral grains typically visible to the naked eye (this is known as coarse-grained) with no glass remaining between the crystals (the glass represents the solidified melt in the magma). Similarly, volcanic rocks cool rapidly at the surface, giving magma less time to crystallize completely. Crystals forming in magma that reached the surface (lava) have less time to develop. The result is a rock with fewer and smaller mineral grains (fine-grained), and large amounts of glass between crystals.

Igneous rocks at the surface of the earth are subjected to weathering and erosional processes. Materials derived from these are deposited to form sediments. The loose sediment consolidates to form a sedimentary rock, this sedimentary rock becomes buried and subjected to heating and deformation and it then becomes metamorphosed. Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure. Metamorphism can be so intense such that the rock begins to melt and form an igneous rock. This can be exposed at the surface of the earth and the process continues like that.

However, the cycle doesnt have to follow a central circular pattern, sedimentary rocks can be weathered and eroded and form new sediments, on the other hand, metamorphic rocks can as well form sedimentary rocks. Rocks can be metamorphosed more than one time. Consequently, igneous rocks can be metamorphosed, while some igneous rocks are formed as a result of the solidification of melts produced by high-grade metamorphism, some of them are formed as a result the partial melting of the mantle. However, metamorphic rocks form a vital part of the rock cycle along the way for instance, from sedimentary rocks to igneous rocks.

The rock cycle still takes place on planet Earth because our core is hot enough to keep the mantle moving, the physical atmosphere is relatively thick, and we have liquid water, ice and air constantly occurring now and then to keep up with the process. The rock cycle may be absent or completely dead on other planets or their satellites, such as the Moon, because the core is not as hot as our to drive mantle convection and there is no atmosphere or liquid water in such places.

Rock cycle diagram