The rock cycle describes the continuous series of changes that rocks undergo. The changes occur over millions of years, destroying old rocks and making new rocks. It is not as clear cut as the very simple water cycle, but is still easy to understand. There are three types of rock in the cycle: igneous, sedimentary, and metamorphic. Each type of rock can become one of the other types of rock under the influence of certain processes. The processes determine what type of the three rocks is formed.
Rocks on the surface of the earth are exposed to wind and water, which result in weathering. This process slowly breaks up the rocks into smaller pieces. The small pieces are called sediment and they are easily moved around by erosion. Water, wind, ice, and gravity can erode rock taking pieces to new places and making the sediments smaller and smaller. After a while the sediments build up, creating layers on top of layers. Each layer is heavy and presses down on the layers beneath it, compacting it and making it hard again. Sedimentary rock is then formed, and it receives its name from those sediments that compose it.
Sedimentary rock can be weathered again if it is forced to the surface in a process called uplift. The earth’s crust is under a great deal of pressure and sometimes moves to accomodate it. Parts of the crust are forced together and pulled apart, moving underground rocks upward, beginning the weathering process again. Sedimentary rock can also be exposed to two new processes. Under more extreme pressure and exposure to heat from magma or from the natural temperature gradient in the earth, sedimentary rock can be metamorphosed. The minerals in the rock will change structures and realign, often forming crystals, sometimes new minerals, and becoming more dense. This rock is known as metamorphic rock. If exposed to greater heat reaching the melting point of the sedimentary rock, the rock turns to magma, which not only contributes to creating metamorphic rock, but when it cools it becomes igneous rock.
Again, both metamorphic rock and igneous rock can be forced to the surface or exposed by uplift and then they can be weathered and eventually turned back into sedimentary rock. Igneous rock can also be turned to metamorphic rock if it encounters pressure and heat, so long as the heat does not melt the rock again.
In the rock cycle, the three types of rocks can become all of the other types of rocks, depending upon what they are subjected to. It is happening everyday as rocks are worn down and other rocks experience uplift. Sedimentary rock is composed of sediments that come from all three types of rock. Igneous rock results when any of the other rock types melt and then cool. Metamorphic rock forms when any of the other rocks are exposed to extreme heat, pressure, or both.