In geology, the concept of the rock cycle is used to describe the different changes a rock may undergo during its existence. Simply put, the rock cycle can be thought of as a qualitative, hypothetical description of the changes undergone by a rock in its lifetime.
As existing rock is exposed to the elements and acted upon by the rain, wind, sunlight, animals, etcetera, it may be physically broken apart. This process is referred to as physical weathering (or mechanical weathering). Rock may also be broken apart and/or altered by chemical reactions, a process known as chemical weathering.
As weathering breaks the rock apart, erosion may occur. That is, water, wind or gravity may transport these pieces, known as sediment, away from the source. Note that weathering and erosion do not refer to the same thing but instead name two complimentary and interrelated processes.
At some point, transportation of the sediment ceases and it is deposited. Most often, this occurs when whatever mechanism was transporting the sediment no longer has enough force to move the sediment. (An example of this is the deposition of sediment at the mouth of a river. As the river reaches the ocean, the velocity of the water drops and it no longer has enough energy to carry the sediment along.) Deposition can occur almost immediately, or it can happen only after a very long time (with possibly more than one mechanism responsible for transportation).
Eventually, older sediment is buried and compacted by newer layers. This compaction of the sediment, in addition to cementation– dissolved materials and minerals in the water between the grains of the sediment acting to cement the grains together- converts sediment to sedimentary rock. Compaction and cementation together are both responsible for lithification.
After sediment has been lithified and turned into sedimentary rock, it may be uplifted due to geological activity and begin this process anew. However, if it is buried deeply enough, strong forces may act to deform the rock by squeezing, folding and reshaping its layers. At depth, the rock may also be heated and subjected to high temperatures. The combination of deformation and heating may convert the sedimentary rock to metamorphic rock through metamorphism.
If the temperature is high enough, the rock may also melt and become molten. (Molten rock below the surface of the earth is known as magma. Once it has reached the surface of the earth, it is known as lava.) If this liquid rock cools, it may crystallize and harden, a process known as solidification. The rock that forms as a result is known as igneous rock. Like sedimentary rocks, metamorphic and igneous rocks may also be uplifted to the surface to begin the cycle again.