The rock cycle explains the changes rocks undergo from the time they are expelled from the earth's mantle in the form of magma to the time that they return to the mantle and become magma once more. This process takes place over millions to billions of years, depending on the conditions the rocks are subject to. Weather, heat, water and pressure are all forces that affect the rock cycle.
The youngest rocks in the cycle are igneous rocks, which are made of magma that recently cooled. Over time, some igneous rocks become buried under the earth, where the heat and pressure transform them into metamorphic rocks. Other igneous rocks are carried away by water, where they are ground down into fine particles that form layers of sediment. As new layers of sediment settle over the old ones, the bottom layers are compacted together until the fine particles fuse into solid rocks called sedimentary rocks.
Both metamorphic and sedimentary rocks are gradually buried by subsequent layers of rock. Over time, these top layers produce so much heat and pressure that they melt the layers below into magma. This magma joins the magma in the earth's mantle and molten crust, beginning the rock cycle over again.