The diagram illustrates how rocks are formed due to erosion, heat and pressure. Overall, there are three stages in the cyclical process which creates sedimentary, metamorphic and igneous rock.
Rock on the surface of the Earth erode by the weather into sand and small stones which are then transported to the sea. These fall to the bottom of the sea where they form a layer of sediment. Over time, more sediment builds up on top and the pressure turns the sediment below into sedimentary rock. As the sedimentary rock pushes down, the pressure and heat increases. Consequently, the sedimentary rock changes into metamorphic rock.
After metamorphic rock forms, it may slowly pass up to the Earth’s surface and the process of erosion and sedimentation repeats. Alternatively, the metamorphic rock may continue down into the earth. When it reaches a certain level of pressure and heat, it melts, becoming magma. This magma may rise to the surface through a volcano. When this cools on the Earth’s surface, igneous rock form and the process of erosion begins again.