Igneous rocks are formed when magma from the Earth's core is pushed to the surface. It cools, crystallizes and hardens into igneous rock. There are two types of igneous rock: extrusive, formed when magma flows directly onto the Earth's surface in liquid form and hardens, and intrusive, where the magma hardens and is pushed to the surface by the shifting of tectonic plates.Extrusive igneous rocks tend to have a finer grain texture than intrusive, while intrusive rocks have larger crystals embedded in their surface than do extrusive rocks.
When igneous rocks make it to the surface, they become exposed to wind, rain and other erosive factors. Over time, these rocks break down into smaller pieces called sediments. Water and wind then carry these sediments to other locations where they are deposited into layers. More and more sediment is deposited into these layers, which applies pressure and forces the sediment to harden into sedimentary rock.
Sedimentary rock is formed by sediments compressed by natural forces. As more sediment, and later, sedimentary rock, builds up on top, the lower layers of rock are pressed deeper into the Earth where they are exposed to heat and yet more pressure. The natural shifting of tectonic plates, the temperature and pressure increase further and push the rock into the next station of the rock cycle.
The final stage of the rock cycle is metamorphic rock, which is formed when sedimentary rocks are pressurized and heated until they recrystallize. This change, called metamorphosis, takes place when the sedimentary rock reaches 300 to 700 degrees Celsius. Metamorphic rock is then eroded in much the same way as igneous rock, turning it into sediment, and beginning the rock cycle again.