The rock cycle is a continuous process which transforms old rocks into new ones. There are three types of rock: igneous, sedimentary, and metamorphic. Igneous rocks are created when underground magma cools and solidifies. Once exposed on the surface, these rocks are broken down into rock particles by weathering. These particles are then transported by rivers and the wind and are deposited as sediments in lakes, deltas, and on the ocean floor. Over time, these sediments solidify into sedimentary rocks. Sometimes, when heated and pressurised underground, these sedimentary rocks are then hardened into metamorphic rocks. At any time, underground rock may melt into magma again, thus completing the cycle.