Rock Cycle: Topic PageThe recycling of the Earth's outer layers. Rocks are continually being formed, destroyed, and re-formed in an endless cycle of change that takes millions of years. The processes involved include the formation of igneous rock from magma (molten rock); surface weathering and erosion; the compaction and cementation of sediments into sedimentary rock; and metamorphism, chemical and physical changes brought about by heat and pressure, producing metamorphic rock.