Rock Cycle

Three rock types and the geological processes that transform them

The rock cycle describes the continuous transformation of rocks among three types. Igneous rocks form when magma or lava cools and crystallizes — slow cooling underground (intrusive) produces coarse-grained granite, while rapid surface cooling (extrusive) produces fine-grained basalt. Sedimentary rocks form when weathering breaks existing rock into fragments that are transported, deposited, compacted, and cemented — examples include sandstone, limestone, and shale. Metamorphic rocks form when existing rocks are subjected to elevated temperature (>200°C) and/or pressure (>0.3 GPa) without melting — slate from shale, marble from limestone, gneiss from granite. Any rock type can become any other: igneous can weather into sedimentary, be metamorphosed, or re-melt. The cycle is driven by Earth's internal heat (mantle convection, radioactive decay) and external energy (solar-driven weathering). Plate tectonics is the engine: subduction carries rocks to depth (metamorphism, melting), while uplift and volcanism bring material back to the surface.