Rock cycle: how Earth's rocks form, change, and interact

Main rock types and where they come from

Geologists divide solid Earth materials into three broad classes, each produced and altered by different conditions:

• Igneous rock — forms when molten rock cools and crystallizes. Cooling can occur below the surface (intrusive) or at the surface after volcanic eruption (extrusive).
• Sedimentary rock — made of compacted and cemented fragments (sediment) of older rocks, mineral precipitates, or biological debris deposited in layers.
• Metamorphic rock — produced when pre-existing rocks are altered by elevated temperature and pressure without melting, changing mineralogy and texture.

Key processes in the rock cycle

Several basic physical and chemical processes move material between rock types. Important processes include:

• Melting — solid rock turns to magma under extreme heat, often deep in the crust or mantle.
• Cooling and crystallization — magma or lava solidifies into igneous rock.
• Weathering and erosion — break down rock at the surface to produce sediment, which can be transported by water, wind, ice or gravity.
• Compaction and cementation — burying sediment compresses it and mineral-rich fluids bind particles into sedimentary rock.
• Metamorphism — burial and tectonic forces impose pressure and heat that recrystallize minerals and form foliated or non‑foliated metamorphic rocks.
• Subduction and deep crustal recycling — plates carry surface rocks into the mantle, promoting melting and large‑scale recycling.

These processes can repeat in different orders. For example, an igneous rock can be weathered into sediment, become a sedimentary rock, then be buried and metamorphosed, and finally melt to form new magma.

Environments that favor particular steps include volcanic arcs and mid‑ocean ridges (where melting and igneous activity dominate), river basins and continental shelves (where sediment accumulates), and continental collision zones (where high pressures and temperatures create metamorphic rocks). Plate tectonics provides the large-scale engines that drive many transformations by creating heat, directing burial and producing uplift and erosion.

Understanding the rock cycle is important for interpreting Earth’s history, locating mineral and energy resources, and predicting landscape evolution. Rocks preserve records of past climates, life and tectonic events; their textures and compositions reveal formation conditions. Though the cycle is continuous across deep time, individual changes may take from thousands to hundreds of millions of years depending on the process and setting.

Conceptually, the rock cycle emphasizes that Earth’s crust is dynamic: materials are never permanently fixed in one state. The idea grew from early modern geology and remains a central organizing principle for teaching how surface and internal processes interact to shape the planet.