The continuous transformation of rock from one type to another, in which igneous, sedimentary, and metamorphic rocks are formed, broken down, and reformed through melting, weathering, deposition, and heat-and-pressure.
- Igneous rocks form from cooled magma or lava; intrusive (plutonic, coarse-grained) like granite, and extrusive (volcanic, fine-grained) like basalt. These are the original or "primary" rocks.
- Sedimentary rocks form from compaction and cementation of sediments, often in layers (strata) and may contain fossils; examples are sandstone, limestone, and shale.
- Metamorphic rocks form when existing rocks are altered by heat and pressure: limestone becomes marble, sandstone becomes quartzite, shale becomes slate, granite becomes gneiss, and coal becomes graphite (and ultimately diamond under extreme conditions).
- The cycle is driven by internal heat (forming igneous and metamorphic rocks) and surface processes such as weathering and erosion (forming sediments).
- Coal and petroleum are organic sedimentary deposits; most mineral and fossil-fuel resources are tied to particular rock types.
The three rock families with examples, and especially the parent-to-metamorphic pairs (limestone to marble, coal to graphite), are very frequently asked matching items.
Igneous (from magma) versus sedimentary (from sediments, may have fossils) versus metamorphic (altered by heat and pressure); intrusive granite versus extrusive basalt; remember limestone gives marble, not the reverse.
Igneous, sedimentary, and metamorphic rocks continuously transform; limestone becomes marble and coal becomes graphite under heat and pressure.