The Rock Cycle: Igneous, Sedimentary and Metamorphic Transformations

Three rock families, one loop

Every rock on Earth's surface belongs to one of three families, and the rock cycle is the set of processes that convert each family into the others, driven by heat from Earth's interior, gravity, and the water cycle at the surface. Igneous rock crystallises directly from molten rock — magma underground (intrusive, coarse-grained, like granite) or lava at the surface (extrusive, fine-grained or glassy, like basalt or obsidian, which cools too fast for large crystals to grow).

From solid rock to loose sediment

Weathering breaks rock down in place, either mechanically (frost wedging as water freezes and expands in cracks, root growth, thermal expansion and contraction) or chemically (rainwater slightly acidified by dissolved CO2 dissolves minerals like feldspar into clay). Erosion then physically transports the resulting fragments — by rivers, wind, ice or gravity — away from where they formed, wearing them down further and sorting them by size and density along the way. The heaviest, most angular fragments settle out first; the finest clay particles travel farthest, often to the ocean floor.

Turning loose sediment back into rock

Where sediment accumulates and is buried under more sediment, two processes convert it into solid sedimentary rock: compaction, where the weight of overlying layers squeezes out water and air, packing grains tighter, and cementation, where minerals dissolved in groundwater — commonly calcite, silica or iron oxide — precipitate in the pore spaces and glue the grains together. Together these processes are called lithification. Sandstone (cemented sand grains), shale (compacted clay) and limestone (compacted shell and coral fragments, or precipitated calcium carbonate) are the classic products.

Heat and pressure without melting: metamorphism

Bury any rock — igneous, sedimentary, or already metamorphic — deep enough, or place it near a body of intruding magma, and rising heat and pressure will recrystallise its minerals into new, more stable forms without fully melting the rock. This is metamorphism. Shale becomes slate, then phyllite, then schist, then gneiss as temperature and pressure climb; limestone becomes marble; sandstone becomes quartzite. Directed pressure often aligns platy minerals into visible banding, called foliation, which is why gneiss shows stripes and slate splits into flat sheets.

Closing the loop: melting

Push temperature and pressure past a rock's melting threshold — commonly near subduction zones, where one tectonic plate is forced under another and drags water and crust down into the mantle, lowering the melting point of surrounding rock — and any rock type, igneous, sedimentary or metamorphic, melts into magma. That magma can cool underground into new intrusive igneous rock, or erupt and cool at the surface, restarting the cycle from the top.

magma/lava  --cools & crystallises-->        igneous rock
igneous/sedimentary/metamorphic rock
            --weathering & erosion-->        sediment
sediment    --compaction & cementation-->    sedimentary rock
any rock    --heat & pressure, no melting--> metamorphic rock
any rock    --melting-->                     magma  (loop closes)

No step is guaranteed or one-way — a rock can cycle through weathering and lithification repeatedly without ever melting, or melt and re-solidify without ever being buried and metamorphosed. The loop describes possible pathways driven by plate tectonics and surface processes operating over anything from thousands to hundreds of millions of years, not a fixed schedule any individual rock follows.

Frequently asked questions

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