The three rock families, igneous, sedimentary and metamorphic, are linked. Rock is never used up; it is recycled from one kind into another. The pathways together are the rock cycle.
- Melting turns any rock into magma; cooling turns magma into igneous rock.
- Weathering and erosion break any rock that reaches the surface into sediment; deposition, compaction and cementation turn sediment into sedimentary rock.
- Heat and pressure turn any rock into metamorphic rock, without melting it.
Key idea
Any rock can become any other kind. Earth's inner heat drives melting, metamorphism and uplift; sunlight (through wind, rain and ice) and gravity drive weathering and erosion.
Why it works
Two engines run the cycle. Heat from inside Earth melts rock, bakes it and pushes it up into mountains. At the surface, the Sun's energy drives the water cycle and wind, which break rock down and carry the pieces downhill under gravity. Because both engines keep running, rock keeps moving around the cycle, by whatever path the conditions allow.
Worked example: A shortcut
Gneiss (metamorphic) is pushed up into a mountain and worn down by rain and ice. What does it become next?
- Weathering and erosion break any exposed rock into sediment.
- So the gneiss becomes sediment, skipping magma and igneous rock completely.
Answer: Sediment
Worked example: A long trip
Sand from weathered granite is buried and cemented, then squeezed and heated deep in a mountain belt. Name each rock along the way.
- Granite is igneous. Weathering makes sand (sediment).
- Burial and cementation make sandstone (sedimentary).
- Heat and pressure turn sandstone into quartzite (metamorphic).
Answer: Granite → sand → sandstone → quartzite
Common mistake
Learning the cycle as one fixed loop, igneous to sedimentary to metamorphic and back. Rocks can skip steps or go backward: metamorphic rock can weather straight into sediment, and igneous rock can be metamorphosed without ever becoming sediment. Check: for any rock, ask what would happen if it were melted, buried or uplifted, and follow that arrow.