Definition · Plain-language
Rock cycle
The rock cycle is the never-ending series of processes that transform rock between igneous, sedimentary and metamorphic forms over vast spans of time.
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A cycle with no beginning or end
The rock cycle describes how the three rock families — igneous, sedimentary and metamorphic — are continually recycled into one another. It is not a one-way path but a web of possible transformations: any rock type can, given the right conditions, become any other. The cycle is driven by two great engines. Earth’s internal heat powers melting, mountain-building and the movement of tectonic plates, while energy from the Sun drives the weather, water and wind that wear rock down at the surface. Together they keep the rock cycle turning over millions of years.
The main pathways
Several key routes link the rock types. Molten magma cools and crystallises into igneous rock. At the surface, any rock can be weathered and eroded into sediment, which is transported, deposited, then compacted and cemented into sedimentary rock. Burial under heat and pressure can change igneous, sedimentary or older metamorphic rock into metamorphic rock without melting it. And if any rock is heated enough to melt, it returns to magma, ready to cool into new igneous rock. Uplift and erosion can also bring deep rock back to the surface to start again.
A timescale beyond human experience
The rock cycle operates over geological time — typically thousands to millions of years for a single transformation — far slower than a human life. This deep-time pace is why solid rock can seem permanent to us even though it is constantly, if imperceptibly, being remade. The cycle also recycles Earth’s materials and helps regulate the planet’s chemistry: for instance, weathering of rock draws carbon dioxide out of the air over long periods. Understanding the rock cycle is fundamental to geology because it connects volcanoes, mountains, rivers and the deep Earth into one system.
Key facts
At a glance
- Definition: the continuous transformation of rock between the three main types
- Three types: igneous, sedimentary and metamorphic
- Key processes: melting, cooling, weathering, erosion, deposition, heat, pressure
- Driven by: Earth’s internal heat and energy from the Sun
- Direction: no fixed start or end — any rock can become any other
- Timescale: geological — thousands to millions of years
Common misconceptions
What people often get wrong
Often heard: The rock cycle starts with igneous rock and ends with metamorphic rock.
Actually: The rock cycle has no fixed beginning or end. Any rock type can transform into any other along several pathways, so it is a continuous loop rather than a one-way sequence.
Often heard: Rocks never really change once they have formed.
Actually: Rocks change constantly, just extremely slowly. Over geological time, weathering, burial, heat, pressure and melting recycle every rock through the cycle into new forms.
Often heard: The rock cycle is driven only by heat inside the Earth.
Actually: Two engines drive it: Earth’s internal heat powers melting and plate movement, while solar energy drives the weather and water that weather and erode rock at the surface.
