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The rock cycle
The rock cycle as a model linking igneous, sedimentary and metamorphic rocks, the processes and energy sources that drive it, plate tectonics as its engine, and James Hutton's discovery of deep time.
Some of the atoms in the sand on Bondi Beach have probably been part of a granite, a sandstone, a schist and a magma, several times over, in the last few billion years.
A model of how rocks change
Rocks are not permanent. They are made, destroyed and made again from the same materials. The rock cycle is a scientific model that shows how the three types of rock are linked by the processes you've met in this chapter.
- Magma cools and crystallises to form igneous rock, either slowly underground (intrusive) or quickly at the surface (extrusive).
- Rock that is uplifted and exposed at the surface is weathered and eroded into sediment.
- Sediment is deposited in layers, then compacted and cemented into sedimentary rock.
- Rock that is buried deep or squeezed between plates is changed by heat and pressure into metamorphic rock.
- If it gets hot enough, rock melts back into magma, and the cycle continues.
Unlike a simple circle, the rock cycle has many short cuts. Igneous rock can be metamorphosed without ever becoming sediment. Sedimentary rock can be uplifted and weathered straight back into sediment. Metamorphic rock can be squeezed again into a higher-grade metamorphic rock. A rock can stay in one stage for billions of years, or move on in a few million.
What drives the cycle?
The rock cycle is powered by two energy sources:
- The Earth's internal heat (left over from its formation, plus radioactive decay) melts rock to make magma, drives metamorphism and powers the mantle convection that moves plates. Moving plates provide the forces that bury rocks, squeeze them and push them back up into mountains.
- The Sun's energy drives the weather and the water cycle: rain, rivers, wind, waves and ice. These do the weathering, erosion and transport at the surface.
Gravity plays a part everywhere: it pulls sediment downhill and makes it settle, compacts layers under the weight of rock above, and pulls cold, dense plates down into the mantle at subduction zones.
Plate tectonics is the engine that keeps the cycle running. At subduction zones, rock is dragged down to be melted and metamorphosed. At collisions, rock is buried, metamorphosed and lifted into mountains to be eroded. Without moving plates, erosion would eventually wear the continents flat.
- Complete all five missions. Which one takes the most steps?
- Track how many millions of years your rock's journey takes. Which processes are slowest?
- Can granite turn into slate? What has to happen first?
"No vestige of a beginning"
In the 1700s most people in Europe believed the Earth was only a few thousand years old. The Scottish farmer and naturalist James Hutton watched soil wash off his fields into streams and realised that the same slow processes, acting over enormous time, could explain the rocks he saw. At Siccar Point in Scotland in 1788 he found nearly vertical layers of old sedimentary rock, worn flat, with almost horizontal layers of younger sandstone lying across their tops. To explain it, the old layers must have been deposited, buried, cemented, tilted, uplifted and eroded, and then new sediment deposited on top: a full turn of the rock cycle, and an unimaginably long time. Hutton wrote that he could find "no vestige of a beginning, no prospect of an end". This idea of deep time later helped Charles Darwin, who needed a very old Earth for evolution.
Hutton's key idea was that the present is the key to the past: the processes we can watch today, like rivers carrying sand, also made the rocks of the past.
Test yourself
Which process turns sediment into sedimentary rock?
The grains are squeezed and glued.
Which energy source mainly drives weathering and erosion?
It powers the water cycle and wind.
A metamorphic rock is heated until it melts and then cools. It becomes
Anything that cools from magma is igneous.
Which process can change igneous rock directly into metamorphic rock?
It doesn't need to become sediment first.
James Hutton's main conclusion was that
Slow processes need enormous time.
Complete the rock cycle.
What powers each process?
Granite becomes slate. Put the steps in order.
The rock cycle is a of how rocks change. Rock pushed up to the surface has been . Molten rock cools and to form igneous rock. The force that pulls sediment downhill is . The idea that the Earth is extremely old is called deep .
Explain why the rock cycle needs both the Earth's internal heat and energy from the Sun. Use at least two processes for each energy source.
The Earth's internal heat, which comes from heat left over from its formation and from radioactive decay, powers the deep parts of the cycle. It melts rock to make magma, which cools into igneous rock. It heats buried rock so that, together with pressure, it is metamorphosed. It also drives mantle convection, which moves the plates; plate collisions bury rocks and later lift them up into mountains.
The Sun's energy powers the surface part of the cycle. It evaporates water and drives the weather, so rain, rivers, waves, ice and wind weather rock and erode and transport the pieces. When the water or wind slows, the sediment is deposited, and it later becomes sedimentary rock.
Both are needed. Without internal heat, no new igneous or metamorphic rock would form and no mountains would be pushed up, so erosion would eventually wear the continents flat. Without the Sun, there would be no weathering or erosion and no new sediment, so no sedimentary rock.