The Earth and its atmosphere · Process
The rock cycle
The stone in a cathedral wall was once sand on a beach, and before that a mountain, and before that molten. Nothing about a rock is permanent except the atoms.
Start here
There are marine fossils near the summit of Everest, at 8800 metres.
The rock at the top of the highest mountain on Earth is limestone, full of the shells of sea creatures. Limestone forms on a warm shallow sea floor. Nobody carried it up there — the sea floor itself was lifted, and it is still rising today.
What does that tell you about rock?
That rock moves, and that the surface of the Earth is not fixed. Sea floor was pushed eight kilometres into the sky when two continents collided, and it is being weathered away up there at this moment — the fragments heading down the rivers to become the sediment of somewhere else. Rock is not a thing. It is a stage in a process, and the process has no end.
The three rock types are not three separate worlds. Each one can be turned into either of the others, given the right conditions and enough time, and the whole set of routes is called the rock cycle.
It has no beginning and no end. The atoms in the rock beneath your feet have been through it before.
Your turn · follow one grain
Seven stages, shuffled. Put them in the order they happen.
0 of 7 placed
One grain of quartz, one complete circuit, back to where it started. Tap the stages in order.
That is the journey, and every stage has a name.
Not quite the order. Here is the journey and what each stage is called.
- Frost and rain break a grain of quartz off a granite mountainside. — Weathering. Water gets into cracks, freezes, expands and levers the rock apart; acids in rainwater attack some minerals chemically. The rock is broken down where it stands.
- A stream carries the grain down the valley, rounding it as it goes. — Erosion and transport. Moving water, wind or ice picks the fragments up and moves them, knocking the corners off on the way — which is why beach sand is rounded and freshly broken rock is sharp.
- The river slows where it reaches the sea and drops the grain on the sea floor. — Deposition. Moving water can only carry sediment while it is moving fast enough; when it slows, the heaviest particles are dropped first. Layer settles on layer, oldest at the bottom.
- Buried under later layers, the grain is squeezed and cemented into sandstone. — Compaction and cementation. The weight above squeezes the water out, and dissolved minerals crystallise between the grains and glue them together. This is now sedimentary rock.
- Two continents collide, and the sandstone is pushed deep and cooked without melting. — Metamorphism. Heat and pressure make the crystals grow and rearrange while the rock stays solid. The sandstone becomes quartzite — a metamorphic rock.
- Pushed deeper still, the rock passes its melting point and becomes magma. — Melting. Deep in the root of a mountain range the rock is hot enough, and wet enough, to pass its melting point. Depth on its own does not do it — the mantle below is hotter still and is solid rock. Everything about the old rock, its layers, its crystals and its history, is erased.
- The magma stalls underground, cools slowly, and crystallises into granite. — Crystallisation. Slow cooling deep underground grows large interlocking crystals. The grain is back in granite, and once the rock above it has been worn away it will be a mountainside again.
And then it starts again, because the granite at the end is the granite at the beginning. That is why it is called a cycle rather than a sequence.
Reference · the six processes
Tap a process. Each one is an arrow on the diagram.
1 of 6 opened
Each one takes a rock from one state to another, and the time it takes is part of the answer.
Weathering
any rock at the surfaceloose fragments
Breaking rock down where it sits. Physical weathering is water freezing in cracks and levering them open; chemical weathering is slightly acidic rain reacting with the minerals. No transport is involved — that is the next process along.
Millimetres per human lifetime
Erosion and transport
fragmentssediment somewhere else
Rivers, glaciers, wind and waves pick the fragments up and carry them. Particles are rounded and sorted by size on the journey — which is why you can tell a beach sand from a scree slope by looking at a handful.
Days to thousands of years
Deposition
sediment in motionlayers of sediment
When the water or wind slows down it can no longer carry its load, so it drops it — heaviest first. Each layer is laid on top of the last, which is why the bottom of a cliff is older than the top.
Continuous, layer by layer
Compaction and cementation
layers of sedimentsedimentary rock
The weight of later layers squeezes the water out of the sediment below, and minerals dissolved in that water crystallise between the grains, cementing them together into solid rock.
Thousands to millions of years
Heat and pressure
any buried rockmetamorphic rock
Deep burial, or the heat of a nearby intrusion of magma, changes the rock without melting it. Crystals grow, minerals rearrange and grains line up — but the rock stays solid throughout.
Millions of years
Melting and cooling
any rock, deep and hot enoughigneous rock
Past its melting point the rock becomes magma and all of its structure is erased. Depth on its own is not what does it — the mantle is deeper and hotter and is solid rock — but where a collision drives rock and water down into the hottest part of the crust, the melting point is passed. Cooling reverses it: slowly and deep underground gives large crystals, quickly at the surface gives tiny ones.
Cooling takes days at the surface, up to millions of years at depth
Key fact
Weathering, erosion, transport, deposition, compaction, heat and pressure, melting and cooling turn every rock type into every other. The cycle has no starting point and no end.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Sedimentary rock becomes metamorphic, and metamorphic becomes igneous. It goes round one way, like a clock.”
The diagram in most books is drawn as a circle with arrows. Commit before you read on.
There are arrows across the middle as well as round the edge. A metamorphic rock exposed at the surface is weathered into sediment without ever melting. An igneous rock can be buried and metamorphosed without ever becoming sediment. A sedimentary rock can be pushed down deep enough to melt and skip the metamorphic stage entirely.
What decides the route is not an order of stages — it is where the rock ends up. At the surface: weathering. Buried deep: heat and pressure. Deeper and hotter still: melting. The cycle is a map of possible journeys, not a timetable.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What turns layers of sediment into sedimentary rock?
Rung 2 · The one that catches people
Can an igneous rock become a sedimentary rock without ever becoming metamorphic?
Rung 3 · Explain
Describe how a grain of rock on a mountainside could end up as part of a sedimentary rock on a sea floor, naming each process in order.
Rung 4 · Take it somewhere new
A geologist says the rock cycle explains why almost none of the Earth's original crust still exists. Explain what they mean, and how it fits with rocks that are billions of years old still being found.
Key note
Rocks are broken down by weathering, carried away by erosion and transport, and deposited as sediment, which is compacted and cemented into sedimentary rock. Burial brings heat and pressure, making metamorphic rock; deeper and hotter still, some of it melts, and cooling magma makes igneous rock. Any rock type can become any other, and the cycle has no beginning or end.
Going further
The engine driving all of this is heat from inside the Earth. Slow currents in the solid mantle drag the rigid plates at the surface around; where plates collide, rock is forced down and melted or pushed up into mountains; where they pull apart, the pressure drops and new igneous rock forms. Without that internal heat the continents would have been worn flat by rain long ago, and the Earth would be a smooth, wet, geologically dead ball.
The timescales are the hardest part to hold in your head. Weathering removes a few millimetres of exposed rock in a human lifetime. The Himalayas took fifty million years to rise and are still rising. A complete circuit of the rock cycle takes hundreds of millions of years, and the oldest crustal rocks found so far are around four billion years old — meaning most of the original crust has already been recycled out of existence.
Before this lesson
At GCSE this becomes
- Plate tectonics as the driver of the cycle, and radiometric dating to put real numbers on how long each stage takes.
Where to next
Ask Mr Badmus AI
Still not sure how a rock gets back to being magma?
Lesson content © MrBadmusAI.