The rock cycle

The rock under your feet has been through this loop many times already. There is no genuinely new rock on Earth and no truly old rock either, because the same material has been melted, crushed and rebuilt for four and a half billion years.

Magma

Magma is rock melted by the heat of the deep Earth, mostly somewhere between seven hundred and thirteen hundred degrees Celsius. Melted rock is lighter than the solid rock around it, so it rises the way a bubble climbs through syrup.

Lava at the surface

When magma reaches the surface it gets a new name, lava. Same material, different address. The moment the pressure drops, its dissolved gas comes fizzing out, and whether that gas escapes gently or all at once decides between a lava flow and an explosion.

Extrusive igneous rock

Lava cools in minutes or hours, far too quickly for crystals to grow, so the rock ends up dark and fine grained. That rock is basalt, and it is the commonest rock on the planet, because the entire floor of every ocean is made of it.

Intrusive igneous rock

Magma that never reaches the surface cools slowly instead, sometimes taking a million years, and slow cooling grows crystals big enough to see. That is granite, and the black, white and pink speckles in a granite worktop are those crystals themselves.

Uplift and exposure

Granite forms kilometres underground, so something has to lift it and strip away everything above before you can stand on it. Mountains rise a few millimetres a year and rivers grind them down about as fast, so the deep rock eventually finds daylight.

Weathering

Now the attack begins. Water seeps into cracks and freezes, and because water expands as it turns to ice it levers the rock apart. Acid from rain, roots and lichen dissolves the rest, chemically taking the rock to pieces without moving it anywhere.

Erosion and transport

Weathering breaks the rock up, erosion carries it away, and the two are not the same thing. Rivers, glaciers and wind move the fragments and round them off as they tumble. The Mississippi alone still delivers well over a hundred million tonnes of sediment to the sea every year.

Sediment deposited

Wherever the water slows down it drops its load, heaviest grains first and the finest mud last. Layers pile up on lake beds, river mouths and the deep sea floor, and every layer is a page of time, with the youngest always on top.

Burial and cementation

Bury those layers under hundreds of metres of later sediment and the sheer weight squeezes the water out from between the grains. Minerals dissolved in that water crystallise in the gaps and glue everything together, a process geologists call cementation.

Sedimentary rock

The result is sandstone, limestone and shale, and these are the only rocks that carry fossils, because they are the only ones made gently enough to preserve a shape. Roughly three quarters of the land surface is covered by them.

Heat and pressure

Push that rock deeper, under a rising mountain range, and it gets hot and hugely squeezed but still does not melt. The atoms rearrange while the rock stays solid, and the new crystals grow lined up at right angles to the squeeze.

Metamorphic rock

Limestone becomes marble, shale becomes slate, and slate splits into flat sheets precisely because its crystals all lie the same way. That is why roofs are made of it. Metamorphic rock is the same atoms rearranged into a tougher pattern.

Melting

Deeper still, past about seven hundred degrees, the rock finally gives in and melts. Every trace of what it used to be is erased, and the loop begins again. Any rock can become any other rock, and that is the whole point of the cycle.