A rock is a natural, solid aggregate of one or more minerals, the crystalline compounds such as quartz and feldspar that make up almost everything underfoot. Geologists sort rocks into three families. The sorting is not by colour or hardness but by how the rock formed, because the way a rock formed is written into its texture and tells you what the place was like at the time.

The textures in the swatches are the clues a geologist reads in the field. Interlocking crystals with no gaps mean the rock grew from a melt, or recrystallised while solid. Separate rounded grains held together by cement mean it was once loose sediment. Minerals lined up like the pages of a book mean it was squeezed while hot.

Why is basalt so fine-grained and granite so coarse? The answer comes from two competing processes that run whenever a liquid freezes into crystals.

A melt only starts to crystallise once it is a little below its melting point; how far below is called the undercooling. Then two things happen. Nucleation is the birth of new crystals: a few atoms happen to line up into a tiny seed stable enough to survive. Growth is atoms in the melt joining crystals that already exist. Both speed up as undercooling increases, but differently. Growth gets going at a small undercooling, because joining an existing crystal is easy. Nucleation needs a larger undercooling, because making a new seed is hard. At very large undercooling both stall, because the melt becomes too stiff for atoms to move.

So the cooling rate decides the texture. Cool a magma slowly and it stays only slightly undercooled for a long time: the few crystals that form keep growing, and you get a coarse rock. Cool it quickly and it races to a large undercooling where seeds form everywhere at once: you get a fine-grained rock of countless tiny crystals. Cool it fast enough and atoms are frozen in place before they can arrange themselves at all, giving volcanic glass such as obsidian.

This one idea explains a lot of what you see in igneous rocks. Granite, and gabbro (the coarse-grained equivalent of basalt), are coarse because they cooled over thousands to hundreds of thousands of years under kilometres of insulating rock. Basalt lava, which cools in days to years, is fine-grained. The outside of a lava flow, or a pillow of lava erupted under the sea, is often glassy because it was chilled in seconds. Some igneous rocks have two crystal sizes, large crystals set in a fine-grained matrix. They record two stages: slow cooling at depth, then fast cooling after the magma moved up. The Minerals & lithology module uses these textures to name igneous rocks.

The three families are linked by the rock cycle: the set of processes that turn any rock into any other. It is easiest to understand in a real landscape, because each process happens in a particular place. Melting and metamorphism happen deep down, weathering and erosion at the surface, and deposition wherever moving water slows down. Uplift, the slow raising of rock by the forces that build mountains, is what connects the deep and shallow halves. Without it, buried rock would never get back to the surface.

Quartz is a good grain to follow because it is so tough. It barely dissolves in rain, so it survives weathering when most other minerals in granite rot to clay, which is why sand on most beaches is mainly quartz. A single quartz grain can go round the sedimentary part of the cycle several times before it is ever melted.

Rocks don't have to go all the way round. Any rock that is lifted to the surface can be weathered straight back into sediment. Any rock buried deep enough can be metamorphosed, and any rock that gets hot enough can melt. The diagram below shows the main loop and the commonest shortcuts.

How long does a lap take? Usually tens to hundreds of millions of years, which is why the rock cycle only makes sense against the backdrop of Deep time. Individual steps vary enormously. Lava can become basalt in days, and a flood can lay down a metre of sand in hours. Turning that sand into sandstone takes millions of years of burial, and lifting rock from 20 km down back to the surface takes tens of millions. The Himalaya show the whole cycle in action: limestone that formed on an ocean floor, packed with marine fossils, now sits at the summit of Everest, lifted there in the collision between India and Asia.

The cycle is powered by two engines. The Earth's internal heat drives melting, metamorphism, volcanoes and uplift, through Plate tectonics. The Sun drives the weather, rain and rivers that wear rock down at the surface. Without both, the cycle would stop.