Rocks & the rock cycle

Pick up a stone from the roadside. It looks like it has always been this way, but that is deceiving. Maybe it was once glowing magma, maybe sand at the bottom of a sea. Every stone tells a story, and once you know the three rock types, you can read it.

Three types of rock

Igneous rock forms when hot magma cools down and turns solid. Granite is such a rock, you find it in many mountain ranges and on kitchen countertops. How big the crystals get depends on the pace: deep down magma cools slowly, so large crystals grow, as in granite. When the magma reaches the surface as lava, it solidifies so fast that the grains stay tiny, for example in dark basalt. Sedimentary rock forms in a completely different way: sand, mud and tiny remains settle layer by layer and get pressed together over a long time. That is how loose sand becomes solid sandstone. The third type is metamorphic rock, which means transformed rock. When a rock deep inside the Earth is exposed to great pressure and heat, it changes without melting. Limestone, for example, turns into marble this way.

The rock cycle

No rock type stays what it is forever. Wind and water wear mountains down, and the debris becomes sedimentary rock. If a rock sinks deep into the Earth, pressure and heat turn it into metamorphic rock. If it melts completely, it becomes magma, and when the magma solidifies again, everything starts over. This journey is called the rock cycle, and every rock can reach every station in it.

Unimaginably slow

The rock cycle runs at a pace we can barely imagine. A single round can take many millions of years. The sandstone in an old city wall may already have been seabed when dinosaurs still existed. That is exactly what makes rocks so exciting: they are the memory of the Earth.