Plate tectonics

Look at a world map: the east coast of South America fits the west coast of Africa almost perfectly, like two puzzle pieces. Coincidence? No. The two continents really were joined once and have been drifting apart ever since. The ground beneath you is moving right now, at roughly the speed your fingernails grow.

A broken shell

The Earth's crust is not one closed shell but broken into about 7 large plates and many smaller ones. These plates float on the hot, viscous mantle below, a bit like ice floes on a lake. That picture has a limit, though: the mantle is not liquid magma but solid rock that flows over millions of years like extremely thick honey. Slow currents in that rock drive the plates: hot material rises, cools down and sinks again. A plate's own weight is part of that circulation: where its cold, heavy edge sinks into the depths, it drags the rest along. This moves the plates a few centimetres per year.

The action is at the edges

Where two continents meet, the crust is pushed together and folds up into mountains: the Himalayas are growing because India is pressing into Asia. Where a heavy ocean plate dives beneath a neighbouring plate instead, deep sea trenches form, like the Mariana Trench, at around 11 km the deepest spot in the oceans. Where plates drift apart, magma wells up and creates new ocean floor: this makes the Atlantic about 2 cm wider every year. And where plates grind past each other, they get stuck and trigger earthquakes. Most volcanoes also sit along such boundaries.

Wandering continents

About 200 million years ago the supercontinent Pangaea, which united all of today's continents, began to break apart. The fragments have been wandering across the globe ever since. When Alfred Wegener proposed this idea in 1912, people laughed at him. Today satellites can measure plate motion to within a few millimetres, and the continents keep moving: in the distant future the world map will look completely different again.