The Earth's rigid outer shell is broken into about a dozen large plates and many smaller ones. A plate is a slab of lithosphere: the crust plus the cold, stiff top of the mantle stuck to its underside. Together they are roughly 100 km thick, thinner under young ocean floor and up to ~200 km under old continents. Beneath the plates lies the asthenosphere, mantle rock that is still solid but hot enough to creep slowly, so the plates can slide over it. (The deep Earth explains the difference between these layers and the crust and mantle.)

Plates move about 2–10 cm a year, roughly as fast as your fingernails grow. That sounds like nothing, but it adds up. At 5 cm a year a plate travels 500 km in 10 million years, and an ocean the width of the Atlantic can open in under 200 million years.

What drives them? Heat escaping from the Earth's interior keeps the mantle slowly churning. The biggest single force, though, is slab pull. Where old ocean plate sinks into the mantle, the weight of the sinking slab drags the rest of the plate after it, like a tablecloth sliding off a table. Plates with long sinking edges, such as the Pacific, are the fastest. A smaller push, ridge push, comes from new plate sliding off the raised mid-ocean ridge where it formed.

Plates meet in three basic ways. At divergent boundaries they pull apart and new plate is made. At convergent boundaries they push together and one plate usually sinks back into the mantle, a process called subduction. At transform boundaries they slide past each other. What happens at a convergent boundary depends on what is colliding: ocean against continent, ocean against ocean, or continent against continent. Try each one below.

Whether a plate can sink comes down to buoyancy: something floats if it is less dense than what lies beneath it, and sinks if it is denser. Here are the rough densities involved.

Notice that oceanic crust on its own is lighter than the mantle. So why does ocean floor sink at all? Because a plate is more than its crust. A new ocean plate is born hot and thin at a ridge. As it moves away it cools, and mantle rock beneath it cools too and becomes part of the plate. After about 80 million years the plate is roughly 100 km thick, and almost all of that is cold mantle. Cold rock is denser than the same rock hot. After a few tens of millions of years the whole plate, a thin light crust on a thick, cold, dense base, ends up denser than the hot asthenosphere beneath it. It is ready to sink. Once one edge goes down at a trench, the weight of the sinking slab pulls the rest after it.

That is why the oldest ocean floor anywhere is only about 180–200 million years old: older ocean floor has all been recycled back into the mantle. Continental crust is different. It is about 35 km thick on average and much lighter, enough to keep its plate afloat however cold it gets. A continent's edge can be dragged down, sometimes more than 100 km, but it does not keep sinking, and much of it bobs back up. So continents survive for billions of years, and they hold almost all of the Earth's old rocks.

When an ocean closes completely and two continents meet, neither can sink for good. The result is the highest mountains on Earth. This is what lies beneath the Himalaya.

In 1912 Alfred Wegener argued that the continents had once been joined and had drifted apart. Most geologists rejected the idea for decades, because nobody could explain how solid continents could plough through solid ocean floor. The answer, that continents ride along on plates that include the ocean floor, came in the 1960s, when evidence from the oceans piled up. Four kinds of evidence are shown below.

Today the motion can be measured directly. GPS stations in India move north-north-east by about 4–5 cm a year, exactly as plate tectonics predicts.

The ground we work on has had a dramatic trip. For hundreds of millions of years India was part of Gondwana, wedged between Africa, Madagascar, Antarctica and Australia. About 130–120 million years ago it began to break away, taking Madagascar with it.

Its route is written in lava. A mantle plume is a column of unusually hot rock rising from deep in the mantle. Where it reaches the base of a plate it melts and feeds volcanoes at a hotspot. When a new plume first arrives, its huge head can produce a flood basalt: lava flow after lava flow, covering a vast area. While India was still close to Antarctica, the Kerguelen plume erupted beneath its eastern edge and built the Rajmahal Traps of Jharkhand, about 118 million years ago.

About 88 million years ago India split from Madagascar and sped up, eventually reaching some 15–20 cm a year, one of the fastest plate speeds known. A likely reason is slab pull: a long slab of Tethys ocean floor was sinking ahead of it. About 66 million years ago India passed over the plume that now lies beneath Réunion island. Lava flooded western India and built the vast Deccan Traps, around the time the dinosaurs died out. As India moved on, the plume left a trail of volcanoes on the sea floor, now the Lakshadweep, Maldive and Chagos islands.

About 50 million years ago, once the Tethys Ocean had closed, India hit Asia. Continental crust is too buoyant to sink for good, so India slowed to ~4–5 cm a year and the crust crumpled into the Himalaya and the Tibetan Plateau. India is still pushing north today, and the mountains are still rising. Drag the slider to follow the journey.