The ground underfoot is not a single shell. Earth’s outer layer is broken into plates of crust and rigid upper mantle that move a few centimeters a year, about as fast as a fingernail grows. Heat from the interior keeps the deeper mantle slowly convecting. Plates ride that motion. Where they pull apart, magma rises and new ocean floor forms. Where they collide, one slab may dive beneath another, or continents crumple into mountains. Where they slide past, faults store and release earthquakes.

This picture, plate tectonics, explains why volcanoes and quakes cluster in belts rather than at random, why fossils of tropical plants sit in polar rocks, and why the same mountain chains reappear on opposite sides of an ocean. The Atlantic is widening. The Pacific is, in places, shrinking. India once sat far south and drove into Asia, lifting the Himalaya. Africa is splitting along the East African Rift.

Continents are thick and buoyant, so they rarely sink whole. Ocean crust is denser and recycles. Over hundreds of millions of years the map of the world is redrawn, even though a human lifetime sees only tiny shifts, a raised curb after a quake, a new island after an eruption. The planet’s surface is a slow machine, and every coastline is a temporary line.
