plate tectonics

Plate tectonics is the discovery that Earth's hard outer shell isn't one solid piece — it's cracked into a couple dozen enormous slabs, called plates, that drift slowly over the hotter, softer rock beneath. Picture the cracked shell of a hard-boiled egg, except the pieces are continents and oceans, and they creep along about as fast as your fingernails grow — a few centimeters a year.

Slow as that sounds, the action all happens at the edges where plates meet. Where two plates shove together, they crumple the ground upward into mountain ranges; where one dives beneath another, it melts and feeds volcanoes; and where they grind past each other, the rock locks, strains, and suddenly slips — and that jolt is an earthquake. So the same patient drift carves the Himalayas, lines the Pacific with volcanoes, and rattles California.

A common mix-up: the plates aren't the same as the continents. A single plate can carry a whole continent plus a slab of seafloor riding along with it, all in one piece. And the ground isn't floating on a sea of liquid magma — the rock below is solid, just hot enough to ooze and flow over thousands of years, like cold honey or a glacier of stone.