The continent you're standing on is drifting roughly as fast as your fingernails grow — about 2 to 10 centimeters per year. In the time it takes you to read this story, North America will have moved noticeably relative to Europe. Given enough time, this motion rearranges the entire planet.

Until the 1960s, this idea was ridiculed. When German meteorologist Alfred Wegener proposed in 1912 that continents had once been joined and drifted apart, geologists called him a fantasist. He noticed how the coasts of South America and Africa fit together like puzzle pieces, and that identical fossil reptiles were found on both sides of the Atlantic. How else could a freshwater lizard have crossed thousands of miles of ocean?

The establishment's objection was simple: how? Continents were supposedly fixed on the Earth's crust. No known force could push a continent across an ocean. Wegener died in 1930, on an expedition to Greenland, his theory still dismissed.

Then came the Cold War. In the 1950s and 60s, the U.S. Navy mapped the ocean floor in unprecedented detail, looking for places to hide submarines. What they found shocked geologists. The ocean bottom wasn't flat and ancient — it was crossed by massive underwater mountain ranges, each split down the middle by a rift. Along these rifts, molten rock was rising from below, cooling, and spreading outward in both directions.

This was seafloor spreading. The oceans weren't passive basins. They were conveyor belts, with new crust being created in the middle and old crust being consumed at the edges. Continents weren't plowing through the ocean floor. They were riding on top of it, like debris on a slow-moving river.

We now know Earth's surface is divided into about 15 major plates, all in constant motion. Their collisions and separations create everything dramatic about our planet. The Himalayas are rising because India is ramming into Asia. Iceland is growing because it sits on a spreading ridge. San Francisco and Los Angeles, on different plates, are slowly moving toward each other and will eventually collide.

Geology, once a science of static rocks, turned out to be the study of a living planet — breathing, flexing, and reshaping itself on timescales almost impossible to perceive.