Once upon a time, a very long time ago, India was all by itself, a very large island floating along in a sea it liked to think of as the Indian Sea. It was minding its own business, when all of a sudden—
India slammed into a huge continent. The biggest continent in the world, as a matter of fact: Asia.
India's collision with Asia pushed up the highest mountains on the planet, the Himalayas, just like an auto collision might push up and rumple a car hood. And the slo-mo collision is still ongoing! India is still pushing into the larger continental mass, still pushing up mountains. The Himalayas are getting higher—2 centimeters per year higher.
Who'd have thunk?
The ideas that continental and sub-continental chunks of land drift around, that they “float” above and move around on the inner mantle, that continents can move from the poles to the equator or vice versa, that seas can open up or shrink and disappear as land masses pull away from each other or collide with each other—all these unlikely-sounding ideas are the fruits of Alfred Wegener's brain.
Our birthday boy was born in Germany in 1880. He grew up to be a geophysicist (a scientist who studies the physics of the Earth, such as its shape, its gravitational and magnetic fields, and its internal structure) and a meteorologist (a scientist who studies climate and weather).
Wegener proposed continental drift in 1912. He'd noticed that the continents seemed as if they fit together like a jigsaw puzzle, He studied rock formations and fossils in the matching sides of the continents, and he found a lot of evidence to back up his idea that they were once joined together. He even came up with the idea that the Mid-Atlantic Ridge was a place in which the Atlantic Ocean was spreading, as he put it, “continuously tearing open and making space for fresh, relatively fluid and hot” rock rising from the Earth's interior.
We now know that Wegener was right! The continents once were glommed together in one huge mass we call Pangea (from the Greek, for “all-lands”). And the Mid-Atlantic Ridge is formed by continuous seafloor spreading and upwelling of fresh magma.
However, in 1912, nobody could come up with ideas of how this could possibly happen, so mosts scientists did the sensible thing: They were skeptical of Wegener's theory. (Some scientists even forcefully attacked the idea of continental drift and promoted “permanentist” views that the continents have always been permanently in the places they are now.)
However, decades later, in the 1950s, paleomagnetic data was discovered—information about Earth's magnetic fields recorded in ancient rocks. Long story short, this new data joined the geologic and fossil evidence in favor of continental drift.
So most scientists did the sensible thing: They changed their minds. By the 1960s, Wegener was recognized as a founding father of the new science of plate tectonics.
As always, I am impressed by both the initial skepticism and the later acceptance of this radical, counter-intuitive idea. Because scientists demonstrated that they don't accept any cool sounding idea, just willy nilly—they look for a cause or mechanism, they look for more evidence. However, scientists also demonstrated that they don't cling to old ideas “just 'cuz.” When the “more evidence” arrives, they can accept the new idea. This is why science and scientific thinking are the best ways to find out about reality!
For more on continental drift and lots of cool links to websites and animations, see this earlier post.