Our planet may look like a solid ball of rock, but if you could crack it like an egg (not actually something I’d recommend, although it would make for a fun scene in a science fiction novel or movie) you’d find it’s quite fluid inside.
And, in fact, the Earth’s solid shell, called the lithosphere, is cracked: broken up into numerous “tectonic plates” that scoot around on top of the more fluid layer beneath, called the asthenosphere. Well, “scoot” might be an overstatement: plate speeds range from around 10 mm a year all the way up to a whopping 160 mm a year (about as fast as your hair grows).
This means Earth’s oceans and continents are constantly changing shape, and if that sounds just a little bit crazy even today, think how it sounded a century ago when meteorologist Alfred Wegener put forth his notion of “continental drift,” a theory he expanded in a 1915 book, The Origin of Continents and Oceans.
Wegener wasn’t the first person to posit something of the kind (after all, he was hardly the first to notice that South America and Africa look uncannily like two pieces of a jigsaw puzzle separated by a few thousand miles of ocean), but he put forward as solid an argument as could be managed at the time, pointing to the similarity of rock formations on the east coast of South America and the west coast of Africa, fossils shared by South America, Antarctica, India and Australia, and more.
But it took a slow accumulation of additional evidence over the next few decades to convince the bulk of the geological community of the fact of continental drift.
One reason was that nobody could quite figure out how, exactly, pieces of crust could move around. Certainly Wegener’s concept of the continents as “icebergs” of low-density granite floating in a “sea” of denser basalt didn’t seem adequate to explain the phenomenon.
Eventually, though, the evidence became overwhelming, and plate tectonics, as it became known, took its rightful place as central to the proper understanding of earth’s geological history. (Though not, alas, in time for Wegener to see his “crazy” notion properly vindicated: he died in 1930.)
The wholesale shifting around of continents appears to have gotten underway about three billion years ago, and three times since then, all of the continents have mushed together into a single supercontinent.
The most recent of these, called Pangaea, only broke apart about 250 million years ago, eventually creating the familiar shapes you memorized in geography.
In some places (“divergent boundaries”), the plates are pulling apart. Rock from the asthenosphere rises to the surface and becomes new lithosphere, mostly in the middle of the oceans, where this forms enormous rifts.