Continental Drift Evidence: Wegener's Proof

The first clue is the one anyone can see. Trace the outline of the eastern Americas and the western edge of Europe and Africa, and the coastlines fit together like pieces of a single torn sheet of paper. Wegener named this reconstructed supercontinent Pangea (Greek for "all lands"), and he dated its breakup to roughly 175 million years ago.

Coastline fit is a good hook but a weak proof on its own — coastlines erode and change. So Wegener went hunting for evidence that the continents had once been physically connected, evidence that could not be explained by coincidence or by animals swimming across an ocean.

Mesosaurus was a small freshwater reptile — it could neither fly nor survive long in salt water. Yet its fossils turn up in only two places on Earth: southern South America and southern Africa. For the same animal to live on both shores of the Atlantic, without leaving a trail across it, those shores must once have been joined.

Even more striking is Glossopteris, a seed fern. Its fossils carpet the southern continents — South America, Africa, India, Antarctica, and Australia — and only those. A wind-dispersed seed could in principle cross water, but the same distinctive plant dominating five now-separated landmasses (including icy Antarctica) makes most sense if they were one continuous land.

If continents were torn apart, the rock layers on either side of the tear should match like the two halves of a broken biscuit — and they do. Distinctive rock sequences of the same age and type line up across the Atlantic. The folded Appalachian Mountains of North America continue, with no gap in age or structure, into the Caledonian Mountains of Scotland and the mountains of Scandinavia. A single mountain belt, split by the opening of an ocean.

Earth's climate leaves fingerprints in rock. Glaciers scratch the bedrock beneath them and pile up distinctive rubble (tillite). Coal forms in warm, swampy forests. Evaporite minerals form in hot, dry, shallow seas. Each deposit tells you the climate where it formed.

Wegener found glacial deposits, all of roughly the same age (about 300 million years old), scattered across South America, Africa, India, Antarctica, and Australia. Some of those places sit near the equator today — where glaciers are impossible. But reassemble the southern continents into Pangea and group them near the South Pole, and all those glacial scars merge into one ice cap. The rocks record a climate the continents' present positions cannot explain.

The evidence was strong, but Wegener could not explain what force could move entire continents. He guessed at the pull of the Moon or Earth's rotation ("pole-fleeing force"), but physicists showed those forces were far too weak. Without a workable mechanism, most geologists in the 1920s–40s filed the idea under "interesting but unproven."

The breakthrough came from the ocean floor, which Wegener could not study. When mid-ocean ridges, magnetic stripes, and the youth of seafloor crust were discovered in the 1950s–60s, the missing mechanism appeared — and it is the subject of the next lessons.