Look at a world map and South America's east coast seems to fit into Africa's west coast like puzzle pieces. In 1915 the German scientist Alfred Wegener argued that the continents were once joined in one supercontinent, which he named Pangaea, and have slowly drifted apart. He gathered several kinds of evidence.
- Matching coastlines: the edges of the continental shelves fit even better than the shorelines.
- Matching fossils: the small freshwater reptile Mesosaurus is found only in southern Africa and South America; the seed fern Glossopteris in South America, Africa, India, Antarctica and Australia. Neither could have crossed an ocean.
- Matching rocks and mountains: rock layers and old mountain belts line up across the Atlantic, such as the Appalachians and the mountains of Scotland and Norway.
- Ancient glaciers: scratches left by ice sheets about 300 million years ago are found in now-tropical Africa, India and South America, and they point back toward a single ice cap.
Key idea
Matching coastlines, fossils, rocks and signs of ancient glaciers show the continents were once joined. Today we know whole plates move, ocean floor and all, carrying the continents with them.
Why it works
Each clue alone could be a coincidence. Together they tell one story: put the continents back together, and the fossil ranges join up, the mountain belts become one belt, and the glacier scratches all point away from one ice cap near the South Pole. Wegener could not explain what force moved the continents, so many scientists doubted him. In the 1960s, maps of the ocean floor showed how the lithosphere moves, and the idea became plate tectonics.
Worked example: A fossil puzzle
Fossils of Mesosaurus, a small reptile that lived in fresh water, are found in Brazil and South Africa, but nowhere else. Why is this evidence for drift?
- A small freshwater reptile could not swim across a salty ocean thousands of kilometers wide.
- If South America and Africa were joined, its home was one stretch of land and lakes.
Answer: The two continents were once joined
Worked example: Ice in the tropics
Rocks in India, which is now near the equator, show deep scratches from glaciers about 300 million years old. What does this suggest?
- Huge ice sheets don't form at the equator.
- So India was once much closer to the South Pole, joined to other southern continents, and has since moved.
Answer: India has moved far from where it was
Common mistake
Picturing the continents plowing through the ocean floor like ships through water. Whole plates move, and a plate can carry both ocean floor and a continent, like the North American Plate, which reaches from California to the middle of the Atlantic. Check: if continents plowed through the sea floor, it would be crumpled in front of them, and it isn't.