Science Puzzle
How Do We Know the Continents Move?
Look at a map and the east coast of South America seems to fit into the west coast of Africa like two jigsaw pieces. In 1912 the German scientist Alfred Wegener claimed the continents had once been joined and had slowly drifted apart. Most geologists dismissed the idea: nobody could imagine whole continents moving.
What evidence shows that South America and Africa really were once joined?
The Answer
Many separate clues all point the same way: the same fossils, the same rocks and the marks of the same ancient ice sheets turn up on both sides of the ocean, lining up as if the coasts were once joined. Today satellites can measure the continents moving.
Fossils of Mesosaurus, a small reptile that lived in fresh water, are found in Brazil and in southern Africa, and nowhere else. It couldn't have swum thousands of kilometres across a salty ocean. The seed fern Glossopteris turns up in South America, Africa, India, Australia and even Antarctica, yet its seeds were far too heavy to blow across oceans.
The rocks match too. Belts of rock of the same type and age, and the remains of old mountain ranges, run up to the coast on one side and carry on at the matching point on the other. Scratches left by ancient glaciers in places that are now tropical line up as if made by one ice sheet when the continents sat together near the South Pole.
Wegener couldn't explain what moved the continents, which is a big reason he wasn't believed. The answer, sea-floor spreading and plate tectonics, came in the 1960s, three decades after he died on an expedition in Greenland. Today GPS shows the Atlantic still widening by a few centimetres a year.
The principle: Converging evidence. Matching coastlines, fossils, rocks and ancient glacier marks on continents now oceans apart all point the same way: they were once joined and have drifted apart.