Continental Drift: Wegener's Theory and Its Evidence

1A torn page

Look at a world map and follow the east coast of South America: the bulge of Brazil looks as if it would slot into the Gulf of Guinea on the coast of Africa, like two halves of a torn page. Map-makers noticed this within a century of the first good maps of the Atlantic. In 1912 a German scientist, Alfred Wegener, turned the observation into a theory: the continents had once been joined and had since drifted apart. In S1 you studied the layers inside the Earth, and in S2 the folding, faulting and volcanoes that shape the crust. This unit explains the movement behind them.

2Wegener's theory

• About 300 million years ago all the continents formed one supercontinent, Pangaea.
• Pangaea broke up, first into Laurasia and Gondwana, then into the continents we know.
• The continents moved apart to their present positions, and they are still moving.
• Wegener suggested that the continents ploughed through the ocean floor, pushed by forces from the Earth's rotation and by tides.

Modern dating has confirmed the outline of the story and made it more precise (Figure 2), but Wegener's proposed forces proved far too weak. That weakness is why most geologists rejected drift for forty years.

Figure 2: the life of Pangaea. Dates are approximate.

3The evidence

1. The fit of the continents

The coastlines of South America and Africa match, but the true edge of a continent is not the shoreline: it is the continental slope, where the shallow continental shelf drops to the deep ocean floor. In 1965 Edward Bullard, Jim Everett and Alan Smith used a computer to fit the continents around the Atlantic along the slope, at a depth of about 900 m. The fit was remarkably close, with only small gaps and overlaps. Figure 1 repeats their rotation for South America using today's coastlines.

Figure 1: South America moved back against Africa using Bullard's rotation. Even with shorelines instead of the continental slope, the two coasts fit, and the Mesosaurus fossil areas line up across the join.

2. Fossils

Identical fossils of plants and land animals are found on continents now thousands of kilometres apart. Mesosaurus was a small reptile of fresh or brackish water, about a metre long; it could not have crossed an ocean. Glossopteris had seeds too large and heavy to be blown across the sea. The simplest explanation is that the continents were joined when these organisms lived, around 300 to 250 million years ago.

3. Rocks and mountain belts

When the continents are fitted together, rock formations and mountain belts continue from one to the next. Old mountains in the eastern United States (the Appalachians) line up with those of Scotland and Scandinavia (the Caledonian mountains). Very old rocks in north-eastern Brazil match those of West Africa in age and type.

4. Past climates

Rocks deposited by glaciers about 300 million years ago, with scratches (striations) cut by moving ice, are found in South America, southern Africa, India, Antarctica and Australia, some of them now close to the equator. Put the continents back into Gondwana and the glaciated areas form one ice sheet near the South Pole of that time. Coal, which forms from the remains of swamp forests, is found in Antarctica, where no forest could grow today.

4From rejection to acceptance

Wegener's evidence was strong, but he could not show what moved the continents. In the late 1920s Arthur Holmes suggested that slow convection currents in the mantle could carry them. The breakthrough came in the 1950s and 1960s, when surveys of the ocean floor showed that it is young, spreading and magnetically striped. Continental drift then became part of a larger theory, plate tectonics, which is the subject of the next lesson.

5Effects of continental drift on physical features