Pangaea and the evidence

Coal, which forms from swamp plants, has been found in Antarctica.

A map that looks like a puzzle

Look at a world map and the coasts on either side of the Atlantic seem to fit. People noticed early. USGS records that the Dutch mapmaker Abraham Ortelius suggested in 1596 that the Americas had been torn away from Europe and Africa, and in 1858 the geographer Antonio Snider-Pellegrini drew maps of the two continents fitted together and then separated. A fitting shape is only a hint, though. Coastlines change with sea level and erosion, and a good puzzle fit could be a coincidence.

Four kinds of evidence

The first is fossils. Wegener found identical fossil species of plants and animals on the matching coasts of South America and Africa, now separated by the Atlantic. He reasoned that most of these living things could not have swum or been carried across a wide ocean, and to him this was the most convincing evidence that the two continents had once been joined. The fossil fern Glossopteris is found in regions that are now near the poles, spread across the southern continents.

The second is rock. Unusual geological structures on one side of the Atlantic continue on the other, as if a page had been torn in two and the lines of print still matched. The third is past climate. Coal, which forms from lush swamp plants, has been found in Antarctica, so that frozen land must once have lain closer to the equator. And there are deposits left by glaciers in places that are now hot and dry, such as the Vaal River valley in South Africa. A single continent that later drifted apart explains these clues together.

The fourth kind came after Wegener: the ocean floor itself. Mirror-image magnetic stripes, young rock at the ridges and old rock near the trenches, and the deep earthquakes along sinking slabs gave the missing mechanism. Continents do not plough through the sea floor; they ride on plates that are made at ridges and destroyed at trenches. By the late 1960s these ideas had grown into the theory of plate tectonics. USGS dates the start of Pangaea's break-up to about 225 to 200 million years ago. Wegener's supporter Alexander Du Toit proposed that it first split into Laurasia in the north and Gondwanaland in the south, which then broke into today's continents.