Alfred Wegener, a German meteorologist and geophysicist, introduced his theory of continental drift in the early 20th century. He proposed that Earth’s continents were not static but had once been connected as a single supercontinent, Pangea. Over millions of years, this landmass fragmented, and its pieces moved to their current positions. Wegener’s idea challenged the prevailing scientific views of his time, which assumed the permanence of continents and ocean basins.
The Continental Puzzle
Wegener observed the striking fit between continent coastlines, particularly the eastern edge of South America and the western edge of Africa. These landmasses interlocked like jigsaw puzzle pieces, suggesting a past physical connection. While others had noted this geographical resemblance before, Wegener was the first to gather evidence that these continents were once joined. He argued that if the continental margins, including the submerged continental shelves, were considered, the fit became even more precise. This visual correlation indicated that the continents had since drifted apart.
Fossil Footprints Across Continents
Wegener presented paleontological evidence: identical plant and animal fossils found on continents now separated by vast oceans. The presence of these fossils on widely separated landmasses made it improbable for these species to have crossed such large water bodies. For example, fossils of the freshwater reptile Mesosaurus, which lived approximately 286 to 258 million years ago, were discovered exclusively in southern Africa and eastern South America. This small, paddle-limbed reptile was adapted to shallow, freshwater environments and could not have survived a journey across the vast saltwater Atlantic Ocean.
Fossils of the land reptile Lystrosaurus, whose fossils are found in Africa, India, and Antarctica. This pig-like herbivore, which lived around 250 million years ago, was unable to swim, making its widespread distribution a strong indicator of a past land connection. The fern Glossopteris, a woody, seed-bearing plant that flourished during the Permian period, left fossils across all southern continents, including South America, Africa, Australia, India, and Antarctica. The seeds of Glossopteris were too heavy to be dispersed across oceans by wind, suggesting these landmasses were once a single habitat.
Geological Similarities
Wegener also pointed to matching geological features across continents. He observed that mountain ranges of similar age and composition aligned when the continents were reassembled. For instance, the Appalachian Mountains in eastern North America showed striking resemblances to mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway. These geological structures, formed around the same time, suggested a shared history and formation process if these landmasses were once a single mountain belt.
Identical rock types and geological structures were found on opposite sides of the Atlantic Ocean. He argued that these rocks must have formed side-by-side and then drifted apart with the continents. The distinctive rock strata of the Karroo system in South Africa, for example, were found to be identical to those of the Santa Catarina system in Brazil. This consistency provided a strong argument that these landmasses were once connected.
Ancient Climate Clues
Wegener gathered paleoclimatic evidence that contradicted the current geographical locations of continents. He found extensive glacial deposits and evidence of glacial striations in regions that are now tropical or subtropical, such as parts of India, Africa, and South America. These ancient glacial features indicated that these areas were once situated closer to the South Pole, where such large-scale glaciation could occur. The presence of these deposits in warm climates made sense only if the continents had moved from different latitudes over geological time.
Conversely, Wegener discovered coal deposits in Antarctica, a continent currently covered in ice and known for its frigid climate. Coal forms from the accumulation of dense vegetation in warm, moist, swampy environments. This indicated Antarctica once experienced a tropical or temperate climate, suggesting it was located closer to the equator in the distant past. These climatic anomalies, where evidence of ancient tropical conditions was found in polar regions and glacial conditions in tropical areas, showed that the continents had shifted.