During the 1600s, as better world maps became available, people noticed that continents, particularly South America and Africa, could be fit together like pieces of a jigsaw puzzle. However, little significance was given to this observation until 1915, when Alfred Wegener (1880–1930), a German meteorologist and geophysicist, wrote The Origin of Continents and Oceans. This book outlined Wegener’s hypothesis, called continental drift, which dared to challenge the long-held assumption that the continents and ocean basins had fixed geographic positions.

Wegener suggested that a single supercontinent consisting of all Earth’s landmasses once existed.* He named this giant landmass Pangaea (pronounced “Pan-jee-ah,” meaning “all lands”) (Figure 2). Wegener further hypothesized that about  million years ago, during a time period called the Mesozoic era, this supercontinent began to fragment into smaller landmasses. These continental blocks then “drifted” to their present positions over a span of millions of years.

Wegener and others who advocated the continental drift hypothesis collected substantial evidence to support their point of view. The fit of South America, and Africa, and the geographic distribution of fossils and ancient climates, all seemed to buttress the idea that these now separate landmasses had once been joined. Let us examine some of this evidence.

Evidence:
The Continental Jigsaw Puzzle

Like a few others before him, Wegener suspected that the continents might once have been joined when he noticed the remarkable similarity between the coastlines on opposite sides of the Atlantic Ocean. However, other Earth scientists challenged Wegener’s use of present-day shorelines to “fit” these continents together. These opponents correctly argued that wave erosion and depositional processes continually modify shorelines. Even if continental displacement had taken place, a good fit today would be unlikely. Because Wegener’s original jigsaw fit of the continents was crude, it is assumed that he was aware of this problem (refer to Figure 2).

Scientists later determined that a much better approximation of the outer boundary of a continent is the seaward edge of its continental shelf, which lies submerged a few hundred meters below sea level. In the early 1960s, Sir Edward Bullard and two associates constructed a map that pieced together the edges of the continental shelves of South America and Africa at a depth of about  meters (3000 feet) (Figure 3). The remarkable fit obtained was more precise than even these researchers had expected.

Evidence:
Fossils Matching Across the Seas

Although the seed for Wegener’s hypothesis came from the remarkable similarities of the continental margins on opposite sides of the Atlantic, it was when he learned that identical fossil organisms had been discovered in rocks from both South America and Africa that his pursuit of continental drift became more focused. Wegener learned that most paleontologists (scientists who study the fossilized remains of ancient organisms) agreed that some type of land connection was needed to explain the existence of similar Paleozoic-age life-forms on widely separated landmasses. Just as modern life-forms native to North America are not the same as those of Africa and Australia, Paleozoic-age organisms on widely separated continents should have been distinctly different.

Mesosaurus and Lystrosaurus

To add credibility to his argument, Wegener documented several cases in which the same fossil organism is found on landmasses that are now widely separated, even though it is unlikely that the living organism could have crossed the barrier of a broad ocean (Figure 4). A classic example is Mesosaurus, a small aquatic freshwater reptile whose fossil remains are mainly found in shales of Permian age (about  million years ago) in eastern South America and southwestern Africa. If Mesosaurus had been able to make the long journey across the South Atlantic, its remains should be more widely distributed. Because this is not the case, Wegener asserted that South America and Africa must have been joined during that period of Earth history. Similarly, fossils of Lystrosaurus, a land reptile, were found in Antarctica, India, and South America. Like the Mesosaurus, the Lystrosaurus would not be capable of swimming across an ocean, and as is the case with the Mesosaurus fossils, the Lystrosaurus fossils provide evidence that the continents were once joined.

How did opponents of continental drift explain the existence of identical fossil organisms in places separated by thousands of kilometers of open ocean? Rafting, transoceanic land bridges (isthmian links), and island stepping stones were the most widely invoked explanations for these migrations (Figure 5). We know, for example, that during the Ice Age that ended about 10,000 years ago, the lowering of sea level allowed mammals (including humans) to cross the narrow Bering Strait that separates Russia and Alaska. Was it possible that land bridges once connected Africa and South America but later subsided below sea level? Modern maps of the seafloor substantiate Wegener’s Pangaea view and show no sunken land bridges.

Glossopteris

Wegener also cited the distribution of the fossil “seed fern” Glossopteris as evidence for Pangaea’s existence (refer to Figure 4). With tongue-shaped leaves and seeds too large to be carried by the wind, this plant was known to be widely dispersed throughout Africa, Australia, India, and South America. Later, fossil remains of Glossopteris were also discovered in Antarctica. Wegener also learned that these seed ferns and associated flora grew only in cool climates—similar to central Canada. Therefore, he concluded that when these landmasses were joined, they were located much closer to the South Pole.

Evidence:
Rock Types and Geologic Features

You know that successfully completing a jigsaw puzzle requires maintaining the continuity of the picture while fitting the pieces together. Similarly, in the case of continental drift, the rocks on either side of the Atlantic that predate the proposed Mesozoic split should match up to form a continuous “picture” when the continents are fitted together, as Wegener proposed.

Indeed, Wegener found such “matches” across the Atlantic. For instance, highly deformed igneous rocks in Brazil closely resemble similar rocks of the same age in Africa. Also, the mountain belt that includes the Appalachians trends northeastward through the eastern United States and disappears off the coast of Newfoundland (Figure 6A). Mountains of comparable age and structure are found in the British Isles and Scandinavia. When these landmasses are positioned as Wegener proposed (Figure 4.6B), the mountain chains form a nearly continuous belt. As Wegener wrote, “It is just as if we were to refit the torn pieces of a newspaper by matching their edges and then check whether the lines of print run smoothly across. If they do, there is nothing left but to conclude that the pieces were in fact joined in this way.”

Evidence:
Ancient Climates

Because Wegener was a student of world climates, he suspected that paleoclimatic (paleo = ancient) data might also support the idea of mobile continents. His assertion was bolstered by the discovery of evidence for a glacial period dating to the late Paleozoic era in southern Africa, South America, Australia, and India. This meant that about  million years ago, vast ice sheets covered extensive portions of the Southern Hemisphere as well as India (Figure 7A). Much of the land area that contains evidence of this Paleozoic glaciation presently lies within 30° of the equator, in subtropical or tropical climates.

How could extensive ice sheets form near the equator? One proposal suggested that our planet experienced a period of extreme global cooling. Wegener rejected this explanation because during the same span of geologic time, large tropical swamps existed in several locations in the Northern Hemisphere. The lush vegetation in those swamps was eventually buried and converted to coal (Figure 7B). Today, these deposits comprise major coal fields in the eastern United States and Northern Europe. Many of the fossils found in these coal-bearing rocks were produced by tree ferns with large fronds—ferns that would have grown in warm, moist climates.* The existence of these large tropical swamps, Wegener argued, was inconsistent with the proposal that extreme global cooling caused glaciers to form in areas that are currently tropical.

Wegener suggested a more plausible explanation for the late Paleozoic glaciation: The southern continents were joined together in the supercontinent of Pangaea and located near the South Pole (refer to Figure 7B). This would account for the polar conditions required to generate extensive expanses of glacial ice over much of these landmasses. At the same time, this geography places the northern continents nearer the equator and accounts for the tropical swamps that generated the vast coal deposits that we see today with continental drift.

As compelling as this evidence may have been, 50 years passed before most of the scientific community accepted the concept of continental drift. A number of questions needed answers, including these: How does a glacier develop in hot, arid central Australia? And how do land animals cross what are now broad oceans?

The Great Debate

From 1924, when Wegener’s book was translated into English, French, Spanish, and Russian, until his death in 1930, his proposed drift hypothesis encountered a great deal of hostile criticism. The respected American geologist R. T. Chamberlain stated, “Wegener’s hypothesis in general is of the foot-loose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories.”

One of the main objections to Wegener’s hypothesis stemmed from his inability to identify a credible mechanism for continental drift. Wegener proposed that gravitational forces of the Moon and Sun that produce Earth’s tides were also capable of gradually moving the continents across the globe. However, the prominent physicist Sir Harold Jeffreys correctly argued that tidal forces strong enough to move Earth’s continents would have resulted in halting our planet’s rotation, which, of course, has not happened.

Wegener also incorrectly suggested that the larger and sturdier continents broke through thinner oceanic crust, much as icebreakers cut through ice. However, no evidence existed to suggest that the ocean floor was weak enough to permit passage of the continents without the continents being appreciably deformed in the process.

In 1930, Wegener made his fourth and final trip to the Greenland Ice Sheet. Although the primary focus of this expedition was to study this great ice cap and its climate, Wegener continued to test his continental drift hypothesis. While returning from Eismitte, an experimental station located in the center of Greenland, Wegener perished along with his Greenland companion. His intriguing idea, however, did not die.

Why was Wegener unable to overturn the established scientific views of his day? Foremost was the fact that, although the central theme of Wegener’s drift hypothesis was correct, some details were incorrect. For example, continents do not break through the ocean floor, and tidal energy is much too weak to move continents. Moreover, for any comprehensive scientific hypothesis to gain wide acceptance, it must withstand critical testing from all areas of science. Despite Wegener’s great contribution to our understanding of Earth, not all of the evidence supported the continental drift hypothesis as he had proposed it. As a result, most of the scientific community (particularly in North America) rejected continental drift or at least treated it with considerable skepticism. However, some scientists recognized the strength of the evidence Wegener had accumulated and continued to pursue the idea.

• German meteorologist Alfred Wegener formulated the continental drift hypothesis in 1915. He suggested that Earth’s continents are not fixed in place but move slowly over geologic time.

• Wegener proposed a supercontinent called Pangaea that existed about 200 million years ago, during the late Paleozoic and early Mesozoic eras.

• Wegener’s evidence that Pangaea existed and later broke into pieces that drifted apart included (1) the shape of the continents, (2) continental fossil organisms that matched across oceans, (3) matching rock types and modern mountain belts on separate continents, and (4) sedimentary rocks that recorded ancient climates, including glaciers on the southern portion of Pangaea.

• Wegener’s hypothesis suffered from two flaws: It proposed tidal forces as the mechanism for the motion of continents, and it implied that the continents would have plowed their way through weaker oceanic crust, like boats cutting through a thin layer of sea ice. Most geologists rejected the idea of continental drift when Wegener proposed it, and it wasn’t resurrected for another 50 years.

• continental drift: A hypothesis which proposed that continents drifted to their present position after fragmenting from a single supercontinent. This hypothesis has been replaced by the plate tectonics theory.

• Pangaea: The name given to a proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.

• supercontinent: A large landmass of the geologic past that contained all, or nearly all, of the existing continents.