II: Historical Perspective

Continental Drift

It was in 1912 that the idea of moving continents was seriously considered as a full-blown scientific theory -- called Continental Drift-- introduced in two articles published by a 32-year-old German meteorologist named Alfred Lothar Wegener. He contended that, around 200 million years ago, the supercontinent Pangaea began to split apart. Alexander Du Toit, Professor of Geology at Johannesburg University and one of Wegener's staunchest supporters, proposed that Pangaea first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. Laurasia and Gondwanaland then continued to break apart into the various smaller continents that exist today. Wegener's theory was based in part on what appeared to him to be the remarkable fit of the South American and African continents, first noted by Abraham Ortelius three centuries earlier. Wegener was also intrigued by the occurrences of unusual geologic structures and of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean. He reasoned that it was
physically impossible for most of these organisms to have swum or have been transported across the vast oceans. To him, the presence of identical fossil species along the coastal parts of Africa and South America was the most compelling evidence that the two continents were once joined.

In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching fossil occurrences but also the evidence of
dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to
the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation
could grow. Other mismatches of geology and climate included distinctive fossil ferns (Glossopteris) discovered in now-polar regions, and the
occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa.

The theory of continental drift would become the spark that ignited a new way of viewing the Earth. But at the time Wegener introduced his theory,
the scientific community firmly believed the continents and oceans to be permanent features on the Earth's surface. Not surprisingly, his proposal was not
well received, even though it seemed to agree with the scientific information available at the time. A fatal weakness in Wegener's theory was that it could
not satisfactorily answer the most fundamental question raised by his critics: What kind of forces could be strong enough to move such large masses of
solid rock over such great distances? Wegener suggested that the continents simply plowed through the ocean floor, but Harold Jeffreys, a noted English
geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plow through the ocean floor without breaking up.
Undaunted by rejection, Wegener devoted the rest of his life to doggedly pursuing additional evidence to defend his theory. He froze to death in 1930
during an expedition crossing the Greenland ice cap, but the controversy he spawned raged on. However, after his death, new evidence from ocean
floor exploration and other studies rekindled interest in Wegener's theory, ultimately leading to the development of the theory of plate tectonics.

Pangea


[ interactive pangea animation ]

Point out original continental mass (Pangea) and how the continents have moved into their current positions

Drifting Continents Theory TODAY

Modern day scientists collect data confirming plate movement using an orbiting satellite called LAGEOS. The LAGEOS satellites are passive vehicles covered with retroreflectors designed to reflect laser beams transmitted from ground stations. By measuring the time between transmission of the beam and reception of the reflected signal from the satellite, stations can precisely measure the distance between themselves and the satellite. These distances can be used to calculate station positions to within 1-3 cm. Long term data sets can be used to monitor the motion of the Earth's tectonic plates, measure the Earth's gravitational field, measure the "wobble" in the Earth's axis of rotation, and better determine the length of an Earth day.


[ LAGEOS summary ]