Title

Dedicated to the teaching of Earth Sciences

GEOLOGY RESOURCES FOR STUDENTS,
TEACHERS, AND RESEARCHERS

Prof. Sean Tvelia Suffolk County Community College

 

Continental Drift

The notion that the continents have moved across the surface of the planet has been tossed around for quite some time. It is no doubt that even the earliest coastal maps must have alluded to the similarities between coastlines. Although the idea had been suggested many times before none of the early hypothesis was ever capable of explaining how continents might be capable of drfited to their present location.

Then, in the early 1900s debate was reignited when Alfred Wegner published his book: "The Origin of Continents and Oceans" In this book Wegner alleges that originally all continents were combined in one giant "supercontinent." He called this supercontinent Pangea (all world). Furthermore, Wegner proposed that around 200 million years ago Pangea began breaking apart and the continents slowly drifted to their present position. So what made Wegner so different from everyone else? As I stated previously this notion isn't anything new and had been proposed many times in the past. What made Wegner's hypothesis different was not that he had finally discovered the process but the vast ammount of evidence he collected.

Some of the first evidence collected to support the idea of continental drift was that collected by Edward Seuss and this evidence had nothing to do with geology. What Seuss noticed was that fossils of an extinct plant known as glossopteris could be found in many locations, including: India, Australia, South Africa, and South America.

On the surface this may not seem that odd but glossopteris was a seedplant; meaning that in order for it to have grown in all four areas where its fossil is found, the seeds would have to be distributed in those areas. But if glossopteris seeds were able to be distributed thousands of miles away across vast oceans, why werent they distributed everywhere else? This puzzle lead Seuss to surmise that at one point the these four continents must have been together as one super continent. Seuss called this super continent Gondawanaland, after the Indian province in which he discovered the glossopteris fossil.

Unfortunately, Seuss, like those before him, could not provide a mechanism to show how these continents could have moved over time. Then in 1908 Frank Taylor and his colleague Howard Baker showed how, when lined up, rock formations within the Caledonian and Appalachian mountain ranges seemed to match. Taylor knew that this was evidence of the break up of a super continent but if his evidence for continental drift was to survive he needed to provide a mechanism. Taylor hypothesised that when the Eath captured the moon, roughly 100 million years ago, the tidal forces generated pulled the continents that were then situated near the poles to equatorial positions. Unfortunately for Taylor none of these ideas were testable and, like those before him, Taylor's theory recieved little attention. (Today, scientist believe that the moon formed during a cataclysmic collision that nearly destroyed the Earth 4.6 billion years ago)

In 1912 Alfred Wegner presented a paper titled, "The formation of the Major Features of the Earth's Crust" to the Geological Association in Frankfurt. In the talk Wegener challenges the belief that land bridges (a hypothesis promoted by other scientist to explain similar species on distant lands, could have existed. However, Wegener was a meteorologist not a geologist and so his theories were met with much contempt. In fact Wegner was considered, by some, a trespasser of science. This did not deter him.

Three years later Wegener published "On the Origin of Continents and Oceans " with three other volumes printed in 1920, 1922, and 1929. Each text provided more and more evidence to support continental drift. After all, Wegner argued, if fossil evidence suggested that continents were once linked and sunken land bridges do not obey accepted theory of isostacy only one conclusion remains.

One of the greatest questions concerning continental drift was how continents, made of solid rock, could move throughout the Earth's solid rock crust. To begin answering this Wegner likened the Earth to a piece of pitch (natural asphalt). At normal condition of temperature and pressure pitch behaves like a brittle solid; if it is struck by a hammer it will shatter. However, if placed under a continued pressure pitch will begin to deform plastically. Perhaps, Wegener hypothesised, the earth behaved in a similar manner. Under constant pressure from the continents maybe the ocean floors behaved more plastically and continents were able to move through the ocean crust much like an icebreaker moves through ice.

Chapter Contents:

3.0: Continental Drift

3.1: Evidence of Continental

3.2: Developing the Theory

3.3: Paleomagnetism and Continental Drift

3.4: Plate Tectonics