POSTED 3 NOVEMBER 2005
Shifting continents
These days, it's not news that Earth's crust is divided into hefty tectonic
plates, or that the continents move about over geologic time. But a century
ago, anybody who looked at, for example, the coasts of South America and
Africa probably thought the interlocking shapes resulted from mere chance.
Enter
Alfred Wegener, a German meteorologist who had plenty of time to ponder
this kind of issue as he spent the winter in an Arctic
hut recording the weather.
German meteorologist Alfred Wegener first came up with "continental drift," the idea that continents have moved around the globe over geologic time. In 1912, he startled the Geological Association in Frankfurt with his radical theory that Africa and South America had once been connected by a continent-size land bridge that had since sunk into the sea. Photo: NASA
In 1915, Wegener published "The Origin of Continents and Oceans," suggesting that all the continents had once been part of a giant super-continent called Pangea. Arguing that the continents had moved gradually to their present positions, Wegener called his revolutionary theory "continental drift."
The theory was greeted, if "greeted" is the right word, as heresy.
Neither Wegener nor anybody else could figure out what kind of truck had moved the continents (talk about an oversize load!), and his theory was scorned as the junk science of his day. W.B. Scott, former president of the American Philosophical Society, called the theory of continental drift "utter damned rot."(!)
Stepping up to the plates
It took half a century -- almost a geologic epoch -- for an updated version
of continental drift to become accepted by mainstream geologists. Let's
take a look at how the theory developed, and how scientists gathered evidence
to support it.
1858 Antonio Snider-Pellegrini suggests that continents were linked during the Pennsylvanian period (325 million to 286 million years ago), because plant fossils from that era looked alike, even though they were found in Europe and North America. (Here's a handy geologic time line.)
1885 Austrian geologist Edward Seuss sees similarities between plant fossils from South America, India, Australia, Africa and Antarctica, and coins "Gondwanaland" for a proposed ancient super-continent with these land masses.
1910 American physicist F.B. Taylor proposes concept of continental drift to explain formation of mountain belts.
1912-15 German meteorologist Alfred Wegener proposes theory of continental drift, based on evidence from geology, climatology and paleontology. Wegener names one of the ancient super-continents "Pangea," and draws maps showing how the continents moved to today's positions.
1920s-60s Assorted arguments are used to debunk continental drift, most importantly the lack of a mechanism for moving continents across ocean basins.
1937 South African geologist Alexander du Toit maps out a northern super-continent, "Laurasia," to explain coal deposits, which presumably indicate the remains of equatorial plants, in the Northern Hemisphere.
Since 1960, a mass of evidence has been accumulated for plate tectonics, the mechanism of continental drift, as well as earthquakes, and the status of continental drift had changed from heresy to conventional wisdom. We'll just hit some scientific highlights:
Ancient magnetism: British scientists found that magnetic fields recorded in rocks from Europe and North America indicate the rocks had formed far from where they were found. The pattern of continental drift recorded by rocks showed Europe and North America had drifted apart for more than 100 million years as the Atlantic Ocean opened up.
Fossils: Remains the plant genus Glossopteris occur on all five Gondwana continents. The seeds were too heavy to be carried by wind, and would have died quickly in salt water, indicating that the continents were once joined. Similarly, fossils of the same reptiles occur on several continents.
Over the eons, continents move about. This is a
reconstruction of the continents more than 200 million years ago. The
colored blobs show modern locations of fossils found on the modern continents.
Logic says that during a previous epoch, the continents were joined as
shown into the super-continent Pangea. Graphic: NASA
Mountains: The Appalachian Mountains extend to mountains in Greenland, the United Kingdom and Norway, indicating that these land masses were once joined.
Glaciers: Glacial scoring on rocks in Africa, India, South America and Australia make no sense in terms of how the continents are located now. But if these continents were once arrayed around the South Pole, the glacial scars would have all pointed away from the Pole. (Today, glaciers form near the poles and travel away as they melt.)
Ocean floors: Ocean floors are formed by spreading away from the mid-oceanic ridges, indicating that the floor is moving. The oldest rocks on the ocean floor are 220 million years old, while the oldest terrestrial rocks are about 4 billion years old. This indicates that the ocean floor is recycled back into the Earth at subduction zones near the continents.
So where does this leave the earthquake prediction problem?
