Wegener Proposes Idea of Continental Drift
Wegener and Continental Drift
Ever since the continents were all mapped, people had noticed that many coastlines, like those of South America and Africa, looked as though they would fit together if they could be moved like puzzle pieces.
Alfred Wegener was one of those people. Though trained as an astronomer, he was a specialist on Greenland. He noticed that, based on nineteenth-century longitude determinations, it appeared that Greenland had moved a mile away from Europe in a hundred years. And Paris and Washington, D.C., seemed to be moving apart by about 15 feet each year while San Diego and Shanghai got about six feet closer. On top of that, Wegener learned that related species, too small to swim the oceans, were found on different continents, as were similar fossils.
In 1912 he proposed that the continents we know today were once all attached in a single landmass he called Pangaea (Greek for "all earth"). They were surrounded by one global ocean, but then broke apart and somehow "drifted" to their separate places on the globe. Although the calculations of Greenland's movement were found to be due to faulty determinations of longitude, the other evidence seemed to match up: the shape of the continents, fossil evidence, matching rock types and geologic structures, and evidence of ancient climate patterns. But Wegener could not come up with an acceptable way to explain how the continents moved.
He knew that any argument based simply on the jigsaw fit of the continents could easily be explained away as a coincidence. To strengthen his case he drew from the fields of geology, geography, biology and paleontology. Wegener questioned why coal deposits, commonly associated with tropical climates, would be found near the North Pole and why the plains of Africa would show evidence of glaciers. Wegener also presented examples where fossils of exactly the same prehistoric species were distributed where you would expect them to be if there had been Continental Drift (e.g. one species occurred in western Africa and South America, and another in Antarctica, India and central Africa).
Few people accepted Wegener's views in his day, but they became the center of heated debate. The year after Wegener died; Arthur Holmes published his idea that thermal convection currents in the earth's mantle could cause continents to move. Holmes also suggested that the continents didn't move but were "carried" by larger pieces of the earth's crust. The controversy quieted down and fell from prominence until the 1960s, when new evidence was brought forward. The new evidence included the discovery of the mid-ocean Ridge. Because it generally exists only in the darkest and deepest parts of the oceans, the mid-ocean ridge was not discovered until the 1950s.
The mid-ocean ridge is the world's longest mountain range, although it is submerged under the ocean. It is a continuous ocean ridge that stretches through all the world's oceans, including the Pacific, Atlantic, Indian, Arctic, and Southern Ocean. It exists along the boundary of tectonic plates. In fact, the only reason that the mid-ocean ridge exists is due to weaknesses in the Earth's crust at plate interfaces which permit volcanic activity. The volcanic activity generates magma, which cools to form this continuous submarine mountain range.
Now, scientists know that the mid-ocean range is continuously being formed by a phenomenon known as seafloor spreading, where convective currents of magma in the mantle push volcanic material up through divergent boundaries between tectonic plates. Ocean spreading pushes the margins of oceanic tectonic plates beneath continental plates, a process known as continental subduction. The margins of these plates are subducted into the mantle, where they melt. Because of this process, the oceanic tectonic plates are all relatively young, under 100 million years in age.
Though not without problems, the theory of continental drift has gained wide acceptance. It is the most complete theory of global dynamics yet, and its roots lie in the work of Alfred Wegener.
Read “Wegener and Continental Drift” and answer the questions.
Name: ______Date: ______Period:____
1. What evidence did Alfred Wegener use to support his theory of continental drift?
2. Why do you think people didn’t believe the theory when Wegener first explained it?
3. What is the mid-ocean ridge?
4. How is the mid-ocean ridge created?