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How did the continents separate?
Asked by: Mikey Lee
School: St. John the Evangelist
Teacher: Anu Rai
Hobbies/Interests: Football, baseball, basketball
Career Interest: Construction worker
Answer from H. Richard Naslund
Professor of Geological Sciences
Research area: Igneous rocks, ore deposits, volcanoes Interests/hobbies: Traveling, scuba diving, collecting masks
This is a great question, because it is a problem that scientists struggled with for more than 50 years. The Theory of Continental Drift was first proposed by Alfred Wegener in 1912, but it wasn’t until the late 1960s that it was widely accepted. The fit of the continents across the Atlantic had been noticed since the 1600s, but Wegener was the first to suggest that the continents had actually moved. Wegener observed that the ancient fossil assemblages in South America, Africa, India, and Australia were remarkably similar, in spite of the fact that today the plants and animals in these areas are very different. He also found that glaciation had occurred in the Southern Hemisphere 320 million years ago, but appeared to have resulted from ice build-up over the oceans that then flowed onto land. Modern glaciers build up on land and then flow toward the ocean. If the continents had been together in the past, these problems with fossils and glaciation disappear. Wegener’s ideas were not widely accepted because he could not figure out a mechanism that could move a continent! In the early 1960s, studies of the ocean floor revealed that there was a volcanic mountain range called the Mid-Atlantic Ridge that ran down the center of the Atlantic Ocean. Ocean crust along the ridge is less than 1 million years old, and the crust becomes progressively older as you move away from the ridge in either direction. The oldest crust is about 180 million years old, and is located next to the edge of the continents on either side of the Atlantic. It was also found that the deepest parts of the Pacific Ocean occur along the margins of the ocean basin where old ocean crust dives down into the Earth’s mantle, and is "subducted" under the continents. The idea that the continents were moving apart seemed inescapable, but we still did not know the mechanism. Some argued that plates were pushed apart along the ocean ridges, while others argued that plates were pulled apart along the subduction zones. The North American plate is about 60 miles thick and stretches from Iceland to California, a distance of over 4,000 miles. It is not possible for such a thin and brittle sheet of rock to be either pushed or pulled for any distance. We now realize that the crustal plates are carried along on giant convection cells operating in the Earth’s mantle. Although the mantle is solid rock, it is under such tremendous pressure that it is able to flow in response to being heated at the bottom and cooled at the top, much like water in a teapot. Heated water convects at a rate of feet per second, however, while the solid mantle convects at a rate of inches per year. Today we are able to confirm the movement of crustal plates using very accurate GPS measurements. Wegner’s original Theory of Continental Drift has been renamed the Theory of Plate Tectonics to reflect our new understanding of the mechanisms behind it, and the effects that it has on the distribution of earthquakes, volcanoes, mountains and ore deposits.
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