Betty Kern, age 15,of Tucson, Ariz., for her question:
CAN YOU EXPLAIN THE CONTINENTAL DRIFT?
Continental drift is the name of a theory that says the continents of the earth have moved great distances across the world's surface and that they are still moving today. A German meteorologist named Alfred Wegener is considered the father of the theory.
According to the theory introduced in 1912, the continents once formed part of a single land mass. Wegener proposed the name of Pangaea for the former supercontinent.
Although much of the earth's land now lies in the Northern Hemisphere, half of Pangaea was in the Southern Hemisphere. What is now the United States lay near the equator, perhaps far east of its present position. India, which at that time was not part of Asia, lay near the South Pole.
About 200 million years ago, according to the theory, Pangaea started to break up into two large land masses called Gondwanaland and Laurasia. These masses started to break up into the continents which then began to drift toward their present locations.
Some of the continents moved in straight paths but others rotated. Most of them drifted only about one inch a year. But India may have moved an average of two inches each year.
During the mid 1900s, earth scientists gathered information that appears to support the continental drift theory. Geological studies of ancient mountain systems show a connection between the continents.
Some of the studies suggest that the Appalachian Mountains of the eastern United States extended through Newfoundland. The Appalachians may have been connected to the Caledonian mountain system, which runs through Northern Ireland, Scandinavia and Scotland.
Other evidence for such a connection comes from paleontologists, who are scientists involved in the study of fossils. They have found fossils of similar land mammals in 100 million year old rocks in Asia, Europe and North America.
In spite of all the evidence that the continents had moved, scientists could not explain until the 1960s how they might have moved. The answer was closely related to an idea proposed 30 years earlier by a Scottish geologist named Arthur Holmes.
Holmes had noted that hot rock rose from deep within the earth's mantle, the layer beneath the crust. As the rock neared the surface, it cooled and sank back into the mantle.
Holmes suggested that these circulating movements, called convection currents, could cause continental drift.
American oceanographers H.H. Hess and Robert Dietz, working independently in 1961 and 1962. suggested that convection currents carry molten rock up to the oceanic ridges and force it into large cracks in the ridges. As the molten rock hardens, it pushes the ocean floor and the continents away from the ridges.
Earth scientists believe that the movement involves more than just the ocean floor and the continents. They think the entire upper 60 miles of the earth's surface, called the lithosphere, is in motion as a rigid plate.