According to the theory of continental drift, the Earth's continents are moving slowly in relation to each other. Over hundreds of millions of years, this drifting has changed the face of the Earth. A map of the world shows that the continents of North America and South America “fit together” across the Atlantic Ocean with the continents of Europe and Africa. It isn't only the continental outlines that match. The rocks of the regions on the opposite sides of the ocean match as well. A belt of ancient rocks along the Brazilian coast, for example, matches one in West Africa. Fossil plants and coal deposits in North American and Europe also match. Also, the oldest matter deposited along the Atlantic coastlines of both South America and Africa dates from the Jurassic period, about 208 to 144 million years ago. This suggests that the ocean did not exist before that time. Wandering continentsIn about 1800 the German naturalist Alexander von Humboldt noted that the bulge in eastern South America seemed to fit perfectly into the bend of Africa. This led him to form a theory that lands bordering the Atlantic Ocean had once been joined. The German scientist Alfred Wegener was the first scientist to form a detailed theory of continental drift. In 1912 Wegener brought together data from rocks and fossils and suggested that through most of geologic time, there was only one continent. He called this supercontinent Pangaea. He said that in the late Triassic Period, which lasted from about 245 to 208 million years ago, Pangaea began to split apart. The Atlantic Ocean was opened up by the westward drift of the Americas, and the land that is now the Indian subcontinent drifted across the equator to merge with Asia. Spreading seafloorsResearch convinced scientists that the ocean floor is spreading. During the early 1960s a U.S. scientist studied the mid-ocean ridges. These are underwater mountains that follow a winding course of about 37,000 miles (60,000 kilometers) along the bottom of the major ocean basins. He concluded that volcanoes at the crests of the ridges constantly generate new oceanic crusts. Molten rock material from deep in the Earth's mantle rises upward to the crests, cools, and is later pushed aside in both directions. Plate tectonicsBy the late 1960s, several U.S. investigators, among them Jack E. Oliver and Bryan L. Isacks, had combined the notions of drifting continents and seafloor spreading to explain what they call plate tectonics. This means that the Earth's surface is composed of a number of large, rigid plates that float on a soft, partially molten, layer of the Earth's mantle. The mid-ocean ridges occur along some of the plate margins. Due to volcanic activity, the plates separate and the swelling material of the mantle forms a new part of the ocean floor. As the plates move away from the sides of the ridges, the continents are carried with them. On the basis of these factors, it may be assumed that the Americas were joined with Europe and Africa until approximately 190 million years ago. The separation occurred when a rift split them apart along what is now the crest of the Mid-Atlantic Ridge. Scientists believe that the continents are continuing to move at very slow rates. |