A CHANGING FACE FOR THE EARTH
About 200 million years ago the last mass movement of the continents began. Earth then had only one consolidated land mass, today called Pangaea. It is believed that this supercontinent accumulated from migrations produced by previous drifting. Some 20 million years later sea-floor spreading had separated the supercontinent into two segments: Laurasia in the north and Gondwana in the south. About 45 million years later the North Atlantic and I ndian Oceans had widened and South America had begun to separate from Africa, while India drifted northward. During the next 70 million years the South Atlantic Ocean widened into a major ocean, the Mediterranean Sea began to open up, and North America just began to separate from Eurasia.
A computer-generated projection for the next 50 million years suggests that the Atlantic and Indian oceans will enlarge, and the Pacific will contract. Australia will continue drifting northward toward a possible collision with Eurasia. Africa's northward movement will doom the Mediterranean. In 10 million years Los Angeles, which is part of the Pacific plate, will have come abreast of San Francisco, which is sitting on the North American plate, and from there will eventually slide into the Aleutian Trench.
Average plate motions are on the order of 5 to 6 centimeters per year so that the reshaping of the earth's face is quite dramatic when one considers the age of the earth. In about 2 billion years the gradual cooling of the earth from heat loss will mean that the asthenosphere will flow less readily and that the platemotion phase of the earth's evolution will probably come to an end. Thus the earth will enter a new phase, in which the plate motions of the earth's lithosphere are not responsible for most of the large-scale terrain features. Large mountain ranges, like the Himalayas, will no longer be uplifted, and they will erode away over millions of years.
