Mountain Building

Mountain building has operated during the recent geologic past in several locations around the world. These relatively young mountain belts include the American Cordillera, which runs along the western margin of the America from Cape Horn to Alaska: The Alpine Himalayan chain, which extend from the Mediterranean through Iran to northern India and into Indonesia: And the mountains terrain of western Pacific, which include mature island arcs such as Japan, the Philippines, and Sumatra. Most of these young mountain belts have come into existence within the last hundred million years. Some including the Himalayas began growth as recently as 45 million years ago.

In addition to these recently formed complex (folded) mountains, several chains of much older mountains exist on earth as well. Although these stretches are deeply eroded and topographically less prominent, they clearly possess the same structural features found in younger mountains. Typical of this older group are the Appalachians in eastern United States and the Urals in the Soviet Union.

Although complex mountains differ from one another in particular details, all possess the same basic stretches. Mountains belts generally consist of roughly parallel ridges of folded and faulted sedimentary and volcanic rocks, portions of which have been strongly metamorphosed and intruded by somewhat younger igneous bodies.

In most cases the sedimentary rocks formed from enormous accumulations of deep-water marine sediments that occasionally exceeded 15,000 meters in thickness, as well as from thinner shallow-Water deposits. Moreover, these deformed sedimentary rocks are for the most part older than the mountain building event. This fact indicates that long quiescent period of deposition was followed by an episode of deformation. In order to unravel the events that produce mountains, many studies are conducted in regions that exhibit ancient mountain structures as well as sites where upward displacement of the earth's crust is thought to be in progress. Of particular interest is active subduction zone, where plates are converging. Here partial melting of a sub-ducted plates and possibly frictional heating of mantle rocks generate a supply of magma that migrates upward.

At sites where oceanic crust is being sub-ducted, continental blocks are also being rafted towards one another. Recent studies indicate that the most important cause of upward movement of earth crust is the collision of two or more of these crustal fragments. Collision can be occurring between a continental block and a variety of land mass, including archipelagos such as the Aleutian Islands, or small crustal fragments similar in size to Madagascar, or even other continental sized blocks.