Examination of a relief map of the world will reveal a number of major mountain chains arranged in long linear arcs. Notable among these are the Alpine-Himalayan system and the Circum-Pacific system comprising the Andes, the Rockies and island chains of east Asia. These systems each consist of series of fold structures aligned in a roughly parallel manner. They appear to have formed as a result of the movement of plates during the earth's geological history.
These episodes may mark the closure of former oceans and folded. Accumulations of over 8 km. of sediment are involved in the Appalachian mountain system, for a instance, The sedimentary rock here and elsewhere characteristically exhibit signs of being deposited near continental edges and consist of sandstone-shale alternations sometimes known as flysch.
The transformation of these sediments into mountains may have been the result of both compressional forces and isostatic uplift In some mountain systems, the uplift also appears to be associated with the intrusion of large bodies of igneous rock. The granitization of the root zone may lead to a reduced rock density and increased volume, which causes the whole orogenic belt to rise. However, by whatever mechanism the chain is formed, gravitational sliding of the upper central areas often seems to have accompanied uplift, creating nappe structures or complex folds.
Many of the world's largest mountain chains exist beneath the sea Some of these are revealed island arcs, as in the West Indies and in the west and south-west Pacific ocean. Associated with them are deep oceanic trenches on the convex sides of the arc. These features are a direct result of the movement of crustal plates. The ocean deep marks the site of the downward plunging of the lower plate into the mantle. These sites are known as subduction zones. Melting of rocks here in the mantle gives rise to surface volcanic activity which is the basis of the island arcs. The mid--oceanic ridges form the longest mountain chains. The mid-Atlantic ridge rises 3 km above the floor of the Atlantic; it is connected with the Indian Ocean ridge, and hence with the Pacific-Antarctic ridge, resulting in a continuous system some 40,000 km in length.
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