Mountains Block Mountains • Block mountains are created when large areas or blocks of earth are broken and displaced vertically. • The uplifted blocks are termed as horsts and the lowered blocks are called graben. • Block mountains are also called fault block mountains since they are formed due to faulting as a result of tensile and compressive forces. • Block mountains are surrounded by faults on either side of rift valleys or grabens. • The Great African Rift Valley (valley floor is graben), The Rhine Valley and the Vosges mountain in Europe are examples. Compression and Tension • When the earth’s crust bends folding occurs, but when it cracks, faulting takes place. • The faulted edges are very steep, e.g. the Vosges and Black Forest of the Rhineland. • Tension may also cause the central portion to be let down between two adjacent fault blocks forming a graben or rift valley, which will have steep walls. • The East African Rift Valley system is the best example. It is 3,000 miles long, stretching from East Africa through the Red Sea to Syria. • Compressional forces set up by earth movements may produce a thrust or reverse fault and shorten the crust. A block may be raised or lowered in relation to surrounding areas. • In general large-scale block mountains and rift valleys are due to tension rather than compression. • The faults may occur in series and be further complicated by tilting and other irregularities. • Denudation through the ages modifies faulted landforms. • Block mountains may originate when the middle block moves downward and becomes a rift valley while the surrounding blocks stand higher as block mountains. Volcanic mountains • Volcanic mountains are formed due to volcanic activity. • Kilimanjaro in Africa and Mt.Fujiyama in Japan are examples of such mountains. • These are, in fact, volcanoes which are built up from material ejected from fissures in the earth’s crust. • The materials include molten lava, volcanic bombs, cinders, ashes, dust and liquid mud. • They fall around the vent in suc-cessive layers, building up a characteristic volcanic cone. • Volcanic mountains are often called mountains of accumulation. • They are common in the Circum-Pacific belt and include such volcanic peaks as Mt. Fuji (Japan) Mt. Mayon (Philippines), Mt. Merapi (Sumatra) etc. Residual mountains • These are mountains evolv-ed by denudation. • Where the general level of the land has been lowered by the agents of denudation some very resistant areas may remain and these form residual mountains, e.g. Mt. Manodnock in U.S.A. • Residual mountains may also evolve from plateaus which have been dissected by rivers into hills and valleys. • Examples of dissected plateaux, where the down-cutting streams have eroded the uplands into mountains of denudation, are the Highlands of Scotland, Scandinavia and the Deccan Plateau. Fold Mountains • Fold mountains are formed when sedimentary rock strata in geosynclines are subjected to compressive forces. • Formation of Fold Mountains was explained in Continent – Ocean Convergence – Formation of Andes, Rockies And Continent – Continent Convergence: Formation of Himalayas • They are the loftiest mountains and they are generally concentrated along continental margins. Very Old Fold Mountains • They are more than 500 million years old. • Rounded features (due to denudation). • Low elevation. • The Appalachians in North America and the Ural mountains in Russia. Old Fold Mountains • Old fold mountains had their origin before the Tertiary period (70 million years). • The fold mountain systems belonging to Caledonian and Hercynian mountain- building periods fall in this category. • They are also called as Thickening relict fold mountains because of lightly rounded features and medium elevation. • Top layers worn out due to erosional activity. • Example: Aravali Range in India. • The Aravali Range in India are the oldest fold mountain systems in India. The range has considerably worn down due to the processes of erosion. The range rose in post Precambrian event called the Aravalli-Delhi orogeny (Silurian and early Devonian periods) Alpine or young fold mountains • Alpine fold mountains belonging to the Tertiary period can be grouped under the new fold mountains category since they originated in the Tertiary period. • Examples are the Rockies, the Andes, the Alps, the Himalayas, etc. Economic Significance of Mountains • Storehouse of resources: Mountains are the storehouse of natural resources. Large resources of minerals like petroleum, coal, limestone are found in mountains. The mountains are the main source of timber, lac, medical herbs, etc. • Generation of hydro-electricity: Hydro-electricity is mainly generated from the waters of perennial rivers in the mountains. • Abundant source of water: Perennial rivers arising in the snow fed or heavily rain-fed mountains are one of the important sources of water. They help in promoting the irrigation and provide water for many other purposes. • Formation of fertile plains: The rivers that originate from the high mountain ranges bring silt along with water to the lower valleys. This helps in the formation of fertile plains and further the expansion of agriculture and related activities. • Natural political frontiers: The mountains can also act as natural boundaries between two countries. They have a prominent role in protecting the country from external threats. • Effects on climate: They serve as a climatic divide between two adjoining regions. The mountains cause orogenic rainfalls, diversion, and blocking of cold winds, etc. • Tourist centers: The pleasant climate and beautiful sceneries of the mountains have led to their development as centers of tourist attraction. .