Types of Mountain, What is Mountain, Characteristics of Mountains, Fold Mountains, Block Mountains, Volcanic Mountains, Residual Mountains
What is Mountain
Since the dawn of geological time, no less than nine orogenic or mountain building movements have taken place,folding and fracturing the earth’s crust. Some of them occurred in Pre Cambrian times about 600-3,500 million years ago. The three more recent orogenic are the Caledonian, Hercynian and Alpine. The Caledonian about 320million years ago raise the mountains of Scandinavia and Scotland, and is represented in North America. These ancient mountains have been worn down and no longer exhibit the striking forms that they must once have had.
In a later period, during the Hercynian earth movements, about 240 million years ago, were form such ranges as the Ural Mountains, the Pennines and Welsh Highlands in Britain, the Harz Mountains in Germany and the Appalachians in America. These mountains have also been reduced in size by the various sculpturing forces.
The last of the major orogenic movements of the earth, the Alpine, occurred about 30 million years ago. Young fold mountain ranges were formed on a gigantic scale. Being the most recently formed, these ranges, such as the Alps, Himalayas, Andes and Rockies are the loftiest and the most imposing. Their peaks are sometimes several miles high.
Types of Mountains
Based on their mode of formation, four main types of mountains can be distinguished:
1. Fold Mountains
These mountains are the most widespread and also the most important. They are caused by large-scale earth movements, when stresses are set up in the earth’s crust. Such stresses may be due to:
– the increased load of the overlying rocks,
– flow movements in the mantle,
– magmatic intrusions into the crust, or • the expansion or contraction of some part of the earth.
When such stress initiates, the rocks are subject to compressive forces that produce wrinkling or folding along the lines of weakness. As illustrated, folding effectively shortens the earth’s crust, creating from the original level surface a series of ‘waves’.
The upfold waves are call as anticlines and the troughs or downfolds are call as synclines. Due to the complexity of the compressional forces, the folds may develop much more complicated forms. When the crest of a fold is pushed too far, an overfold is formed. If it is pushed still further, it becomes a recumbent fold. In extreme cases, fractures may occur in the crust, so that the upper part of the recumbent fold slides forward over the lower part along a thrust plane forming an over thrust fold. The over-riding portion of the thrust fold is termed a nappe.
Since the rock strata have been elevated to great heights, sometimes measurable in miles, fold mountains maybe called mountains of elevation. The fold mountains are also closely associated with volcanic activity. They contain many active volcanoes, especially in the Circum-Pacific fold mountain system. They also contain rich mineral resources such as tin, copper, gold and petroleum.
Characteristics of Fold Mountains
– Fold mountains are the youngest mountains on the surface of the Earth. • Young folded mountains represent the highest mountains on the earth. They also have the highest mountain summits. Mt. Everest is the most typical example (8848m).
– Fold mountains have been formed due to the folding of sedimentary rocks formed due to the deposition and consolidation of sediments in water bodies mainly in the oceanic environment.
– The sedimentary rocks of the fold mountains were deposited in shallow seas. The greater thickness of sediments is possible due to the continuous sedimentation and subsidence.
– The length of the fold mountains is much more than their width. The east-west extent of the Himalayas is about 2400 km, but their north-south width is only 400 km. Thus the fold mountains must have been form in long, narrow and shallow seas.
– Fold mountains are generally arc-shaped, having a concave slope on one side and convex on the other.
– Fold mountains are found along the margins of the continents facing ocean such as the Andes and the Rockies. If we consider the former Tethys Sea, then the Himalayas are also located along the margins of the continent.
– Fold mountains are mostly located in two directions. In the north-south direction lie the Rockies and the Andes, while in the west-east direction lie the Himalayas and the Alps.
Human activity surrounding fold mountains
– Winter sports such as skiing in resorts.
– Climbing and hiking in the summer months.
– Agriculture – takes place mainly on south facing slopes and includes cereals, sugar beet, vines and fruits.
– Forestry – coniferous forests for fuel and building.
– Communications – roads and railways follow valleys.
– Hydroelectric power (HEP) – steep slopes and glacial melt water are ideal for generating HEP. Hydroelectric accounts for 60 per cent of Switzerland’s electricity production.
2. Block Mountains
When the earth’s crust bends, folding occurs, but when it cracks, faulting takes place. Faulting may be cause by tension or compression, forces which lengthen or shorten the earth’s crust, causing a section of it to subside or to rise above the surrounding level. Earth movements generate tensional forces that tend to pull the crust apart and faults are develope. If the block enclosed by the faults remains as it is or rises, and the land on either side subsides, the upstanding block becomes the horst or block mountain. The faulted edges are very steep, with scarp slopes and the summit is almost level, e.g. the Hunsruck Mountains, the Vosges and Black Forest of the Rhineland.
Tension 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 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 raise or lower in relation to surrounding areas. It illustrates a rift valley formed in this way. In general large-scale block mountains and rift valleys are due to tension rather than compression.
3. Volcanic Mountains
These form from material which ejects 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 successive layers, building up a characteristic volcanic cone. Volcanic mountains are often call as 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), Mt. Agung (Bali) and Mt. Catopaxi (Ecuador).
4. Residual Mountains
These are mountains evolve by denudation. Where the general level of the land has been lower 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 plateaux which have been dissect by rivers into hills and valleys like. In these type of mountains, the ridges and peaks are all very similar in height.