What are the main characteristics of the folded mountains?

Folded Mountains, especially the young folded mountains, represent the world's highest and most rugged mountains, and belong to the recently formed Alpine sys­tem.

The young folded mountains of the Alpine system circle the Pacific, and cross Eurasia from the Mediterranean to the Pacific, including such high ranges as the Andes, Sierra Nevada of California, the Cascades, the Canadian Rockies, the Alaska Range, the Alps, the Karakorum Ranges, and the Himalaya.

Characteristics of the folded mountains

(i) Folded mountains, young or old, were formed in a special type of narrow and elongated sea, called geosyncline.

(ii) These mountains are composed of sedimentary, igneous as well as metamorphic rocks, but the sedimentary rocks predominate and contain fossils which are marine is nature.

(iii) All folded mountains grew out of the sediments laid down is the geosyncline of a great geosynclinal sea in Mesozoic times.

(iv) Folded mountains are characterized by longer length and much shorter width. The Himalayas, for example, are 2400 km long and only 400 km wide.

(v) Young folded mountains represent the highest mountains on the earth. They have also highest mountain summits. Mt. Everest is the most typical example (8848m).

(vi) These mountains are generally arcuate in shape, and their length far exceeds their width.

(vii)The sediments deposited is the sea of Tethys were subjected to great pressure exerted by forelands, so young folded mountains have been characterized by widespread folds, faults and over-thrust masses.

(viii) Having been subjected to great pressure exerted by the tectonic forces, the rocks were subjected to intensive heat and pressure, so that they were metamorphosed on a large scale.

(ix) The great fractures created by faults provided suitable places for the deposits of valuable minerals such as gold, silver, tungsten, etc.

(x) Generally folded mountains are situated along the margins of the continents. Rockies and Andes, the two young folded mountains are found along the western margins of North and South Americas respectively. The Alpine-Himalayan Mountains however, are also located along the margins of continent flanked by the Tethys Sea.

(xi) As regards the structural characteristics of Fold Mountains, they include alternations of more or less open anticlines and synclines; closely packed isoclinal folds; and recumbent folds and thrusts.

Geosynclines:

Geosynclines are the birth places of the folded mountains on the earth. The geosynclinals concept is associated with the work done on the Appalachians.

It was discovered by James Hall in 1859 that the folded Paleozoic sediments of the mountain ranges are shallow-water marine types and their thickness far exceeded that of the unfolded strata of the same ages in the interior lowlands to the west.

Hall arrived at the conclusion that such a thick accumulation of sediments forming shales, limestones and sandstones clearly shows that the floor of the older underlying rocks must have subsided proportionately.

There was a long period of down-warping. During this period a perfect balance was reached between subsidence and sedimentation. In 1873, Dana coined the term 'geosynclinals' for such elongated belts of long-continued subsidence and sedimentation.

However, later on the term 'geosynclinals' was modified as 'geosyncline' which is being used even today. Thus, it is clear why the concept of geosynclines is ascribed to Hall and Dana.

In fact, Hall was the person who first of all suggested that mountain ranges and geosynclines are closely connected.

Keeping in view the huge thickness of sediments forming the Fold Mountains the suggestion was made that, as the sedimentation continued, the sea floor subsided so as to keep pace with it.

Geosyncline may be defined as a major structural down-fold in the earth's crust on a sub-continental scale.

It comprises a long and narrow basin in which considerable thick masses of sediment derived from neighbouring land masses are deposited. As the load of sediments continued to grow, there was progressive subsidence of the geosyncline floor.

Now, the process of mountain building starts and is completed in three stages as discussed below:-

1. Lithogenesis:

This stage marks the beginning of the rocks due to accumulation of sediments in the narrow and shallow geosyncline. It is interesting to note that there is perfect balance in the rate of deposition and subsidence, so that depth of the shallow sea remains the same.

2. Orogenesis:

Lateral compression caused by the movement of forelands causes folding in the sediments accumulated in the geosyncline. Here and there rocks tend to undergo faulting also. Blocks of rocks overlie each other creating recumbent folds and nappes. Ultimately a time comes when the sediments are compressed and folded. Because of the movements of foreland, uplift begins to take place and sediments are squelched out.

3. Glyptogenesis:

In the third stage, the orogenesis continues for a long time with the result that the mountains begin to gain height. At this stage of the growth of mountains, the process of sub-aerial denudation starts working.

The debris of eroded material from the newly born mountains is deposited in the adjacent depression or seas. It leads to the disturbance in the isostatic equilibrium. The mountains are forced to rise to re-establish the balance.

Thus, uplift and erosion continue simultaneously. As a fold mountain is being formed, there may be granite emplacement at the deeper level.

The deeper strata of the sedimentary accumulation often become highly metamorphosed due to very high pressure and heat.

Young folded mountains:

The highest mountain systems of the present day are: the Himalayas of India, the Andes of South America, and the Alps of Southern Europe.

These mountains generally exceed heights of 3000 meters above sea level and the highest peaks rise above 8000 meters (Mt. Everest).

The main belts of young folded mountains are found on the western margins of the North and South America. They also occur in a zone from Southern Europe to northern India.

The young folded mountains consist of great thickness of low-density continental sial, including granite, gneisses, and sedimentary rocks. The Alps have a sial thickness of 10 km.

The complex structures into which the rocks have been deformed include large overturned folds, several hundred kilometers across and thrust faults, showing the intense compression that has taken place.

The rocks in the cores of these mountain ranges, exposed by erosion, include highly metamorphosed rocks and huge granite batholiths aligned with the trend of the mountain ranges. These rocks indicate deep burial of large sections of the earth's crust at the time of mountain building.

Types of Orogenies

Orogenesis literally means the birth of mountains ('oros' comes from the Greek meaning mountain). An orogeny is a mountain - building event occurring over millions of years that thickens the continental crust. It can occur through large scale deformation and uplift of the crust.

(i) Oceanic plate and continental Plate collision orogenesis occurring along the Pacific coast of the Americas has formed the Sierras of Central America, the Andes, the Rockies, and other western mountains. There are folded sedimentary rocks and intrusions of magma forming granitic plutons at the heart of these mountains.

(ii) Oceanic plate and Oceanic plate collision occurs where portions of oceanic crust collide. Such collisions produce either simple volcanic island arcs or more complex areas like Japan that include deformation and metamorphism of rocks, and granitic intrusions.

These processes have formed the chains of island arcs and volcanoes that continue from the South­western Pacific to the Western Pacific, the Philippines, the Kuriles, and through parts of the Aleutians.

Originally, the mechanism of lateral movements and resultant folding was considered to be the direct cause of mountain building and hence was given the name orogenesis.

However, there is evidence to suggest that subsequent to folding, vertical uplift may often take place to create mountains, as in the case of the Appalachians. Hence, orogenesis as usually defined is not necessarily sufficient for mountain building to occur.

Besides horizontal forces which are important in controlling the configuration of rocks found within mountains, there core certain other factors which also contribute in the generation of relief.

It may be pointed out that both the oceanic-continental and oceanic-oceanic plate collision types are active around the Pacific Rim. Both are thermal is nature, because the diving plate melts and migrates back toward the surface as magma.

This region around the Pacific is known as the Circum-Pacific belt or the ring of fire for its number of volcanoes.

(iii) Continental plate and Conti­nental plate collision occurs when two large continental masses collide. In this case the orogenesis is mechanical. Large masses of continental crust are subjected to intense folding, over-thrusting, faulting and uplifting.

Deformation of shallow and deep marine sediments and basaltic oceanic crust is pro­duced by crushing as the plates collide. The collision of India with the Eurasian land- mass produced the Himalayan Mountains.

The disruption created by that collision has reached far under China, and frequent earth­quakes that occur there indicate the conti­nuation of this collision. As is well known, the Himalayas are the tallest above-sea level mountains on earth including Mount Everest at 8848 m elevation.

As orogenic belts increase is elevation, weathering and erosion operate to reduce the mountains. The mountain mass is in conti­nued isostatic adjustment as it builds and wears away simultaneously.