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MOUNTAINS
Mount Ararat, as seen from Khor Virap, Armenia
PLATE TECTONIC THEORY – MOUNTAIN FORMATION
Plate tectonic theory is a comprehensive theory that offers explanations for various relief features and tectonic events viz. Mountain building, folding, and faulting, continental drift, vulcanicity, seismic events (earthquakes), etc. It envisages the formation of mountains due to the collision of plate boundaries.
Three types of plate boundaries have been identified e.g.
(i) destructive plate boundaries.
(ii) Constructive plate boundaries and
(iii) conservative plate
Two plates moving together under the impact of thermal convective currents collide against each other and the plate boundary having relatively denser materials is subducted under the other plate boundary of relatively lighter material.
This subduction of plate boundary causes lateral compressive force which ultimately squeezes and folds the sediments and materials of the margins of the plates and thus mountains are formed.
EARTH MOVEMENTS AND THE MAJOR LANDFORMS
The face of the earth is constantly being reshaped by the agents of denudation-running water, rain, frost, sun, wind, glaciers, and waves so that our present landforms are very varied and diverse.
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 between 600- 3,500 million years ago.
The three more recent orogenies are Caledonian, Hercynian, and Alpine.
The Caledonian, about 320 million years ago raised 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 formed such ranges as the Ural Mountains.
The Pennine and Welsh Highlands in Britain, the Harz Mountains in Germany, the Appalachians in America as well as the high plateau of Siberia and The Mountains have also been reduced in size by the various sculpturing forces.
We are now living in an era very close to the last of the major orogenic movement of the earth, the Alpine, about 30 million years Young fold mountain ranges were buckled up and overthrust 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 meters high.
But the time will come when these lofty ranges will be lowered like those that existed before them.
From the eroded materials, new rocks will be formed, later to be uplifted to form the next generation of mountains.
TYPES OF MOUNTAINS
Mountains make up a large proportion of the earth’s surface. Based on their mode of formation, four main types of mountains can be distinguished
FOLD MOUNTAINS.
The formation of fold mountains
These mountains are by far the most widespread and also the most important.
They are caused by large-scale earth movement 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 stresses are initiated, the rocks are subjected 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 series of ‘waves.
The unfolded waves are called anticlines and the troughs or down folds are synclines.
The formation of up-and-down folds closely resembles that of the wrinkles of a tablecloth when it pushed from either one or both sides of the table.
In the great fold mountains of the world such as the Himalayas, Rockies, Andes, and Alps, due to the complexity of the compressional forces, the foIds developed 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 overthrust 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 may be 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.
When the earth’s crust bends folding occurs, but when it cracks, faulting takes
Faulting may be caused 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.
In earth, movements generate tensional forces that tend to pull the crust apart, and faults are developed.
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 the 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 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.
A block may be raised or lowered in relation to surrounding areas, illustrates a rift valley formed in this way.
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 Denudation through the ages modifies faulted landforms.
VOLCANIC MOUNTAINS
The subducted part of the plate after reaching a depth of 100 km or more in the mantle is liquefied and thus expands in volume because of conversion of the portion of the plate into magma.
This expansion of molten materials causes a further rise in the mountain.
These are, in fact, volcanoes that are built up from material ejected from fissures in the earth’s crust.
The materials include molten lava, volcanic bombs, cinder, ashes, dust, and liquid mud.
They fall around the vent in successive layers, building up a characteristic volcanic clone.
Volcanic mountains are often called mountains of accumulation.
They are common in the Circum- Pacific belt and include such volcanic peaks as Fuji (Japan) Mt. Mayon (Philippines), Mt. Merapi (Sumatra), Mt. Agung (Bali), and Mt. Cotopaxi (Ecuador).
RESIDUAL MOUNTAINS
These are mountains evolved 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 the U.S.A. Residual mountains may also evolve from the plateau which has been dissected by rivers into hills and valleys like the ones illustrated.
Here the ridges and peaks are all very similar in height.
Examples of the dissected plateau, where the down-cutting streams have eroded the uplands into mountains of denudation, are the Highlands of Scotland, Scandinavia, and the Deccan Plateau.
ISLAND ARC FORMATION
When one tectonic plate meets another and sinks underneath it is known as the subduction phenomenon.
There are many subduction zones in the Ring of Fire, and it is in these zones that island arcs can form.
Subduction occurs when the oceanic lithosphere meets the continental lithosphere.
The lithosphere under the oceans is denser and heavier than that under the continent.
When the two run into each other, the oceanic lithosphere, therefore, sinks under the continent.
When two oceanic plates meet, one will sink under the other.
The oceanic lithosphere melts into the asthenosphere and turns into magma.
It’s like a recycling of the rocks that make up the crust and lithosphere.
When the oceanic rock sinks under the continental lithosphere and melts into the magma of the asthenosphere, some of it may leak into the crust and bubble up to the surface.
When magma bubbles up to the surface of the earth’s crust, we get volcanoes.
As the volcanoes forming at a subduction zone erupt, they build up the rock at the surface.
Over time enough builds up to create a volcanic island that rises above the surface of the ocean. Because a subduction zone can create several volcanoes in a row, these types of islands tend to form in chains or clusters, which we call island arcs.
OCEAN TRENCHES
Ocean trenches are a result of tectonic activity, which describes the movement of the Earth’s lithosphere.
In particular, ocean trenches are a feature of convergent plate boundaries, where two or more tectonic plates meet.
At many convergent plate boundaries, dense lithosphere melts or slides beneath less-dense lithosphere in a process called subduction, creating a trench.
Ocean trenches occupy the deepest layer of the ocean, the hadalpelagic zone.
The intense pressure, lack of sunlight, and frigid temperatures of the hadalpelagic zone make ocean trenches some of the most unique habitats on Earth.
MID ATLANTIC RIDGE FORMATION
The North American and Eurasian Plates are moving away from each other along the line of the Mid Atlantic Ridge.
The Ridge extends into the South Atlantic Ocean between the South American and African Plates.
The ocean ridge rises between 2 to 3 km above the ocean floor and has a rift valley its crest marking the location at which the two plates are moving apart.
The Mid Atlantic Ridge, like other ocean ridge systems, has developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American Plates.
As the mantle rises towards the surface below the ridge the pressure is lowered (decompression) and the hot rock starts to partially melt.
This produces basaltic volcanoes when an eruption occurs above the surface and characteristic basalt “pillow lava” in underwater eruptions. In this way, as the plates move further apart new ocean lithosphere is formed at the ridge and the ocean basin gets water.
This process is known as “seafloor spreading” and results in a symmetrical alignment of the rocks of the ocean floor which get older with distance from the ridge crest.
