MOUNTAINS

A mountain can be defined as an area of land that rises abruptly from the surrounding region. A mountain range is a succession of many closely spaced mountains covering a particular region of the Earth. Mountain belts consist of several mountain ranges that run roughly parallel to each other.

The North American Cordillera, Himalayas, Alps and Appalachians are all examples of mountain belts that are composed of numerous mountain ranges.

The Earth’s mountain ranges have various ages of formation. Parts of the Himalayas are relatively young. Mountain building in this region of the world began about 45 million year ago. When the continental plates of India and Eurasia converged on each other. The Himalaya mountain are still actively being uplifted.

The Appalachian belt along the last coast of North America is quite old. Mountain building in this region of the world started about 450 million years ago. Orogeny stopped in the Appalachians about 250 million year ago.

Mountains can be classified on the basis of their age as- Young or new mountains, which have come into being after the continental drift started, with the break –up of the large landmass of Pangaea. The Himalayas, Andes, Rockies and Alps are examples of new or young mountains. Old mountains are those that were formed in the pre-drift era, long before the continental masses came together to form Pangaea. The Pennines (Europe), Appalachians (America) and Aravallis (India) are examples of old mountains.

Types:

Some mountains are volcanic in origin, forming where rising magma breaks through the Earth’s surface. Volcanic mountains tend to have sporadic distributions within a mountains mountain range (Mount St Helena, Rainier and Baker) or can occur alone because of a localized hot spot (Hawaiian Islands).

Most mountains were created from tectonic forces that elevate, fold and fault rock materials. Tectonic mountains can occur as a single range (the Urals) or as a belt of several mountain ranges (North American Cordillera). These major mountain systems include the North American Cordillera Belt and the Tasman Belt. There are broadly four types of mountains, which are differentiated according to their origin or formations, they are:

FOLD MOUNTAINS

Fold mountains are created where two or more of Earth’s tectonic plates are pushed together. At these colliding, compressing boundaries, rocks and debris are warped and folded into rocky outcrops, hills, mountains, and entire mountain ranges. Folding Is a type of earth movement resulting from the horizontal compression of rock layers by internal forces of the earth along plate boundaries. A upfold are termed as anticlines. The downfolds are termed synclines.

Fold mountains are created through a process called orogeny.At a compression zone, tectonic activity forces crustal compression at the leading edge of the crust formation.

Fold mountains are often associated with continental crust. They are created at convergent plate boundaries, sometimes called continental collision zones or compression zones. Convergent plate boundaries are sites of collisions, where tectonic plates crash into each other. Compression describes a set of stresses directed at one point in a rock or rock formation.

BLOCK MOUNTAINS (OR HORST) A mountain mass formed by the lifting up of land between faults (cracks in the rock strata) or by the sinking of land outside the faults, are called block mountains. They are formed when a mass of elevated land under strain, cracks, leaving a higher elevation standing between two areas of lower elevation. It may happen that the outer part sinks, leaving an elevated central part, a crust block or block mountain. These are usually steep-sided. The Vosges in France and the Black Forest mountains in western Germany come under this class of mountains.

VOLCANIC MOUTAINS  A volcano is a mountain formed of material that has erupted from inside the Earth, through the opening in the Earth’s crust. Superheated molten rock matter called a lava is ejected, forming a hill, conical in shape, with a funnel-shaped hollow at its top called a crater. It is estimated that there are about 1500 active volcanoes. Mt Fujiyama in Japan, Mt Vesuvius in Italy and Chimborazo and Cotopaxi in Andes (South America) are examples of volcanic mountains. They are also called mountains of accumulation. About two-third of all active volcanoes occur at the boundaries between tectonic plates. A good percentage is found along a belt encircling the Pacific Ocean. This region in the interior plates of area called ‘hot spots’ (like those that from the Hawaiian Islands). Although, most of active volcanoes on the earth seem to occur on land where plates collide, however, greater numbers of the Earth’s volcanoes occur on Ocean floor along spreading ridges.

RESIDUAL MOUNTAINS Mountains that are deeply dissected and reduced by weathering and river action are called residual mountains. Aravalis are an example of residual mountains.

Evolution of Mountains

Geologists have developed a general model to explain how most mountain ranges form.

This model suggests that mountain building involves three stages –(i) accumulation of sediments, (ii) an orogenic period of rock deformation and crustal uplift, and (iii)  a period of crustal uplift caused by isolation rebound and block-faulting. The latter two stages of this model involve tectonic convergence of crustal plates, which provide the compressional and tensional stresses that produce rock deformation, uplift and faulting.

In the organic stage of mountain building, the accumulated sediments become deformed by compressional forces from the collision of tectonic plates.

This tectonic convergence can be of three types-ocean-continent, arc continent, or continent-continent. In an ocean-continent convergence, the collision of ocean and continental plates causes the accretion of marine sedimentary deposits to the edge of the continent. Arc-continent convergence occurs when an islands arc collides with the edge of a continental plate. In this convergence, the ocean plate area between the arc and the continent is subducted into the asthenosphere and the volcanic rocks and sediments associated with the island arc become accreted to the margin of the continent over time. This type of collision may have been responsible for the creations of the Sierra Nevada Mountains in California, during the Mesozoic era. The final type of convergence called continent-continent convergence occurs when an ocean basin closes and two continental plates collide. This convergence leads to mountain building activities responsible for the formation of the Himalayas, Ural and Appalachian mountains systems.

In all three types of tectonic convergence, layered rocks that were once located in the ocean basin are squeezed into a smaller area. This compression causes the once flat sedimentary beds to be folded and uplifted. When the compressional forces become greater than the cocks ability to deform, faulting occurs. Compressional forces typically result in reverses and overthrust faulting. Another consequence of the orogenic stage is regional metamorphism and the incursion of magma plumes, plutons, and volcanoes into the growing mountain range. Compressional forces squished the existing sedimentary deposits, upwards between the converging continental plates and rocks at the margin of the Eurasian and Indian plates. These forces also created a number of overthrust faults.

At the end of plate convergence, mountain building enters its final stage. This stage is characterized by crustal uplift because of isostatic rebound and block-faulting. Isostatic rebound involves the vertical movement of the continental crust that is floating in the plastic upper continental crust that is floating in the plastic upper mantle. As erosion removes surface materials from mountains, the weight of the crust in this region becomes progressively less. With less weight, the continental crust makes as isostatic adjustment, causing it to rise vertically (float higher) in the mantle. This process also causes tensional forces to exist in a horizontal direction, breaking the continental crust into a number of blocks. Each block moves vertically to compensate for the forces of tension, producing normal and graben faults.

Crustal Deformation Processes-Folding and Faulting

The Earth’s surface is deformed. This deformation is the result of forces that are strong enough to move ocean sediments to an elevation many thousand metres above sea level. In previous sections, we have seen that this displacement of rock can be caused by tectonic plate movement and subduction, volcanic activity and intrusive igneous activity. Deformation of rock involves changes in the shape and/or volume of these substances. Changes in shape and volume occur when stress and strain causes rock to buckle and fracture or crumple into folds.

FOLDS A fold can be defined as a bend in rock that is a response to compressional forces. Folds are most visible in rocks that contain layering. For plastic deformation of rock to occur a number of conditions must be met, including:

• The rock material must have the ability to deform under pressure and heat.
• The higher the temperature of the rocks is, the more plants it becomes.
• Pressure must not exceed the internal strength of the rock if it does, fracturing occurs
• Deformation must be applied slowly

A number of different folds have been recognised and classifieds by geologist. The simplest type of fold in called a monocline. This fold involves a slight bend in otherwise parallel layers of rock. An anticline is a convex up fold in rock that resembles an arch-like structure with the rock beds (or limbs) dipping way from the centre of the structure. A syncline is a fold where the layers are warped downwards. Both anticlines and synclines are the result of compression stress.

Types of fold

Anticline

This anticline is in Alberta, Canadia in the Rocky Mountains

Syncline

Syncline sidling hill

Chevron fold 

 

Chevron folds with flat-lying axial planes, Millook Haven, North Cornwall, UK

Chevron folds are a structural feature characterized by repeated well behaved folded beds with straight limbs and sharp hinges. Well developed, these folds develop repeated set of v-shaped beds. They develop in response to regional or local compressive stress. Inter-limb angles are generally 60 degrees or less. Chevron folding preferentially occurs when the bedding regularly alternates between contrasting competences.

Recumbent fold

Recumbent fold Bahrain

Recumbent fold has an essentially horizontal axial plane. linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold.

Plunging fold

Plunging chevron folds

A fold whose axis plane is not horizontal (not Parallel to sea level). Direction of plunge – the direction in which the axis is inclined nose – indicate the direction of plunge. In anticline, plunge is directed towards nose and in syncline it is directed away from nose.

Dome and Basin

Desert of Mauritania. dome.

 

We also have domes, which are like anticlines but instead of an arch, the fold is in a dome shape, like an inverted bowl. Similarly, there are also basins, which are like synclines but again, instead of a sinking arch, the fold is in a shape of a bowl sinking down into the ground. Dome: nonlinear, strata dip away from center in all directions, oldest strata in center. Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.

Ptygmatic fold

 Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones. Ptygmatic folds generally represent conditions where the folded material is of a much greater viscosity than the surrounding medium.

FAULTS There are several kinds of faults, each named according to the type of stress that acts on the rock and by the nature of the movement of the rock blocks on either side of the fault plane. Normal faults occur when tensional forces act in opposite directions and cause one slab of the rock to be displaced up and the other slab down. Reverse fault develops when compressional forces exist. Compression causes one block to be pushed up and over the other block. A graben fault is produced when tension stresses result in the subsidence of a block of rock. On a large scale these features are known as rift valleys. A horst fault is the development of two reverse faults, causing a block of rock to be pushed up. The final, major type of fault is the strike-slip or transform fault. These faults are vertical in nature and are produced where stresses are exerted parallel to each other. A well-known example of this type of fault is the San-Andreas fault in California.