Excellent study material for all civil services aspirants - being learning - Kar ke dikhayenge!
SHARE:
Faults, isostasy, mountains
& landform development - Part 1
1.0 FORMATION OF MOUNTAINS
The process of mountain formation is one of the most remarkable geological processes. These processes are caused due to large-scale movements of the earth's crust (plate tectonics). The orogenic process of mountain building includes processes like folding, faulting, volcanic activity, igneous intrusion and metamorphism. The understanding of specific landscape features in terms of the underlying tectonic processes is called tectonic geomorphology, and the study of geologically young or ongoing processes is called neotectonics.
Mountains form along the boundaries where the tectonic plates move towards each other (convergent boundaries). The tectonic plates collide triggering deformation and thickening of the crust. This in turn leads to crustal uplift and mountain formation. This process is a horizontal compression that leads to deformation folding and faulting of layers into folds or wrinkles along the convergent plate boundaries. This crustal uplift can be either a hill or a mountain depending upon the height and slope of the formation. But also to balance the weight of the earth surface, much of the compressed rock is forced downward, producing deep mountain roots making mountains for both upward and downward.
The Himalaya in Asia formed from one such massive collision that started about 55 million years ago. Thirty of the world's highest mountains are in the Himalaya. The summit of Mount Everest, at 29,035 feet (8,850 meters), is the highest point on Earth.
The tallest mountain measured from top to bottom is Mauna Kea, an inactive volcano on the island of Hawaii in the Pacific Ocean. Measured from the base, Mauna Kea stands 33,474 feet (10,203 meters) tall, though it only rises 13,796 feet (4,205 meters) above the sea.
Mountains can also be formed along fault lines. Blocks of Earth are uplifted and tilted over as two plates grind together. The uplifted part forms a mountain, and the lowered parts are filled in with eroded material. An example of this is the Sierra Nevada mountain range in California.
Another way that mountains are formed is when magma from beneath the Earth's surface is pushed up, but doesn't actually crack through. This bulge of magma eventually cools and hardens into hard rock, like granite. The layers of softer rock above the magma erode away and you're left with a large dome-shaped mountain. If the magma actually cracks through the surface a volcano is formed. Regular eruptions of lava, ash and rock build up a volcano to large heights. In fact, some of the largest, tallest mountains in the world are volcanoes. For example, Mauna Loa and Mauna Kea are examples of volcanoes. Measured from the bottom of the sea floor, they're actually taller than Mount Everest.
The final way to form a mountain is through erosion. If you have a high plateau, rivers will carve deep channels into the area. Eventually, you have mountains in between the river valleys.
1.1 Types of mountains
1.1.1 Fold Mountains: The most common type of mountain in the world are called fold mountains. Fold mountains occur near convergent or compressional plate boundaries. Where an area of sea separates two plates, sediments settle on the sea floor in depressions called geosynclines. These sediments gradually become compressed into sedimentary rock. When the two plates move towards each other again, the layers of sedimentary rock on the sea floor become crumpled and folded.
Eventually the sedimentary rock appears above sea level as a range of fold mountains. Where the rocks are folded upwards, they are called anticlines. Where the rocks are folded downwards, they are called synclines. Severely folded and faulted rocks are called nappes.
Some examples of fold mountain ranges include the Rocky Mountains in North America, and the Himalayan Mountains in Asia.
1.1.2 Fault-Block Mountains: Fault-Block mountains are formed by folded rock strata broken along young fault lines into blocks that are uplifted to different heights. They usually arise in folded zones that once had a mountain relief but have lost their plasticity and been smoothed by denudation. Due to repeated tectonic action sections of the earth's core do not form folds but break into separate blocks, some of which rise as horsts and begin to form ranges ("reborn mountains"), and others sink as grabens, forming depressions. Sometimes with repeated orogeny, the smoothed surface of the earth is subjected to plicate deformation, which leads to the formation of broad and gently sloping folds accompanied by faults. Instead of folding, like the plate collision we get with fold mountains, block mountains break up into chunks and move up or down. Fault-block mountains usually have a steep front side and then a sloping back side.
Examples of fault-block mountains include the Sierra Nevada mountains.
1.1.3 Dome Mountains: Dome mountains are formed due to volcanism. Melted rock in the interior of the Earth squeezes together into vast pools of magma beneath the ground. As it is less dense than the surrounding rock, it makes its way upward to the surface. If the magma crashes through the surface it results in a volcano. But if the magma pushes up but doesn't actually crack through the surface dome mountains are formed.
Dome mountains don't usually get as high as folded mountains because the force of the magma underneath doesn't push hard enough. Over a long period, the magma cools to become cold, hard rock. The result is a dome-shaped mountain.
1.1.4 Volcanic Mountains: These mountains are formed due expulsion of materials from deep within the earth in the form of enormous amounts of lava or cinders This material builds up around the volcanic vent and piles up building a mountain. Some of the largest mountains in the world were created this way, including Mauna Loa and Mauna Kea on the Big Island of Hawaii. Other familiar volcanoes are Mt. Fuji in Japan and Mt. Rainier in the US. A recent example of creation of a volcaninc mountain occurred on February 20, 1943, when a farmer's cornfield in Mexico suddenly began to erupt. By the second day, the cone had risen to 100 feet (30.5 m.). By two weeks it was 450 feet high (137 m.), and when the eruptions finally ceased in 1952 the cone had risen to 1,350 feet (411 m.). The nearby villages of Paricutin and Parangaricutiro had been completely buried under debris from the new volcano. Lava flows extended six miles from the crater and all vegetation for miles around had been choked out from the accumulations of dust and rock. The volcano itself was named Paricutin from one of the nearby villages it destroyed.
1.1.5 Plateau Mountains: Plateau mountains are not formed by internal activity. Instead, these mountains are formed by erosion. Plateaus are large flat areas that have been pushed above sea level by forces within the Earth, or have been formed by layers of lava. Plateau mountains are created when running water carves deep channels into a region, creating mountains. Over billions of years, the rivers can cut deep into a plateau and make tall mountains. Plateau mountains are usually found near folded mountains
Mountain ranges of the World
2.0 LANDFORMS
Landforms are the result of interaction between geological, climatic and biological forces. The resulting changes in the Earth's surface which may be classified as
- Long Period Changes (Man Is unable to notice this change) and
- Short Period Changes.
The forces which affect the crust of the earth are broadly classified into endogenetic and exogenetic forces
2.1 Endogenetic Forces
The forces originating from within the earth are called as Endogenetic forces. They cause two types of movements in the earth - horizontal movements andvertical movements. These forces derive their energy from changes such as radioactivity, chemical recombination, expansion or contraction or displacement of molten materials which occur in the interior of the earth. This group of forces may be also becalled as tectonic forces which manifest through processes called Diastrophism and Volcanism.
Varying thermal conditions and temperatures inside the Earth are the causes of the origin of Endogenetic forces. The related horizontal and vertical movements occur due to contraction and expansion of rocks. Volcanic eruptions and seismic events are also expressions of Endogenetic forces. The displacement and readjustment of geomaterials sometimes take place so rapidly that earth's movements are caused below the crust. On the basis of intensity, the Endogenetic forces and movements are divided into sudden forces and diastrophic forces.
2.1.1 Sudden Forces
Movements, caused by sudden-Endogenetic forces coming from deep within the earth, cause sudden and rapid events that cause massive destructions at and below the earth's surfaces. These forces work very quickly and their results are seen within minutes. Events like volcanic eruptions and earthquakes, are called 'EXTREME EVENTS' and become disastrous hazards when they occur in densely populated localities. These forces are the result of long period of preparation. Only their cumulative effects on the earth's surface are quick and sudden. Geologically, these sudden forces are termed as 'constructive forces' because these create certain relief features on the earth's surface. For example, volcanic eruptions result in the formation of volcanic cones and mountains while fissure flows of lavas form extensive lava plateaus and lava plains. Earthquakes create faults, fractures, lakes etc.
2.1.2 Diastrophic Forces and Movements
Diastrophic forces operate very slowly and their effects become discernible after thousands and millions of years. They include both vertical and horizontal movements which are caused due to forces deep within the earth. These forces are also termed as constructive forces, affect large areas of earth and produce meso-level reliefs such as mountains, Plains, Plateaus, lakes, big faults etc. These diastrophic forces and movements further subdivided in epeirogenetic and orogenetic movements.
Epeirogenetic movements: These movements cause upliftment and subsidence of continents masses through upward and downward movements respectively. Upward movement causes upliftment of continental masses in two ways - the upliftment of whole continent or part thereof and/or the upliftment of coastal land of the continents. Such type of an upliftment is called emergence. Downward movement causes subsidence of continent in two ways - the subsidence of the land area or alternatively, the land area near the sea coast moves downwards or subsides below sea level and is thus submerged under the seawater. Such type of downward movement is called submergence.
Orogenetic movements: Orogenetic movements result in the formation of mountains which are caused due to the Endogenetic forces working in a horizontal manner. The horizontal forces and the resultant movements are also called as 'tangential forces'. These forces work in two ways - fensional Force and compressional force. Tensional forces operate in opposite direction and thus create ruptures, cracks, fractures and faults in the crustal parts of the earth. Such type of forces and movements are also called Divergent forces and movements. Compressional forces operate towards each other or face to face and they cause crustal bending leading to the formation of folds or crustal warping leading to the local rise or subsidence of crustal parts. They are also called Convergent forces.
When horizontal forces work face to face the crustal rocks are bent due to the resultant compressional and tangential forces, and the crustal rocks undergo the process of 'crustal bending' in two ways - warping and folding.
The process of crustal warping affects larger areas of the crust where in crustal parts are warped upward or downward. The upward rise of the crustal part due to the compressive force resulting in the convergent horizontal movement is called upwarping while bending of the crustal parts downward in the form of a basin or depression is called downwarping. When the process of upwarping and downwarping affects larger areas, the resultant mechanism is called BROADWARPING. When the compressive horizontal forces and the resulting convergent movements cause buckling and squeezing of crustal rocks, the resultant mechanisms is called folding in which wavelike bends are formed in crustal rocks called folds.
Crustal fracture refers to displacement of rocks along a plane due to tensional and compressional forces acting either horizontally or vertically or sometimes even in both ways. Crustal fracture depends on the strength of the rocks and intensity of tensional forces. The crustal rocks suffer only cracks when the tensional force is moderate but when the rocks are subjected to intense tensional force, the rock beds are subjected to dislocation and displacement resulting in to the formation of faults. Joints and faults are two major types of crustal fractures.
2.1.3 Anticlines and Synclines
The up folded rock strata in arch-like form are called 'anticlines' while the down folded feature forming trough-like feature is called 'synclines'. The two sides of the fold are called limbs of the fold. The limb which is shared between an anticline and its companion syncline is called middle limb. The planes which bisects the angle between the two limbs of the anticline or middle limb of like syncline is called the axis of fold or axial plane. On the basis of anticline and syncline these axial planes are called as axis of anticline and axis of syncline respectively.
Unfolded rock beds are called anticlines. In simple fold, the rock strata of both the limbs dip in opposite directions. Sometimes, folding becomes so acute that the dip angle of the anticline is accentuated and the fold becomes almost vertical. When the slopes of both the (limbs or the sides) of the anticline are uniform, the anticline is called 'symmetrical anticline' but when the slopes are unequal, the anticline is called as 'asymmetrical anticline'. Anticlines are divided in to two types on the basis of dip angle - gentle anticline when the dip angle is less than 40°, sometimes 1° or 2° and steep anticline when the dip angle ranges between 40° and 90°
The down folded rock beds due to compressive forces caused by horizontal tangential forces are called synclines. These are trough like forms in which beds on either side 'incline together' towards the middle part. If folded intensely, the synclines assume the form of a canoe.
2.1.4 Anticlinorium and Synclinorium
Anticlinorium refers to those folded structures in the regions of folded mountains where there are series of minor anticlines and synclines with one extensive anticline. They are formed when the horizontal compressive tangential forces do not wok regularly. Such type of a fold is also called as a fan fold.
Synclinorium represents such folded structure which includes an extensive syncline having numerous minor anticlines and synclines which formed due to irregular folding of irregular compressive forces.
2.2 Types of folds
The nature of the folds depends on various factors such as the nature of rocks, the nature and the intensity of compressive forces, duration of the operation of the compressive forces etc. Based on the inclinations of the limbs the folds are classified in five types - symmetrical, asymmetrical, monoclinal, isoclinal & recumbent.
Symmetrical folds: If both the limbs incline uniformly then they are called as symmetrical folds. These folds are an example of open folds and are formed when Compressive forces work regularly but with moderate intensity.
Asymmetrical folds: These are characterized by unequal irregular limbs which incline at different angles. One limb is relatively larger and the inclination is moderate and regular while the other limb is relatively shorter with Steep inclination.
Monoclinal folds: These are the folds in which one limb inclines moderately with regular slope while the other limb inclines steeply at right angle and the slope is almost vertical.
Isoclinals folds: These folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal.
Recumbent folds:These folds are formed when compressive forces are so strong that both the limbs of the folds become parallel as well as horizontal.
2.3 Nappes
Nappes are the result of complex folding mechanism caused by intense horizontal movement and resultant compressive forces. Both the limbs of the recumbent fold are parallel and horizontal. Due to further increase in the continued compressive force one limb of the recumbent folds slides forward and overrides the Other fold This process is called 'thrust' and the Plane along which one Part of the fold is thrust is called 'thrust plane'. The upthrust part of the fold is called 'Overthrust fold'. When the compressive forces become so acute that it crosses the limit of elasticity of the rock beds, the limbs of the fold are so acutely folded that these break at the axis of the fold and the lower rock beds come upward. Thus the resultant structure becomes reverse to the normal structure. Due to continued horizontal movement and compressive force the broken limb of the fold is thrown several kilometers away from the original structure and overrides the rock beds of the distant Place. Such type of structure becomes unconformal to the original structure of the place where the broken limb of the fold of other place overrides the rock beds. Such broken limb of the fold is called 'napple'.
Several examples of nappes are traceable in the present folded mountains. The four major groups of nappes are Helvetic nappe, Pennine nappe, Austride nappe and Dinaride nappe.
2.4 Rift Valley
Rift valley is a major relief feature resulting from faulting activities. It represents a trough, depression or basin between two crustal Parts. Rift valleys are formed due to displacement of crustal parts and subsidence of middle portion between two normal faults by horizontal and vertical movements motored by Endogenetic forces Rift valley are generally also called as 'graben' which is a German word meaning a trough-like depression. A rift valley may be formed in two ways;
- When the middle portion of the crust between two normal faults is dropped downward while the two blocks on the either side of the down dropped block remain stable
- When the middle portion between two normal remains stable and the two side blocks on the either side of the middle position are raised upward
Rhine rift valley is the best example of a rift valley. It is bounded by Vosges and Hardt mountains (block mountains-horst) one one side while the other side is bordered by Black forest and Odenwald mountains. Some of the other rift valleys are Jordan River valley, Death Valley of southern Californian and Dead Sea in Asia. The rift valleys are not only confined to continental crustal surfaces but they are also found on the sea floor. The deepest grabens are found in the form of 'ocean deeps' and trenches.
2.4.1 Origin of rift valleys
The hypothesis regarding the origin of rift valleys are generally grouped in to two categories
- Tensional hypothesis - based on tensional forces
- Compressional hypothesis - based on compressional forces
However, both these hypothesis have lot of limitations and was not able to solve many of the intricate problems of the origin of rift valley.
Hypothesis of E.C Bullard: E.C. Bullard postulated his concept of the origin of the rift valleys during a survey of gravity. According to his theory the formation of rift valley is completed through a series of sequential phases of compressional forces coming from both the sides of the land. The horizontal compressive forces work face to face from both the sides of the land. This lateral compression becomes so enormous that it exceeds the strength of the rocks and a crack is developed at a place in the crustal rocks. This crack is gradually enlarged due to continuous increase in the compressive force. Due to the formation of crack, one portion overrides the other portion and this portion is called thrusting. On the other hand, the second part is thrown downward relative to the first part. This process is called down thrusting.
The crack developed at downthrust block place becomes enlarged due to increased compression and becomes a rift valley.
2.5 Exogenetic Forces
The exogenetic forces or processes, also called the denudational processes, or 'destructional forces or processes' originate from the atmosphere. These forces are continuously engaged in the destruction of the relief features created by Endogenetic forces through their weathering, erosional and depositional activities. Denudation includes both weathering and erosion where weathering being a static process includes the disintegration and decomposition of rocks in situ whereas erosion is dynamic process which includes both, removal of materials and their transportation to different destinations.
2.5.1 Weathering
The weathering of parent rocks results in the formation of soils which are very essential for the sustenance of the biotic lives in the biosphere. Hence, these processes are very important for the biosphere ecosystem. Weathering is basically of three types.
Physical or Mechanical weathering: It causes rocks to crumble. Water seeps into cracks and crevices in rock. If the temperature drops low enough, the water will freeze. When water freezes, it expands. The ice then works as a wedge. It slowly widens the cracks and splits the rock. When ice melts, water performs the act of erosion by carrying away the tiny rock fragments lost in the split.
Mechanical weathering also occurs as the rock heats up and cools down. The changes in temperature cause the rock to expand and contract. As this happens over and over again, the rock weakens. Over time, it crumbles.
Another type of mechanical weathering occurs when clay or other materials near hard rock absorb water. The clay swells with the water, breaking apart the surrounding rock.
Salt also works to weather rock. Saltwater sometimes gets into the cracks and pores of rock. If the saltwater evaporates, salt crystals are left behind. As the crystals grow, they put pressure on the rock, slowly breaking it apart.
Plants and animals are agents of mechanical weathering. The seed of a tree may sprout in soil that has collected in a cracked rock. As the roots grow, they widen the cracks, eventually breaking the rock into pieces. Over time, trees can break apart even large rocks. Even small plants, such as mosses, can enlarge tiny cracks as they grow.
Animals that tunnel underground, such as moles and prairie dogs, also work to break apart rock and soil. Other animals dig and trample rock aboveground, causing rock to slowly crumble.
Chemical weathering: Chemical weathering changes the materials that make up rocks and soil. Sometimes, carbon dioxide from the air or soil combines with water. This produces a weak acid, called carbonic acid, that can dissolve rock.
Carbonic acid is especially effective at dissolving limestone. When the carbonic acid seeps through limestone underground, it can open up huge cracks or hollow out vast networks of caves. Carlsbad Caverns National Park, in the U.S. state of New Mexico, includes more than 110 limestone caves. The largest is called the Big Room. At about 1,200 meters (4,000 feet) long and 190 meters (625 feet) wide, it is the size of six football fields.
Sometimes, chemical weathering dissolves large regions of limestone or other rock on the surface of the Earth to form a landscape called karst. In these dramatic areas, the surface rock is pockmarked with holes, sinkholes, and caves. One of the worlds most spectacular examples of karst is Shilin, or the Stone Forest, near Kunming, China. Hundreds of slender, sharp towers of limestone rise from the landscape.
Another type of chemical weathering works on rocks that contain iron. These rocks rust in a process called oxidation. As the rust expands, it weakens the rock and helps break it apart.
Biological weathering: Biological weathering is the weakening and subsequent disintegration of rock by plants, animals and microbes. Growing plant roots can exert stress or pressure on rock. Although the process is physical, the pressure is exerted by a biological process (i.e., growing roots). Biological processes can also produce chemical weathering, for example where plant roots or microorganisms produce organic acids which help to dissolve minerals.
Microbial activity breaks down rock minerals by altering the rock's chemical composition, thus making it more susceptible to weathering. One example of microbial activity is lichen; lichen is fungi and algae, living together in a symbiotic relationship. Fungi release chemicals that break down rock minerals; the minerals thus released from rock are consumed by the algae. As this process continues, holes and gaps continue to develop on the rock, exposing the rock further to physical and chemical weathering.
Weathering and People: Weathering is a natural process, but human activities can speed it up. For example, certain kinds of air pollution increase the rate of weathering. Burning coal, natural gas, and oil releases chemicals such as nitrogen oxide and sulfur dioxide into the atmosphere. When these chemicals combine with sunlight and moisture, they change into acids. They then fall back to Earth as acid rain.
Acid rain rapidly weathers limestone, marble, and other kinds of stone. The effects of acid rain can be seen on gravestones. Names and other inscriptions can be impossible to read.
Acid rain has also damaged many historic buildings and monuments. At 71 meters (233 feet) tall, the Leshan Giant Buddha at Mount Emei in China is the worlds largest statue of the Buddha. It was carved 1,300 years ago and sat unharmed for centuries. But in recent years, acid rain has turned its nose black and made some of its hair crumble and fall.
The type, rate and extent of weathering depends upon several controlling factors:
Climate dictates the type of weathering processes that operate, largely by determining the amount of water available and the temperature at which the processes occur. Chemical reactions are faster at higher temperatures, while frost wedging occurs in colder climates.
Rock type determines the resistance of the rock to the weathering processes that operate in that particular environment. Each rock type is composed of a particular set of minerals, which are joined together by crystallisation, chemical bonding or cementing. When the forces of plate tectonics move these rocks from the environment in which they formed and expose them to the atmosphere they begin to weather.
Rock Structure: highly jointed or faulted rocks present many planes of weakness along which weathering agents (e.g. water) can penetrate into the rock mass.
Topography: the slope angle determines the energy of the weathering system by controlling the rate at which water passes through the rock mass. Generally, higher, or tectonically active areas with steeper slopes have more dynamic weathering systems, whereas flat plains have slower weathering systems.
Erosion: the dynamism and efficiency of erosion determines how rapidly any weathered material is removed, how frequently fresh rock is exposed to weathering, and if deeply weathered profiles are preserved.
Time: the duration of the period that the same type of weathering has been operating, uninterrupted by climatic change, earth movements, and other factors, determines the degree and depth to which the rocks have been weathered.
2.5.2 Erosion
Erosion is the general name for the processes that break down rocks (weathering) and the processes that carry away the breakdown products (transportation). The erosional processes include running water or river, ground water, sea -waves, glaciers, periglacial processes and wind. These erosional processes erode the rocks, transport the eroded materials and deposit them in suitable places and thus form several types of erosional and depositional landforms of different magnitudes and dimensions.
The physical processes of erosion are called corrasion or mechanical erosion; the chemical processes are called corrosion or chemical erosion. But most examples of erosion include some of both corrasion and corrosion.
The agents of erosion are gravity, ice, water (eluvian erosion) and wind (eolian erosion). Erosion is sometimes restricted to transportation, excluding weathering. Erosion does not include mass wasting, unless erosion is being discussed in a tectonic context, in which case exhumation may be the better term.
2.6 davisian Theory of Landscape Development
William Morris Davis often called "the father of modern geography" developed a theory of landform creation and erosion, which he called the "geographical cycle." This theory is more commonly known as the "cycle of erosion," or more properly, the "geomorphic cycle." His theory explained that mountains and landforms are created, mature, and then become old.
He explained that the cycle begins with the uplift of mountains. Rivers and streams begin to create V-shaped valleys among the mountains (the stage called "youth"). During this first stage, the relief is steepest and most illregular. Over time, the streams are able to carve wider valleys ("maturity") and then begin to meander, leaving only gently rolling hills ("old age"). Finally, all that is left is a flat, level plain at the lowest elevation possible (called the "base level.") This plain was called by Davis a "peneplain," which means "almost a plain" for a plain is actually a completely flat surface. Then, "rejuvenation" occurs and there is another uplift of mountains and the cycle continues.
2.7 Penck’s Theory
Walther Penck, the German geologist, put forward his thesis of landform evolution in 1924. Penck criticised and opposed the cycle concept of Davis. Penck did not agree with Davis' assumption of very rapid uplift of a land surface followed by a long period of crustal stability. Penck, contrary to Davis' concept, built his theory on the alternative assumption of uplift so prolonged that the landscape would be eroded at the same time as it was being elevated.
The characteristic feature of Penck's hypothesis is that landform evolution of a given region is dependant on the tectonic activity of the region concerned. Opposed to the view of Davis, he was convinced that landforms were shaped and developed by the rate of uplift and that of erosion.
COMMENTS