A Break or Weakness in the Continuity of the Crust of the Earth is Called a

After reading this article you will learn about:- 1. Types of Weathering 2. Works of Glacier 3. Works of Sea Waves 4. Works of Wind.

We have found that folds and faults have developed on the crust of the earth due to internal forces. As soon as a landmass assumes a certain height, it is subjected to different forces which destroy the elevations and fill up the depressions of the earth's surface.

As a result of these two opposite processes, the surface ultimately attains a common level or grade. All the processes that tend to bring the surface of lithosphere to such a common level are known as process of gradation.

The general wearing out of the elevated land surfaces by the external forces and transforming them to plain lands is known as denudation or degradation. Denudation includes processes of weathering, erosion and transportation.

The process of weathering includes weakening, breaking up, rotting and disintegration of rocks. This process starts as soon as rock is exposed to the influence of weather and in this process the weathered materials remain at the place of disintegration.

Weathering is followed by erosion. Erosion is a process of scraping, scratching and grinding of the surface rock and it includes the process of transportation of the weathered materials from the places of origin to other places.

The erosion is performed by agents like running water, ground water, moving ice, winds, waves and currents. Erosion is a mobile process and weathering is a static one.

Erosion = weathering + transportation

The weathered materials when started collecting in the depressions over the earth surface by the process of transportation are known as deposition or aggradation. The agents of deposition are also same as agents of erosion. From this, it is seen that the agents of denudation bring various changes over the surface of the earth through the processes of erosion and deposition.

Types of Weathering :

On the basis of the characteristics of the agents, the process of weathering can be classified into three types:

1. Mechanical or physical weathering

2. Chemical weathering

3. Biological weathering.

The process of disintegration of rocks by mechanical or physical process is known as—Mechanical or Physical weathering.

In this process, the chemical composition of the disintegrated rock is not changed. The minerals in the rock are simply separated from each other. There are several agents of mechanical weathering and the major ones are—temperature, water and ice action. Weathering is common in cold and hot deserts, where there is high fluctuation of daily temperature.

During the daytime, the exposed rocks become hot and expand by the intense amount of sunshine. During winter nights temperature may fall close to freezing point and the rocks contract. Different minerals expand and contract at different rates. As a result, a series of cracks or joints develop at the outer surface of the rock and ultimately split them into fragments and pieces. This type of weathering is called Disintegration.

Sometimes, the disintegrated materials move down the mountains slopes and gather at the foot of the mountain. These types of collections are called Talus and Scree. Sometimes, due to alternate heating and cooling, the expansion and contraction of outer rock layers being more than inner ones, ultimately the outer layers peel off from the inner layers. This kind of weathering is called Exfoliation.

Otherwise, when the minerals of the parent rock are broken down into pieces it is called Granular Disintegration. Sometimes, the parent rock (especially the limestone) may be broken into pieces in blocks. Such disintegration is called Block Disintegration.

It is known that the conversion of water into ice increases the volume. It has been proved that 9 cubic centimetres of water when freezed, turns into 10 cubic centimetres of ice. In the areas of severe cold climate water freezes and forms ice during night and again melts into water during day time.

Rocks get disintegrated through this type of action also. The process is known as Frost weathering. When the weathered materials spread over a very large area it is called Felsenmer or Boulder clay.

The processes that cause rocks to decay or decompose are known as chemical weathering. The chief agents of chemical weathering are air and water. By the action of these two agents, the minerals in rock undergo chemical changes and form a new chemical substance. Chemical weathering takes place through the process of oxidation, carbonation and hydration.

When the atmospheric oxygen in rain water meets with mineral like iron, rusting of iron takes place and changes the colour of the rock into red, yellow or brown. The process is called oxidation. The chemical union of water with minerals like potash, feldspar and aluminium decompose the outer shell of the rock and the process is called hydration.

When the carbon dioxide present in water reacts with rocks new chemical substances like calcium carbonate and iron carbonate are formed. This process is called carbonation. Some other minerals like lime, salt and gypsum are removed by the process of solution in water.

In addition to the above, rocks may be disintegrated directly or indirectly by animals, plants and even by man. This type of the process of weathering is called Biological weathering. Biological weathering may be either mechanical or chemical.

Agents of Gradation :

The natural agents that remove the weathered materials from one place to another are known as Agents of Gradation. There are many agents of gradation. However, works of running water, wind, glacier, underground water and sea waves are most important.

These agents bring many changes over the surface of the earth through erosion, transportation and deposition. Through these actions the highlands are eroded, the eroded materials are deposited in the depressions and consequently transform the landform into plains.

Running Water :

Running water is one of the most significant agents of gradation that brings considerable changes over the surface of the earth. After a heavy rain the water generally flows along the slope in small channels and gullies. In course of time streams originate by the combination of a number of small channels.

The slopes of the streams in the mountain region are higher than that of the plain region. On the other hand, the amount of sediments in the mountain region is smaller than that of the plain region. As a result, when stream flows along the lesser slopes with higher sediments, its speed of flow starts slowing down and the sediments start collecting in certain places.

A river is formed by the combination of many streams. Generally, river gets water from rain, glacier, lake, swamps and sub-surface water. A river performs the works of erosion, transportation and deposition. These three works are interrelated and the works of erosion and transportation depend upon the energy of the river. The energy of the river generally depends upon the volume of water and the speed of its movement.

The river, in course of journey over the surface of the earth carries various sediments like—sand, silt, pebbles and other material. These materials are collectively called as Load of a river. The river collects its load mainly from the two sides and forms the bed. The particles get smoothened, fragmented and rounded by hitting each other, against the side and bottom of the river bed.

Through this process the two sides get widened and the river bed gets deepened. It is called as Erosive work of the river. As the river flows towards the plains, the load increases and speed of flow of water diminishes. As a result, the load starts collecting along the two sides in certain places and in the mouth of river. It is called as the Depositional work of the river.

It has been observed that the erosive power of the river increases according to the increase of its load. The erosive power of clear water is less. The load actually works as a cutting tool. However, it is to be remembered that the erosive power will be lowered again when more load is transported along lesser slopes.

In this situation, the energy of the river is spent to carry the load and as a result, deposition starts instead of erosion. The erosive power of the river depends on a specific amount of slope and load.

The river carries its load in three ways:

1. In suspension,

2. In traction, and

3. In solution.

The sand, silt and pebbles are carried within the stream in suspension while the stones and boulders are rolled along the bed (traction) and some materials like salt, lime, potash, sulphur, etc., are carried in solution.

Again, a loaded river performs its erosive work in three ways:

1. Corrosion,

2. Solution, and

3. Hydraulic action.

Corrosion is a mechanical process of erosion. The materials of the load collide with each other on the two sides and on the bed of the river. As a result, erosion takes place, and the process is called corrosion.

Through corrosion, the river valley widens and the river bed deepens. It is proved that when the flow of water is doubled, the load carrying capacity of the water is increased by 64 times. Similarly, when the volume of water is doubled, the load carrying capacity becomes doubled.

The carrying capacity also depends on the size of the materials of load. Smaller the size, greater will be the carrying power. That is why the erosional activities of the river increase or decrease during certain months of the year.

Some rock forming minerals dissolve in water. The dissolving power of water increases with the increase of carbonic acid in water. The erosion that takes place in this way is called solution. River flows with very high speed in the mountain regions and due to this, holes are produced on the rocks over which water flows. The holes are generally round and are called pot holes. The erosion by this process is called hydraulic action.

Cycle of Erosion :

We have already found that certain portion of the earth's surface may be uplifted by the endogenetic forces. As soon as a landmass is lifted up compared to the surrounding areas exogenetic forces like—running water, wind, glacier, underground water, sea waves, etc., begin the work upon it.

These forces through the process of weathering and erosion try to level down the area and ultimately create a plane surface without much irregularity on it. All landscape that we see over the surface of the earth is actually the product of the works of the endogenetic and exogenetic forces. While endogenetic forces are responsible for raising an area, the exogenetic forces always try to level it to a featureless plain.

The period of time in which an uplifted area through the process of weathering and erosion is converted to almost a featureless plain is known as geomorphic cycle. The concept of geomorphic cycle was first formulated by W.M. Davis in 1902.

According to Davis, any landform is a function of structure (i.e., type of rock, hardness, folds, faults, etc.), process (the factors which bring about weathering and erosion) and stage (the time through which the landform is currently passing). The cycle concept first postulated with reference to river erosion is mid latitude, which is generally known as Cycle of Normal Erosion or Normal Cycle of Erosion.

Works of Glacier :

The running water, glacier is also another major agent of gradation. Glacier brings many changes over the surface of the earth through erosion, transportation and deposition.

On the basis of location glaciers are grouped into four types:

1. Continental glacier,

2. Mountain of valley glacier,

3. Piedmont glacier, and

4. Ice-caps.

In those areas over the surface of the earth where temperature of the air falls below freezing point, the precipitation occurs in the form of snowfall. In this situation, if the rate of melting of snow is less than the rate of snowfall, the snow starts accumulating. In course of time, the snow becomes hard, its weights increase and due to force of gravity it starts moving down the slope very slowly.

This type of river of moving ice and snow is generally known as glacier. For the first time, in 1834, a Swiss named Louis Agassiz has proved the movement of ice in glacier. Glaciers are found in all continents except Australia.

The places where snow lies for the whole year are called snow fields and the line above which there is perpetual snow is known as the snowline. The snowline differs according to latitude, amount of snowfall and physical feature of the region. The height of the snowline is about 5000 metres in the equatorial region and the sea level at the polar region.

The favourable conditions for formation of snow fields above the snowline are:

(1) The gentle slope, and

(2) Localities which do not receive direct sunlight.

The continental glaciers are the largest among all other types. The biggest continental glacier of Iceland has an area of 8450 sq km and has a thickness of nearly 1000 metres. The movement of such a glacier is so slow that it may take thousands of years to identify the features developed by it. Therefore, the features that are developed by the mountain or valley glacier have been described here.

The mountain glaciers originate above the snowline on the high mountain ranges of the world. In our Himalayas also, there are many such mountain glaciers. The biggest glacier of Himalayas is Sianchin located in the Karakoram Mountain (72 km long) and the second biggest is the Gangotri (25.5 km) in Uttarakhand. There are many others that have 5-10 km length.

The mountain glacier moves along the mountain valley towards the warm climate. Under this condition, the snow of the front side of the glacier starts melting and the glacier takes the shape of a human tongue. A glacier has two types of movements/onward and backward. When the snowfall in the upper portion of the glacier is more, the glacier moves forwards and when the snowfall is less it retreats.

The glacier carries huge quantities of material like soil, boulders, plants and other debris when it moves downward along the mountain slope. These are called load of the glacier. These materials increase the erosional power of the glacier. The erosional activity of glacier is called abrasion.

By abrasion the valley of the glacier becomes U-shaped. The daily average speed of movement of glacier may be from as slow as 3 cm to as high as 18 metres. The speed of movement depends upon the slope and the amount of snow. Generally, the speed of movement is more in the middle portion and relatively less on the two sides.

Due to this type of unequal speed of movement, innumerable cracks-small and big, as well as, shallow and deep develop over the glacier. The small and shallow cracks are called crevasses and the bigger and deeper cracks are called bergschrund.

The number and depth of the cracks become larger and deeper when glacier moves downwards along the slope and the cracks join together when glacier moves upward along the slope. These jointed cracks are generally covered with soft snow and they become very dangerous for the mountaineers, as some of the crevasses may be 50-200 metres deep.

From the study of glaciated regions, the glacier has been found to perform the following works:

(1) Erosion in the mountain region,

(2) Deposition of the rock materials in the lowland, and

(3) Interference with drainage.

The erosional work done by glacier is quite different from that of a river because of the following special characteristics:

(a) As compared to water the rate of movement of ice is negligible.

(b) The lower portion of a glacier does not come in direct contact with the rock floor.

(c) The speed, thickness and abrading power of glacier varies according to slope along which it moves.

The major erosional features of a glacier in the mountain region are—cirques, U-shaped valley, hanging valley, cols, horns, roche moutonnes, tarn, arete, pyramidal peak, combridges, etc.

A mother glacier may have numerous affluent glaciers on both sides. The erosional power of the small glaciers is less than the main glacier. Therefore, the valleys formed by the sub- glaciers remain at some height from the valley of the main glacier. The valleys formed by the sub-glaciers are called hanging valley. The glacial valleys—whether hanging or main—are U-shaped valleys.

Cirques are arm-chair type of hollows formed at the head of the glaciated valley. These are open at one end and have flat bottoms with steep slopes on three sides. They are also known as corrie and cwm.

When two cirques develop on the opposite sides of a ridge at same level, the cut back action reduces the ridge and in course of time a shaddle like formation develops. This is known as col. Cols are used as mountain passes. Sometimes, it may so happen that a number of cirques may develop round a common ridge.

In course of time by the cut back activities, they approach to each other and the height is transformed into a three or four-sided pyramid. This type of formation is called horn or pyramidal peak. Matter-horn in Switzerland is the famous example. At the floor of cirque basin, some lakes are formed. These mountain lakes are called tarns.

Unlike water, glacier is not diverted from its path by slight barriers. The moving ice tries to level down whatever comes in its way. In this situation, the glacier pushes the debris upwards on the slope of it. And, due to upstream abrasion, this side becomes very smooth and assumes gentle slope.

On the other side of the barrier it falls downwards and due to downstream plucking, the slope becomes steep and rough. This type of rock formation is called Roches Moutonnes. Many roche moutonnes can be seen in the Alps, southern Sweden and Finland.

When glaciers melt or recede they deposit the rock materials brought by them in heaps of various shapes and sizes. These are known as moraines. Moraines are depositional features and consist of rock materials of heterogeneous shapes and sizes. The pile of unsorted rock materials is called till. Till is more or less uniform and in Europe it is called glacial drift.

The moraines may be of various types according to the place where they have been deposited. The materials dropped at the end of a mountain glacier are in the form of a ridge called terminal moraine. The materials deposited at either of the two sides of a glacier are called lateral moraine. When two glaciers join, the two lateral moraines also join and form a single moraine at the confluence of the two glaciers. It is called medial moraine.

Sometimes, the load of glacier becomes excessive and cannot be carried forward. The materials are deposited at the bed or at the base. This type of moraine is called ground moraine. Because of its boulder nature it is also called boulder clay.

When the ground moraine is deposited in a rough topography, depressions are filled and the tops of hills smoothened and rounded. These low hills are called drumlins. Generally drumlins are found in groups with average heights from 60-100 metres. Where drumlins are found, the topography resembles "eggs in a basket" when viewed from above.

The streams that originate from the end of the glacier are heavily loaded with rock materials and start depositing their load just outside the margin of the glacier. Such stream deposits form outwash plain.

When the ice of the front portion of the glacier melts at different times, it leaves behind a lake between the terminal moraine and the outwash plain. Such a lake is called fosse. Another typical feature made of sand and gravel is also formed by currents of water which flow beneath the ice and make tunnel for them. These are known as eskers.

Sometimes, a number of ridges and mounds are formed along the ice front. These are called kames. Kames consist of assorted material-gravel and sands. Kames follow the direction of the icefront and have heights between 30-60 metres.

Outwash plains, Eskers, Kames and Fosses are fluvio-glacial deposits.

All these deposits ire found to exist in a series as follows:

Boulder clay or Glacial drift or Tilt plain followed by

Drumlin-Terminal moraine-Kame-Outwash plain.

In this way, the glacier brings about many changes over the surface of the earth and creates various erosional and depositional features.

Works of Sea Waves:

Like running water and moving ice, sea waves also act as agents of gradation and bring many changes especially along the sea coasts. When wild comes into contact with water surface, waves form. Each wave has two portions—crest and trough. Each crest is followed by a trough. The length from the middle of a wave crest to the middle of the next crest or from the middle of a trough to the middle of the next trough is called wavelength.

The waves that move from deeper water towards the coast, come into contact with the bottom of the sea floor and are retarded. The crest however goes on moving forward. Such waves are called breaker. A succession of breakers one after another is called surf.

A breaker generally has three stages: plunge, swash and backwash. When waves advance through shallow water, its waters are piled up on the shore, which is called plunge. This is followed by uprush of water known as swash. After this, begins the descent of waters which is known as backwash.

The sea waves perform the works of erosion and deposition.

The erosive power of sea waves depends upon:

(а) Size and strength of waves,

(b) Slope of the coastal area,

(c) Rock composition,

(d) Depth of water, and

(e) Types of load carried by waves.

As the water rushes along, it exerts tremendous mechanical force. It has been estimated that waves can exert pressure up to 30,000 kg per sq metre. The rushing of water forward and backward compresses and expands the air in the cracks of the rock and ultimately the rocks break.

The waters also exert hydraulic action in the opening of the rock. In addition, erosion also takes place as a result of the chemical action of sea waves particularly in limestone and chalk regions.

Sea cliffs, sea-waves, stacks, arches, spouting horns and chasms are some of the important erosional features produced by sea waves.

By continual wave action, the shore is cut into a notch. The upper portion of the notch is called a cliff. If the rock is of varied resistance, or there is constant wave attack under the cliff, the result will be the formation of pockets. These pockets are known as sea-caves.

Sea- caves are formed when overlaying rocks are more resistant. When sea-caves attack from opposite direction they may ultimately join together and a new feature may be formed. It is called sea-arch or a natural bridge.

When the sea-arches collapse, the remaining portions are called stacks. If the widths of sea-caves remain small and their length increase, the rock cavities are called spouting horns. Sometimes rock cavities may form in vertical direction. These vertically formed hollows are called chasms.

The sediments produced by wave erosion and the amount carried by rivers are redeposited by waves and currents. The marine deposits along the sea shore also produce various depositional features. Beach, sand bars and spits are some of the important depositional features of sea-waves.

The eroded materials gather in different formations as the intensity of the wave force declines. The most common accumulation is the semi-permanent embankments called beaches. The beaches consist of large and small stones and pebbles. Depending upon the supply of eroded materials and wave intensity the beaches differ in size and extent.

On certain low-lying shores, beaches are formed as a border parallel to the coast line. These are known as off shore bars or reefs. Another important depositional formation, spit is a ridge or embankment of sediments attached to the land at one end and open at the other end.

Sometimes, two spits may be formed one from the mainland to the sea and another from some island towards the mainland. Such spits may join together to form looped bar or tombolos.

The sea-water enclosed between the shore and the bat is called lagoon. Lagoons have openings to the sea through narrow gaps between the bars. The chilka and the pulicat are two important lagoons of India.

Works of Wind:

Wind is the chief agent of gradation in the hot and temperate deserts of the world. In the desert, the soil is loose, vegetation is scanty and as such, the wind carries with it thousands of dust and sand particles. These sand particles help the wind to erode the rock surface by the process of mechanical erosion.

As soon as the speed of the wind slows down, it becomes difficult to carry the load and ultimately they are deposited in the form of hillrocks. Thus, wind also performs the work of erosion, transportation and deposition.

The lifting of the loose materials from the surface and blowing away by the wind is known as deflation. The sand grains help polishing the rock surfaces like a sand paper and the process is called abrasion.

Therefore, the erosive power of wind depends on:

(i) The load of the wind (amount, size and shape)

(ii) The energy of the wind (speed, time and surface)

The smallest-sized load of the wind may be lifted to a greater height and may be flown to a very long distance. However, medium sized sand particles cannot be lifted beyond certain height and the biggest ones may travel along with the wind by rolling or jumping.

As a result, the erosive power of wind becomes very high at certain height from the ground level. The resistant rocks take the shape of mushroom having large top and a small neck. The formation is called Mushroom Rock. Many such formations can be seen near Jodhpur in Rajasthan.

The total effect of wind action is to reduce the bedrocks into a level plain with a smooth surface so that large and small rock masses stand out here and there from the surface of the desert. Such a feature is known as inselberg. They are quite common in African deserts.

When the speed of wind decreases, it starts dropping its load. The process is called deposition. Some of the important depositional features are-sand dunes and loess. Sand dunes are hillocks of coarse sands. They do not have vegetation and move gradually forward in the direction of the wind.

A sand dune may advance at the rate of 5-30 metres a year. The sand dunes can be made stable by planting trees or grass of quick growing varieties. The form and shape of dunes depend upon the speed of wind and the supply of sand. Their form may be either parallel to the direction of wind or perpendicular to it.

Therefore, sand dunes may be of two types:

1. Transverse

2. Longitudinal

In the areas of desert where the speed of wind is less and amount of load is more, sand dunes are formed like sea-waves in perpendicular position to the direction of wind. These are called Transverse sand dunes. If the load is more and speed of wind is moderate, sickle shaped or crescentric dunes are formed. These are known as barchans.

Barchans (Barkhans) have convex or flat slopes towards the windward side and the leeward slopes are concave and steep. The two ends take the shape of horns. Barkhans attain maximum height at the central position and sometimes it attains a height as much as 30 metres. Barkhans are quite common in Turkey. Barkhans are special types of Transverse dunes.

Longitudinal sand dunes are formed by modification of the shape of transverse dunes. When the speed of wind becomes faster, it carves out channel through the depressions and the transverse dune is divided into two separate dunes parallel to the direction of the wind. This type of dune is also called seif dune.

Loess deposits are found on the borders of desert areas. Loess is formed of fine dust and the average thickness may vary from 120-300 metres. The loessic soils are porous and usually very fertile. Loess are sometimes formed at very distant places from the desert.

The loss of China is a typical example. The dusts of this loess have been brought from the Gobi desert of Central Asia. Loess soil does not have any strata like other sedimentary rock. The loess soil transforms to a sticky mud when it gets wet and becomes very dusty when dry. The loess soil of China is yellowish in colour.

larsonerepaing00.blogspot.com

Source: https://www.geographynotes.com/earth/crust/changes-in-the-earths-crust-earth-geography/1694