Weathering and Denudation

A is false but R is true.

Explanation — Barkhans are not stationary. They migrate slowly in the direction of the prevailing wind as more sand accumulates on the windward side and slips down the leeward side.

A is true but R is false.

Explanation — In the lower course, the main work of the river is deposition, not erosion. The river slows down and deposits the sediments it has carried from the upper and middle courses, leading to the formation of deltas.

Weathering is a static process and it is defined as disintegration and decomposition of rocks due to several agents like wind, rain, animals, plants etc. It leads to formation of soil.

Weathering occurs because earths surface remains exposed to various elements of weather. These elements of weather like rain, air, heat, frost and cold wear away the rocks.

The three types of weathering are:

1. Physical weathering
2. Chemical weathering
3. Biological weathering

Denudation is the long-term effect of processes that cause the wearing away of the earth's surface leading to a reduction in elevation and relief of landforms and landscapes. Denudation is two fold process, i.e. both destructive and constructive. It causes wearing away of rocks and then deposition of the eroded materials forming plains.

When water enters the rocks and turns into ice, its volume increases. During the day, the ice thaws and the water enters deep into rocks. When temperature drops during the night, the water again freezes, widening th cracks. This repeated freezing and thawing results in breaking up of rocks or weathering of rocks.

Atmosphere assists in weathering due to its various elements that participate in disintegration of rocks. The factors of atmosphere that affect weathering are:

1. Temperature - The diurnal and seasonal variation of temperature weakens the rocks and finally leads to their weathering by peeling.
2. Air(wind) - Winds as source of weathering are more effective in arid areas. The sand particles strike the soft rocks and slowly erode them.
3. Rain and flowing water - Water and frost are also important agents of mechanical weathering. The atmosphere contains oxides of sulphur and nitrogen that result in acid rain. Rain water dissolves some of minerals and cause chemical weathering.

The decomposition of rocks is called chemical weathering. It involves the breaking down of rocks by altering or dissolving the rock minerals due to chemical action or changes. It is more common in hot and humid regions. It is a slow and gradual process which occurs due to exposure to air and water. For example, the iron present in rocks reacts with oxygen and water to form rust.

Biological weathering takes place due to biotic factors like plants and animals. It takes place in following ways:

1. The roots of plants grow into cracks and crevices in search of water and nourishment. As they grow, they exert great pressure on the rocks and break them.
2. Sometimes the roots produce carbonic acid which changes the composition of minerals.
3. Animals also loosen and weaken the rocks by burrowing.
4. Human activities such as road construction, mining, farming and deforestation cause weathering and disintegration of rocks.

The breakdown of rocks into smaller fragments by physical processes such as frost wedging is called mechanical weathering or disintegration.

The physical or mechanical weathering occurs due to factors such as the extremes of temperature, nature of rocks, structure of rocks, frost, wind and slope of land. It does not involve any chemical change in the composition of rock. It is more effective in dry regions.

The rocks are of different types. They may be hard or soft, soluble or insoluble. The mechanical weathering takes place in the softer or soluble rocks more easily than in the hard and insoluble rocks. Therefore, composition of rock affects mechanical weathering.

The factors responsible for chemical weathering are:

1. Oxidation
2. Hydration
3. Carbonation
4. Solution

Carbonation is the reaction of carbonic acid on minerals. Rainwater, when mixed with carbon dioxide in the atmosphere forms a weak solution of carbonic acid. It acts on rocks containing limestone and chalk. The reaction results in calcium bicarbonate which is highly soluble and gets easily washed away in solution as it flows through the rocks or seeps below the surface.

Two examples of the landforms formed by carbonation are stalactites and stalagmites.

The decomposition of rocks is called chemical weathering. It involves the breaking down of rocks by altering or dissolving the rock minerals due to chemical action or changes. It involves processes like oxidation, hydration, carbonation and solution. It is more common in hot and humid regions. It is a slow and gradual process which occurs due to exposure to air and water.

Deposition

Barchans

Hydration

Exfoliation

Physical weathering

Delta

Deflation Hollows

Middle course

Carbonation

Meander

River

Weathering

• Image 1 depicts Barkhans.
  Barkhan dunes are crescent shaped with horns of crescent trailing downwards. They are caused by accumulation of sand at an obstacle by the prevailing winds, giving them convex and gentle slope at the windward side and are concave and steep at the leeward side. Hence, Barkhan dunes are formed transversely to the direction of winds.

• Image 2 depicts Seif dunes.
  These are long narrow ridges of sand, lying parallel to the direction of prevailing winds. The prevailing dominant winds blow the corridor between the lines of dunes so that they are swept clean of sand and remain smooth. Hence, Seif dunes are formed longitudinally with the direction of winds.

Carbonation is responsible for the formation of limestone topography.

When the rainwater mixes with carbon dioxides in the atmosphere it forms carbonic acid, which after reacting with limestone (CaCO3) forms Calcium bicarbonate which is highly soluble and easily washed away in solution as it flows through rocks or seeps below the surface.

The process by which oxygen present in air and water react with minerals in the rock is called oxidation. For example, most rocks contain a certain amount of iron. Oxygen and water react with iron to form iron oxide which easily erodes in comparison to iron. As it is removed the rocks are weakened and thus decomposed.

The process by which the rainwater reacts with minerals present in the rock and dissolves them causing decomposition of rock is called solution. For example, rock salt dissolves in water.

Exfoliation is the weathering process by which concentric shells, slabs, sheets, or flakes are successively broken loose and stripped away from a rock mass.

Weathering is defined as disintegration and decomposition of rocks due to several agents like wind, rain, animals and plants. It leads to formation of soil.

All processes that cause degradation of landscape, weathering mass movement, erosion and deposition of eroded material are called denudation.

Due to the absence of vegetation the soil is loose & exposed to the action of winds & scanty rainfall.

In the upper course of the river the gradient of the slope is high, therefore, the velocity of the river is also high. So the main action of the river is erosion.

As there is no involvement in the movement of disintegrated rock fragments in weathering process.

As the river approaches the sea or ocean, its gradient decreases, resulting in a reduction of the river's velocity. It's volume also increases due to addition of tributaries along the way. Due to this, it cannot carry the sand particles any more and deposits all the sediments at its mouth. Hence, deposition is the main work of river in the lower course.

Chemical weathering may dissolve the soluble components of the soils. This may cause the soils to become infertile and may affect farming activities.