Water – A Powerful Agent Of Weathering

Weathering is caused in rocks by water when within the small cracks pressure is exerted by humic acid. When plant materials break down, the rocks crack and weathering takes place. Rocks can also break down due to presence of acidic water in the ground. Thawing and freezing is another cause of weathering of rocks in colder regions. Glacial erosion is caused due to glaciers. Hydraulic erosion is caused due to water. When water freezes it leads to fluvial erosion. In the biological weathering process, the plant roots get into the cracks in rocks. Earthworms and other small insects also make tunnels under the ground leading to physical weathering of rocks. In a physical weathering process, no chemicals are used in rock disintegration. It is due to abrasion that rocks disintegrate. In areas where temperature fluctuation and lesser vegetation exist weathering takes place.

Weathering Caused Due To Precipitation
Rain water is acidic by nature since it contains carbon dioxide dissolved in it from the air. When it falls on rocks chemical weathering takes place. Chemicals can cause weathering of rocks like chalk and limestone. A chemical reaction occurs, new substances get formed and when they get washed, weathering of rocks occurs. Cliffs fall out and caves become hollow due to chemical weathering. Some rocks weather at a slow pace while other rocks weather fast. When rainwater acids react with minerals in rocks, new substances are formed and the weaker one fall away after crumbling. Natural gas, oil and coal on burning release sulfur dioxide and carbon dioxide in air which dissolves in clouds to make the rain water acidic leading to acid rain.
Weathering Due To River Action And Flooding
If the river path is seen, right from its source to the mouth, features in a range can be seen. Different kind of processes affects the path of the river. A river has upland and lowland features or environments. At the higher altitude or the source the processes and upland features occur. At the lower altitude the lowland features occur. Hence the course of the river gets divided into different stages like old stage, young stage and the mature stage.

The river undergoes different processes like erosion, then transportation and then deposition. For each of these processes there are sub processes too. Erosion of rocks takes place by 4 sub processes called abrasion, corrosion, attrition, solution caused erosion or corrosion and hydraulic erosion. In the same water the transportation sub processes include, traction, saltation, suspension and solution. Rivers get affected by the erosive process and the abrasion or corrosion process. In this the suspended small particles in the river rub against the banks of rivers mostly in the low lying areas. The gradient is low at this altitude.

In the attrition process, along the bed of the river, transportation of boulders takes places, where they collide and get smashed to pieces that are smaller. In the upland regions where higher velocity of the river exists, this mainly occurs. In the case of erosion or corrosion by solution, the rocks that are softer in nature like limestone dissolve due to the acids dissolving in the water. When hydraulic action takes place, the bed of the river dislodges and breaks due to sheer force. From the bed of the river and its banks, the particles and rocks get dislodged.

Transportation Of Different Kinds

The different kinds of transportation processes include solution, traction, suspension and saltation, and processes of other kinds affect the river’s landscaping greatly. Effect of humans and weathering are also involved in processes of such kinds. Interlocking spurs and narrow valleys in V shape, waterfalls and rapids are the main features in the valley of a young river. In case of rivers that are older the included features are deltas, ox box lakes, levees, meanders and flood plains.

In the river environments upland the processes and features include the interlocking spurs where around hill sides that protrude, a young river keeps winding. Valleys of a younger river have steep sides and are in a V and narrow shape. At the river source, due to presence of high energy, huge stones and boulders get transported along the river bed. The river cuts down at a rapid pace. Sides of the valley are steep since the loosened rocks and soil get washed down the hill when it rains heavily. The river then gets loaded with material of this kind.

Waterfalls and rapids also lead to weathering of rocks. When in the bed of the river, a sudden drop or an interruption is met suddenly then a waterfall is created. At times, in the sea level there is a drop and this could be the result. Movement of earth or ice movements could result in a sea level drop. When a hard rock layer is met with by the river, formation of a waterfall takes place after which it flows over a layer of rock that is softer.

Erosion of the softer rocks takes place more as compared to the harder rock. Increase in undercutting takes place over time and a overhanging of a hard rock formation is caused. Due to its own weight it collapses, subsequently. Swirling of rock and collision along the pool sides occurs due to high discharge, leading to gorges with steep sides when this process gets repeated many times. In the case of rapids the slope does not break where the soft rock and hard rock layers are very thin.

Ox box lakes and meanders are caused in environments of lowland rivers. In a river where there is a bend it is termed as a meander which increases to form an ox bow lake over a period of time. The meander bend is in a banana shape. When on one side of a straight river bank, a hard rock lies, a meander is formed. The softer rock breaks down as the water flows against the harder rock. On one side of the river due to constant rock erosion a path is formed with a slight bend. It is at this point that velocity increases and more suspended material is transported due to this.

Meanders and Ox-bow lakes

Against the bank due to constant corrosion, rock material erode that under cuts the bank. This leads to a collapse, a backwards retreat occurs, just like a waterfall and hence a river cliff is formed. A slip off slope is created due to decreased velocity in the inside bend and higher velocity in the outer bend. On the meander loops outside area, there is continual erosion causing narrowing down of the meander neck during a high discharge or floods. Since the necks join, the river course flows at a shorter course. The ox box lake or a pool of water is left as the original meander and could dry up if there is no heavy rainfall. What are levees and floodplains? The surrounding area of a river is termed as floodplain and is formed when flooding of a river occurs. In the same way when the river floods, a levee is formed and it is an embankment in natural form in the river’s upper bank. It is due to lateral river erosion that widening of a river occurs. Due to high energy discharge, material in bigger amounts suspended in the river, get transported at a faster pace.

Flooding of rives take place in a high discharge and the water spreads across the flat land nearby. Lots of fine silt is deposited due to increased friction first and then decreased velocity. When flooding happens each time, a thin silt layer gets deposited leading to the formation of a floodplain that is flat. Deposits of heavy material occur first on flooding of a river and a levee is formed next to the river embankment. This levee is nothing but a flood defense naturally formed and gets strengthened in time. On breaking of levees huge floods are caused and that too on larger land areas.

When weak and strong currents both meet then due to deposition a small land area called delta is formed. Water currents reduce as the river gets closer to the sea and river mouth leading to deposits of material. The river path gets blocked due to this material and river channels in a series called distributaries are formed to reach the sea. Deltas are formed due to build up of deposited materials over a period of time. When the river flows into a lakes slow or gentle current, then also a delta is formed. As the river flows, along its length a range of features can be seen. Rivers at a younger stage have lots of energy at the source for erosion and transportation of material. When gradient is steeper, the river flows faster leading to increased energy and in turn erosion and transportation. Rivers flow faster due to the river valley shape and sides of the valley.

Glaciers Cause Weathering
Shattering due to frost is a weathering process where the temperature goes below or close to freezing point. During night, the water present in joints and cracks in rocks freezes and thaws during day. This causes the breaking up of rocks to pieces of smaller size. In the crystallization process better known as haloclasty water seepage takes place into joints and cracks of rocks then evaporates thus leaving crystals behind. Expansion of crystals due to heat leads to pressure on the rocks, a process that occurs in arid regions mainly.

In glacial weathering, the erosion is caused due to movement of rocks and soil by the glaciers. Glacial erosion takes place in three processes namely abrasion, plucking and sub glacial melt water erosion. Debris at the glacier’s sides and base get eroded by the movement of glaciers. In a quarrying or plucking process, bedrock is removed. When ice is at pressure melting point, plucking occurs. The bed of rocks moves due to the friction caused by the thick glacial ice. In sub glacial melt water erosion, both physical and chemical erosion takes place. If the bedrock is of limestone and chalk then the meltwater makes the minerals soluble.

Weathering Due To Humidity In The Air

In the case of weathering, changing temperatures, wind, and rain act on the sides of valleys, leading to soil and rock break up on the sides of the valley. This extra material then falls in the rive adding to the river load. Weathering is caused in two types, mainly exfoliation and freeze thawing. Within the rock cracks, water gets filled and freezes in colder regions leading to a freeze-thaw. Water expands on freezing and then again warms up and melts down again. This constant process can break rocks and the rocks roll down the sides of the valley due to gravitational influence into the river.

The other weathering process is termed as ‘onion’ weathering or exfoliation. Due to heat of the sun, the rock gets heated and in the night the rock cools down. This constant cooling and heating causes the rock to split in a number of layers leading to breakage and the rocks roll down into the river. Wearing of valley sides occurs due to continued weathering and the river gets added with more loading of rocks.

In desert areas, the exfoliation process takes place due to huge temperature fluctuations in the day and night. Rocks expand and contract constantly and due to this stress, they break down and peel off. Top layer of rocks gets peeled off due to pressure release. Rocks expand on pressure, stress is caused inside it and then it eventually cracks and gradually the top layer starts breaking off.

Action of humans also causes weathering. In different ways, rivers get affected due to human intervention. Channels of rivers are straightened, the river course and shape is altered, artificial banks are built, channels are dredged, locks bridges and viaducts are built which affects rivers adversely. Activities like cultivation or farming are carried out on the edge of rivers which leads to the erosion of sides of river. Chemicals and waste are dumped into rivers which kills the vegetation and marine life in rivers.

It because a weathering process that has occurred that people get soil to make pots, plant trees and cultivate crops. With time the rocks break down to form particles of soil and sand in a process called weathering. Living organisms, water and air act on boulders and rocks leading to weathering. The minerals get broken down to smaller pieces and with the force of water and air get carried away to different locations. Over time the rock gets converted to clay and sand. Climatic conditions present in the region where the rocks are located are responsible for weathering.

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