Rocks break into smaller pieces and fianlly into fine particles by the atmospheric elements like temperature wind, humdity, rain etc. This process is called weathering. Plants and animals also cause weathering of rocks.
Weathering processes are of two types namely:
(i) Physical or Mechanical weathering
(ii) Chemical Weathering
(i) Physical or Mechanical Weathering : The rocks break into smaller fragments and pieces by physical process which is known as physical weathering.
The chemical composition in this types of weathering does not change.
Due to the heat of the sun outer surfaces of rocks gets heated up but inside remains almost cool. This makes the rock to expand and crack. Thin layer of rocks peel off. This is called exfoliation. Sometimes grain of rocks expand and separate and changed into granules or small particles.
In desert areas heating in day time and cooling in night time cause expansion and contraction constantly making the rocks disintegrate.
In very cold areas, water in the cracks of the rocks changes into ice due to low temperatures. Water expands while turning into ice, widening the cracks between the rocks. The rocks begin to crack along the weaker parts and rock get separated. This is a kind of physical weatheirng.
Plants grow on rocky land surface. Their roots enter the joints of the rocks in search of moisture. As the plant grows the roots go deeper causing the rock to crack and break into pieces.
Animals like mice, rats and rabbits dig holes into the earth weakening the support of rocks. The rocks get loose and eventually fall and break into pieces.
Man is also an agent contributing to weathering. Falling of trees, mining, constructing roads and railways help weathering to take place effectively. The weathering caused by plants, animals and human beings is called biological weathering.
2. Chemical weathering : In the atmosphere near the ground sruface, oxygen, carbon dioxide and water vapour are present. Oxygen and carbon dioxide facilitates chemical reactions to take place on the minerals in the rocks. The minerals get dissolved and the chemical reaction causes the rocks to disintegrate and turn into finer particles and finally into soil.
Iron oxides are formed by the chemical reaction of oxygen and water on iron.
Carbon dioxide and water form carbonic acid. Because of this acid, the minerals in the rocks dissolve and the rocks become brittle. Limestone dissolves under the action of carbonic acid. The dissolving of minerals carries the chemical weathering of rocks.
The ratio of physical and chemical weathering is almost the same in humidity. Physical weathering is more effective in a hot and dry desert climate. Chemical weathering is very effective in the hot and humid conditions of the equatorial areas.
Importance of Weathering
Weathered surfaces of mountains, hills and rocks slide down the slopes. This material help the errosive action of rivers, winds and glaciers.
Running water : Streams and hills coming down the slopes combine together and form a river. The river is a very important weathering agent.
A river does many jobs from its source to mouth, such as disintegration of rocks, erosion, and deposition. The different phases of work of a river are as follows:
Transportation : A river flows down a slope with a speed. On account of this speed, it carries fragments of rocks, gravel stones, sediments and dissolved substances with its current. This is called transportation.
Erosion : River water and the materials flowing rub against the banks and the bed of the river. Similarly, the rocks flowing down also collide with each other, and break into pieces. Some substances from the rocks dissolve in the running water. The banks of the river are cut and the bed gets scoured because of friction and the impact of the river-current. This is called erosion. The erosion is of three types : 1. headward erosion, 2. vertical erosion, and 3. lateral erosion.
1. Headward erosion : The headward erosion is one which takes place near the source of the river.
2. Vertical erosion : The river-water and the material in it cuts down the bed deeper. This down-cutting of the river bed is called vertical erosion.
3. Lateral erosion : The river banks are cut by the friction of river-water and the material in it. This is called lateral erosion.
Deposition : The deposition is the process in which material is carried by a river and is deposited on its banks and bed. In the areas where the river flows slowly, there is more deposition.
Land forms created by the rivers
V-shaped valley : The speed of the river is greater in the mountainous region. This creates a deep valley with steep sides. This valley is known as V-shaped valley.
A deep and narrow valley with steep sides is called a gorge. There are many gorges in the Himalayas and one in Jabalpur.
Waterfall : If both hard and soft rocks come in the way of flowing river the soft rock gets eroded faster. The hard rock does not erode so easily. That is why, the river falls with a great speed from hard rock. This is called a waterfall. The Niagara Falls in North America are well known and famous.
Potholes : The hard stones carried along with the river water turn round and round due to the collisions and impact of water. That is why holes of various shapes are formed in the rocky river bed. Such holes are called potholes. Many such holes are seen in the river beds of the Kukadi, Krishna, Godavari etc. in Maharasthra.
Meanders and ox-bow lakes : Lateral erosion is responsible for meanders.
As the erosion increases over a period of time, the meanders in the river go on increasing. The meander breaks when there are floods, and the river again starts flowing in a straight line.
Fan-shaped plains : Near the source of a river the tributaries joining the main river deposit substances on the banks of the main river. This deposition gives birth to fan-like plains.
Flood plains : In floods, the river-water overflowing its banks spreads in the surrounding regions. The silt carried by the water gets deposited in these areas and creates flat plains on both the banks of the river. The Gangetic Plain is a flood plain.
Natural levees : During floods, the water of the river crosses its banks and the pebbles and stones carried by the river are deposited near the banks. The region near the bank of the river rises higher than the flood plain. This high wall is called a natural levee.
Delta region : The silt carried by the river is deposited on the bed near its mouth. The region near the mouth of the river gets gradually filled up by this deposition. In due course of time, the main course of the river gets split into streams of small channels. These streams are called distributaries. A trinagular region of large number of such distributaries is formed near the mouth of the river. This area is called the delta region. Deltas exist near the mouths of the rivers Godavari, Ganga, Nile, Mississippi etc. Deltas are very fertile.
Glacier : It is a mass of slow moving ice coming down the slope from a snow-bound region.
Glaciers are of two types :
1. Continental Glacier : The pressure of ice makes extensive sheet of ice to move. This is called a continental glacier. Such glaciers are seen in Antarctica and Greenland.
2. Alpine or mountain glacier : There are snow-bound areas in the mountainous regions of the Himalayas, the Alps, the Andes, the Rocky mountains, etc. The ice deposited in these areas starts sliding down the slopes. This sliding down ice is called a mountain glacier or an alpine glacier.
Iceberg : A block of ice floating in the sea is called an iceberg. These icebergs are very large in size. The density of ice is slightly less than that of water, so a very little portion of an iceberg is seen above the water level and the rest of it is submerged under water.
Work of a glacier : Like a river, a glacier works of transportation, erosion and deposition.
1. Transportation : The speed of a glacier is very slow. The speed of a glacier depends on the thickness of the accumulated ice, the temperature of the region, and the slope of the land. On an average a glacier moves 1 to 15 metres a day. It is called transportation.
2. Erosion : The floor and the sides of the glacier get scoured due to the rock fragments that it carries. Sometimes, the rocks on the floor of the glacier are dislodged by the pressure of the moving ice. These dislodged rocks are carried further with the glacier. In the case of a glacier, the work of erosion and transportation takes place simultaneously and as result the glacier collects many rock-fragments, mud, soil and sand on its way.
3. Deposition : When a glacier enters an area of higher temperatures, the ice begins to melt and the materials carried by the glacier get deposited at its bottom in the form of heaps of stone, sand and mud.
Land forms of glaciation : Various land forms are created by glaciers. These are—
Cirque : When a glacier slides, it gets deposited in a hollow, if there is one on any side of the peak. The accumulated snow starts sliding down the slope. This causes friction at the floor and at the sides of the hollow, thus enlarging it further. This is called a cirque.
The back wall of a cirque is like a high cliff and the floor is concave and huge in size. When a glacier melts completely, water accumulates in the cirque and forms a lake which is known as tarn.
U-shaped valley : When a glacier is in a valley in a mountainous area, the erosion of the sides is greater than that of the floor, a valley is formed with vertical sides and a wide floor. This valley is called a U-shaped valley.
Hanging valley : In the mountainous areas many tributaries join the main glacier. The valley of a tributary is at a higher level than a valley of the main glacier. If seen from the floor of the main glacier, the valley of the tributary appears to be hanging. Such a valley is called a hanging valley.
Moraine : The substances transported and deposited by a glacier are known as moraine. These are of the following types—1. lateral moraine, 2. medial moraine and 3. terminal moraine.
Wind : Wind is an agent in desert areas with hot and dry climates. The rainfall in these areas is very low.
Desert regions have very little and scattered vegetation. Hence, the wind does not have any obstruction and therefore blows with a greater velocity. Also, as the physical weathering in these regions is greater, the transportation and erosional work of the wind is more effective.
(a) Transportation : The wind carries fine dust and sand particles from where it blows. That is why rocks become bare, and small holes are formed in their surface. Sand is carried away due to the speed of the wind.
(b) Erosion : The dust and sand particles carried by the wind attack the rocks that obstruct the wind and the rock gets eroded.
When the wind blows with a strong speed bigger sand particles are dragged along the ground surface, eroding it to a greater extent. This gives rise to elongated depressions.
(c) Deposition : When the speed of the wind becomes low the material it carries is deposited in that area.
Land forms created by the work of wind
Mushroom rock : The wind blowing in desert areas erodes the rock near the ground surface. At the same time, the upper part of the rock gets eroded to a lesser extent. As this is a continuous process, the foot of the rock becomes narrow. The top portion of the rock then looks like an umbrella. This land form is called a mushroom rock.
Sand dunes : Sand is transported from one place to another along with the wind. At a place where the wind meets an obstruction, dunes are formed.
The side of the dune towards the wind has a gentle slope and the opposite side has a steep slope.
Barkhan : The fine sand particles carried by the wind get deposited when the speed of the wind is reduced forming crescent shaped dunes. Such hills are called barkhans.
Loess : Loess is a soil finer than sand. Loess is a silt transported by the wind from the desert regions and deposited much farther away.
Groundwater : Some water from the rainfall recieved on the earth’s surface seeps through the ground. This water trickles down until it reaches an impervious rock. Water accumulated in this manner, is called groundwater.
Groundwater is found at various depths from the earth’s surface. The water tables rises nearer the earth’s surface in the monsoon, and sinks down in summer. Groundwater comes out in the form of springs.
The work of groundwater : The work of groundwater is seen mainly in limestone regions.
Transportation : The ground water dissolves minerals in the rocks and get them transported along with it.
Erosion : The presence of carbon dioxide and oxygen in groundwater i.e. carbonic acid causes the minerals in the rocks to dissolve. The dissolved materials form a solution which flows away causing erosion. This erosion results in the weakening of the rock.
Deposition : On account of the extreme heat beneath the earth’s surface, the groundwater evaporates and the minerals it contain get deposited.
Land forms created by groundwater
Sink holes : Water on the ground surface seeps through limestone and gets dissolves in the water. If this process takes place continuosly, it makes holes in these rocks. In a number of years, these holes get enlarged. These holes are called sink holes.
Caves : In limestone region, water goes very deep through sink holes. If there is a layer of impervious and hard rock underneath, water flows horizontally on the impervious rock instead of going deeper. Hence, soft rock gets eroded and a cave is formed.
Stalactites and stalagmites : Inside the cave, water is always seeping through the roof. This water contains calcium carbonate. As the seeping water evaporates, some of the calcium carbonate is deposited on the cave’s roof. This deposition continues to grow very slowly. Hence, a column grows from the roof towards the floor. It is called a stalactite. The water dripping on the floor of the cave also evaporates leaving behind calcium carbonate which accumulates over a period of time. A column then starts growing from the floor towards the roof. This column is called a stalagmite.
Work of sea waves : Sea waves also do the work of transportation, erosion and deposition.
1. Transportation : Movements of sea water is mainly caused by the wind. The wind currents create sea waves. Transportation work is done by the waves near the coastal region. Sand, gravel, fragments of rocks get transported because of the waves.
2. Erosion : The impact of waves makes the rocks on the coast to break. The sand, gravel, etc. brought along with the waves hits against the rocks on the coast and causes erosion.
3. Deposition : Sea waves go towards the coast and their speed gets reduced. When the waves break and water starts receding, the various materials carried by the waves get deposited on the coast.
Land forms created by the work of sea waves
Sea cliff : The rock base on the coast gets eroded and notches develop. These notches in the rocks gradually extend landwards with time. Then the crest falls and a steep cliff is formed.
Sea cave : On the coast, rocks have large number of cracks. The impact of waves make them wider and wider creating small caves. They are called sea caves.
Beach : The fine sand and other material that goes along the waves get deposited. This deposition of sand is called a beach.
Sand bar : A deposition of sand near the coast is called a sand bar.
Lagoon : A shallow lake is created between the sand bar and the sea coast which is called a lagoon.
Shopping Cart