Maharashtra Board Class 11 Geography Solution Chapter 3 Agents of Erosion
Balbharati Maharashtra Board Class 11 Geography Solution Chapter 3: Agents of Erosion. Marathi or English Medium Students of Class 11 get here Agents of Erosion full Exercise Solution.
Std |
Maharashtra Class 11 |
Subject |
Geography |
Chapter |
11 |
Chapter Name |
Agents of Erosion |
1) Complete the table :
Agents | Erosional Landforms | Depositional landforms |
1) River | ||
2) Wind | ||
3) Sea Waves | ||
4) Ground water | ||
5) Glacier |
Answer – The completed table would be as follows : –
Agents | Erosional Landforms | Depositional landforms |
1) River | Waterfalls, Canyons, Gorhes, Potholes | Natural levees, deltas, flood plains, alluvial fans, meanders |
2) Wind | Yardang, deflation hollow, ventifacts mushroom rocks | Sand dunes, loess, Barcalys |
3) Sea Waves | Sea arches, sea stack, wave cut platform | Beach, sand bar, lagoon, sandpits |
4) Ground water | Caves, sink holes | Pillars, stalactites, salagmites |
5) Glacier | Roche mountaineers, cirque, arête horne, hanging valley, U-shaped valley | Drumlins, eskers, terminal moraines, medial moraines, lateral moraines, end moraines, erratic rocks |
Q.2) Choose the correct option by identifying the correct correlation in the sentences:
1) Water or snow enters the cracks in the rocks and makes it weak. When the glacier passes on these rocks, it pulls the rocks at the bottom along with it. This process is called
a) Plucking b) Abrasion
c) Attrition d) Transportation
Answer – Water or snow enters the cracks in the rocks and makes it weak. When the glacier passes on these rocks, it pulls the rocks at the bottom along with it. This process is called a) Plucking
2) Sometimes, the river starts erosion upstream. This happens when the head stream gets a lot of water in the early stages of river’s flow.
a) Downcutting b) Headward erosion
c) Lateral erosion d) Vertical erosion
Answer – b) Headward erosion.
3) Soft rock erodes beneath the hard rock due to sea waves. This results into landforms which further develop as sea arches. The landform is
a) Sea cave b) sea stack
c) sea cliff d) wave cut platform
Answer – Soft rock erodes beneath the hard rock due to sea waves. This results into landforms which further develop as sea arches. The landform is a) Sea cave
4) This landform develops due to depositional work of wind. The windward slope of this landform is gentle.
a) Loess plains b) barchans
c) Seif d) Sand hills
Answer – This landform develops due to depositional work of wind. The windward slope of this landform is gentle – b) Barchans
5) River, glacier, wind, sea waves and groundwater are the agents of erosion.
Following work in the correct order is responsible to form various landforms.
a) Disintegration, picking up, transportation, weathering
b) picking up, disintegration, deposition, weathering
c) deposition, transportation, picking up, disintegration
d) disintegration, picking up, transportation, deposition
Answer – d) disintegration, picking up, transportation, deposition
- 3) Give geographical reasons :
1) The Eastern coast of India have deltas formed by the rivers but the Western coast has estuaries.
Answer – Deltas are commonly seen to have been formed at the mouth of those rivers where there is a large amount of sediment brought down by the river and also the depth of the sea is not too much.
On the other hand if the sea is deep and also the sediment supplied is not large enough rivers tend to form estuaries, while on its journey to the sea.
With the above in mind, on the eastern coast of India, the rivers flow into the Bay of Bengal. These rivers that flow to the eastern coast move through a gentle slope thus attain a lower velocity and on their way to the sea, deposit large quantities of sediments forming deltas.
The western coast on the other hand meets with the Arabian sea, which has a rocky coast. The rivers flowing into the western coast are relatively faster and shorter, moving swiftly with greater velocity, thereby carrying lesser amount of sediments. These rivers therefore form estuary on their way to the sea.
2) There is direct relationship between the velocity of the agents and the process of deposition.
Answer – The velocity of the agent such as stream, river, wind etc has a direct relationship with the process of deposition.
The higher velocity of the river or stream, more will be its carrying capacity. It will be able to carry bigger quantities of sediments and even larger particles to greater distances and vice versa. This can be seen during floods or when the river moves down the slopes of mountains the velocity is high and so the carrying capacity or power is largely increased. However, on the other hand in the plains where the slope is gentle or sometimes even becomes negligible, sediments get deposited due to lower velocity of the river.
3) Compared to all the agents, sea waves work ceaselessly.
Answer – Sea waves work ceaselessly, continuously is the coastal region causing erosion at some place and deposition at some other adjoining areas simultaneously.
Power of the sea waves are an effect of creation of sea beaches and sea bars. Energy of the sea waves depend on the velocity of the winds over the ocean, the temperature and the gravitational forces of sun and moon. Coastal erosion is mainly due to the effect of the sea waves with respect to hydraulic action, velocity causing impact, abrasion, corrosion (due to presence of salt in sea water) and thereby erosion. The softer rocks are prone to erosion first in this case.
4) One finds many sheep rocks, horns, arêtes and hanging valleys in the Himalayas.
Answer – Himalayas are high altitude mountains, hills, plateaus and passes where glaciers are predominantly active due to snow covered peaks with steep slopes.
Glaciers too are agents of erosion that can be seen only in high altitude mountains and polar regions and they are responsible for producing structures like sheep rocks, horns, arêtes and hanging valleys. Glaciers have been seen as agents of erosion, by transportation of materials on their slow movement and later on deposition, just like any other agents. They remove rock particles from the surface forming sheep rocks.
Ice movement due to weathering followed by mass wasting makes the head of the glacier steeper creating an arm chair like depression known as cirque. Once, two or more cirques develop, the area between them is called arête and on further erosion in the head ward direction the glacier forms a structure called a horn.
Glaciers like rivers may have tributaries and the rate of erosion could also differ with their velocity, slope etc, from one tributary to another. Erosion is faster at the main valley which becomes deeper than in the glacial tributary valley. They may thus appear to be hanging when seen from the main valley and therefore the name hanging valleys.
5) Karst landforms are seen concealed under the surface of the earth.
Answer – Karst landforms are those structures that are formed underground below the surface of the earth. These landforms are formed due to the acidified water, saturated with carbon dioxide, below the surface of the earth.
Warm, humid climatic conditions with a lot of vegetation, provide for ample amounts of carbon di oxide which dissolves in water. The water when saturated with CO2 gets acidified and as ground water finds way below the surface due to the permeability of rocks or cracks, they form landforms that come to be known as Karst landforms.
These landforms can be seen in parts of Meghalaya, Bora caves of Andhra Pradesh, Kanhur plateau in Maharasthra, as well as some parts of Madhya Pradesh and Chhattisgarh.
Thus, Karst landforms ideally need Carbon-di-oxide provided by warm humid climate and vegetation, an active source of underground water and cracks or permeable rocks for further seepage of water from the ground surface.
6) Snowline decides the limit of glacier work as an agent of erosion.
Answer – In higher altitude mountains and polar regions, glaciers are an important agent of erosion that can be seen in action.
Snowline decides the limit of glacier work as an agent of erosion, because it is the lower limit of permanent snow cover underneath which snow is not present. As glaciers move forward along their way, they remove rock particles and deposit them at another place just as other agents like rivers. Underneath the snowline the snow is no longer present and therefore no erosional features can be seen.
4) Write short notes on :
1) Attrition
Answer – Rivers, streams and other agents of erosion carry rocks, pebbles and particles from one place to another due to their velocity, slope etc. These rocks and pebbles collide with into one another while in motion and gradually end up becoming smaller in size, and further disintegrating into smaller and smaller pieces. Shapes of the rocks also become rounded.
This process where the rocks while on their journey through the rivers, streams, waterfalls etc end up becoming smaller by colliding with one another, is called attrition. It can be seen in rivers, winds and sea waves.
2) The work of rivers in hilly areas and human activities
Answer – Rivers begin their journey from hills or mountains where the altitudes are higher with a steep slope, therefore their velocity is highest there and so begins the act of erosion by moving rock materials. This is the reason that the river beds are deeper than the banks and Gorges are formed with narrow beds and steep banks. Eg. Gorge of Narmada river at Jabalpur, which have become a tourist place.
People living around the ‘V’ shaped valleys mainly practice terrace farming, tea plantations on the slopes.
As the river moves down the steep slopes, people living around are seen to have agro-forestry, agriculture, animal husbandry as their main occupation.
As the river descends further, it may flow down a cliff forming waterfalls which again become a place of natural tourist attraction and these then allow growth of a number of other professions like hotels, shops, food stalls etc. Eg. Venna falls on Mahabelshwar, Jog falls on Sharavati river
3) Conditions necessary for work of wind.
Answer- In deserts, wind plays a significant role in erosion. The conditions necessary for wind to be effective are
- Dry environment or arid to semi arid climatic conditions, where evaporation is faster and very little or almost no rainfall. This makes the soil become loose and wind carries it away easily causing erosion.
- Absence of trees or vegetation – The soil is not held together by the roots and can be swept away easily by the wind. Arid climatic conditions itself means less chance of vegetation.
- Wind velocities need to be enough to carry the soil particles from one place to another.
5) Distinguish between :
1) Attrition and Abrasion
Answer – Attrition – Attrition refers to a process when the rocks and pebbles collide with one another and break up into smaller fragments. On their way down the river, the rock particles get reduced in size and become more rounded shape wise.
Attrition is a process that occurs to moving particles, while in motion.
Abrasion – The process that involves the scratching and polishing of the surface or bedrock by the particles which are moving onto it. It is like when a sandpaper is used for smoothening, here the rock particles involved in abrasion rub against and wear away the surface. The eroded bedrock or surface will have a smooth side. Abrasion increases as velocity increases.
The difference between abrasion and attrition is that abrasion affects the surface along which the material moves and attrition relates to the material that moves.
2) U shaped valley and V shaped valley
Answer – U Shaped valley – U-shaped valleys are formed by glaciers as the erosional agent from an existing valley.
A pre-existing valley through which the glacier passed is broadened at the bottom by further lateral erosion by the glacier and thereby the U-shaped valley is developed.
V-shaped valleys – Rivers are the erosional agent for V-shaped valleys.
No pre-existing valley is seen here and the river is known to carve out its own valley, and with time and further erosion and downcutting the valley deepens.
3) Stalactite and stalagmite
Answer – In caves, present in areas where there are permeable or water soluble rocks like gypsum and limestone, it is often seen that water gets saturated with calcium carbonate and starts dripping onto the floor of the cave to build up massive structures. These structures are seen to be growing from the ceiling are called stalactites.
However, some other such structures are formed when water saturated with calcium carbonate starts building formations that are growing from the ground of the cave to the roof. These are called stalagmites.
Stalactites and stalagmites often simultaneously continue the growing process and may eventually meet to form columns or pillars.
Eg, in one of the caves in Meghalaya in India is 23 km long and one can see both stalactites and stalagmites in plenty to have been formed there.
4) Tributaries and distributaries
Answer – Tributaries – Tributaries is the term given to a number of smaller rivers that come together to form the main larger river. Therefore, it could be seen that the tributaries lie at the point of origination and their source is either glacier, springs or lakes. Eg. Yamuna and Saraswati are tributaries of the river Ganga.
Distributaries – Distributaries are branches of the main river and are formed closer to the coastal areas when the river is on the verge of ending its journey into the sea.
When the main river before flowing into the sea, breaks up into several smaller rivers or sub branches, these smaller rivers are called distributaries of the main river, since they are originating from the main river which is their source. Eg Hooghly and Padma are distributaries of river Ganga.
6) Answer in detail :
1) Explain the landforms formed by different agents through the process of abrasion
Answer – Abrasion is a process that mainly involves scratching or polishing of the surface or bedrock by particles that are moving onto it. In a manner, similar to a sandpaper being used for smoothening in our day to day activities, the rock particles here are involved in abrasion, for rubbing against and wear away of the surface. The eroded bedrock or surface eventually will have a smoothened side. Abrasion increases with increasing velocity.
Briefly, the following landforms are formed by the process of abrasion –
Sea cliffs & caves– Sea cliffs are consequently carved where sea waves strike directly against rocks. Softer rocks along the coastline are the ones that get eroded first. Sometimes, waves can even erode the softer part to such an extent that it becomes hollow enough to be called a cave.
Sea arch & sea stack– A sea arch is a structure that is formed when the sea waves erode a layer of soft rock underneath a layer of hard rock. With time, if after a lot of abrasion, the sea arch collapses, it creates a sea stack, which resembles a large rock in the middle of water. Sea arches are also formed when two caves meet from each side of a headland.
Wave-cut planes – Surfaces formed at the base of the cliffs are referred to as wave-cut platforms. Extensive platforms are formed where the rocks are most vulnerable to wave erosion, due to the process of abrasion. These platforms at low tide and are also proof of cliff recession.
Work of winds :
Ventifacts – In arid regions, where the wind speeds are high, due to low rainfall and due to lack of vegetation, the soil is dry and loose, wind polishes the rock surface through abrasion. Abrasion eventually carves the windward side (side facing the wind) of rock into smooth sloping surface and these rocks are called ventifacts.
Mushroom rocks – Larger high rising individual rocks that come in the path of the wind are attacked by the sand particles and mud they carry. In this process of mushroom rock formation, the base of the high rising individual rock is affected the most, because the wind carrying the sand and other particles which are heavy in nature remain grounded as they cannot be lifted because of their weight.
The top of the rock is not eroded much due to absence of any particles there, but the middle part of rock at medium height is also impacted quite seriously because the particles which are of medium or lighter weight are present in this zone of the wind and they travel faster due to high velocity of the wind.
Thus, upon erosion by abrasion, the rock forms the shape of a mushroom and hence has got such a name.
Yardang – In those areas where both hard and soft rocks are found together, the softer rocks naturally get more eroded faster. These eroded portions of softer rocks appear like elongated ridges while the harder rocks look like elevated portions. The remaining portion of the ridge thus formed, where rocks have been eroded is called a Yardang.
Work of Glaciers –
Roche moutonnee – Roche moutonnee are the bedrock hills that appear to be smoothly rounded by abrasion on the upper side and on the lower side by plucking .
Cirque – Ice movement, when accompanied by weathering and mass wasting, steepens the wall towards the head of the glacier. The shape formed thus resembles an armchair-shaped depression which is called cirque.
Arete – Often it is found that two or more cirques are formed side-by side leaving and the area between any two of them into a narrow wall which is known as arête.
Horn – When we see that three or more cirques are formed, headward erosion of the glacier erodes the top or the summit. The resulting structure that is formed is called a horn. Eg. the Matterhorn in Swiss Alps.
U shaped valleys – As glaciers move ahead, eroding the sides as well as the bottom of the valleys, through which they move along, both lateral and vertical erosion is prominent, thereby making the valley broader at the bottom. This is known as a U-shaped valley.
2) Explain how the depositional work done by river Ganga has been beneficial to human activities.
Answer – Throughout history we have seen that human civilisations have grown and thrived by the banks of the rivers, especially in the plains. Occupation of the people in the plains were mainly cultivation, animal husbandry and this was possible due to the presence of highly rich and fertile soil which was most suited for their crops.
The alluvial plains – The richness of the soil, formed by material brought by the rivers (alluvium) is beneficial for agriculture to thrive here. During floods, these floodplains become inundated with sediment-filled water that deposits sediments on the sides of the river. This leads to formation of natural levees on the sides of the river banks. The river also provides for another occupation of fishing and serves for an easy mechanism for transportation. Fertile plains are seen in states of Bihar, West Bengal, Uttar Pradesh, Haryana and Punjab.
Deltas – The Sunderbans in west Bengal or the Ganga Brahmaputra delta is the world’s largest delta at the mouth of River Ganga. It is considered to be one of the most fertile regions containing mostly alluvial soil. Agriculture is therefore the most practiced occupation in the deltas.
Fishing is also an occupation that serves to the livelihood of the people living in these regions, since the fishermen have access to both marine and fresh water fish, being adjacent to the sea. Also, some of the marine fish like Hilsa comes to riverine water just to lay eggs, as these eggs will break open due to osmotic pressure if laid in sea.
3) Which agents of erosion can you see on the cover page of the textbook? Which landforms can you see there? Write the process of formation of any one.
Answer – The cover page of the textbook have pictures showing agents of erosion like glaciers and rivers originating from the mountains and sea waves. Also, different landforms are visible like the meanders, oxbow lakes, alluvial fans, deltas, sea and beach.
As river comes down to the plains, it uses much of its energy (that it acquired while traversing the slopes in hills or mountains) for transporting heavy load. Speed of erosion is reduced with lower speed of flow. Erosion now starts on the outer banks and deposition of materials on the inner bank of the channel begins forming serpentine bends called meanders. However, during floods as the volume of water and speed increases, streams look for a shorter and straight forward path and may not meander, forming a new flow channel. This isolates the meander bend from the new flow channel of the river. The cut-off meander if it remains filled with water, forms a newly formed lake referred to as an oxbow lake.
Further down, as the river approaches the sea, minimum gradient and close proximity to the sea makes erosion impossible in this region. Heavy load of the river and reduced velocity makes it break into various channels or distributaries. The river starts depositing the sediments within its channel and as a result flow gets separated further. Islands are formed which takes a somewhat triangular shape. This distinct landform is called a delta.