Geography - Soil Erosion, Soil Types Study Notes for SSC CGL 2016

Definition: Soil erosion is a form of soil degradation, which is naturally occurring process that happens on all land.

Synopsis:

  1. The primary agents of soil erosion are wind and water, each accounting for a considerable amount of soil loss each year. 
  2. Soil erosion could be a gradual process or may also occur at an alarming rate, causing serious loss of topsoil.
  3. Loss of soil from farmland leads to reduced agricultural production, lower quality of surface water and damaged drainage networks.
  4. Although erosion is a natural process, human activities have increased it globally by at least 10-40 times.
  5. More than average erosion causes both “on-site” and “off-site” problems. On-site problems include decrease in agricultural production and ecological collapse. The end result is that the land turns into a desert due to loss of nutrient-rich upper layer of soil.
  6. Off-set effects of erosion include sedimentation of waterways, eutrophication of water bodies, as well as sediment related damages to houses and roads.
  7. Water and wind, the primary causes of erosion account for about 84 per cent of degraded land across the globe. Excessive erosion is the biggest environmental issue world-wide.
  8. Human activities like intensive agriculture, deforestation, climate change and large scale urbanization are amongst the significant causes contributing towards increasing erosion.

Types of soil erosion

The broader categories of soil erosion are, rainfall or surface runoff, erosion by rivers and streams, coastal erosion, chemical erosion, erosion by glaciers, erosion by floods, erosion by wind and erosion by mass movement.

1. Erosion by rainfall

  1. Rainfall produces four main types of erosion, namely splash erosion, sheet erosion, rill erosion and gully erosion.
  2. Splash erosion is the mildest form of erosion, followed by sheet erosion, rill erosion and then rill erosion, in order of their severities.
  3. In splash erosion, falling raindrops create a crater which dislodges soil particles. The maximum distance these soil particles can travel is 0.6 meters (2 feet) vertically and 1.5 meters (five feet) horizontally.
  4. In sheet erosion, if the soil is saturated, or if the rainfall is more than the rate at which the water can infiltrate into the soil, surface runoff occurs. If this runoff has sufficient flow of energy, the loosened soil particles are transported down the slope, which is known as sheet erosion.
  5. Rill erosion means development of small, ephemeral concentrated flow paths which function both as sediment sources and sedimentary delivery systems for erosion on hilltops. Generally, rills are active where water erosion rates on disturbed upland areas are the most.
  6. When runoff water accumulates and rapidly flows into narrow channels during or immediately after heavy rains or melting snow, removing soil to considerable depths is known as gully erosion.

2. Erosion by rivers and streams

  1. Continued water flow along a linear feature causes valley or stream erosion. This erosion is downwards, which deepens the valley and also headwards, extending the valley into the hillside, creating head cuts and steep banks.
  2. Bank erosion happens by wearing away of the banks of a stream or river that is distinguished by changes on the river bed of the water course, which is called scour. Inserting metal rods into the banks and marking the position of banks at different times helps to measure the level of erosion.
  3. Melting and weakening of permafrost due to moving water results in thermal erosion. Thermal erosion can occur both along rivers and the coast.

3. Coastal erosion

  1. Erosion along the shores, which occurs on both exposed and sheltered coasts, primarily occurring through the action of waves and currents also contributes to a change in landscape, and is known as shoreline erosion.
  2. Hydraulic action occurs when air in a joint is suddenly compressed by a wave closing the entrance of the joint, which cracks it.
  3. Wave pounding happens when sheer energy of the wave hits the cliffs or rocks, breaking them down. Waves also cause abrasions and corrosion, launching sea load at the cliff. Sea water also dissolves rocks by carbonic acid contained in it, which specifically affects limestone cliffs.

4. Chemical erosion

  1. Loss of matter in a landscape in the form of solutes is known as chemical erosion. The quantum of chemical erosion is usually determined from the solutes found in a stream.

5. Erosion by glaciers

  1. Glaciers are predominantly responsible for three types of erosion, namely abrasion, plucking and ice thrusting.
  2. Abrasion process occurs when debris in the basal ice scrapes along the bed, polishing the underlying rocks, similar to the action of a sand paper.
  3. Plucking occurs when glaciers cause cracking of pieces of rock off the bedrock.
  4. In ice thrusting, the glacier freezes to its bed, and then it surges forward, moving along large sheets of frozen sediment at the base along with the glacier.

6. Erosion by floods

  1. Large volumes of rapidly rushing water forms kolks or vortices. These kolks cause extreme local erosion, which plucks bedrocks, creating geographical features called rock-cut basins.

7. Erosion by wind

  1. Wind erosion is a major force is arid and semi-arid zones. There are two types of wind erosion, namely deflation and abrasion.
  2. In deflation wind picks up and carries away loose particles. In abrasion, surfaces are worn down after being struck down by airborne particles carried by winds.

8. Erosion by mass movement

  1. The downward and outward movement of rocks and sediments on a sloped surface, mainly due to gravitational force causes mass movement. Mass movement is the first stage in the breakdown and transportation of weathered rocks in mountainous regions.
1. What happens to the soil when salinisation occurs?
[A] During storms, sea water washes in and contaminates plants
[B] Salts and other minerals are washed out of the soil during heavy rains
[C] Water evaporates in high temperatures drawing salts from the soil to the surface
[D] Flood water removes all the other soil particles except salt
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2. On many islands and in some nature reserves, goats are being removed and
banned. How does removing goats and similar grazing animals reduce soil
erosion?
[A] Goats tend to move around a lot, stirring up soil and leaving it open to erosion
[B] Goats will often overgraze areas and remove all vegetation, leaving bare earth
[C] The excess organic matter from the goats' droppings leads to pockets of rapid plant growth
[D] Goats discourage other animals that may help the plants to grow
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3. Heavy machinery often leads to soil compaction. How does soil compaction lead to soil erosion?
[A] The solid soil is removed in blocks rather than in small amounts
[B] The solid soil prevents water from percolating into the soil and increases surface runoff
[C] The compacted soils will be lower and encourage water to pool and soil to be washed away
[D] The machines remove large amounts of soil on their tyre treads
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4. When a field is ploughed, why is it best to plough at right angles to the slope?
[A] Ploughing parallel can lead to machines toppling over
[B] Ploughing at right angles to the slope improves plants' chances at getting equal sunlight
[C] Ploughing parallel to the slope draws the soil downhill
[D] Ploughing parallel to the slope can increase gully erosion
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5. What form of erosion occurs when intense rainfall cuts small streams into slopes in areas with little or no vegetation cover?
[A] Sheet Erosion
[B] Gully Erosion
[C] Wind Erosion
[D] Salinisation
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6. What type of erosion occurs when moderate rainfall on bare soil removes the topsoil down-slope?
[A] Sheet Erosion
[B] Gully Erosion
[C] Wind Erosion
[D] Salinisation
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7. Where do most nutrients in soils come from?
[A] Broken down rocks
[B] Decaying organic matter
[C] Rain fall
[D] Volcanic eruptions
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8. Which of the following is not a method of soil conservation?
[A] Planting hedges
[B] Terraces
[C] Contour ploughing
[D] Slash and burn agriculture
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9. Nomadic tribes people are becoming more sedentary, leading to over cultivation. How is over cultivation defined?
[A] Not planting enough crops to allow the soil to be protected from the rain
[B] Planting more crops than can be watered, leading to death of some of the crops
[C] The excessive use of farmland to the point where productivity falls due to soil exhaustion or land degradation
[D] Adding too much fertiliser, leading to eutrophication of the streams and waterways.
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10.
What is the danger of soil being left bare in hot, dry zones?
[A] Desertification as the soil blows away, leaving only the heavier sand grains
[B] Weeds will grow rampant, preventing crops from growing
[C] Build up of dry soils as the bare patch spreads
[D] The nutrients will be leached away by the Sun
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