Erosion and deposition

Detail

1. What erosion is

• Erosion means picking up rock debris and carrying it away.
• Weathering and other processes first break rocks into pieces.

• Erosional agents then remove the pieces and carry them away. Each agent does this in its own way (its "dynamics").

• The five erosional agents are running water, groundwater, glaciers, wind and waves.

• Abrasion is the scraping and grinding of rock surfaces by the debris an agent carries.
• Pebbles in a river, sand in the wind and boulders frozen into a glacier act like sandpaper.

• Debris adds "tools" to the agent, so abrasion greatly increases its erosive power.

• Result: relief degrades. Highlands are worn down over time.

• Weathering helps erosion but is not a precondition for it.
• Weathering loosens rock, so agents can lift it easily.

• Erosion can still happen without weathering. Abrasion by debris-laden water, ice or wind can cut fresh, unweathered rock.

• Degradational processes (processes that wear the land down): weathering, mass wasting and erosion.

• Of these three, erosion causes most of the continuous change on the earth's surface.

2. Energy behind erosion

• Kinetic energy (energy of motion) controls both erosion and transport.
• Faster or heavier moving water, ice or air → more kinetic energy → more and bigger debris carried.
• Speed comes from slope (gravity) or from pressure differences (wind).

3. What controls each agent

Agent	State of matter	Main control	What it means
Wind	Gas	Climate	Strongest in dry, bare lands, e.g. the Thar Desert (Rajasthan)
Running water	Liquid	Climate	Most widespread agent in humid and monsoon lands, e.g. the Ganga basin
Glaciers	Solid	Climate	Work only in cold high-latitude or high-altitude areas, e.g. Himalaya, Antarctica
Waves	Liquid (sea)	Location at the land-sea interface (the line where land meets sea)	Work only along coasts
Groundwater	Liquid (underground)	Lithology (the type of rock)	Forms karst only where rock is permeable and soluble and water is available

• Wind, running water and glaciers represent the three states of matter: gas, liquid and solid. All three are controlled by climate.
• Karst topography is landscape made when groundwater dissolves soluble rock such as limestone or dolomite. It has caves, sinkholes and underground streams.
• Three conditions: (i) rock lets water pass through it (permeable), (ii) rock dissolves in water (soluble), (iii) water is available.

4. Deposition

• Deposition is the dropping of the load that an agent carries.
• Cause → process → result:
• The agent reaches a gentler slope → it loses speed and kinetic energy.
• It can no longer hold its load → the material settles.
• Coarse material settles first. Fine material settles later.

• Depressions (hollows) fill up, and the land is built up.

• Deposition is a consequence of erosion. It is not really the "work" of any agent.

• The same agents (running water, glaciers, wind, waves, groundwater) therefore also act as aggradational agents, which build up the land.
• Gradation means the overall levelling of the land: degradation (wearing down) + aggradation (building up).

5. Landforms made by each agent (preview)

Agent	Erosional landforms	Depositional landforms
Running water	Valleys, waterfalls	Meanders/floodplains, deltas (e.g. Ganga-Brahmaputra delta)
Sea waves	Sea cliffs	Beaches
Glaciers	U-shaped valleys	Moraines (heaps of glacial debris)
Wind	Mushroom rocks	Sand dunes (e.g. Thar Desert)
Groundwater	Caves (e.g. limestone caves)	Stalactites and stalagmites

• Meanders (bends in a river on a flat plain) form through both erosion on the outer bank and deposition on the inner bank.

6. Human dimension: why erosion matters

• Farms: erosion removes fertile topsoil, so crop yields fall.
• Rivers and coasts: it washes away land, houses and roads.
• Construction and mining sites: loose ground becomes unstable.
• Tourism and fishing: both suffer when beaches and coastal land are lost.
• Scale of land loss in India:
• Land degradation and desertification covered 97.84 million hectares in 2018-19, according to the Desertification and Land Degradation Atlas of India by SAC-ISRO, Ahmedabad [2].
• It was 96.4 million hectares (29.32% of total geographical area) in 2011-13 [2].
• Soil erosion that strips topsoil, together with shifting sand dunes, has degraded about 17.77 million hectares (5.34% of total geographical area) [2].

• India's target is to restore 26 million hectares of degraded land by 2030, under its commitment to Land Degradation Neutrality (when new land degradation is balanced by restoring an equal area) [2].

• Coastal erosion in India:

• The National Centre for Coastal Research (NCCR), under the Ministry of Earth Sciences, studied the whole coastline for 1990-2018 using satellite images and field surveys [3][4].
• Results: 33.6% eroding, 26.9% accreting (growing, because the sea deposits sediment), 39.6% stable [3][4].
• NCCR prepared 526 maps at 1:25,000 scale, plus 69 district maps and maps for 9 states and 2 Union Territories [3].
• The report "National Assessment of Shoreline Changes along Indian Coast" came out in July 2018. An updated atlas was released on 25 March 2022 [3].
• The shoreline changes come from both natural causes and human activity. As the coast retreats, fishers lose land and space to park boats and mend nets [3].

7. Traditional conservation methods

• Contour ploughing / continuous contour trenches: trenches dug along lines of equal height on a slope. They slow run-off and hold back soil.
• Bunding: low earthen walls (bunds) around fields that stop water and soil from running off.
• Terracing: cutting a hill slope into flat steps. Common in the Himalaya and the North-East.
• Check dams: small barriers across streams. They slow the water, trap silt and recharge groundwater.
• Zabo system (Nagaland):
• The name comes from zabö, which means "impounding run-off water" in the Chokri dialect [5].
• The Chakhesang tribe of Kikruma village, Phek district, Nagaland, developed it about 80-100 years ago [5].
• It joins forestry, horticulture, agriculture, fishery and animal husbandry in one system [5].
• How it works: forest on the hilltop holds rainwater → the water flows into a pond → livestock pens sit below the pond → the water flows through the pens, picks up manure and then irrigates the paddy fields lower down.

• Paddy yield is about 1.95 tonnes/ha, compared with 1.0 tonne/ha under jhum (shifting cultivation) [5].

• Sindhu-Sarasvati (Harappan) civilisation: it managed water carefully, with planned drains and reservoirs, which reduced run-off damage.

8. Standard short-answer distinctions

Basis	Erosion	Mass movement
Driving force	A moving agent (water, ice, wind, waves)	Gravity alone
Carrying medium	Yes. Debris is carried by an agent	No. Debris moves by itself down the slope
Distance	Can be very long (e.g. river to sea)	Usually short (down a slope)
Abrasion	Yes	No

• Is appreciable erosion possible without weathering? Yes.
• Weathering speeds up erosion, but it is not needed for it.
• Debris-laden rivers, glaciers and wind can abrade unweathered rock.
• In practice, deep weathering (as in humid tropics) supplies much more loose material, so erosion is faster there.

Prelims Hooks

• Agents that represent the three states of matter: wind (gas), running water (liquid), glaciers (solid). All three are controlled by climate.
• Waves are controlled by location (the land-sea interface). Groundwater is controlled by lithology (rock type). They are NOT controlled mainly by climate.
• Karst needs three things: permeable rock, soluble rock and available water.
• In deposition, coarse material settles first and fine material settles last.
• Weathering aids erosion but is not a precondition for it. This is a frequent trap statement.
• Zabo system: Nagaland, Chakhesang tribe, Phek district. It means "impounding run-off" [5].
• NCCR shoreline study (1990-2018): 33.6% eroding, 26.9% accreting, 39.6% stable [3][4].
• The Desertification and Land Degradation Atlas of India is published by SAC-ISRO, Ahmedabad. Degraded land: 97.84 million ha in 2018-19 [2].
• Pairs of agent and depositional landform: river → delta; glacier → moraine; wind → dune; waves → beach.

Mains Points

• Erosion as a development problem (GS-III):

• Topsoil loss → lower farm yields → threat to food security. India had 97.84 million ha of degraded land in 2018-19 and aims to restore 26 million ha by 2030 [2].

• Coastal erosion (GS-I/III):

• About one-third of India's coast is eroding (NCCR, 1990-2018) [3].
• Causes include natural wave action and human actions such as ports, sand mining and dams that trap river sediment.

• Effects: loss of homes and fishing space. Responses: shoreline mapping and coastal protection by states.

• Traditional knowledge as a solution:

• Zabo, terracing, bunding and check dams control run-off at the source.

• They are cheap, run by the community, and fit with watershed programmes and Land Degradation Neutrality.

• Conceptual answer:

• Weathering and erosion work together. Weathering supplies the debris; abrasion by that debris gives erosion its power.
• Erosion, not weathering, is the main sculptor of continuous surface change.