Special effects and significance of weathering: exfoliation, tors and ore enrichment

Detail

1. Exfoliation: what it is

• Exfoliation: curved sheets or shells of rock flake off a rock or bedrock. It leaves smooth, rounded surfaces (NCERT Figure 5.3).

• It is sometimes called onion-skin weathering, because the rock loses layers like an onion.

• Exfoliation is a result, not a process. It is the end product of other weathering processes. Three causes produce it:

• Unloading (also called pressure release): the rock above is removed, so the rock below expands upward.
• Thermal expansion and contraction: the rock heats up by day and cools at night, again and again.

• Salt weathering: salt crystals grow inside pores and cracks and push the rock apart.

• Exam trap: Exfoliation is often listed as a type of physical weathering. NCERT puts it more exactly: it is the effect of unloading, temperature change and salt weathering.

2. Exfoliation dome: formed by unloading

• Exfoliation dome: a large, smooth, rounded dome of rock (usually granite) that has lost curved sheets from its surface.
• How it forms (cause → process → result):
• Cause: granite cools slowly deep underground as a batholith (a huge body of magma that cooled and hardened deep in the crust) or a laccolith (a dome-shaped body of magma that cooled under the surface and pushed the layers above it up). It is under heavy pressure from the rocks above.
• Process: erosion removes the rocks above. The pressure falls (unloading). The granite expands, and cracks open parallel to the surface. These are called sheet joints.

• Result: curved slabs peel away one after another. Sharp edges are cut off and a smooth dome is left.

• Thermal stress also matters. A USGS-led study published in Nature Communications on 22 February 2018 watched a granite dome in California, USA, during the summers of 2014–2016. Repeated daily heating and cooling, plus heat building up at the dome surface, caused slow crack growth (subcritical cracking). This ended in exfoliation that looked "spontaneous". The study said thermal stresses help shape rock domes worldwide [4].

• Indian example: the domal granite hills of the Karnataka plateau. They are exfoliated laccoliths or batholiths, formed when magma cooled inside the crust and was later exposed by erosion (NCERT Class 11, Interior of the Earth).

3. Tor: formed by thermal expansion along joints

• Tor: a rounded pile of boulders sitting on a hilltop.
• How it forms (cause → process → result):
• Cause: granite has joints (natural cracks, often at right angles to each other).
• Process: weathering works fastest along the joints, mainly through thermal expansion and contraction. The corners and edges of each block are attacked from more than one side, so they round off fastest.

• Result: the weathered material is washed away. The hard, rounded core-stones are left piled on the hilltop.

• Indian example: the boulder landscape of Hampi, Karnataka.

• The Hampi World Heritage property covers 4,187.24 hectares in the Tungabhadra basin, central Karnataka, in Bellary (Ballari) district. Its setting is described as the Tungabhadra river, craggy hill ranges and open plains [5].

• Other well-known areas: tors are classic features of granite uplands in Dartmoor (England) and in granite terrains of the Deccan.

Feature	Shape	Main cause (NCERT)	Typical rock	Indian example
Exfoliation dome	One large, smooth, rounded dome	Unloading (pressure release)	Granite (batholith/laccolith)	Domal granite hills, Karnataka plateau
Tor	Pile of rounded boulders on a hilltop	Thermal expansion along joints	Jointed granite	Hampi boulder landscape, Karnataka

4. Significance of weathering

• It makes regolith and soil.
• Regolith: the layer of loose, broken rock that covers solid bedrock.

• Weathering breaks rock into fragments. This prepares the ground for regolith and soils, mass movements (downslope movement of rock and soil under gravity) and erosion.

• Link with erosion:

• If rocks are not weathered, erosion cannot be significant. Weathering helps mass wasting, erosion and the lowering of relief (the height difference between high and low land).

• But weathering is not a precondition for erosion. Wind, water and ice can erode rock without it being weathered first.

• Link with biodiversity:

• Weathering builds a weathering mantle (the thick layer of weathered material above bedrock).
• Forests depend on how deep the weathering mantle is.
• Biomes and biodiversity depend on forests.
• So weathering supports biodiversity (NCERT exercise 2(i)).

5. Enrichment: weathering concentrates ores

• Enrichment: weathering removes some material and leaves valuable ores concentrated. This happens for ores of iron, manganese, aluminium and copper.
• How it works:
• Groundwater carries away some material by chemical or physical leaching.
• The valuable material that stays behind becomes richer in metal.

• When it is rich enough, it becomes economically viable (worth mining for profit).

• Such ores are called residual deposits, because they are what is left behind after weathering (NCERT Class 10).

6. Leaching, laterite and bauxite

• Leaching: water seeping downward through soil removes soluble minerals (minerals that dissolve in water).
• In the wet tropics (cause → process → result):
• Cause: high temperature and heavy rain, with wet and dry seasons.
• Process: heavy leaching removes bases (calcium, magnesium, sodium, potassium) and silica.

• Result: iron and aluminium oxides are left behind. This forms laterite (a red, iron-rich, leached soil or rock) and bauxite.

• Bauxite: an aluminous rock. Its main part is hydrated aluminium oxide. Smaller amounts of iron oxide, silica and titania are also present [2].

• Its aluminium minerals are gibbsite (Al₂O₃·3H₂O), and boehmite and diaspore (Al₂O₃·H₂O) [2].

• It is the main ore of aluminium. It is also used in the refractory (heat-resistant material) and chemical industries [2].

• Reserves in India (as on 1 April 2020, UNFC system) [2]:

• Total 4,958 million tonnes: 646 million tonnes of reserves and 4,311 million tonnes of remaining resources.
• About 79% is metallurgical grade (used to make aluminium). Refractory and chemical grades together make up only about 4%.
• The main bauxite resources are in the East Coast bauxite deposits of Odisha and Andhra Pradesh. These are laterite cappings on the Eastern Ghats.

Rank	State	Share of India's bauxite resources (1.4.2020) [2]
1	Odisha	41%
2	Chhattisgarh	20%
3	Andhra Pradesh	12%
4	Gujarat	8%
5	Jharkhand	6%
6	Maharashtra	5%
7	Madhya Pradesh	4%

• Production [2]:
• India produced 22,494 thousand tonnes of bauxite in 2021-22, about 10% more than in the year before.
• Odisha was the leading producing state, with about 73% of production in 2021-22.

• NALCO (National Aluminium Company) was the leading producer, with 33% of production in 2021-22. Its mine is in Koraput district, Odisha.

• Panchpatmali (Koraput district, Odisha): NALCO's laterite bauxite mine on an Eastern Ghats plateau top. It is a textbook example of a residual weathering deposit.

• Panchpatmali gave 31% of India's bauxite production in 2018-19 [3].

Prelims Hooks

• Exfoliation is a result, not a process. Its causes are unloading, thermal expansion and contraction, and salt weathering.
• Process–landform pairs: exfoliation dome → unloading. Tor → thermal expansion along joints in granite.
• Feature–location: exfoliated domal granite hills → Karnataka plateau (laccoliths or batholiths). Boulder landscape → Hampi, Karnataka (Tungabhadra basin) [5].
• Trap: "Weathering is a precondition for erosion." False. Erosion is not significant without weathering, but weathering is not a precondition.
• Trap: "Weathering is responsible for biodiversity." True (weathering mantle → forests → biodiversity).
• Weathering enriches ores of iron, manganese, aluminium and copper.
• Leaching in the wet tropics removes bases and silica. It leaves iron and aluminium oxides, forming laterite and bauxite.
• Panchpatmali bauxite mine → Koraput district, Odisha (NALCO).
• Bauxite resources, highest to lowest (1.4.2020): Odisha (41%) > Chhattisgarh (20%) > Andhra Pradesh (12%) > Gujarat (8%) [2].
• Odisha produced about 73% of India's bauxite in 2021-22 [2].

Mains Points

• Weathering as the base of landscape and life (GS-I): it makes regolith and soil, prepares material for mass movement and erosion, and lowers relief. A deep weathering mantle supports forests, and forests support biodiversity. Example: deep tropical weathering supports the forests of the Western Ghats and the north-east.
• Climate controls weathering landforms (GS-I): in dry, hot areas with big day–night temperature swings, temperature change and salt weathering cause exfoliation. Unloading of deep-seated granite gives domes (Karnataka plateau) and tors (Hampi). Thermal stress in summer can trigger exfoliation sheets to break off, a rockfall hazard on granite domes [4].
• Residual ores and resource geography (GS-III): leaching in the wet tropics makes laterite bauxite on the Eastern Ghats. Odisha has 41% of India's resources (1.4.2020) and produced about 73% of output (2021-22) [2]. This shapes where the alumina–aluminium industry is located (NALCO, Koraput–Damanjodi–Angul).
• Mining vs ecology (GS-III): bauxite lies on plateau tops in forested, tribal areas of Odisha and Andhra Pradesh. The same weathering mantle holds forests, springs and biodiversity. Mining it means weighing mineral wealth against forest loss and tribal rights, as in debates over Eastern Ghats hill-top mining.