Open-cast mining

Also called: Surface mining · Topic: Primary Activities: Gathering, Pastoralism, Agriculture and Mining · NCERT: Class 12, Ch 4 "Primary Activities"

Meaning

Open-cast mining (also called surface mining) means digging out minerals that lie close to the surface. First the overburden is removed. Overburden is the layer of soil and rock that covers the mineral. Then the mineral is dug out of a wide, open pit. It matters because it is the easiest and cheapest way to mine and it gives large output quickly. This is why India now gets about 96–97% of its coal from open-cast mines [2]. The cost is scarred land and people forced to move.

Explanation

How it works (cause → process → result)

• Cause: the mineral lies near the surface.
• The method is chosen by the mode of occurrence, meaning where and how the mineral lies: near the surface or deep down, in thick beds or in thin veins.

• When the bed is shallow, it is cheaper to dig it out in the open than to sink shafts.

• Process:

• The overburden is removed first.
• The mineral bed is then dug out from a wide, open pit.

• Large machines do the work: shovels, dumpers (trucks that carry and tip loads) and draglines (huge cranes that drag a bucket to scoop earth).

• Result: large amounts of mineral come out fast. Big pits and heaps of waste rock are left behind.

Advantages

• It is the easiest and cheapest method.
• Safety precautions and equipment cost little compared with underground mines. There are no shafts, lifts or ventilation systems to pay for.
• Output is large and quick.
• The safety risk is low. Underground mines face poisonous gases, fires, explosions, floods and roof collapse.

Problems

• It scars the land.

• It leaves big pits and dumps of waste rock.

• It displaces people.

• Villages, farms and forests sitting over the deposit have to be cleared.

• In India, this often affects tribal communities and forest land.

• The land needs reclamation, which means bringing mined land back into use:

• filling the pits
• making the slopes stable
• planting trees

When open-cast mining pays (factors)

• Physical factors: the size of the deposit, the grade of the ore (how much metal is in the rock) and its mode of occurrence. A shallow deposit suits open-cast mining.
• Economic factors: demand, technology, capital, and the costs of labour and transport.
• A deposit counts as "workable" only in economic terms. A low-grade, shallow deposit may be left alone today. If prices rise or technology improves, it can become profitable to dig.

Where it occurs

• India: coal. Open-cast mining is now the main method for coal.
• The share of underground mining in coal output fell every year: 5.54% (2019-20) → 4.50% (2020-21) → 4.26% (2021-22) → 3.90% (2022-23) → 3.44% (2023-24) [2].

• So about 96.6% of Indian coal (2023-24) came from open-cast mines [2].

• India: mine closure and reclamation.

• Mine closure means closing a mine and making it safe once mining ends. The closure plan fills open pits, makes slopes stable, manages waste dumps and plants trees. Afforestation (planting trees) is the main part of this work [3].

• Coal companies have found 179 pre-2009 and 162 post-2009 abandoned or discontinued mines. Of these, 147 are marked for closure. The rest are being joined to working mines and started again [3].

• The contrast case in India: underground mining in Dhanbad (Jharkhand).

• The Jharia coalfield fires and the Chasnala disaster (1975), a mine flood, show the dangers that open-cast mining mostly avoids.

• The world: mining is moving from developed countries to labour-rich developing countries in Africa, South America and Asia. The main reason is that labour costs are high in developed countries.

Don't confuse with

• Underground (shaft) mining: used when ore lies deep. Vertical shafts are sunk, and horizontal galleries branch out from them. It is costly, slower and high-risk (gas, fire, flood, roof collapse). Open-cast mining works near the surface, costs little and is low-risk.
• Placer mining: this works on a placer, a deposit of heavy minerals (gold, tin, diamonds) that rivers have washed down into sand and gravel. Miners wash the sand so the heavy minerals settle out. Open-cast mining instead strips off the overburden and digs out a mineral bed.
• Dredging: a dredge (a floating machine) scoops minerals out of river beds or shallow sea floors. It works under water, not in a dry open pit.
• Solution mining (lithium brines, e.g. Salar de Atacama, Chile): salty underground water (brine) is pumped up from as deep as 30 m, and lithium is taken out of it in evaporation ponds. No rock is dug [4]. Open-cast mining removes rock and soil.

Prelims Hooks

• Open-cast mining = removing the overburden, then digging from an open pit. It is the cheapest method, gives large, quick output and has low safety costs.
• Main harms of open-cast mining: land scarring and people displaced. Main harms of underground mining: accidents, subsidence (land sinking) and underground fires.
• India's underground share of coal output was 3.44% in 2023-24, down from 5.54% in 2019-20. It fell every year in between. So about 96.6% came from open-cast mines [2].
• Trap: size, grade and mode of occurrence are physical factors. Demand, technology, capital, labour and transport are economic factors.
• Mine closure work: filling pits, making slopes stable, managing dumps and planting trees. Afforestation is the main part [3]. 147 abandoned coal mines are marked for closure [3].
• Methods of mining are shown in NCERT Class 12, Ch 4, Fig. 4.19.

Mains Points

• The trade-off between cost and land (GS-III): India gets about 97% of its coal from open-cast mines because they are cheap and fast [2].
• The price is scarred land, displaced tribal communities and lost forest.

• Underground mining is being promoted again because it disturbs less land.

• The policy response through mine closure and reclamation (GS-III, environment): closure plans fill pits, make slopes stable, manage waste dumps and focus on afforestation [3].

• Of 341 abandoned or discontinued coal mines (179 pre-2009 plus 162 post-2009), 147 are marked for closure. The rest are being joined to working mines and restarted [3].

• The changing geography of mining (GS-I): surface deposits are profitable only when demand, prices and technology allow it.

• Mining is shifting from developed countries, where labour costs are high, to labour-rich developing countries.
• Many of these countries become mineral-dependent economies, earning over 50% of their income from minerals. They then face price volatility (sharp rises and falls in prices) and the resource curse (slow growth despite mineral wealth).

Related concepts

• Mining
• Factors affecting mining activity
• Underground mining
• Mineral-dependent economies
• Katanga-Zambia copper belt

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