Minerals and Rocks

Minerals and rocks form the physical foundation of India's economic geography — from the iron ore of Jharkhand to the granite of the Deccan and the coal of Gondwana basins. Understanding rock types and their formation also explains why certain landscapes look the way they do, why some areas are fertile and others barren, and why particular minerals are found in particular regions.

UPSC tests this chapter through Prelims questions on mineral classification (silicates vs carbonates), rock identification (granite vs basalt, sandstone vs limestone), and the rock cycle. Mains questions on India's mineral wealth, industrial location, and soil formation all require a solid grasp of this material.

PART 1 — Quick Reference Tables

Table 1: Major Mineral Groups

Table 2: Types of Igneous Rocks

Table 3: Types of Sedimentary Rocks

Table 4: Types of Metamorphic Rocks

Table 5: The Rock Cycle — Key Processes

PART 2 — Detailed Notes

What is a Mineral?

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and crystalline structure. Note what this excludes:

• Organic compounds (oil, coal) — not minerals in the strict sense, though sometimes called "mineral fuels"
• Synthetic materials — must be naturally occurring
• Liquids and gases — must be solid

There are about 3,000 known minerals, but only about 30 are common. The most abundant minerals in the crust are silicates (quartz, feldspars, micas) — together they make up the bulk of all rocks.

Silicates: The Most Important Group

Silicates are built around the SiO₄ tetrahedron — one silicon atom surrounded by four oxygen atoms. Different silicate minerals form by different arrangements of these tetrahedra.

Key silicates:

• Quartz (SiOâ‚‚): Very resistant to weathering; forms sandstone and sandy soils
• Feldspar: Most abundant mineral in crust; weathers to clay minerals (important for soil formation)
• Mica: Flexible flakes; used in electrical insulation
• Olivine: Dense; dominant in the upper mantle

Igneous Rocks

Igneous rocks (Latin: ignis = fire) form from the cooling and solidification of magma (below ground) or lava (above ground).

Texture depends on cooling rate:

• Slow cooling (intrusive): Large crystals form — coarse-grained (granite)
• Fast cooling (extrusive): Small crystals — fine-grained (basalt)
• Very fast cooling (quenched in water): No crystals — glassy (obsidian)
• Gas-rich lava: Vesicular (pumice — so light it floats on water)

💡 Explainer: Granite vs Basalt — India Context

Granite is the classic intrusive igneous rock — coarse-grained, light-coloured (quartz + feldspar + mica). It forms the base of continents. Much of the Peninsular Plateau (Deccan) is underlain by ancient granite and gneiss (metamorphosed granite). Granite is used extensively in India for building and infrastructure.

Basalt is the extrusive equivalent — fine-grained, dark, dense. The Deccan Traps — a massive basaltic lava plateau covering parts of Maharashtra, Karnataka, Madhya Pradesh, and Gujarat — formed ~66 million years ago from enormous volcanic eruptions. These basalt flows weathered into the extremely fertile black cotton soil (regur) that makes Maharashtra a major cotton-growing region.

Sedimentary Rocks

Sedimentary rocks form through the WELD cycle: Weathering → Erosion → Liftoff (transport) → Deposition, followed by compaction and cementation (lithification).

They are important because:

1. They occur in layers (strata) — useful for geological dating and reading Earth's history
2. They contain fossils — evidence of past life
3. Most economically important minerals and fuels are associated with sedimentary rocks: coal, petroleum, natural gas (organic sedimentary); rock salt, gypsum (chemical sedimentary); limestone (organic/chemical, used in cement and steel making)
4. Sedimentary basins are the primary target for oil and gas exploration

India's sedimentary basins: The Gondwana coal fields (Damodar Valley, Son Valley) preserve Permian-age Gondwana sediments with ~98% of India's coal reserves. The Mumbai High, Krishna-Godavari, and Cauvery basins are offshore sedimentary basins holding oil and gas.

Metamorphic Rocks

Metamorphic rocks form when existing rocks (igneous, sedimentary, or other metamorphic) are subjected to high temperature, high pressure, or chemically active fluids — causing mineralogical and textural changes without melting.

Two types:

• Regional metamorphism: Large-scale, associated with mountain-building events; produces schist, gneiss, slate (vast areas)
• Contact metamorphism: Local, due to heat from magma intrusions; produces marble and hornfels (limited area)

🎯 UPSC Connect: The Rock Cycle

The rock cycle is the continuous process by which rocks are transformed from one type to another over geological time:

Magma → (cools) → Igneous rock → (weathered, eroded, deposited) → Sedimentary rock → (heated, pressured) → Metamorphic rock → (melted) → Magma again

No rock type is permanent. Granite can become gneiss (metamorphism), which can weather to form sand (sediment), which becomes sandstone (sedimentary), which can melt to magma (then igneous) again. This cycle operates over millions of years but is continuous.

UPSC relevance: The rock cycle connects to:

• Soil formation (weathering of parent rocks)
• River sediment load and delta formation
• Coal and petroleum formation (organic sedimentary)
• Mineral ore concentration (hydrothermal processes near igneous intrusions)

📌 Key Fact: Deccan Traps

The Deccan Traps are one of the largest volcanic features on Earth — ~500,000 km² of basaltic lava flows, up to 2 km thick. They erupted ~66 mya (coinciding with the K-Pg extinction event). The weathered basalt produces black cotton soil (regur), which:

• Has high clay content (swells when wet, cracks when dry)
• Is self-ploughing due to expansion/contraction
• Retains moisture well — suitable for rain-fed cotton cultivation
• Covers much of the Deccan Plateau (Maharashtra, Madhya Pradesh, Karnataka, Gujarat)

PART 3 — Frameworks & Analysis

Rock Types: Comparative Summary

Mineral Ores: Rock Type Association

Exam Strategy

Prelims Traps:

• Granite is intrusive (coarse-grained); basalt is extrusive (fine-grained) — same composition broadly, different cooling rates.
• Coal is organic sedimentary, not igneous (it is not a mineral in the strict sense — it is a rock).
• Marble is metamorphosed limestone; quartzite is metamorphosed sandstone.
• Haematite is an oxide mineral (iron ore); do not confuse with calcite (carbonate).
• Bauxite (aluminium ore) is formed by intense chemical weathering of silicate rocks in tropical conditions — it is a lateritic/residual deposit.

Mains Frameworks:

• For "distribution of minerals in India" questions: link ore type to rock type to geological formation (Gondwana basins for coal; Peninsular metamorphic rocks for iron/manganese).
• Rock cycle is useful for explaining how landscapes evolve over time and why certain areas have certain soils.

Practice Questions

1. UPSC Prelims 2020: Which of the following is an example of an organically formed sedimentary rock? (Limestone, coal — tests rock classification)
2. UPSC Prelims 2018: Haematite is an ore of which element? (Iron — tests mineral knowledge)
3. UPSC Mains GS1 2017: How does the rock cycle explain the formation and transformation of rocks? Discuss with suitable examples from India.
4. UPSC Mains GS3 2019: "India's coal reserves are among the largest in the world but India still imports coal." Examine. (Requires knowledge of coal formation and quality differences)