UPSC Mains 2022 GS Paper 1 Q4: model answer
Describe the characteristics and types of primary rocks.
What the question wants
Describe what primary (igneous) rocks are, their characteristics and types.
How to structure it
Describe the characteristics and types: a classification answer.
- Meaning
- Characteristics
- Types
- Importance
How to think it through
Primary rocks are the 'first' rocks, born from cooled magma before any other kind. Slow cooling deep down gives big crystals (granite); fast cooling on the surface gives fine ones (basalt). That single rule, cooling speed controls crystal size, makes you sound like a geographer.
Model answer
The Deccan Traps cover about 5 lakh sq km of India with basalt from lava eruptions some 66 million years ago. Basalt is a primary or igneous rock, formed by the cooling and solidification of magma or lava, and the parent of all other rocks.
Characteristics
- Crystalline, hard and compact; usually without layers or fossils.
- Crystal size depends on cooling speed: slow cooling gives coarse crystals, fast cooling fine ones.
- Rich in minerals such as feldspar, quartz and mica, and associated with metallic ores.
- Weathering produces fertile soils (black regur soils from basalt).
Types
- Intrusive (plutonic): cooled slowly below the surface; coarse-grained (granite, gabbro); seen in batholiths, laccoliths, dykes and sills.
- Extrusive (volcanic): cooled quickly on the surface; fine-grained (basalt, pumice, obsidian).
- By chemistry: acidic (silica-rich, light-coloured, granite) and basic (silica-poor, dark, basalt).
Importance
Granite is used as building stone (from Rajasthan and Karnataka); igneous bodies host ores such as gold at Kolar; and basalt weathers into black soils for cotton farming in Maharashtra.
Draw: Cross-section of a volcano with intrusive bodies (batholith, dyke, sill) and lava flows.
Keywords, cases and data to drop in
Igneous, intrusive, extrusive, batholith, dyke, sill, granite, basalt, regur.
Where most answers lose marks
- Mixing in sedimentary or metamorphic rocks. Stay with primary rocks.
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Question text as printed in the official UPSC Civil Services (Main) question paper. The model answer is original Battle IAS work; use it to learn structure, not to memorise.
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