Describe the characteristics and types of primary rocks.
Approach · directive: “describe”
What it asks · Define primary (igneous) rocks, list their characteristics, and classify them by mode of formation and by silica content, with examples.
The question has 2 parts — answer each
- Describe the characteristics of primary (igneous) rocks
- Describe their types, by mode of formation and by silica content, with examples
Open with · Igneous rocks, formed by cooling and solidification of magma or lava, are called primary rocks because sedimentary and metamorphic rocks are derived from them.
Cover
- Characteristics: crystalline, hard and massive; without layers or fossils; mostly impermeable, jointed and resistant to weathering; source of many mineral ores.
- Intrusive (plutonic) rocks: cooled slowly below the surface, coarse-grained, such as granite, gabbro and diorite; found as batholiths, laccoliths, sills and dykes.
- Extrusive (volcanic) rocks: cooled quickly at the surface, fine-grained or glassy, such as basalt, rhyolite, obsidian and pumice; the Deccan Trap is basaltic.
- Hypabyssal (intermediate) rocks: cooled in fissures at moderate depth, forming dykes and sills, for example dolerite.
- By silica: acidic (granite, over 65 per cent), intermediate (diorite), basic (gabbro, basalt) and ultrabasic (peridotite); colour darkens and density rises as silica falls.
- Significance: parent material of other rocks and soils, and a source of building stone and minerals.
Close with · Crystalline texture, absence of fossils and origin from magma set them apart, while their variety reflects the depth and rate of cooling.
Question: UPSC's CS (Main) 2022, GS Paper I — paper ↗. Approach: Minimalist IAS, checked 30 Sept 2026 (how we verify) — UPSC publishes no model answers. ·
Model answer · 211 words (UPSC limit 150) · Minimalist IAS
Igneous rocks form when magma or lava cools and solidifies; they are called primary rocks because sedimentary and metamorphic rocks are ultimately derived from them.
Characteristics
- Crystalline and massive: interlocking mineral crystals with no strata, bedding planes or fossils.
- Texture follows cooling rate: slow cooling at depth gives coarse grains, rapid cooling at the surface gives fine or glassy texture.
- Hard, compact and generally impermeable, though joints admit water and weathering; resistant enough to form plateaus such as the Deccan.
- Economically valuable: host many metallic ores, supply building stone and weather into fertile soils such as regur.
Types by mode of formation
- Intrusive (plutonic): cooled slowly at depth as batholiths and laccoliths; granite, gabbro, diorite.
- Hypabyssal: cooled at moderate depth in fissures as dykes and sills; dolerite.
- Extrusive (volcanic): cooled quickly at the surface as lava or ash; basalt, rhyolite, obsidian, pumice. The Deccan Trap is basaltic flood lava.
Types by silica content
- Acidic (silica above 65 per cent): granite, rhyolite; light-coloured, low density.
- Intermediate: diorite, andesite.
- Basic: gabbro, basalt; dark and dense.
- Ultrabasic: peridotite; darkest and densest, since colour deepens and density rises as silica falls.
Crystalline texture, absence of fossils and magmatic origin set primary rocks apart, while depth and rate of cooling and silica content explain their variety.
Written by Minimalist IAS from facts checked at source (how we verify) — a little fuller than exam length, so every part of the question is covered; in the hall, keep the structure and trim the detail. UPSC publishes no model answers: compare your structure and coverage with this, then write your own.