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.
It asks 2 things — answer each
- Describe the characteristics of primary (igneous) rocks
- Describe their types: by mode of formation (intrusive, hypabyssal, extrusive) and by composition (acid to ultrabasic), with examples
Open with · Igneous rocks form when magma or lava cools and solidifies; they are called primary rocks because sedimentary and metamorphic rocks derive from them. Depth and rate of cooling shape their character.
Cover
- Crystalline and massive: interlocking crystals, no strata or bedding planes, no fossils.
- Texture follows cooling: slow cooling at depth gives large grains; quick cooling at the surface gives fine or glassy texture.
- Hard and resistant: compact, mostly impermeable but jointed; basalt flows build plateaus such as the Deccan.
- Intrusive: granite and gabbro in batholiths (cooled magma chambers) and dome-shaped laccoliths.
- Hypabyssal: dolerite in dykes and sills; extrusive: basalt, obsidian, pumice, with flood basalts such as the Deccan Traps.
- By composition: acid (silica-rich, light: granite), intermediate (diorite), basic (silica-poor, dark: basalt, gabbro), ultrabasic (peridotite).
Close with · Weathering carries primary rocks into India's farms: Deccan basalt yields black regur, and crystalline igneous rocks the red soils of the eastern and southern Deccan.
Question: UPSC's CS (Main) 2022, GS Paper I — paper ↗. Approach: Minimalist IAS, checked 16 Oct 2026 (how we verify) — UPSC publishes no model answers. ·
Model answer· 221words · Minimalist IAS
Igneous rocks form when magma or lava cools and solidifies; they are called primary rocks because sedimentary and metamorphic rocks derive from them. Depth and rate of cooling shape their character.
Characteristics
- Crystalline and massive: interlocking mineral crystals, with no strata, bedding planes or fossils.
- Texture follows cooling: slow cooling at depth gives large grains; quick cooling at the surface gives fine or glassy texture.
- Hard and resistant: compact and mostly impermeable, though joints admit water; basalt flows build plateaus such as the Deccan.
- Useful: they host metallic ores and supply building stone.
Types by mode of formation
| Type | How it cools | Examples |
|---|---|---|
| Intrusive (plutonic) | slowly, deep in the crust; coarse | granite, gabbro |
| Hypabyssal | in fissures at moderate depth | dolerite |
| Extrusive (volcanic) | quickly at the surface; fine or glassy | basalt, obsidian, pumice |
- Intrusive bodies: batholiths are granitic, cooled magma chambers; laccoliths are level-based domes; dykes and sills fill fissures.
- Extrusive flows: flood basalts such as the Deccan Traps spread highly fluid lava over long distances.
Types by composition
- Acid: silica-rich, light-coloured, less dense: granite, rhyolite.
- Intermediate: diorite, andesite.
- Basic: silica-poor, rich in iron and magnesium; dark and dense: basalt, gabbro.
- Ultrabasic: peridotite, the darkest and densest.
Weathering carries primary rocks into India's farms: Deccan basalt yields black regur, and crystalline igneous rocks the red soils of the eastern and southern Deccan.
Prepared by Minimalist IAS to the answer standard of Shivam Singh (mentor, 7 years) · facts checked against official sources (how we verify) · updated 16 Oct 2026. UPSC publishes no model answers: compare your structure and coverage with this, then write your own.
