Rock Types and the Rock Cycle

5.1 Three Rock Types

1. Igneous Rocks — "Fire Rocks"

Formed by cooling and crystallisation of magma (below surface) or lava (at surface).

Sub-type	Formation	Texture	Examples	Significance
Intrusive/Plutonic	Slow cooling deep underground	Coarse-grained (large crystals)	Granite, Gabbro, Diorite	Foundations of continents; major mineral deposits
Extrusive/Volcanic	Fast cooling at surface	Fine-grained or glassy	Basalt, Rhyolite, Obsidian	Deccan Traps (basalt); ocean floors

RPSC significance: Granite forms the ancient Aravalli basement (some of the world's oldest rocks, ~2.5 billion years). Basalt covers Rajasthan's Hadoti region (eastern Rajasthan — Kota, Bundi, Jhalawar).

2. Sedimentary Rocks — "Layered Rocks"

Formed by deposition, compaction, and cementation of sediments (from weathering and erosion of pre-existing rocks, biological material, or chemical precipitation).

Sub-type	Process	Examples	Economic Use
Clastic	Detrital particles	Sandstone, Shale, Conglomerate	Building stone, aquifers
Chemical	Precipitation from solution	Limestone, Rock Salt, Gypsum	Cement, road salt; Rajasthan gypsum
Organic/Biogenic	Accumulation of organic matter	Coal, Petroleum, Chalk	Fossil fuels

Key for exams: All fossil fuels (coal, petroleum, natural gas) are found exclusively in sedimentary rock sequences. Limestone is used for cement production. India's Thar Desert sand = aeolian sedimentary deposits.

3. Metamorphic Rocks — "Changed Rocks"

Formed when pre-existing rocks are subjected to intense heat and/or pressure without melting, causing mineralogical and textural changes.

Parent Rock	Metamorphic Product	Process
Limestone	Marble	Heat + Pressure
Shale	Slate → Phyllite → Schist → Gneiss	Progressive metamorphism
Sandstone	Quartzite	Recrystallisation
Granite	Gneiss	High-grade metamorphism

RPSC significance: Makrana Marble (Rajasthan, Nagaur district) is a famous metamorphic rock used in the Taj Mahal. Rajasthan is a major producer of marble, quartzite, and slate.

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