🪨 ROCKS World Geography · Ch 08

SSC / Railway / UPSC / BPSC — World Geography Mind Map Series

🌋 1. Igneous Rocks
  • Formed by the cooling and solidification of lava or magma inside or outside the Earth
  • Examples: Granite, Basalt, Pegmatite, Diorite, Gabbro, Pumice

Intrusive (Inside the Earth)

  • Forms the Continental crust
  • Cools slowly under the surface (e.g., Granite)

Extrusive (Outside the Earth)

  • Forms the Oceanic crust
  • Cools rapidly on the surface (e.g., Basalt)
  • Weathering of basaltic Deccan Trap lava → Black (Regur) Soil
🔄 Transformation of Igneous Rocks
Igneous RockTransformed (Metamorphic)
GraniteGneiss
BasaltSchist / Amphibolite
GabbroAmphibolite
📌 Mica of Koderma (Jharkhand) Obtained from Pegmatite rock
⚠ Exam Trap
Black (Regur) Soil comes from the weathering of Basalt — an Extrusive igneous rock — not from Intrusive rocks. This pairing is frequently mixed up in MCQs.
🏖️ 2. Sedimentary Rocks (Layered Rocks)
  • Formed by weathering and erosion of existing rocks
  • Process – Lithification
  • Examples: Sandstone, Limestone, Conglomerate, Rock salt, Shale, etc.
📌 Famous monuments Agra Fort & Red Fort of Delhi built using sedimentary rocks (sandstone)
🔄 Transformation of Sedimentary Rocks
SedimentaryMetamorphic
SandstoneQuartzite
LimestoneMarble
ShaleSlate
DolomiteMarble
Lignite CoalHigher coal rank: lignite → bituminous → anthracite
CoalGraphite under intense metamorphism; natural diamonds form mainly deep in the mantle.
📌 Wind-deposited sediment Loess
📌 Glacial deposit Moraine
⚡ Quick Recall
Both Sandstone → Quartzite and Shale → Slate — fine-grained sediments turn into hard, compact metamorphic rocks under pressure.
📂 Classification of Sedimentary Rocks

Mechanically / Inorganically Formed

  • Sand-dominant rocks (Sandstone)
  • Clay-dominant rocks (Shale)
  • Gravel-dominant rocks (Conglomerate)

Biologically / Organically Formed

  • Lime-dominant rocks (Limestone, Chalk)
  • Carbon-dominant rocks (Coal)

Chemically Formed

  • Rock Salt, Gypsum, Dolomite, Halite
  • Iron deposits, Stalactites
⚠ Exam Trap
Limestone: carbonate rock, often biochemical but sometimes chemically precipitated; shale: fine-grained clastic rock.
🔥 3. Metamorphic Rocks (Transformed Rocks)
  • Pre-existing rocks transform through heat, pressure and/or chemically active fluids without complete melting.
  • Examples: Gneiss, Amphibolite, Greenstone, Serpentinite, Phyllite
🔄 Transformation of Metamorphic Rocks
Metamorphic RockTransformed into
SlatePhyllite
PhylliteSchist
📌 Petrology Scientific study of rock composition, structure, etc.
⚡ Quick Recall
Progressive metamorphism chain: Shale → Slate → Phyllite → Schist (increasing heat & pressure)
📝 Quick Summary — All 3 Rock Types
Rock TypeFormationExamplesTransforms To
IgneousCooling & solidification of lava/magmaGranite, Basalt, Gabbro, Pumice, PegmatiteMetamorphic (Gneiss, Greenschist/Amphibolite; peridotite/dunite → serpentinite)
SedimentaryWeathering & erosion (Lithification)Sandstone, Limestone, Shale, Coal, Rock saltMetamorphic (Quartzite, Marble, Slate; carbonaceous rocks → anthracite/graphite)
MetamorphicHeat, pressure and/or active fluids; without complete meltingGneiss, Marble, Slate, Schist, PhylliteIgneous (via melting → Magma)
⚡ Quick Recall
All three rock types are interconvertible through the Rock Cycle — none is a permanent "final" form.
🔁 The Rock Cycle
Igneous
▸
Sedimentary
▸
Metamorphic
▸
Magma (Melting)
▸
Igneous
  • Weathering and Erosion drive the transition from Igneous → Sedimentary
  • Heat and Pressure drive the transition from Sedimentary → Metamorphic
  • Melting drives the transition from Metamorphic → Magma → Igneous (cycle repeats)
⚠ Exam Trap
The Rock Cycle has no fixed starting point — any rock type can transform into any other given the right conditions; it is not a strictly one-directional sequence.
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