Why this chapter matters for UPSC: The interior structure of the Earth, types of rocks and their formation, and the rock cycle are foundational for GS1 Physical Geography. Rocks and their formation are directly linked to mineral distribution, soil types, landforms, and earthquake/volcanic activity.

PART 1 — Quick Reference Tables

Interior Layers of the Earth

Layer Depth Composition State Temperature
Crust (Sial + Sima) 0–70 km (continental), 0–10 km (oceanic) Continental: granite (silicon + aluminium = Sial); Oceanic: basalt (silicon + magnesium = Sima) Solid ~0–1,000°C
Mantle 70–2,900 km Silicates of iron and magnesium (peridotite) Solid but plastic (asthenosphere can flow) 1,000–3,700°C
Outer Core 2,900–5,100 km Iron and nickel Liquid ~3,700–5,000°C
Inner Core 5,100–6,371 km (centre) Iron and nickel Solid (despite high temperature — extreme pressure keeps it solid) ~5,000–6,000°C

Types of Rocks

Rock Type How Formed Examples Economic Uses
Igneous ("fire rocks") Magma/lava cools and solidifies Granite, basalt, pumice, obsidian Granite = construction; basalt = road construction; pumice = abrasive
Sedimentary ("layered rocks") Sediments deposited and compressed over time Sandstone, limestone, coal, shale Coal = fuel; limestone = cement; sandstone = buildings
Metamorphic ("changed rocks") Existing rocks transformed by heat and pressure Marble (from limestone), slate (from shale), quartzite (from sandstone), diamond (from coal — extreme pressure) Marble = buildings/sculpture; slate = roofing; quartzite = flooring

PART 2 — Detailed Notes

The Earth's Interior

Key Term

How do we know what's inside the Earth?

We cannot drill to the Earth's core — the deepest borehole (Kola Superdeep Borehole, Russia) went only 12 km. We learn about the interior through:

  1. Seismic waves: Earthquake waves that travel through the Earth; P-waves (primary/pressure waves) travel through solid and liquid; S-waves (secondary/shear waves) travel only through solid. The way these waves change speed and direction tells us about internal structure.
  2. Volcanic eruptions: Bring material from the mantle to the surface
  3. Meteorites: Similar composition to Earth's core material
  4. Gravity measurements: Variation in gravitational pull reveals density differences inside

Key facts:

  • Earth's radius: ~6,371 km
  • Total layers: Crust → Mantle → Outer Core → Inner Core
  • Asthenosphere: Upper part of mantle (~100–700 km depth); semi-molten, plastic; tectonic plates "float" on this layer
  • Discontinuities:
    • Mohorovicic Discontinuity (Moho): Boundary between crust and mantle
    • Gutenberg Discontinuity: Boundary between mantle and outer core
    • Lehmann Discontinuity: Boundary between outer and inner core

Types of Rocks

UPSC Connect

UPSC GS1 — Rocks and their significance:

Igneous Rocks (Primary rocks):

  • Formed when molten rock (magma underground; lava when it reaches surface) cools and solidifies
  • Intrusive igneous rocks: Magma cools slowly underground → large crystals → e.g., granite (coarse-grained)
  • Extrusive igneous rocks: Lava cools quickly on surface → fine crystals or glassy → e.g., basalt (fine-grained), obsidian (glassy)
  • India: Deccan Plateau = world's largest basaltic lava plateau (Deccan Traps); formed ~66 million years ago; black cotton soil (Regur) developed on basalt — best for cotton cultivation
  • Granite: Rajasthan, Andhra Pradesh, Karnataka — major source of India's granite exports

Sedimentary Rocks:

  • Formed from sediments (fragments of other rocks, minerals, organic material) deposited by wind, water, ice; compressed over millions of years
  • Fossils are found ONLY in sedimentary rocks — because only sedimentary deposition can preserve organisms
  • Coal = compressed plant material (carboniferous period); India's coal deposits in sedimentary formations of Gondwanaland (Jharkhand, Odisha, Chhattisgarh)
  • Limestone = marine shells and coral accumulated; most of India's cement industry uses limestone from Rajasthan, Andhra Pradesh
  • Shale = most common sedimentary rock; source rock for shale oil/gas (fracking)
  • Sandstone: Rajasthan (Red Fort and many Mughal monuments built of red sandstone from Fatehpur Sikri/Dhaulpur)

Metamorphic Rocks:

  • Existing rocks (igneous or sedimentary) transformed by heat, pressure, or both
  • Marble: From limestone; Rajasthan (Makrana marble — used in Taj Mahal); white and pure
  • Diamond: Carbon under extreme pressure and temperature in the mantle; brought to surface by kimberlite pipes (volcanic conduits)
  • Slate: Split into thin layers → excellent roofing material; Himachal Pradesh

The Rock Cycle

Explainer

Rock cycle: The continuous process by which rocks are formed, broken down, and reformed:

Magma/Lava → (cooling) → Igneous Rock
Igneous Rock → (weathering + erosion) → Sediments
Sediments → (deposition + compaction) → Sedimentary Rock
Any Rock → (heat + pressure) → Metamorphic Rock
Metamorphic Rock → (melting) → Magma (cycle repeats)

Key point: No rock type is permanent. Over geological time (millions of years), all rocks cycle through different forms. This means:

  • Minerals in one rock type can end up in another
  • Fossils in sedimentary rock can be destroyed if the rock becomes metamorphic
  • The same atoms have been recycled through countless rock forms over 4.5 billion years

Practical significance:

  • Soil formation: Rocks weather into parent material → soil; the type of rock determines the type of soil (basalt → black cotton soil; granite → red laterite soil)
  • Mineral deposits: Many economically valuable minerals are concentrated by geological processes in specific rock types (iron ore in banded iron formations; gold in igneous/metamorphic)
  • Groundwater: Sedimentary rocks (especially limestone) form important aquifers

Exam Strategy

Prelims traps:

  • Fossils found ONLY in sedimentary rocks — NOT igneous or metamorphic
  • Marble = metamorphic (NOT sedimentary); comes from limestone under heat and pressure
  • Deccan Plateau = basaltic (igneous) rock → black cotton soil (Regur) = best for cotton
  • Moho discontinuity = boundary between crust and mantle (NOT mantle and core)
  • Gutenberg discontinuity = between mantle and outer core
  • Diamond = metamorphic (extreme pressure transforms carbon/coal) — often tested as trick question
  • Granite = intrusive igneous (cools slowly underground, coarse-grained); Basalt = extrusive igneous (cools fast, fine-grained)
  • Red Fort, Fatehpur Sikri = red sandstone (Rajasthan) — NOT granite or marble

Previous Year Questions

Prelims:

  1. Which of the following types of rocks is most likely to contain fossils?
    (a) Igneous
    (b) Sedimentary
    (c) Metamorphic
    (d) All of the above equally

  2. The "Deccan Traps" of the Indian subcontinent are primarily composed of which type of rock?
    (a) Sandstone
    (b) Granite
    (c) Basalt
    (d) Limestone

  3. Which discontinuity marks the boundary between the Earth's crust and the mantle?
    (a) Mohorovicic Discontinuity (Moho)
    (b) Gutenberg Discontinuity
    (c) Lehmann Discontinuity
    (d) Conrad Discontinuity