Quick Revision Notes

Class 11 Geography: Interior of the Earth (Prithvi ki Aantarik Sanrachna)

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1. Introduction

Understanding the earth's interior is crucial because we cannot directly access it due to immense temperature and pressure increasing with depth. Our knowledge is derived from two main sources:

• Direct Sources: Rocks from mining, volcanic eruptions, and deep drilling projects (e.g., Kola Superdeep Borehole in Arctic ocean, ~12 km deep).
• Indirect Sources: Temperature/pressure changes, meteors, gravitation, magnetic field, and Seismic Waves (most important).

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2. Seismic Waves (Bhoomikampiya Tarangein)

Earthquakes generate waves that travel through the earth's interior, helping scientists map its layers. These are recorded by a Seismograph.

• Body Waves (Sharirik Tarangein): Generated due to the release of energy at the focus and move in all directions travelling through the body of the earth.

  • P-Waves (Primary Waves / Prathamik Tarangein):
    • Fastest moving waves, arrive first.
    • Vibrate parallel to the wave direction (Longitudinal / Anudairghya).
    • Can travel through Solid, Liquid, and Gases.
  • S-Waves (Secondary Waves / Dwitiyak Tarangein):
    • Arrive after P-waves.
    • Vibrate perpendicular to the wave direction (Transverse / Anupashtha).
    • Can travel ONLY through Solids. (This helped scientists understand the liquid outer core).

• Surface Waves (Pristhiya Tarangein):

  • Body waves interact with surface rocks and generate new waves called surface waves.
  • Last to report on seismograph.
  • Most destructive waves (Rayleigh and Love waves).

• Shadow Zone (Chhaya Kshetra):

  • Specific areas where seismographs remain silent or do not record earthquake waves.
  • P-Wave Shadow Zone: Between 105° and 145° from the epicenter forms a wide shadow zone for P-waves.
  • S-Wave Shadow Zone: Beyond 105° from the epicenter, completely devoid of S-waves. Since S-waves cannot pass through liquids, this proves the outer core is liquid. The S-wave shadow zone is larger than the P-wave shadow zone (covers nearly 40% of the earth).

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3. Structure of the Earth (Prithvi ki Sanrachna)

The earth's interior is broadly divided into three concentric layers: Crust, Mantle, and Core.

A. The Crust (Bhoopurpapti)

• Outermost solid part of the earth.
• Brittle in nature.
• Thickness varies: Under oceanic areas (~5 km) vs. Continental areas (~30 km). Under high mountain ranges like Himalayas, it can be up to 70 km.
• Oceanic Crust: Mainly composed of silica and magnesium (SIMA), denser, basaltic rocks.
• Continental Crust: Mainly composed of silica and aluminium (SIAL), lighter, granitic rocks.
• Average Density: 2.7 to 3.0 g/cm³.

B. The Mantle (Mantl)

• The portion interior to the crust is called the mantle.
• Extends from Moho’s discontinuity to a depth of 2,900 km.
• Asthenosphere (Durbalata Mandal):
  • Upper portion of the mantle (extends up to ~400 km).
  • Main source of magma that finds its way to the surface.
  • Semi-fluid / plastic state, allowing tectonic plates to float.
• Density: Higher than crust, ranges from 3.3 to 5.7 g/cm³.
• Composed heavily of silicates of iron and magnesium.

C. The Core (Krodh)

• Innermost layer surrounding the earth's center.
• Extends from 2,900 km to 6,400 km depth.
• Outer Core: Liquid state (proven by S-wave shadow zone).
• Inner Core: Solid state (due to immense pressure).
• Composed mainly of Nickel and Iron (NIFE).
• Density: Very high, ranges from 9.9 to 13.0 g/cm³ at the center.
• Responsible for the Earth's Magnetic Field.

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4. Discontinuities in the Earth's Interior (Asamantarikta)

Transition zones between different layers of the earth are called discontinuities.

Discontinuity Name	Located Between
Conrad Discontinuity	Upper Crust and Lower Crust
Mohorovicic (Moho) Discontinuity	Crust and Mantle
Repetti Discontinuity	Upper Mantle and Lower Mantle
Gutenberg Discontinuity	Mantle and Outer Core
Lehmann Discontinuity	Outer Core and Inner Core

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5. Volcanoes and Volcanic Landforms (Jwalamukhi)

• Volcano: A place where magma forces its way up to the surface through a vent or fissure.
• Magma vs. Lava: Molten rock inside the earth is Magma; when it reaches the surface, it is called Lava.
• Types of Volcanoes:
  • Shield Volcanoes: Broad, gently sloping shields made of fluid basaltic lava (e.g., Mauna Loa, Hawaii).
  • Composite Volcanoes: Viscous, cooler lava with pyroclastic material and ash layers, very explosive (e.g., Mt. Fuji, Vesuvius).
  • Caldera: Extremely explosive volcanoes that collapse on themselves, forming massive depressions called calderas.
  • Flood Basalt Provinces: Highly fluid lava flowing over vast areas (e.g., Deccan Traps, India).
  • Mid-Ocean Ridge Volcanoes: Occur in oceanic areas along divergent plate boundaries.

Intrusive Volcanic Landforms (Aantarik Stharoop)

When magma cools inside the crust, it forms plutonic intrusive landforms:

• Batholiths: Large domoid masses of solidified magma (magma chambers).
• Lacoliths: Large dome-shaped intrusive bodies with a level base connected to a pipe-like conduit.
• Lapolith: Saucer-shaped, concave intrusive body.
• Phacoliths: Wavy mass of magma deposited in anticlines and synclines of mountains.
• Sills: Horizontal sheets or thick layers of intrusive magma.
• Dykes: Wall-like perpendicular/steep structures formed when magma forces itself through cracks.