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Interior of the Earth (UPSC Geography Notes)
Overview: Understanding the interior of the Earth helps us explain earthquakes, volcanoes, mountain building, and plate tectonics. Direct observation is not possible, so scientists depend on indirect evidence such as seismic waves, density, and magnetic data.
Table of Contents
- 1. Sources of Information
- 2. Structure of the Earth
- 3. Composition and Characteristics of Each Layer
- 4. Earthquake Waves and What They Reveal
- 5. Quick Revision
- 6. UPSC Previous Year Question
1. Sources of Information about Earth's Interior
Direct Sources
- Drilling and mining give limited samples (deepest borehole ~12 km).
- Volcanic eruptions bring material (magma, gases) from deeper layers.
Indirect Sources
- Seismic waves: Study of how earthquake waves travel through different materials reveals density and state of matter inside the Earth.
- Earth’s magnetic field: Indicates the presence of a molten metallic core.
- Meteorites: Believed to have similar composition to Earth’s interior.
- Gravitational and density data: Help estimate mass and distribution of material inside the planet.
2. Structure of the Earth
The Earth is made up of three main layers: Crust, Mantle, and Core.
| Layer | Depth (Approx.) | Main Characteristics |
|---|---|---|
| Crust | 0 – 35 km | Outer solid layer; composed mainly of silica and alumina (sial) in continents and silica with magnesium (sima) in oceans. |
| Mantle | 35 – 2900 km | Composed of silicate rocks rich in iron and magnesium. Semi-solid and capable of slow flow. Upper part is the asthenosphere where convection currents occur. |
| Core | 2900 – 6371 km | Mainly nickel and iron (nife). Divided into outer liquid core and inner solid core. Source of Earth’s magnetic field. |
3. Composition and Characteristics of Each Layer
A. Crust
- Thinnest layer, forms continents and ocean basins.
- Average thickness: 35 km under continents, 5 km under oceans.
- Continental crust is mostly granite; oceanic crust is basalt.
B. Mantle
- Makes up about 84% of Earth’s volume.
- Temperature increases with depth (up to 4000°C near core).
- Movement in the upper mantle drives plate tectonics.
C. Core
- Extends from 2900 km to the center (6371 km).
- Outer core is molten; inner core is solid due to immense pressure.
- Temperatures reach about 6000°C.
- Movement of molten iron in outer core generates Earth’s magnetic field.
4. Earthquake Waves and What They Reveal
- P (Primary) Waves: Travel through solids, liquids, and gases; fastest seismic waves.
- S (Secondary) Waves: Travel only through solids; do not pass through liquid core, proving that the outer core is molten.
- L (Surface) Waves: Move along Earth’s surface; cause most damage during earthquakes.
By analyzing how these waves change speed and direction, scientists identified boundaries such as the Moho Discontinuity (between crust and mantle) and the Gutenberg Discontinuity (between mantle and core).
5. Quick Revision
Mantle – 35–2900 km, semi-solid, convection currents
Core – 2900–6371 km, nickel-iron, outer liquid + inner solid
Main Discontinuities – Mohorovičić (crust–mantle), Gutenberg (mantle–core)
S-waves do not pass through liquid → outer core is molten
6. UPSC Previous Year Question
Q. Which one of the following gives evidence that the outer core of the Earth is in a liquid state?
(a) P-waves travel through it
(b) S-waves do not pass through it
(c) Surface waves disappear within it
(d) Earth’s magnetic field is absent there
Answer: (b) S-waves do not pass through it
Related Keywords
Interior of the Earth UPSC Notes, Structure of the Earth, Earth Layers Geography, Physical Geography for IAS, Seismic Waves, Mantle Core Crust, NCERT Geography Simplified.
Sources: NCERT Class XI – Physical Geography, Geological Survey of India, Britannica
