Earth’s Structure and Composition for UPSC Prelims
1. Introduction to Earth’s Structure
The Earth’s structure comprises distinct layers—crust, mantle, and core—each with unique physical and chemical properties. These layers interact to drive geological processes like plate tectonics, earthquakes, and volcanism. Understanding Earth’s composition is crucial for UPSC Prelims, as it forms the foundation of physical geography and explains phenomena like seismic activity and mineral distribution.
Key Facts
- Main Layers: Crust, mantle, core (outer and inner)
- Earth’s Radius: Approximately 6,371 km
- Significance: Influences geological processes, resource distribution
- Study Methods: Seismic waves, drilling, meteorite analysis
Important Facts for UPSC
- The crust is the thinnest layer, varying from 5–70 km in thickness.
- The core is primarily composed of iron and nickel.
- Seismic discontinuities (e.g., Moho, Gutenberg) mark layer boundaries.
2. Overview of Earth’s Layers
Earth is divided into concentric layers based on physical and chemical properties. The crust forms the outermost solid layer, the mantle is a semi-solid intermediate layer, and the core is the innermost, densest layer. These layers are identified through seismic wave studies.
| Layer |
Depth (km) |
State |
Primary Composition |
| Crust |
0–70 |
Solid |
Silica, aluminum (continental); silica, magnesium (oceanic) |
| Mantle |
70–2,900 |
Semi-solid |
Silica, magnesium, iron |
| Outer Core |
2,900–5,150 |
Liquid |
Iron, nickel |
| Inner Core |
5,150–6,371 |
Solid |
Iron, nickel |
Important Facts for UPSC
- The mantle constitutes about 84% of Earth’s volume.
- The outer core’s liquid state generates Earth’s magnetic field.
- The inner core’s solid state is due to immense pressure despite high temperatures.
3. The Crust
Overview
The crust is Earth’s outermost, thinnest layer, varying in thickness from 5 km (oceanic) to 70 km (continental). It is rigid and brittle, forming the lithosphere when combined with the uppermost mantle.
Types
- Continental Crust: Thicker (30–70 km), less dense, granitic composition (rich in silica and aluminum).
- Oceanic Crust: Thinner (5–10 km), denser, basaltic composition (rich in silica and magnesium).
Properties
- Density: 2.7 g/cm³ (continental), 3.0 g/cm³ (oceanic).
- Hosts all known life and mineral resources.
- Subject to weathering, erosion, and tectonic activity.
Important Facts for UPSC
- Continental crust is older (up to 4 billion years) than oceanic crust (up to 200 million years).
- The crust is divided into tectonic plates, driving plate tectonics.
- SIAL (silica + aluminum) and SIMA (silica + magnesium) describe crust compositions.
4. The Mantle
Overview
The mantle extends from the Moho discontinuity (beneath the crust) to about 2,900 km depth, making up 67% of Earth’s mass. It is semi-solid, with a plastic-like behavior due to high temperature and pressure.
Structure
- Upper Mantle: Up to 410 km, includes the asthenosphere (partially molten, enables plate movement).
- Lower Mantle: 410–2,900 km, more rigid due to increased pressure.
Properties
- Composition: Peridotite (silica, magnesium, iron).
- Temperature: 1,000–3,700°C.
- Convection currents drive plate tectonics and volcanic activity.
Important Facts for UPSC
- The asthenosphere is critical for tectonic plate movement.
- Mantle convection causes earthquakes and volcanism.
- The mantle’s density ranges from 3.3–5.7 g/cm³.
5. The Core
Overview
The core, extending from 2,900 km to Earth’s center (6,371 km), is divided into a liquid outer core and a solid inner core. It is the densest layer, primarily composed of iron and nickel.
Structure
- Outer Core: 2,900–5,150 km, liquid, generates Earth’s magnetic field via dynamo effect.
- Inner Core: 5,150–6,371 km, solid due to extreme pressure, radius ~1,220 km.
Properties
- Composition: Iron (~85%), nickel (~10%), trace elements (sulfur, oxygen).
- Temperature: 4,000–6,000°C.
- Density: 9.9–12.2 g/cm³ (outer), 12.6–13.0 g/cm³ (inner).
Important Facts for UPSC
- The outer core’s motion creates the geomagnetic field, protecting Earth from solar radiation.
- The inner core is as hot as the Sun’s surface (~5,500°C).
- Core’s high density contributes to Earth’s gravitational pull.
6. Discontinuities Between Layers
Overview
Discontinuities are boundaries where seismic wave velocities change, indicating transitions between Earth’s layers.
Key Discontinuities
| Discontinuity |
Location |
Separates |
Discovered By |
| Mohorovičić (Moho) |
5–70 km |
Crust and mantle |
Andrija Mohorovičić (1909) |
| Gutenberg |
~2,900 km |
Mantle and outer core |
Beno Gutenberg (1913) |
| Lehmann |
~5,150 km |
Outer and inner core |
Inge Lehmann (1936) |
Significance
Discontinuities help scientists map Earth’s interior using seismic data, revealing layer transitions.
Important Facts for UPSC
- The Moho is deeper under continents (~70 km) than oceans (~10 km).
- Gutenberg discontinuity marks a sharp density increase from mantle to core.
- Lehmann’s discovery confirmed the inner core’s solid nature.
7. Chemical Composition of Earth
Overview
Earth’s chemical composition varies by layer, with lighter elements dominating the crust and heavier ones in the core.
Composition by Layer
- Crust: Oxygen (46.6%), silicon (27.7%), aluminum (8.1%), iron (5%).
- Mantle: Oxygen (44.8%), silicon (21.5%), magnesium (22.8%), iron (5.8%).
- Core: Iron (~85%), nickel (~5%), sulfur, oxygen (trace).
Significance
Composition influences density, mineral formation, and geological processes, affecting resource availability.
Important Facts for UPSC
- The crust is rich in silicates, forming rocks like granite and basalt.
- The core’s iron-nickel alloy is inferred from meteorite studies.
- Earth’s overall composition is estimated from chondritic meteorites.
8. Geological Processes and Earth’s Layers
Overview
Interactions between Earth’s layers drive geological processes, shaping the planet’s surface and interior.
Key Processes
- Plate Tectonics: Driven by mantle convection, causes earthquakes and mountain formation.
- Volcanism: Mantle-derived magma erupts through the crust, forming volcanoes.
- Magnetic Field: Generated by outer core’s liquid motion, protects Earth’s atmosphere.
Impact
These processes create landforms, recycle materials, and sustain life by maintaining a habitable environment.
Important Facts for UPSC
- Plate tectonics explains continental drift and ocean basin formation.
- Volcanic eruptions release mantle-derived gases, influencing climate.
- The magnetic field shields Earth from solar wind, preserving the atmosphere.
9. Methods of Studying Earth’s Interior
Overview
Direct access to Earth’s interior is limited, so indirect methods like seismic studies and geophysical techniques are used.
Key Methods
- Seismic Waves: P-waves (primary) and S-waves (secondary) reveal layer properties.
- Drilling: Kola Superdeep Borehole (12.3 km) provides shallow crust data.
- Meteorites: Chondrites offer clues to Earth’s core and mantle composition.
- Gravity and Magnetism: Variations indicate density and composition differences.
Significance
These methods enable accurate models of Earth’s structure, aiding resource exploration and hazard prediction.
Important Facts for UPSC
- P-waves travel through solids and liquids; S-waves only through solids.
- Seismic shadow zones confirm the core’s liquid outer layer.
- Deepest drilling reaches only 0.2% of Earth’s radius.
10. Significance for Geography
Geological Significance
Earth’s layered structure drives tectonic activity, shaping continents, oceans, and mountains, and influencing mineral deposits.
Environmental Significance
The magnetic field and atmospheric recycling via volcanism sustain life. Layer interactions regulate climate and ecosystems.
UPSC Relevance
Understanding Earth’s structure is foundational for topics like geomorphology, seismology, and resource geography, frequently tested in Prelims.
Important Facts for UPSC
- Earth’s structure explains India’s Himalayan formation and seismic zones.
- Mineral resources (e.g., coal, oil) are linked to crustal processes.
- Knowledge of seismic waves aids in earthquake-prone region planning.
UPSC Preparation Tips
- Memorize the layers (crust, Mantle, Core) and their properties (thickness, composition, state).
- Focus on discontinuities (Moho, Gutenberg, Lehmann) and their significance.
- Understand the role of mantle convection in plate tectonics and volcanism.
- Link core properties to the magnetic field and its environmental role.
- Practice diagram-based questions on Earth’s layers and seismic wave paths.
- Revise key terms (SIAL, SIMA, asthenosphere, dynamo effect).