Geography Class 11 – CBSE – Course A – Chapter 3

📘 CHAPTER 3 — INTERIOR OF THE EARTH

1. INTRODUCTION

• The Earth is made up of several internal layers with different physical and chemical properties.
• Direct observation of these layers is not possible beyond a few kilometers.
• Therefore, scientists rely on indirect sources such as seismic waves, gravity, magnetic field, and meteorites to understand Earth’s interior.

2. INDIRECT SOURCES

2.1 Mining

• People have drilled only up to 12 km into the crust.
• Useful for understanding crustal characteristics but not deeper layers.

2.2 Volcanic Eruptions

• Magma coming to the surface gives clues about the mantle’s composition.

2.3 Meteorites

• Remnants of early solar system.
• Composition often similar to Earth’s interior → helps understand core and mantle structure.

2.4 Gravitation

• Variations in gravitational force indicate density differences below the surface.

2.5 Magnetic Field

• Earth’s magnetic field is generated by motion of molten iron in the outer core.

2.6 Seismic Waves (Most Important Source)

• Produced during earthquakes and travel through the Earth.
• Their speed and path reveal density and composition of inner layers.

3. EARTHQUAKE

3.1 Definition

• A sudden release of energy in the Earth’s crust causing ground shaking.
• Energy travels as seismic waves.

3.2 Focus

• The point inside the Earth where the earthquake originates.

3.3 Epicenter

• The point on the surface vertically above the focus.

3.4 Magnitude

• Measured using Richter Scale.

3.5 Intensity

• Observed effects on land, structures, and people measured on Modified Mercalli Scale.

4. EARTHQUAKE WAVES

4.1 Body Waves

Travel through the Earth’s interior.

(a) P-Waves (Primary Waves)

• Fastest seismic waves.
• Travel through solids, liquids, and gases.
• Cause compression and expansion.

(b) S-Waves (Secondary Waves)

• Slower than P-waves.
• Travel only through solids → proves outer core is liquid.
• Cause shear motion.

4.2 Surface Waves

Travel along the Earth’s surface.

(a) Love Waves

• Cause horizontal shaking → very destructive.

(b) Rayleigh Waves

• Cause rolling movement → also highly damaging.

4.3 Behaviour of Waves (Important for Structure of Earth)

• P-waves slow down in liquids → indicates molten outer core.
• S-waves disappear in outer core → confirms it is liquid.
• Sudden changes in seismic velocity indicate different layers.

5. EFFECTS OF EARTHQUAKE

5.1 Primary Effects

• Ground shaking
• Surface rupture
• Cracks and fissures
• Collapse of buildings

5.2 Secondary Effects

• Landslides
• Fires
• Tsunamis
• Soil liquefaction
• Floods due to dam failure

6. LAYERS OF THE EARTH

The Earth is divided into Crust, Mantle, and Core based on chemical composition.

7. THE CRUST

7.1 Characteristics

• Outermost and thinnest layer.
• Thickness varies:
  • Oceanic crust: ~5 km
  • Continental crust: ~30–40 km

7.2 Composition

• Oceanic crust → basaltic, rich in silica & magnesium (SIMA)
• Continental crust → granitic, rich in silica & aluminium (SIAL)

7.3 Density

• Oceanic crust is denser than continental crust.

8. THE MANTLE

8.1 Characteristics

• Extends from 30 km to 2900 km depth.
• Makes up 84% of Earth’s total volume.

8.2 Composition

• Silicates of magnesium and iron.

8.3 Upper Mantle

• Partially molten layer known as asthenosphere.
• Important for plate tectonics.

8.4 Lower Mantle

• Solid and denser due to high pressure.

9. THE CORE

9.1 Characteristics

• Extends from 2900 km to 6370 km.
• Divided into:
  • Outer Core: Liquid
  • Inner Core: Solid

9.2 Composition

• Mostly iron and nickel (NIFE).

9.3 Outer Core

• Liquid metal generates Earth’s magnetic field.

9.4 Inner Core

• Solid due to immense pressure despite high temperature.

10. VOLCANOES

10.1 Definition

• A volcano is an opening in Earth’s crust through which magma, gases, and ash are expelled.

10.2 Causes of Volcanism

• Movement of tectonic plates
• Formation of magma due to:
  • Heat
  • Pressure reduction
  • Chemical changes

11. VOLCANOES AND VOLCANIC LANDFORMS

11.1 Types of Volcanoes

(a) Active Volcano

• Erupts frequently
• Example: Mount Etna, Mount Stromboli

(b) Dormant Volcano

• Erupted in the past, may erupt again
• Example: Mount Vesuvius

(c) Extinct Volcano

• No eruption for thousands of years
• Example: Kilimanjaro (considered inactive)

11.2 Types of Volcanic Landforms

A. Extrusive Landforms

Formed when lava cools on the surface.

1. Lava Plateaus

• Broad, flat surfaces of solidified lava.

2. Shield Volcanoes

• Gentle slopes; lava flows easily.
• Example: Mauna Loa (Hawaii)

3. Composite Volcanoes

• Layers of lava + ash; steep slopes.
• Example: Mount Fuji

4. Cinder Cones

• Small, steep cones made of volcanic fragments.

B. Intrusive Landforms

Formed when magma cools inside the Earth.

1. Batholiths

• Large, dome-like bodies of cooled magma.

2. Laccoliths

• Magma pushes layers upward forming a dome.

3. Dykes

• Magma fills vertical cracks.

4. Sills

• Magma fills horizontal layers.

12. CONCLUSION

• The Earth’s interior is complex and dynamic.
• Most knowledge comes from seismic wave studies, which reveal distinct layers.
• Crust, mantle, and core differ in composition, density, and physical state.
• Earthquakes and volcanoes are natural outcomes of internal heat and plate movements.
• Understanding Earth’s interior helps in predicting hazards, ensuring safety, and studying planetary evolution.