Class 11 Geography Notes Chapter 3 (Interior of the earth) – Fundamental of Physical Geography Book

Alright students, let's dive into Chapter 3, "Interior of the Earth," from our Fundamentals of Physical Geography textbook. This is a foundational chapter, crucial for understanding many other geographical phenomena, and frequently tested in government exams. Pay close attention to the sources of information and the structure derived from them.

Chapter 3: Interior of the Earth - Detailed Notes for Exam Preparation

1. Introduction

• Understanding the Earth's interior is essential as it influences surface topography, resource distribution, earthquakes, volcanism, and the planet's magnetic field.
• Direct observation is impossible due to immense heat and pressure. Hence, our knowledge is largely based on indirect evidence.

2. Sources of Information about the Earth's Interior

• A. Direct Sources:

  • Surface Rocks: Rocks available at the surface or obtained through mining provide limited information about shallow depths.
  • Mining & Drilling: Deep mines (e.g., gold mines in South Africa, approx. 3-4 km deep) and drilling projects provide samples.
    • Key Projects: "Deep Ocean Drilling Project" and "Integrated Ocean Drilling Project." The deepest drill so far is at Kola, Arctic Ocean (approx. 12 km deep). These are still shallow compared to Earth's radius (~6371 km).
  • Volcanic Eruptions: Molten material (magma) thrown onto the surface (lava) provides samples from considerable depths (Asthenosphere), but the exact depth isn't always certain.

• B. Indirect Sources: (More significant for deeper understanding)

  • Temperature & Pressure Gradients: Temperature and pressure increase with depth. The rate of temperature increase decreases with depth. This gradient provides clues about the state of matter inside.
  • Density: The average density of Earth is ~5.5 g/cm³, while surface rocks average ~2.7-3.0 g/cm³. This implies the core must be composed of very dense materials. Density increases significantly with depth.
  • Meteorites: These are fragments of extraterrestrial bodies, believed to have formed similarly to Earth. Studying their structure and composition (often iron and nickel) gives clues about Earth's core composition.
  • Gravitation: The gravitational force (g) is not uniform across the Earth's surface. Variations (gravity anomalies) depend on the distribution of mass within the Earth, providing insights into material distribution.
  • Magnetic Field: The existence of Earth's magnetic field suggests the presence of a dynamic, metallic (likely iron), liquid outer core where convection currents generate the field (dynamo effect).
  • Seismic Waves (Most Important Source):
    • Earthquakes generate seismic waves that travel through the Earth.
    • Their paths and velocities change as they encounter layers of different densities and states (solid/liquid).
    • Studying these changes using seismographs helps map the Earth's interior structure.

3. Earthquakes & Seismic Waves

• Earthquake: Shaking of the Earth's surface caused by a sudden release of energy in the Earth's lithosphere, creating seismic waves.

• Focus (Hypocentre): The point inside the Earth where the energy is released.

• Epicentre: The point on the Earth's surface directly above the focus. Seismic waves reach the epicentre first.

• Seismic Waves:

  • Body Waves: Travel through the Earth's interior. Generated at the focus.
    • P-waves (Primary Waves):
      • Fastest waves. Arrive first at seismographs.
      • Longitudinal waves (particle motion is parallel to wave propagation direction - like a slinky).
      • Can travel through solids, liquids, and gases.
      • Velocity changes based on material density (faster in denser material, generally). Refract when crossing boundaries.
    • S-waves (Secondary Waves):
      • Slower than P-waves. Arrive second.
      • Transverse waves (particle motion is perpendicular to wave propagation direction - like a rope wave).
      • Can only travel through solids. They cannot pass through liquids. This property is crucial evidence for the liquid outer core.
  • Surface Waves: Travel along the Earth's surface. Generated when body waves reach the surface.
    • Slower than body waves. Arrive last.
    • Cause most of the damage during an earthquake.
    • Include Love waves (L-waves, faster surface wave, side-to-side motion) and Rayleigh waves (R-waves, rolling motion).

• Shadow Zones: Areas on the Earth's surface where seismographs do not detect certain seismic waves from a specific earthquake.

  • P-wave Shadow Zone: An annular (ring-like) zone between 105° and 145° (angular distance) from the epicentre. Caused by refraction of P-waves as they enter the liquid outer core. A faint P-wave signal can sometimes be detected within this zone due to refraction at the inner core boundary.
  • S-wave Shadow Zone: A much larger zone beyond 105° from the epicentre. S-waves cannot travel through the liquid outer core, creating this vast shadow zone. This is the primary evidence for the existence of a liquid outer core.

4. Structure of the Earth

Based primarily on seismic wave analysis, the Earth is divided into three main concentric layers: Crust, Mantle, and Core.

• A. The Crust:

  • Outermost, solid, brittle layer.
  • Thickness varies significantly:
    • Oceanic Crust: Thinner (approx. 5-10 km), denser, composed mainly of basaltic rocks (SIMA: Silica + Magnesium).
    • Continental Crust: Thicker (approx. 30-70 km, thickest under mountains), less dense, composed mainly of granitic rocks (SIAL: Silica + Aluminium).
  • Mohorovičić Discontinuity (Moho): The boundary between the Crust and the Mantle, identified by a sharp increase in seismic wave velocity.

• B. The Mantle:

  • Extends from the Moho discontinuity down to approx. 2900 km depth.
  • Comprises about 84% of Earth's volume and 67% of its mass.
  • Density is higher than the crust. Composed mainly of silicate rocks rich in iron and magnesium.
  • Upper Mantle:
    • Asthenosphere: Upper part of the Upper Mantle (approx. 100-400 km depth, though variable). It's a highly viscous, mechanically weak, ductile (plastic-like) region. Source of magma for volcanic eruptions. Lithospheric plates move over the asthenosphere.
    • Lithosphere: Includes the Crust and the uppermost solid part of the Mantle. It's the rigid outer part of the Earth.
  • Lower Mantle: Extends from the end of the Asthenosphere/Upper Mantle transition zone down to the Core boundary. It is solid due to immense pressure, despite high temperatures.
  • Gutenberg Discontinuity: The boundary between the Mantle and the Outer Core. Marked by a significant decrease in P-wave velocity and the disappearance of S-waves.

• C. The Core:

  • Innermost layer, extending from 2900 km depth to the centre (approx. 6371 km).
  • Accounts for about 15% of Earth's volume but 32% of its mass due to high density.
  • Composed primarily of Iron (Fe) and Nickel (Ni), hence often called NIFE.
  • Outer Core:
    • Depth: 2900 km to 5150 km.
    • State: Liquid. Inferred from the inability of S-waves to pass through it and the significant slowing of P-waves.
    • Responsible for generating Earth's magnetic field through convection currents.
  • Inner Core:
    • Depth: 5150 km to 6371 km (Earth's centre).
    • State: Solid. Despite extremely high temperatures (estimated similar to the Sun's surface), it's solid due to immense pressure. Inferred from the increase in P-wave velocity passing through it.
  • Lehmann Discontinuity: Boundary between the liquid Outer Core and the solid Inner Core (though not always explicitly named in basic NCERT texts, the transition is key).

5. Volcanoes and Volcanic Landforms

• Volcano: A vent or opening in the Earth's crust through which molten rock (magma), gases, and ash erupt onto the surface (where magma becomes lava).

• Types of Volcanoes (Based on Eruption Style & Form):

  • Shield Volcanoes: Largest type (except flood basalts). Gentle slopes, formed by fluid basaltic lava flows (e.g., Hawaiian volcanoes). Generally less explosive.
  • Composite Volcanoes (Stratovolcanoes): Steep-sided cones formed by alternating layers of viscous lava flows, ash, and pyroclastic material. Often associated with explosive eruptions (e.g., Mt. Fuji, Mt. Vesuvius).
  • Calderas: Most explosive type. Form when a volcano collapses into itself following a massive eruption, leaving a large depression.
  • Flood Basalt Provinces: Huge volumes of highly fluid basaltic lava erupting from fissures, covering vast areas (e.g., Deccan Traps in India).
  • Mid-Ocean Ridge Volcanoes: Occur at divergent plate boundaries underwater. Characterized by fissure eruptions of basaltic lava, forming new oceanic crust.

• Volcanic Landforms (Intrusive): Formed when magma cools and solidifies beneath the Earth's surface. Exposed later by erosion.

  • Batholiths: Large, irregular masses of cooled magma; form the core of mountain ranges.
  • Laccoliths: Dome-shaped intrusions fed by a pipe-like conduit from below.
  • Lopoliths: Saucer-shaped intrusions, concave upwards.
  • Phacoliths: Lens-shaped intrusions found along anticlines/synclines.
  • Sills: Horizontal or near-horizontal sheets of magma injected between older rock layers.
  • Dykes: Vertical or near-vertical sheets of magma cutting across older rock layers.

Multiple Choice Questions (MCQs)

1. Which of the following is considered the MOST reliable source of information about the deep interior of the Earth?
   (a) Volcanic Eruptions
   (b) Deep Mining Projects
   (c) Analysis of Seismic Waves
   (d) Study of Meteorites

2. The point within the Earth where an earthquake rupture starts is called the:
   (a) Epicentre
   (b) Focus (Hypocentre)
   (c) Fault Line
   (d) Seismic Centre

3. Which type of seismic wave can travel through solids, liquids, and gases?
   (a) S-waves (Secondary Waves)
   (b) L-waves (Love Waves)
   (c) P-waves (Primary Waves)
   (d) R-waves (Rayleigh Waves)

4. The inability of S-waves to pass through the outer core provides strong evidence that the outer core is:
   (a) Solid
   (b) Liquid
   (c) Gaseous
   (d) Plastic

5. The boundary separating the Earth's Crust from the Mantle is known as the:
   (a) Gutenberg Discontinuity
   (b) Lehmann Discontinuity
   (c) Mohorovičić Discontinuity (Moho)
   (d) Conrad Discontinuity

6. The Asthenosphere, the source region for most magma, is part of the:
   (a) Crust
   (b) Upper Mantle
   (c) Lower Mantle
   (d) Outer Core

7. The Earth's core is primarily composed of which elements?
   (a) Silicon and Aluminium (SIAL)
   (b) Silicon and Magnesium (SIMA)
   (c) Iron and Nickel (NIFE)
   (d) Iron and Silicon

8. Which type of volcano is characterized by gentle slopes and eruptions of fluid basaltic lava?
   (a) Composite Volcano
   (b) Shield Volcano
   (c) Caldera
   (d) Cinder Cone

9. A horizontal sheet of magma that solidifies between older layers of rock is called a:
   (a) Dyke
   (b) Sill
   (c) Batholith
   (d) Laccolith

10. The shadow zone for P-waves, located between 105° and 145° from the epicentre, is primarily caused by:
    (a) Absorption of P-waves by the mantle
    (b) Reflection of P-waves at the Moho discontinuity
    (c) Refraction of P-waves as they enter the outer core
    (d) Complete blockage of P-waves by the inner core

Answer Key for MCQs:

1. (c)
2. (b)
3. (c)
4. (b)
5. (c)
6. (b)
7. (c)
8. (b)
9. (b)
10. (c)

Make sure you understand not just the facts, but the reasons behind them – like why S-waves indicate a liquid outer core, or how seismic velocities help define layer boundaries. This deeper understanding is key for tackling analytical questions in exams. Good luck with your preparation!