A.1] Earth and its Structure

1. Earth’s Layers

1.1 Crust

• Definition: The outermost layer of the Earth, composed mainly of solid rock.
• Thickness: Varies from about 5–70 km.
• Types:
  • Continental Crust: Thicker (30–70 km), composed mainly of granitic rocks (low density).
  • Oceanic Crust: Thinner (5–10 km), composed mainly of basaltic rocks (high density).
• Composition: Silica and aluminum-rich (Sial).
• Key Fact: The crust is the thinnest layer of the Earth.

1.2 Asthenosphere

• Definition: A ductile, partially molten layer in the upper mantle.
• Location: Lies below the lithosphere, approximately 100–200 km deep.
• Properties:
  • Plastic and semi-fluid.
  • Allows for the movement of tectonic plates.
• Key Fact: The asthenosphere is crucial for plate tectonics and isothermal in nature.

1.3 Mantle

• Definition: The largest layer of the Earth, composed mainly of silicate rocks.
• Thickness: Approximately 2,900 km.
• Composition:
  • Upper Mantle: Includes the asthenosphere and the lithosphere.
  • Lower Mantle: Denser, composed of silicate minerals under high pressure.
• Key Fact: The mantle is responsible for most of the Earth’s internal heat and is the source of magma.

1.4 Core

• Definition: The innermost layer of the Earth, composed mainly of iron and nickel.
• Thickness: Approximately 3,480 km.
• Sub-layers:
  • Outer Core: Liquid, 2,200–5,150 km deep.
  • Inner Core: Solid, 5,150–6,371 km deep.
• Properties:
  • Outer Core: Generates Earth’s magnetic field due to convection currents.
  • Inner Core: Solid due to immense pressure, despite high temperature.
• Key Fact: The core is the densest layer and is responsible for the Earth’s magnetic field.

2. Discontinuities Separating Layers

2.1 Mohorovicic Discontinuity (Moho)

• Definition: The boundary between the crust and the mantle.
• Discovery: Identified by Andrija Mohorovičić in 1909.
• Location: Typically 5–70 km below the Earth’s surface.
• Properties:
  • Seismic waves increase in velocity at this boundary.
  • Indicates a change in composition and density.
• Key Fact: Moho is a critical boundary for understanding crustal structure.

2.2 Gutenberg Discontinuity

• Definition: The boundary between the mantle and the outer core.
• Discovery: Identified by Beno Gutenberg in 1914.
• Location: Approximately 2,900 km below the Earth’s surface.
• Properties:
  • Seismic waves (P-waves) slow down significantly at this boundary.
  • S-waves disappear beyond this point, indicating a liquid layer.
• Key Fact: Gutenberg discontinuity marks the transition to the liquid outer core.

2.3 Lehmann Discontinuity

• Definition: The boundary between the outer core and the inner core.
• Discovery: Identified by Inge Lehmann in 1936.
• Location: Approximately 5,150 km below the Earth’s surface.
• Properties:
  • Seismic waves (P-waves) change velocity at this boundary.
  • Indicates a solid inner core despite high temperature due to immense pressure.
• Key Fact: Lehmann’s discovery was pivotal in understanding the Earth’s core structure.

3. Comparison Table of Earth’s Layers

4. Key Facts for Competitive Exams (SSC, RRB)

• Mohorovicic Discontinuity (Moho): Separates crust and mantle, discovered by Andrija Mohorovičić.
• Gutenberg Discontinuity: Separates mantle and outer core, discovered by Beno Gutenberg.
• Lehmann Discontinuity: Separates outer core and inner core, discovered by Inge Lehmann.
• Crust types: Continental (granitic, thick) and oceanic (basaltic, thin).
• Core composition: Iron and nickel, with outer core liquid and inner core solid.
• Mantle: Composed of silicate minerals, responsible for most of Earth’s internal heat.
• Asthenosphere: Ductile, partially molten, allows for plate tectonics.

5. Frequently Asked Questions (FAQs)

• Q: What is the thinnest layer of the Earth?
  A: The crust.

• Q: Which layer is responsible for the Earth’s magnetic field?
  A: The outer core.

• Q: What is the Moho?
  A: The boundary between the crust and the mantle.

• Q: What is the Gutenberg discontinuity?
  A: The boundary between the mantle and the outer core.

• Q: What is the Lehmann discontinuity?
  A: The boundary between the outer core and the inner core.

• Q: Why is the inner core solid despite high temperature?
  A: Due to immense pressure, which keeps it solid.