Earth's Interior Structure: Layers, Composition & Discontinuities

Introduction

To answer a 7-mark question on the interior of the Earth, provide a structured response that covers the chemical composition, physical properties, and the seismic discontinuities that separate the layers.

The interior of the Earth is not a uniform mass but is composed of concentric layers, much like an onion. These layers are identified based on two criteria: Chemical composition (what they are made of) and Mechanical properties (how they behave — solid, liquid, or plastic).

The Crust (Outermost Layer)

The crust is the thin, brittle, outermost shell of the Earth.

• Thickness: Varies from 5 km (oceanic) to 70 km (continental).
• Types:
  • Continental crust: Primarily composed of sial (silica and aluminium). It is thicker and less dense (granitic).
  • Oceanic crust: Primarily composed of sima (silica and magnesium). It is thinner and denser (basaltic).
• Volume: Accounts for only about 1% of the Earth’s volume.

The Mantle (Middle Layer)

The mantle extends from the Mohorovičić (Moho) discontinuity to a depth of 2,900 km.

• Composition: Primarily silicate rocks rich in magnesium and iron.
• Volume & mass: It is the largest layer, making up about 84% of Earth's volume and 67% of its mass.
• Key zones:
  • Lithosphere: The rigid outer part including the crust and the uppermost mantle (0–100 km).
  • Asthenosphere: A semi-fluid, plastic layer (100–400 km) where convection currents occur, driving plate tectonics.
  • Mesosphere: The solid lower mantle extending down to the core boundary.

The Core (Innermost Layer)

The core is the "engine room" of the planet, located at the center.

• Composition: Primarily Ni–Fe (nickel and iron).
• Density: The densest layer, with values ranging from 9.5 to 14.5 g/cm³.
• Sub-layers:
  • Outer core: A liquid layer about 2,200 km thick. The movement of metallic iron here generates Earth's magnetic field.
  • Inner core: A solid ball with a radius of about 1,220 km. Despite extreme temperatures (~5,200 °C), it remains solid due to immense pressure.

Major Seismic Discontinuities

To secure high marks, mention the boundaries (discontinuities) where seismic wave velocities change. The study of seismic discontinuities explains sudden changes in seismic wave speed and helps identify layer boundaries.

Conclusion

The study of Earth's interior relies heavily on indirect evidence such as seismic waves (P and S waves) and meteorites. Understanding these layers is crucial for explaining phenomena like earthquakes, volcanoes, and the Earth's magnetic field.