The Earth is composed of concentric layers with distinct physical and chemical properties. These layers are studied using seismic waves generated during earthquakes.

Broad Classification

Mechanical basis

• Lithosphere
• Asthenosphere
• Mesospheric mantle
• Outer core
• Inner core

Chemical basis

• Crust
• Upper mantle
• Lower mantle
• Outer core
• Inner core

The Crust

• Outermost solid layer of the Earth
• Accounts for 5–1% of Earth’s volume and less than 1% of its mass
• Average density: ~2.7 g/cm³
• Thickness:
  • Oceanic crust: 5–30 km
  • Continental crust: 30–70 km
  • Mountain regions (Himalayas): 70–100 km

Composition

• Upper crust: sedimentary rocks over crystalline igneous and metamorphic rocks
• Lower crust: basaltic and ultrabasic rocks
• Continental crust: felsic rocks (rich in silica, aluminium, sodium, potassium) – granite
• Oceanic crust: mafic rocks (rich in iron and magnesium) – basalt

Temperature

• Increases with depth
• About 200–400°C at the crust–mantle boundary
• Average geothermal gradient: ~30°C per km in upper crust

Most Abundant Elements in Earth’s Crust

• Oxygen
• Silicon
• Aluminium
• Iron
• Calcium
• Sodium
• Potassium
• Magnesium

Mnemonic: OS CIA Na K Mg

Mohorovicc Discontinuity (Moho)

• The boundary between the crust and the mantle
• Identified by a sudden increase in seismic wave velocity
• Depth:
  • Under oceans: ~8 km
  • Under continents: ~30 km
• Caused by a change in rock composition from feldspar-rich rocks to feldspar-free rocks

Lithosphere

• Rigid outer layer of Earth
• Includes crust + uppermost mantle
• Thickness: 10–200 km
• Broken into tectonic plates
• Plate movements cause:
  • Earthquakes
  • Volcanism
  • Folding and faulting
• Heat source for plate movement:
  • Primordial heat
  • Radioactive decay of uranium, thorium, and potassium

Mantle

• Extends from Moho to 2900 km depth
• Accounts for:
  • 83% of Earth’s volume
  • 67% of Earth’s mass
• Composition: silicate rocks rich in iron and magnesium

Upper Mantle

• Density: 9–3.3 g/cm³
• Includes asthenosphere

Lower Mantle

• Solid state
• Density: 3–5.7 g/cm³

Temperature

• From ~200°C near crust to ~4000°C near core
• Heat causes mantle convection, driving plate tectonics

Seismicity

• Earthquakes observed up to 670 km depth in subduction zones

Asthenosphere

• Weak, ductile layer of the upper mantle
• Lies below the lithosphere
• Depth: ~80–200 km
• Highly viscous and partially molten
• Functions:
  • Facilitates plate movement
  • Source of magma
  • Enables isostatic adjustment

Outer Core

• Lies between 2900 km and 5100 km
• Composition: iron + nickel (NiFe) with lighter elements
• State: liquid
• Density: 9–12.2 g/cm³
• Temperature: 4400–6000°C

Importance

• Convection currents + Coriolis force generate Earth’s magnetic field
• Explained by Dynamo Theory

Inner Core

• Extends from 5100 km to Earth’s centre
• Composition: ~80% iron, some nickel
• State: solid due to immense pressure
• Density: 6–13 g/cm³
• Temperature: ~6000°C
• Can transmit S-waves, unlike the outer core
• Rotates slightly faster than Earth’s surface
• Does not hold a permanent magnetic field

Core Contribution

• Volume: ~16% of Earth
• Mass: ~33% of Earth

Seismic Discontinuities

Seismic discontinuities are zones where seismic wave velocity changes abruptly due to a change in composition or state.

Major Discontinuities

• Mohorovicc Discontinuity – crust–mantle boundary
• Gutenberg Discontinuity – mantle–outer core boundary
• Lehmann Discontinuity – outer core–inner core boundary

Pointers

• S-waves cannot travel through liquids
• The outer core is responsible for the magnetic field
• Continental crust is thicker but lighter than oceanic crust
• Mantle convection drives plate tectonics
• An increase in pressure generally increases the melting point, except for ice