2.1 Internal structure of the Earth

Earth's internal structure is like a giant onion, with distinct layers each playing a crucial role. From the thin crust we live on to the blazing hot core, these layers shape our planet's behavior and evolution.

Understanding Earth's layers helps explain phenomena like plate tectonics, volcanism, and magnetic fields. By studying seismic waves, scientists have pieced together a detailed picture of our planet's hidden depths.

Earth's Layers

Composition and Density

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• Crust is the outermost layer of the Earth, composed of solid rocks and minerals
  • Varies in thickness from 5-70 km (oceanic crust is thinner than continental crust)
  • Least dense layer of the Earth with an average density of 2.7-3.0 g/cm³ (granite and basalt)
• Mantle is the second layer of the Earth, located between the crust and the core
  • Approximately 2,900 km thick, making up ~84% of Earth's volume
  • Composed of hot, dense rocks rich in iron and magnesium (peridotite)
  • Upper mantle has an average density of 3.4-4.4 g/cm³, while the lower mantle is 4.4-5.6 g/cm³

Core Characteristics

• Outer core is a liquid layer beneath the mantle, composed primarily of iron and nickel
  • Approximately 2,300 km thick, extending from 2,890-5,150 km below Earth's surface
  • Extremely high temperatures ranging from 4,000-6,000°C
  • Density ranges from 9.9-12.2 g/cm³, contributing to Earth's high average density (5.5 g/cm³)
• Inner core is the centermost layer of the Earth, a solid sphere of primarily iron and nickel
  • Radius of approximately 1,220 km, extending from 5,150-6,370 km below Earth's surface
  • Despite extremely high temperatures (5,000-7,000°C), the inner core remains solid due to immense pressure
  • Highest density of all Earth's layers, ranging from 12.8-13.1 g/cm³

Mantle Divisions

Lithosphere and Asthenosphere

• Lithosphere is the rigid, outermost layer of the Earth, including the crust and uppermost mantle
  • Thickness varies from 50-200 km (thicker under continents, thinner under oceans)
  • Broken into several large tectonic plates that move and interact with each other
• Asthenosphere is a highly viscous, ductile layer of the upper mantle beneath the lithosphere
  • Extends from ~80-200 km to ~660 km below Earth's surface
  • Partially molten and capable of gradual flow, allowing tectonic plates to move

Mohorovičić Discontinuity (Moho)

• Mohorovičić discontinuity, or Moho, is the boundary between the crust and the mantle
  • Discovered by Croatian seismologist Andrija Mohorovičić in 1909
  • Marked by a sudden increase in seismic wave velocities (P-waves from ~6.7-7.2 km/s to ~7.6-8.6 km/s)
  • Depth varies from ~5-10 km beneath oceanic crust to ~20-90 km beneath continental crust

Seismic Evidence

Seismic Waves and Earth's Interior

• Seismic waves, generated by earthquakes or artificial explosions, provide crucial evidence for Earth's internal structure
  • P-waves (primary or compressional waves) travel through both solids and liquids
  • S-waves (secondary or shear waves) can only propagate through solids
• Velocity and behavior of seismic waves change with depth, indicating variations in Earth's composition and physical properties
  • Sudden changes in seismic wave velocities at certain depths reveal boundaries between layers (e.g., Moho)
  • S-waves do not pass through the outer core, indicating its liquid state
  • P-wave shadow zone (103-143°) further confirms the liquid outer core