Earth Structure & Convection

Earth Structure Inferred From Seismic Waves

• From below-ground nuclear weapons tests, geophysicists inferred the Earth's structure.
• Seismic waves curve as they travel into the Earth, due to:
  • increased temperature with depth.
  • pressure with depth.
  • varying composition.
• Boundaries of the Earth's structure mapped by reflection and refraction of the seismic waves.
• Secondary waves (S-waves) were blocked by the outer core, suggesting it is molten.
• P-wave shadow zone:
  • zone where P-waves of earthquake not detected.
  • due to refraction.
• S-wave shadow zone:
  • zone where S-waves of earthquake not detected.
  • due to combination of blocking by the outer core and refraction.

Convection Currents

• There are many models to account for the motion of the plates.
• Most are based upon movement of mantle materials based upon temperature-induced density differences (TIDD).
• There are three generally accepted theories.
• Mantle convection current theory
  • First proposed by Arthur Holmes in 1928.
  • As the warm, rising mantle rises then spreads across the Earth's surface, it drags along the lithosphere plates.
  • The cooled mantle then descends.
  • Some scientists hold that the convection loops are large, using the entire mantle.
  • Others suggest a smaller asthenosphere feature.
• Slab-pull theory
  • Depends upon the mound formed at the mid-ocean ridge material by the warm, less-dense oceanic crust.
  • As this crust cools and moves away from the ridge,
  • it contracts and begins to sink into the mantle.
  • This pulls the plate along.
  • An alternative way to view this is as slab-push.
  • The elevated ridge pushes the plate along.
• Hot Spot theory
  • The long, thin plumes of hot spots are a possible convection current based engine for plate motion.