Lecture 8

  • Direct study of rocks
  • Surface rocks
  • Drill holes
  • Lava

Xenoliths - rocks taken from mantle through magma, and carried to the surface


  • Earthquakes occur as a result of built up strained energy in blocks of rock subjected to tectonic forces
  • Earthquakes are therefore shaking ground accompanied from the sudden release of energy
  • Caused by fracturing rocks, due to the strain energy that builds up as a result of plate motion

Fault - rock fracture along which relative movement occurs

Focus - initial point of rupture along a fault

  • Focal depth influences earthquake impacts

Epicenter - point on the surface directly above the focus

  • Seismic body waves move throughout the body of the planet

What Can We Conclude by Tracking These Body Waves as They Make Their Way Throughout the Planet?

  • The waves as they make their way through the planet, it is going to encounter layering,
  • The waves will be bent when passing from the crust to the liquid lower layers

P Waves - primary waves

S Waves - secondary waves

Surface Waves - waves that do not go through the body, instead travel along surface

  • S waves cannot travel through liquids, therefore they are blocked by the outer core
  • This is one of the most powerful evidences that the core is liquid


  • Fe - Ni metal
  • Inner core \rightarrow solid
  • Outer core \rightarrow liquid
  • Peach analogy to regard the layers of the earth, the skin of the peach being the crust


  • Made of relatively dense rock
  • Not as dense as core


  • Weak zone
  • Near melting point

The Crust

  • Oceanic crust
  • Basalt
  • Dense, thin
  • Continental crust
  • Granite
  • Less dense, thick
  • Crust + top mantle = lithosphere

Why Do Continents Move Around?

  • Mantle convection drives the plate motion
  • Hot rock (low density) rises from depth and cool rock (high density) sinks from the surface
  • Hot mobile mantle + thin, cool, brittle lithosphere
  • Breaks into plates

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