The internal structure of the Earth - Seismology: the study of earthquake waves

• Concept of Elastic deformation
• Generation of waves in earthquakes: Stored elastic energy in rocks
• Earthquake focus and Epicenter
• Body Waves: travel through the earth; cf., sound and light waves
  • P - waves: (Primary) alternate compression and expansion, go through all states of matter
  • S - waves: (Secondary) shear, not passed through liquids
• Surface Waves: like water waves, only at surface
• Propagation of waves:
  • transmission : velocity depends on density and temperature
  • reflection
  • refraction
• Wave Paths in the earth-arcuate, reflected and refracted
  • Travel times and distance from epicenter
  • Location of epicenter
  • Discovery of Mohorovicic discontinuity

The layered structure of the Earth

• Crust
• Mohorovicic Discontinuity
• Mantle - upper and lower separated by the 670 km discontinuity
• Core- 2883 km discontinuity to 6371 km
  • Outer core-liquid (2883 km - 5140 km), earth’s magnetic field
  • Inner core - solid( 5140 km- 6371 km)

Details of the chemical structure of the earth

• Crust: the outer most rocky layer in the earth, divided into oceanic and continental.
  • Age contrasts of the continental and oceanic crusts:
  • Continental crust - 30-80 km thick, maximum under mountain ranges. Chemistry - made up of light elements like K, Na, Si, Ca, Al silicates
  • Oceanic crust - 6-8 km above the Moho, mostly of rocks, Fe, Mg silicates
• Mantle: - 68% of the earth by mass, largely made up of Fe, Mg silicates
  • Upper mantle - above the 700 km discontinuity, characterized by the transition zone, fluctuations in P and S wave velocities, and the Low velocity zone (LVZ), where material is close to melting point, has very little rigidity
  • Lower mantle: from ~700 km to 2900 km, homogeneous, made up of Fe, Mg silicates
• Core: 32% by mass of the earth, Major change in P wave velocities at the core mantle boundary, largely made up of Iron, alloyed with small amounts of Ni, and other light elements as deduced from cosmochemical and density considerations and direct laboratory measurements of velocities.
  • outer core: liquid, inferred from the non-passage of S waves.
  • inner core: solid. Probably crystallized from the cooling of a liquid core.

Temperatures inside the Earth and Heat sources

• Primordial heat from accretion
• Heat generation due to internal radioactivity
• Shallow geotherms
• Temperature distribution in the interior

• Rheological structure of the crust and mantle: Brittle and ductile behavior of rocks
  • Lithosphere: cool, rigid and brittle layer, ~ 100 km thick, includes both oceanic and continental crust and the upper parts of the mantle.
  • Asthenosphere: hot, weak, plastic, 100 km to ~350 km, able to flow.
  • Mesosphere: hot, but strong due to the effect of pressure, from about 350 km to the top of the outer core Outer Core

Modes of heat flow

• Conduction: heat is transferred without material transport (e.g. handle of a pot on the stove gets hot)
• Convection: rising of hot material upward and sinking of cold material downward (e.g. water at a rolling boil)
• Radiation: energy emitted due to the temperature, not important in the earth (e.g. energy from the sun)