Lecture 21

• Earth can be divided in to layers based on composition (evidence from gravity, seismology, xenoliths, meteorites)
• crust:

  • radius = 5 - 65 km
  • minerals with light elements
• outer core:

  • radius = 2400 km
  • molten Fe-Ni alloy
• mantle:

  • radius = 2900 km
  • mainly peridotite
• inner core:

  • radius = 1100 km
  • solid Fe-Ni alloy
• non-dynamo hypothesis

  permanently magnetized Earth

  • 50% of rocks would have to be magnetite
  • temperature too high below 30 km
  • cannot account for secular variation
• non-dynamo hypothesis

  rotationally induced magnetism

  • magnetism is associated with angular momentum
  • when Earth is made magnetic dipoles point in random directions. net angular momentum is zero
  • when earth starts to spin, the net angular momentum of the Earth increases and the angular momentum of the atoms are forced to align in the opposite direction to keep the total angular momentum constant
  • horizontal field component should decrease inside the Earth
  • Blackett showed this not to be the source of the Earth's magnetic field
• the self-exciting disk dynamo

  initial seed magnetic field

  • remove the seed field
  • the disk dynamo continues to generate a magnetic field as long as the disk is rotated
  • the magnetic field produces a breaking force on the disk
  • you need to put energy in to keep it rotating
  • coupled disk dynamos fixes some of the issues
• sources of energy for the geodynamo

  1. primordial heat

  • large amounts of heat generated during the accretionary process of bringing small bodies together to make the proto-earth
  • the earth has a large heat capacity and is slowly losing heat through its surface
  • heat transport by convection
• convection

  • heat fluids from below
  • material gets hot and expands
  • fluid becomes lighter than its surroundings and rises
  • at the core mantle boundary it cools and sinks
• sources of energy for the geodynamo

  2. thermal convection

  • driven by radioactive decay of 40K
  • 40K half life 2 x 10^9 years
  • possible addition heat source - latent heat of crystallization of molten metal as it solidifies at the inner core
• sources of energy for the geodynamo

  3. non-thermal (gravitational/compositional) convection

  • dominates only in the outer core
  • driven by fluids rising as they become lighter
  • outer core material consists of iron and lighter elements
  • iron sticks to inner core, leaving a parcel of fluid that is lighter than its surroundings
  • all iron used up in 10^9 years
• has compositional convection always dominated?

  in the early earth the inner core had not formed - thermal convection dominated