Indirect Evidence (Lower Mantle/Core)

• Meteorites

  Iron Meteorites: mostly iron/nickle, about 5%

  Stony Meteorites: mainly silicate minerals
• We infer that as Earth was created at the same time, it would have material of a similar composition to meteorites
• Seismology


  Tells us about theological properties that show us how each layer behaves. No composition details
• Density

  We can measure seismic wave velocities in rocks with different properties therefore we can suggest densities for the different layers taking into account temperature and pressure
• Continental: 2.7g/cm3

  Ocean: 2.9g/cm3

  Mantle: 3.3g/cm3

  Core: 12g/cm3

  Earth: 5.5g/cm3
• Gravity

  Force of attraction between two bodies as a result of their masses
• The Gravimeter

  Uses a mass suspended on a string. The greater force of gravity, the more the mass will pull down on the string. The amount of extension on the spring gives the measure of gravity
• If the measure value is Greater than expected then there is a Positive anomaly so there Is more mass than average and the rock density is relatively high
• If the measured value is Less than expected then there is a negative anomaly and the mass is less than average and the rock density is low
• Gravity Anomalies


  Used widely in oil and mineral exploration as well as studies investigating the interior of the Earth
• Unit is gal which is divided into a thousand milligals, typical continental anomalies are 20 milligals
• Shape of the Earth

  Due to the spin of the earth it actually bulges at the equator and is flattened at the poles. Poles are closer to the centre of mass so gravity is higher
• Altitude

  All measured values are adjusted to sea level. At 10m AMSL the value you read is lower compared to sea level. The correction is added to the value of gravity
• Bouguer Anomaly


  If you are 100m AMSL on a hill you have an extra 100m if rock mass AMSL as well. You subtract the extra mass, about 1 milligal per 10m