Paleomagnetism

1
Paleomagnetism

• The measurement of remnant magnetism can provide
  information important information about where a
  rock may have come from.
• Measuring a paleomagnetic direction
• An individual lava flow may not record an
  average pole (secular variation), so samples
  from a series of flows may be taken
• Oriented (azimuth and dip) rock cores separated
  by up to a few meters are drilled (using
  non-magnetic equipment).
• If the rock has been tilted since its formation,
  this has to be measured.
• The magnetization direction is measured (by
  measuring all three axis of the core) using a
  very sensitive magnetometer.
• The direction, which is relative to the cylinder
  is calculated with respect to north and the
  vertical.
• The magnetization direction is plotted on a
  stereonet.

2
Paleomagnetism

• Magnetic inclination varies from vertical in the
  center to horizontal at the circumference.
• Declination is the angle around the circle
  clockwise from north.
• Downward magnetizations (positive inclination)
  are plotted as open circle. Negative
  magnetizations are plotted as solid circles.
• Plot mean direction and 95 confidence interval
  (95 probability of containing the true
  direction).

From Mussett and Khan, 2000
3
Paleomagnetism

• Now that we now the inclination of the magnetic
  field in the rock sample we can calculate its
  paleolatitude, and from that the relative
  location of the pole at that time.
• If the apparent pole is not at the north
  geographic pole, the rock must have moved
  (assuming secular variations have been averaged
  out).
• If the declination is not due north, the rock has
  rotated about its vertical axis.
• We cannot tell if the rock has changed its
  longitude.
• Uses??
• Climate change

From Taylor et al., 2000
4
Apparent Polar Wander

• If a landmass has moved north or south over
  geological time, the paleopoles of rocks of
  successive ages will change and trace out an
  apparent polar wonder (APW) path. The poles have
  not moved but the continent has.

From Mussett and Khan, 2000

• The APW for North America and Europe are
  different, as the two have moved relative to each
  other.

5
Magnetostratigraphy

• By measuring the polarity of magnetization of a
  rock of know age (radiometric data, sediment on
  ocean floor above basement) we can build up a
  magnetic polarity timescale.
• At even smaller scales we can examine secular
  variation within a series of lava flow (assuming
  a high resolution series of flows).
• If these flows are historic, we could probably
  date them.
• If they are very old, we could use the pattern of
  secular variation to correlate between outcrops.
• Archeological applications dating ancient
  fireplaces.
• The resultant magnetic timescale can be used to
  date sediments and the seafloor by the
  recognition of distinctive reversal patterns.

From Mussett and Khan, 2000
6
Modeling Seafloor Spreading
From Robb et al., 2004
7
Modeling Seafloor Spreading
From Robb et al., 2004
8
Modeling Seafloor Spreading
From Robb et al., 2004
9
References Used

• Mussett, A.E. and M.A. Khan, Looking into the
  Earth An introduction to geological geophysics,
  2000.
• Robb, M., B. Taylor, and A.M. Goodliffe,
  Re0examination of the magnetic lineations of the
  Gascoyne and Cuiver abyssal plains, NW Australia,
  submitted to GJI, 2004.
• Taylor, G.K., J. Gascoyne, and H. Colley, Rapid
  rotation of Fiji Paleomagnetic evidence and
  tectonic implications, Journal of Geophysical
  Research, 105 (B3), 5771-5781, 2000.