Record of Steens Mountain reversal cast in
terms of virtual geomagnetic poles (VGPs). Compare with
VGP path for the second reversal of the tomographic simulation,
depicted below under Simulations.
Record
of the Steens Mountain reversal showing the path of directions
traversed by the field as polarity progressed from reversed
to normal. Successive lavas with essentially the same direction
are averaged to give a directional group (DG), and indicated
as numbered circles along the path. The first attempt of
the field to reverse was unsuccessful; although it attained
normal polarity by DG30, it soon returned to intermediate
directions and only achieved stable normal polarity after
DG15.
The
big gaps in direction between DG31 and 30, 22 and 21, and
21 and 20 are striking. In both the first and second gap
there is a lava flow with individual sample directions smeared
along the path in such a way as to suggest that the field
may have been changing extremely fast during the time the
flow was cooling and becoming magnetized. If this interpretation
is correct, it has significant implications for fluid velocities
in the core, functioning of the geodynamo, and, as an aside,
the mean conductivity of the mantle. If the smeared directions
are an artifact, they signal a virtually invisible remagnetization
mechanism that paleomagnetists must strive to understand
and guard against.
