PHD OPPORTUNITY - please draw this to the attention to any students who
may be thinking of a PhD in geophysics.
Pacific Geomagnetic Secular Variation: the Story from Hawaii
Professor David Gubbins, School of Earth Sciences, University of Leeds;
Dr. Emilio Herrero-Bervera, Hawaii Institute of Geophysics and Planetology,
University of Hawaii.
This project involves geomagnetic surveys and paleomagnetic measurements
on recent lava flows on the Big Island of Hawaii in order to determine
the rate of change of the Earth's magnetic field over the last 5-50 thousand
years, and interpreting the all paleomagnetic data from the region in
terms of current theories of core-mantle interaction.
There is now good evidence that lower mantle anomalies influence the
geomagnetic field. Magnetic flux is concentrated around the Pacific rim,
where persistent plate subduction has produced cold lower mantle which
in turn induces downwelling in the liquid core and concentration of magnetic
flux. Flux concentration can be seen in the modern magnetic field, the
time average of paleomagnetic data over the last 5 million years, paleomagnetic
secular variation, and reversal transition paths.
The secular variation, the rate of change of the geomagnetic field, also
has geographical variations in the last 400 years, the length of the
historical record of direct observation; most of the activity is confined
away from the Pacific and the activity peaks in the southern hemisphere
around Africa, and the Atlantic and Indian Oceans. This hemispheric difference
is difficult to explain in terms of known lower mantle anomalies because
both Africa and the central Pacific have very low seismic velocity and
should therefore be hot.
The difference could be temporary: 400 years is not a long time in terms
of the geomagnetic record. South Atlantic activity appears to be associated
with expulsion of flux from inside the Earth's core, just as sunspots
are manifested by periodic flux expulsion. It could also be a long-term
feature: there is a long-running debate on whether the paleomagnetic record
shows low secular variation in the Pacific, and recent dynamo calculations
that incorporate a model of heat flux through the core-mantle boundary
derived from lower mantle seismic velocities also has low Pacific secular variation.
The question can be answered by a careful examination of the paleomagnetic
record from the last 5-50 thousand years. The Hawaiian Islands chain provide
ideal lavas for this purpose. The Big Island has many recent flows, many
well dated by C14. There are many historical (post 1750AD) lava flows
that can be used to calibrate errors in the paleomagnetic measurements.
A recent survey shows that paleomagnetic inclinations are systematically
low, either because of local magnetic anomalies or the demagnetising effect
of these strongly magnetic basalt layers.
The project has three parts:
1.Revisit the historical flows and conduct magnetic surveys on the
centimetre-to-kilometre scale to determine the effect of the crustal
magnetisation and develop a theoretical model of the demagnetising effect.
2.Refine and add to the paleomagnetic record from the last 50 thousand
years, focussing on intensities and their use to improve dating of the flows.
3.Model the resulting paleomagnetic record of secular variation in Hawaii
in terms of modern ideas of core dynamics, convection, and the dynamo.
Data analysis, theoretical calculations, and dynamo modelling, will be
done in Leeds. The paleomagnetic work will be conducted in two field
seasons involving sampling on the Big Island (and in California where
previously-drilled samples have been stored) and laboratory work at the
Hawaii Institute of Geophysics and Planetology at University of Hawaii,
Manoa. The student will be trained in modern methods of paleomagnetism,
data analysis, and fluid dynamical modelling.
--
David Gubbins
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School of Earth Sciences
University of Leeds
Leeds LS2 9JT
UK
Tel +44 113 233 5255
Fax +44 113 233 5259
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