Summary: Studying Oceanic Plate Motions with Magnetic Data
Eos/Geophysics News (c) 1994 American Geophysical Union. Permission is hereby granted to journ
use this material so long as credit is given, and to teachers to use this material in classrooms.
Gary D. Acton and Katerina E. Petronotis, University of New Mexico, Albuquerque, New Mexico an
University of New England, Armidale, Australia
The geocentric axial dipole hypothesis states that the geomagnetic field, when averaged over tens to
hundreds of thousands of years, corresponds to that of a dipole located at Earth's center and aligned
Earth's rotation axis. Accordingly, paleomagnetic poles, which are estimates of the position of the a
dipole axis, give past positions of the Earth's rotation axis relative to a sample locality from which t
paleolatitude of the sample can be derived. Using this hypothesis, paleomagnetists have established
paleogeographies for the continents that span several hundreds of million years. Such information
essential for understanding plate and terrane motions, mantle convection, paleoclimates, and
geomagnetism, as well as many other subjects.
Paleomagnetic data are typically obtained from the magnetization directions estimated from orient
samples collected from rock exposures, such as the layers of rocks exposed along stream or road cut
layers of volcanic flows exposed along the flanks of volcanos. For oceanic plates, such as the Pacific
rock exposures are limited to a few young islands. Even for continental plates, most of which have
oceanic components, rock exposures may be very limited in the range of ages they span. In these cas
paleomagnetic poles must be estimated by other techniques.
One technique with great promise uses the shapes of marine magnetic anomalies. Marine magneti