The ancient magnetic field: Clues hidden in lava flows from around the world

Lisa Tauxe

University of California San Diego, Scripps Institution of Oceanography

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William Gilbert, the physician to Queen Elizabeth I, realized back in 1600 that the Earth's magnetic field was very much like one that would be produced by a bar magnet in the center of the Earth aligned with the spin axis. This is the heart of what is known as the 'geocentric axial dipole hypothesis', or GAD, and is how we know continents have drifted over the billions of years of Earth's history. Given a GAD field, if we know a field direction at a particular location at a particular age, we can predict where the axis of the centered dipole intersects the surface of the Earth (a paleomagnetic pole position) and conversely, we can use a particular pole and predict the magnetic field direction. The GAD hypothesis has been validated for at least the last billion years or so. 

I am interested not only in the ancient directions of Earth's magnetic field, but in the strength of the magnetic field as well. It turns out that while the directions recovered from ancient rocks are consistent with a GAD field, the strengths are not. Why not?  Is this a failure of the method we use to estimate ancient field strengths? Is this because we have under-sampled a highly variable field? Or is this a failure of the GAD hypothesis itself? The answers may lie beneath our feet in Oregon in the abundant volcanic deposits all around us.  

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