It’s better to burn out than fade away…well maybe

In a recent post on this blog, Prof. Richard Holme guaranteed us that a field reversal won’t affect human life for “many lifetimes to come”.  This raises the question, how fast can Earth’s magnetic field reverse?  A recent article by an Italian-French-American research team (Sagnotti et al., Geophysical Journal International)  has garnered the attention of the global media over the past couple of weeks (Le Monde, Der Spiegel, phys.org).   In this study they calculate that the field could change very quickly, just less than 2 degrees per year, i.e. the field could completely flip polarity within a century.  This is very much quicker than the majority of previous estimates, which suggest it takes a few thousand years for a flip to occur, and quicker than the rate of directional changes observed today (a few degrees per century).  Indeed, there is only sparse evidence for such quick changes in the geological record, the most notable being a reversal recorded ~16 Ma in the Steens Mountain lava flows, SE Oregon, and a recent excursion (the Laschamp, ~41 ka) recorded in sediments from the Black Sea.  The Steens Mountain lavas were initially calculated to record changes in the direction of the field up to a phenomenal 3 degrees per day.  However, continued research on the Steens lavas over the last 30 years, headed by Prof. Rob Coe at the University of California-Santa Cruz, led his research group to recently conclude that an interesting phenomena in how the magnetization was acquired by these lavas gave rise to these apparent rapid changes in direction. The directional changes estimated from sediments from the Black Sea were quick (half a degree per year), but quite a bit slower than those from the Sagnotti et al. study and nothing like as fast as those recorded by the Steens Mountain lavas.

How did they reach their result?

The Sagnotti team investigated exposed lake sediments in the Apennines, Italy, covering the time of the most recent geomagnetic field reversal, the Matuyama-Brunhes reversal.  Some debate exists over the age of this reversal, but it occurred approximately 780 ka (this is an ongoing area of fervent research).  As sediments are continually deposited over time they can provide a detailed record of changes in the geomagnetic field depending on how much material is deposited per unit time.  Within these sediments are a number of tephras (ash deposits from volcanic eruptions), which can contain excellent material for radiometric dating, e.g., crystals of sanidine.  In this case researchers from two different labs used the ratio of argon-40 to argon-39 to estimate the ages of the ashes from a large number of experiments on sanidines.  They found the Matuyama-Brunhes reversal lay between two of the dated tephras and by assuming sedimentation was constant between the average ages of their newly determined tephra, estimated the age and the duration of the reversal.  The result: directional changes of just less than 2 degrees per day.

Does this mean the field could completely reverse in less than a human lifetime?

Changes in direction aren’t the whole story.  Although observations on the rate of directional change are interesting, the Earth’s magnetic field has another important component, namely its strength or intensity.  Evidence to date suggests the intensity of Earth’s magnetic field decreases during a reversal and this decrease brackets the polarity flip, but may last considerably longer than the flip itself.  It is likely the decrease in intensity is more intimately linked to underlying processes in Earth’s outer core than the directions themselves, although we don’t exactly know how the reversal process is initiated.  Work done at the University of Liverpool by me and Richard Holme (find here) suggests the length of the polarity flip could be variable depending on the location of the observation, e.g., the duration of the polarity flip could last twice as long in Australia as in Italy, yet the underlying pattern of intensity decrease could be similar. This observation is supported by numerical and empirical modelling of the reversing field.  Even if the polarity flip is short at some locations, the underlying process generating field reversals could be much longer.

In regards to the ability of Earth’s magnetic field to shield Earth from cosmic radiation, it is the strength of the field as well as its configuration that is important, and the field doesn’t fully vanish during a reversal.  Earth’s atmosphere also does a sterling job of protecting the Earth.  It is important to remember the genus Homo have lived through a number of reversals and excursions of the geomagnetic field, such as the Matuyama-Brunhes reversal and more recently the Laschamp excursion.

Is it better to burn out than fade away?  Some reversals may live hard and fast and others may like the quiet life.

About Maxwell Brown

I am a postdoctoral researcher in the Earth's Magnetic Field group at the German Research Centre for Geosciences, Potsdam, Germany.
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