Reversal of Earth's Magnetic Field

     Since the topic came up in class, I thought a post on the Earth's magnetic field would be appropriate.  As discussed in class, the magnetic poles have reversed in the past.  This happens on average every 200,000 to 300,000 years, although the last reversal was about 800,000 years ago and in geologic history there have been periods up to millions of years with no reversal.  These reversals do not happen instantaneously; the magnetic field lines become entangled and the poles wander.  Scroll down for a link to a computer simulation of this.  During this process, the field weakens temporarily (don't worry, this is not a problem) before the reversal happens.  It is unknown how long this reversal takes; all of the sources I have read so far estimate hundreds to tens of thousands of years.
      It is also unknown why the magnetic field reverses itself and why it happens so irregularly.  The reversals can be seen in the rock record, but there is no evidence to suggest why this happens.  In the computer model, the data was input into a supercomputer, and left to run, the computer model showed periodic reversals, which was an unexpected result.  See http://www.psc.edu/science/glatzmaier.html for more info and the computer simulation at the bottom of the page.  While the magnetic field does weaken during the transition, it does not completely disappear, and according to the USGS (see link in the references), it is not sufficient to cause mass extinctions.  If you are interested in a brief introduction about paleomagnetism and what the geologic record can tell us, read the next section. 

A Little Background and Interesting Facts

     In between the reversals, the magnetic poles are not completely stable; they wander, sometimes more than others.  A lava flow in Oregan indicates that for a brief period, magnetic north moved by about 6 degrees per day.  See http://www.psc.edu/science/glatzmaier.html for details.  Even now, the magnetic declination is changing slightly (declination is the difference between magnetic north and true north). 
     We know when the magnetic reversals happen by looking at the geologic record, lava flows for example.  Minerals in the lava that have magnetic properties align themselves with the magnetic field.  As the lava solidifies, the minerals crystalize with this orientation, creating a record of the magnetic field polarity.  Another, and very important, example of this record is the magnetic striping on the ocean floor.  At mid-ocean ridges, where the seafloor is spreading, magma rises to the surface, flows out symmetrically from the ridge and cools, with the crystals aligning themselves to the magnetic field.  This causes alternating stripes on the seafloor of normal and reverse polarity.  See the papers by Vine and Matthews (1963) and Vine and Wilson (1965) for detail.  This is the original research on magnetic striping and is heavily cited even in modern research.


References:

Information including computer simulation of magnetic reversal (can be played using windows media player)
 http://www.psc.edu/science/glatzmaier.html

This is an excellent, non-technical site with the same computer model, but the animation requires quicktime
http://www.pbs.org/wgbh/nova/earth/when-our-magnetic-field-flips.html

USGS sites
About magnetic reversals and mass extinctions
http://www.usgs.gov/faq/index.php?sid=54684&lang=en&action=artikel&cat=11&id=484&artlang=en

For everything related to the magnetic field
http://www.usgs.gov/science/science.php?term=677


Technical

Vine, F. J., Matthews, D.H.., 1963, Magnetic anomalies over ocean ridges, Nature, no 4897, p. 947-949

Vine, F. J., Wilson, J. T., 1965, Magnetic Anomalies Over A Young Oceanic Ridge Off Vancouver Island: Science, Vol. 150, p. 485-489