|MadSci Network: Earth Sciences|
Reversals in the earth's magnetic field are one of the most interesting of all geophysical phenomenon. There are many ideas, but to my knowledge, no one really understands the mechanism behind them. What we do know better is what they are like, thanks to detailed study of the magnetic direction and intensity in sediments and volcanic rocks deposited while a reversal was occurring. For examples of data, see:
http://www.gps.caltech.edu/MagLab/proj_dv.html and http://www.gps.caltech.edu/MagLab/proj_hsdp.html.
During a reversal, the intensity of the earth's field decreases to zero and then increases in the opposite polarity. There is no evidence that the poles 'migrate' during this process, though they continue the process of 'secular variation'. Secular variation is the wobbling of the magnetic pole in a region around the geographic pole over tens to thousands of years and is a constant feature of the earth's magnetism. Some of the research above suggests that the poles wobble more than previously thought, voving up to 60 degrees away from geographic pole. I just learned this as I looked up info for you! BTW, I found those sites by doing an Alta Vista search on geomagnetic reversals.
Geomagnetic reversals happen rapidly, geologically speaking, typically in much less than a million years and they happen more or less 'randomly' through geologic time, sometimes with only a few million years between and sometimes with 10's of millions of years between. On human time scales, reversals would appear gradual but might be quite complex in detail with fluctuation in the intensity around the general trend of decrease and increase. There is some research that suggests that reversals may not be entirely random, but may follow a fractal distribution instead. The significance of this is not known yet. There is also a suggestion that the earth's field fluctuates more or less continuously up and down but does not reverse on every fluctuation.
For a brief review of some new research on the nature and timing of reversals see:
This article indicates the reversals recently have occurred on the scale of hundreds of thousands of years, not millions. They may have occurred that often in the past as well, but we lack the preserved record and sampling density to detect so many reversals.
The effects of geomagnetic reversals are not well known, since humans have never observed one, but they would definitely not include unusual eartquakes or volcanism. Those events are driven by tectonic motions that are in turn driven by the earth's internal heat and by gravitational forces, neither of which is influenced by earth's magnetism. Life would be affected, since the earth's magnetic field shields us from a lot of solar radiation outside the visible spectrum. Climate would probably not be greatly affected.
There's a lot of work going on in this field and I'm not completely up-to- date myself, but I hope this helps. If you are motivated to do more, you might start with Alan Cox's book on paleomagnetism and geomagnetism. I can get more info, if you want.
Try the links in the MadSci Library for more information on Earth Sciences.