Way back in 5th grade was the first time I learned about this phenomenon. Rocks have been found with southern polarity in the north arctic ocean, and vice versa, dating back around 30,000 years. The theory is that every 30,000 years or so the polarity of the earth shifts, north becomes south and south becomes north.

The big part of this is the median period, where the polarity is nullified. During this period the magnetosphere is basically non existent. This leads to a shower of stellar and solar radiation onto the surface of the planet. This would kill off many species, but those who mutate in a beneficial way would go through rapid evolution.

Almost like a real life example of the X-Men.

The last full magnetic reversal happened 740,000 years ago.

However, since then there have been a few 'magnetic excursions' - when the strength of the magnetic field weakens significantly but the poles don't actually 'flip' - after a period the field regains its strength with the poles still in the same position.

A good Q&A page here:

http://www.geomag.bgs.ac.uk/reversals.html

(note: we're talking about the magnetic poles - nothing physical happens!)

As far as we can determine, neither full reversal nor excursions result in any increase in the rates of species extinction, nor in an upsurge in speciation as a result of mutations. There may be in general increase in the occurance of some cancers though.

If a reversal happens in our lifetime, best stock up on sunscreen, wear sun glasses, ditch the trunks or bikini and look forward to the best aurora displays in history

Not exactly. Besides the magnetic rocks there is the liquid earth core that could be compared to a gigantic electrostatic machine which generates a forcefield much stronger than the magnetic field. This forcefield is more responsible for the deviation of charged particles from the atmosphere than the magnetic field ... which could be a by-product of the electrostatic forecefield.

So let me get this straight, not only do I have to worry about Green House effect, nuclear war, asteriod or comet impact, but also the freaking earth turning upside down. Great place we chose to live ehh>?

Pole flipping is a facisinating topic.

This is because it could mean our current theories of electro-magnetism are encomplete. Imagine taking a bar magnet and flipping the poles. I would like to know how that could be done.

Following the electrostatic engine theories. Sooner or later it will come to a point where the positive load equals the negative load, at which time the system will discharge. If the polar magnets are of the none permanent type (most probably because it is supposed to be iron) they could alter their polarity.

The other possibility is that, because it is in a liquid environment, the electrostatic machine can shift completely. You must imagine this as a ball in a goldfish glass full of water. Turn the glass and the ball does not necessarily turn in the same direction. That would cause currents to shift and first demagnetize the ferrous pole and then magnetize it in another direction. For example, take a piece of steel. Place it in a coil with continuous current and it will magnetize. After that turn the iron around in the coil and it first will demagnetize and afterwards magnetize with another polarity.

If you perform the above you will notice that the magnetized iron does not want to stay in the coil exercising considerable force. If the same happens with the earth magnetic poles it could well be that we have a few little earthquakes along with it.

atom286,

It is fascinating, yes. But the pole flip is not unexplained. The Earth's magnetic field is dynamic, it is not at all like the static field of a bar magnet. It's more like the magnetic field of an electromagnet. And, as I'm sure you know, to instantly reverse the field in an electromagnet, one need only instantly reverse the current.

Of course, it's not that simple with the earth's magnetic field flipping. That's more about varying portions of the field collapsing which causes other portions to be less restrained. The north-south orientation of the Earth's magnetic field is really only a fortunate circumstance. The orientation could as easily be along any other diameter of the Earth. The truth is, the Earth's field has some what we would call "south poles" not far from the geographic north pole. The whole field is constantly in flux and changing around, but only on a local level. The north-south orientation is how the average of all the different orientations come out. If you look much much more closely, you'll see north "poles" and south "poles" all over the Earth.

Harte

which is the reason why magnetic compasses have deviations in changing magnitudes depending on where on the globe they are situated.