I've actually been aware of this for some time, although probably b/c I was a geology major. It's well-documented that the magnetic polarity of the earth flip-flops every so often and, when it does so, the magnetic field that protects us from UV rays (among other things) pretty much disappears. There are rocks that have magnetic crystals in them that set into place when the rock forms/cools. By looking at the crystals, you can tell what the earth's magnetic polarity was at the time. When you look at the rocks across the Atlantic Ocean floor, the crystal orientation occasionally completely reverses (with little to no intermediate orientations). By knowing the age of the rocks, geologists can determine how often the magnetic poles reverse. It's pretty fascinating stuff, especially if you're a dork like me.
Geologically, the polarity reversal occurs pretty quickly, but it probably takes in the 100s to 1000s of years. It is reasonably suggested that we are entering into a polarity reversal, which, of course, has all kinds of interesting repercussions - not the least of which being increased difficulty in bushwhacking in the Whites!
