Pole Shift
Geology tells us that the magnetic fields of the earth have changed many times over the life of the planet. We know this because the iron in lava maintains its magnetic direction as the lava cools and hardens, so lava flows contain a history of the magnetic poles of the earth. The last magnetic pole flip occurred 740,000 years ago.
The sun also undergoes magnetic pole flips, and they happen every 11 years at the peak of the sunspot cycle. The next expected magnetic pole flip of the sun is in 2012; there's that date again! Could the sun's pole reversal help push the earths reversal if the earth's field was already unstable?
Some believe that the magnetic field strength would decrease to zero during the transition time of the pole shift. If that indeed happened, it would be extremely bad for life on the planet, as the magnetic field acts as a shield protecting us from lethal radiation. Of course some would survive, but the vast majority of people and other life forms would perish.
Physical pole shift would be much more violent, and destructive. The physical pole shift theory says that due to some impetus, like a redistribution of polar ice, the earth's crust suddenly slips on the liquid core of the earth. The polar regions may rotate to the equator, and the equator may migrate to the polar regions causing gross changes in the climate of earth. The energy required to do this in a rapid time frame would be enormous, as all of the land mass, and all of the oceans would be in motion. Huge devastating tidal waves would overwhelm the land, and enormous winds (air tries to remain in place) would scour the earth destroying everything in its pass. There is some evidence that this has happened, as deep underneath the ice in Prudhoe Bay Alaska, oil wells have brought up ferns and tropical plants. Also, the woolly mammoths in Siberia were found quick frozen showing the climate changed so quickly that the animals still had tropical vegetation in their guts.
On the other hand, if physical pole shift happened over a period of millennia, no one would probably notice except by scientific measurements.
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