A pole shift refers to the Earth's magnetic field reversing its polarity. If a magnetic reversal occurred today, compasses would point south rather than north.
In the past 15 million years, scientists found that these shifts occurred four times every 1 million years. Though this averages out to once every 250,000 years, switches do not occur at regular intervals. During one period in the Cretaceous, polarity remained constant for as long as 30 million years, though this is believed to be an anomaly. The last pole shift took place 790,000 years ago, causing some scientists to believe that the Earth is due, while others speculate a reversal is already underway.
Dynamic processes taking place deep inside the planet generate Earth's magnetic field. A core of molten iron surrounds the inner core of solid iron, each rotating at different rates. Their interaction, and perhaps other geophysical processes not yet understood, creates what scientists call a "hydromagnetic dynamo." This self-perpetuating electric field acts in some ways like a gigantic bar magnet.
The Earth's magnetic field extends into space for tens of thousands of miles from the planet's poles. It not only protects the planet from solar radiation but plays a fundamental role in overall climate, weather patterns, and migratory habits of animals. If the poles were to reverse instantly, destruction would be global, from earthquakes and volcanic eruptions to melting of Arctic ice and vast flooding. Evidence suggests that pole shifts happen gradually, however, taking anywhere from 1,000 - 28,000 years. The last four flip-flops took about 7,000 years each.
Evidence for these shifts came unexpectedly in the 1950s while exploring seafloor spreading along the mid-Atlantic ridge. Here, molten material wells up, cools and hardens, creating new sea crust, pushing the old crust outwards. Magnetic particles or iron oxides in the lava act like tiny compass needles, aligning themselves with the magnetic field, leaving a permanent record of the Earth's polarity at the time the crust is created. By reading the orientation of the oxides at various distances out from the point of welling, scientists can "look back in time." What they found was striping or alternating bands — periods of reversal throughout history.
Some researchers believe a pole shift is underway today because the magnetic field has decreased in intensity as much as 10% - 15% over the last 150 years, with the rate of decay increasing more significantly in recent years. If this trend continues, the magnetic field will be gone in 1,000 to 2,000 years. A weakening magnetic field is a precursor to the shifting, though it's acknowledged the current decay might also be attributable to other unknown causes, or might reverse itself.
In the case of a pole shift, once the magnetic field weakens enough, the field directions undergo a near-180° switch before strengthening and stabilizing in the new orientation. Scientists don't really know how long this process takes, but what is known is that it takes twice as long at the poles as at the equator. So while compasses at the mid-latitudes might point south after a 3,000-year transition, compasses at the poles would continue to point north for another 3,000 years.
The actual mechanisms behind this phenomena are still unknown. Some theories suggest comet impacts might play a role; others, that the magnetic field is inherently prone to flip-flops. Conclusive answers await a better understanding of the dynamics of this very fascinating geophysical phenomenon.