The Earth’s magnetic field reverses polarity every 250,000 years, with the last shift occurring 790,000 years ago. The magnetic field protects the planet from solar radiation and plays a vital role in climate, weather patterns, and animal migration. Evidence for these changes comes from the seabed along the Mid-Atlantic Ridge. Some scientists believe a pole shift is already underway due to a decrease in the magnetic field’s strength, but the actual mechanisms are still unknown. The process takes anywhere from 1,000 to 28,000 years, with compasses at the poles taking twice as long to change direction.
A pole shift refers to the Earth’s magnetic field reversing its polarity. If a magnetic reversal were to occur today, compasses would point south instead of north.
Over the past 15 million years, scientists have found that these changes have occurred four times every 1 million years. While this averages once every 250,000 years, the changes do not occur at regular intervals. During a Cretaceous period, the polarity remained constant for up to 30 million years, although this is thought to be an anomaly. The last pole shift occurred 790,000 years ago, leading some scientists to believe that Earth’s due, while others speculate that a reversal is already underway.
The dynamic processes that occur in the depths of the planet generate the earth’s magnetic field. A molten iron core surrounds the solid iron inner core, each rotating at different speeds. Their interaction, and possibly other as-yet-ununderstood geophysical processes, creates what scientists call a “hydromagnetic dynamo.” This self-perpetuating electric field acts somewhat like a giant bar magnet.
The Earth’s magnetic field extends into space for tens of thousands of kilometers from the planet’s poles. Not only does it protect the planet from solar radiation, but it plays a vital role in overall climate, weather patterns and animal migration habits. If the poles were to reverse instantaneously, the destruction would be global, from earthquakes and volcanic eruptions to melting Arctic ice and widespread flooding. Evidence suggests that pole shifts occur gradually, however, taking anywhere from 1,000 to 28,000 years. The last four flip-flops each took about 7,000 years.
Evidence for these changes came unexpectedly in the 1950s during exploration of the seabed extending along the Mid-Atlantic Ridge. Here, the molten material bubbles up, cools and hardens, creating new marine crust, pushing the old crust outward. The magnetic particles or iron oxides in the lava act like tiny compass needles, aligning themselves with the magnetic field, leaving a permanent record of Earth’s polarity as the crust is created. By reading the orientation of the oxides at various distances from the point of extraction, scientists can “look back in time”. What they found was alternating streaks or bands – periods of reversal throughout history.
Some researchers believe a pole shift is underway today because the magnetic field has decreased in strength by up to 10% to 15% over the past 150 years, with the rate of decay increasing more significantly in recent years. If this trend continues, the magnetic field will disappear in 1,000 to 2,000 years. A weakened magnetic field is a precursor to displacement, although it is recognized that the current decay could also be attributable to other unknown causes or could reverse itself.
In the case of a pole shift, once the magnetic field has weakened sufficiently, the directions of the field reverse nearly 180° 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 it does at the equator. So while compasses at 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 underlying this phenomenon are still unknown. Some theories suggest that comet impacts could play a role; others, that the magnetic field is inherently prone to flip-flop. Conclusive answers await a better understanding of the dynamics of this very fascinating geophysical phenomenon.
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