It is as constant as the North Star: No matter where a person is on Earth, a compass always points to the Earth's magnetic North Pole. But as scientists have known for a few decades, that has been true only since about 790,000 years ago, when the Earth's magnetic field pulled a 180-degree turnaround. Before that, had people been around to try, compasses would have pointed south.
Samples of geologically ancient rocks show that the Earth's magnetic field has flipped hundreds of times, sometimes 20,000 to 30,000 years after the previous flip and sometimes after periods of stability that lasted as long as 50 million years. Geologic evidence in the form of magnetic rocks has suggested that these reversals, once they begin, happen relatively quickly-taking between 1,000 and 28,000 years to complete. But scientists have had a difficult time pinning the changeover period any more narrowly than that.
Now Florida International University earth scientist Bradford Clement has found an explanation for the disparity: The time it takes for reversals to happen depends on where on Earth it is measured. For example, the most recent reversal took about 2,000 years to become complete near the equator, but about 10,000 years at mid- and high latitudes.
Clement's conclusion, described in the April 8 issue of Nature, is based on his analysis of rock samples obtained from deep-sea drilling projects. Researchers said the recognition that field reversals take longer nearer the poles than near the equator could help them understand the still largely mysterious process that generates the magnetic field.
Current theories hold that it is the result of electrical currents in the planet's iron-rich liquid outer core-and convection currents in that liquid rock. But the details of how that works, which stumped even Albert Einstein, remain largely opaque to this day.
