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To understand magnetic-field reversal, you first have to explore the nature of Earth's magnetosphere. Theories as to its source range from the basic to the more complex. We'll try to keep our explanation short and sweet. Currently, most scientists believe that the magnetosphere is predominately generated by electrical currents deep within Earth's iron core. The heat generated there causes churning in the liquid outer core, which in turn produces a swirling whirlpool around the North and South Poles. This flowing current produces forces that extend beyond our planet and into outer space. Called
Van Allen Radiation Belts, they run along the magnetic north and south poles, which are constantly in flux. These opposing tides of magnetism stretch with the solar winds, forming a magnetotail. External solar forces, internal plate tectonics, and geothermal activity all push and pull on our planet. Generally speaking, scientists believe it is the combination of these forces that causes the magnetic field to shift
in midstream, so to speak. The best evidence of magnetic pole shifts are preserved in the geological record. By studying lava fields in places like Hawaii, scientists "guesstimate" that the most recent shift occurred 780,000 years ago. Beyond that, no one knows how long a shift takes -- could be thousands of years, could be overnight! (Although we think Alaskan salmon may know something we don't...)
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