Handout #19 - Pole Wandering and Pole Reversal
Scientists believe that the positions of the magnetic poles wander over long periods of time (thousands of years) primarily because of irregularities in the motions of the liquid outer core of the earth. In addition, the solar wind causes daily and short-term shifts of up to 80 km (50 miles). The magnetic field of the Earth deflects the solar wind, and in doing so, is distorted by the charged particles that make up the solar wind. The distortion of the magnetic field is the cause of the magnetic pole shifts. Scientists have also discovered that the Earth’s magnetic field occasionally reverses direction. Roughly every 500,000 years, the magnetic field changes direction, or reverses polarity, and the N and S poles switch places. The change is gradual, and is estimated to take between one and five thousand years to complete. When this happens, the north-pointing needle on a compass will point in the opposite direction (now towards the south pole). At least 171 reversals have occurred during the past 75 million years.
We know this because geologists have found evidence of the past orientation of Earth’s magnetic field in ice core samples from Antarctica and in the expanding continental plates at the mid-Atlantic ridge. In both cases, the process that deposited the ice or the rock did so in the presence of the Earth’s magnetic field. Small bits of magnetized rock were solidified into place, all oriented in the Earth’s magnetic field, and stayed solidified even later when the Earth’s field reversed. Today we can look at the record of the Earth’s past magnetic field by observing where in the ice or rock the magnetic field changes.