The thermal and compositional currents moving within the liquid outer core, coupled with the Earth’s rotation, produce electrical currents that are responsible for the Earth’s magnetic field. The shape of the magnetic field is similar to that of a large bar magnet. The ends of the magnet are close to, but not exactly at, the geographic poles on Earth. The north arrow on a compass, therefore, does not point to geographic north but, rather, to the magnetic north. The magnetic field plays a role in making the Earth hospitable to humans. Solar wind sends hot gases called plasma to Earth, and the magnetic field deflects most of this plasma. Without the work of the magnetic field, these damaging rays would harm life on the planet. As the solar wind approaches the Earth, the side of the Earth’s magnetic field closest to the Sun gets pushed in, while the magnetic field on the opposite side away from the sun stretches out (Figure 2.5). You may have heard of the Aurora Borealis or “Northern Lights.” Solar storms can create disturbances within the magnetic field, producing these magnificent light displays (Figure 2.6).
The magnetic field changes constantly and has experienced numerous reversals of polarity within the past, although these reversals are not well understood. Study of past reversals relies on paleomagnetism, the record of remnant magnetism preserved within certain rock types. Iron-bearing minerals that form from lava can align with the Earth’s magnetic field and thus provide a record of the magnetic field in the Earth’s past. However, this preserved magnetism could be lost if the mineral in the rocks has not been heated above a temperature known as the Curie point (a temperature above which minerals lose their magnetism). Essentially, the iron atoms “lock” into position, pointing to the magnetic pole. This records the alignment of the magnetic field at that time (we currently are in a normal polarity, in which north on a compass arrow aligns closely with geographic north, or the North Pole). If the magnetic field was stationary, all of the magnetic minerals would point in the same direction. This is not the case, however. Reversals occur rather frequently on the geologic time scale.
Not only do magnetic poles reverse over geologic time but they also wander. Paleomagnetic data show that the magnetic poles move systematically, wandering across the globe. Polar wandering curves have been created to display the migration of the poles across the Earth’s surface over time. Apparent polar wander refers to the perceived movement of the Earth’s paleomagnetic poles relative to a continent (the continent remains fixed) (Figure 2.7). As you will learn in the Plate Tectonics chapter, polar wandering curves provide excellent evidence of the theory that the plates move, as curves for different continents do not agree on the magnetic pole locations. They all converge on the current pole location at present day, however.