Resource: See a Reversal

Although early explorers relied heavily on their compasses for navigation, they didn't know how or why they worked. In fact, before 1600, most thought compass needles pointed to a particular star, rather than at the North Pole. A geographer named William Gilberts finally tied magnetism directly to the Earth, theorizing that our planet creates its own global magnetic field along which compass needles align themselves. That view is still held today. In fact, scientists now know that the Sun and probably all of the planets in our solar system and many of their moons also create magnetic fields.

The magnetic field created by the Earth is similar to fields created by simple bar magnets. However, in addition to being infinitely larger than simpler magnetic fields, Earth's magnetic field also differs from them in that it undergoes dramatic changes in its intensity and direction over time. This variability is probably a reflection of the source of the magnetic field: electric currents generated by Earth's outer core. Because the outer core is a liquid, the nature of those currents changes over time, and so too does the resulting magnetic field.

The discovery that Earth's magnetic field can change polarity was first made in the early 1900s when scientists found rocks whose polarity was opposite that of Earth's present polarity. They surmised that rocks permanently record what the Earth's polarity was at the time that they were formed. Indeed, lava flows on the surface of continents and along the ocean floor show an alternating pattern of magnetic polarity that corresponds to their age. This rock record provides an accurate history of polarity reversals dating back some 300 million years. In fact, Earth's inconstant magnetic field is not unusual in the universe. It is likely that any rotating body, such as the planet Jupiter or even the Milky Way Galaxy, may experience magnetic field reversals. For example, regular changes of polarity can be seen in the Sun's magnetic field approximately every 11 years.

Earth's long record of rock magnetism has revealed that its magnetic reversals have occurred, on the average, approximately every 250,000 years. Interestingly, the Earth has held its present polarity for the past 730,000 years. Some scientists think we are long overdue for a reversal. However, the paleohistorical rock magnetism record demonstrates that no two polarity reversals seem to occur in exactly the same way. For this reason, ascribing a physical mechanism for magnetic reversals and predicting reversal events is extremely difficult.

The computer simulation presented in this resource is based on a single set of parameters and illustrates just one way in which Earth's polarity can reverse. The geological record shows that the intensity and duration of polarity shifts can vary dramatically from place to place and from event to event. Only time will tell if a reversal is in our near future.

To learn about magnetic fields generated by the Sun, check out Solar Magnetism.

• Why does a compass needle point to the north?
• How does the composition of the interior of the Earth affect the magnetic poles?
• In the computer simulation, what does each color, blue and orange, represent? What does each oval represent? Do the patterns of blue and orange in the two ovals seem to be connected or related? Why might this be so?
• How do scientists know that the magnetic field reversal occurs on average every 250,000 years?
• When was the last magnetic field reversal? Are we due for another one?
• Even though a reversal may take hundreds of years, imagine if one occurred in the next decade. Try to describe any consequences of this reversal and how it would affect the world as we know it.