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Magnetic Reversals: Overview

Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Everybody knows that a compass points north. But do you know why? And would you be surprised to learn that it might point south some day? You will learn about our planet's changeable magnetic field in this lesson.

A Less Than Permanent Magnet

The earth's magnetic field allows compasses to be used for navigation. And it protects us from the high-energy particles streaming from the sun. But we now know that our useful and protective magnetic field is a whole lot less constant than we might like.

Earth's Dynamo

The magnetic field is generated in the outer core of the earth. Source: U.S. Geological Survey
Earth structure 2

The magnetic field is created by electrical currents generated by movement of molten iron and nickel in the planet's rotating outer core. That mechanism, called a dynamo, is still being studied, but the movements seem to be caused by interactions at the boundary between the outer core and the mantle.

Scientists recognized early on that the magnetic poles of the earth's magnetic field were not aligned with its rotational axis. But recent research indicates that the field is even more variable, in both strength and position, than once believed due to periodic oscillations in the flow within the outer core.

What is more interesting is that not only do the magnetic poles shift position, they actually switch places. Those events are called magnetic reversals.

Polarity of the Magnetic Field

The magnetic field of Earth has the same north and south poles as a bar magnet. Source: Wikimedia user Geek3, Creative Commons license
bar magnet

Just like the familiar bar magnet, Earth's magnetic field has a north pole and a south pole. These names are actually arbitrary and are used only to distinguish the opposite magnetic force found at each end of a magnet. At the present time, our magnetic field possesses what is known as normal polarity. Simply put, that means it is the polarity of the field that has existed since humans have been using compass needles to point north. The opposite condition is called reverse polarity.

Ancient Magnetic Fields

The polarity of Earth's magnetic field can be determined for times far in the geologic past by studying rocks that contain iron-bearing minerals. Iron atoms (or iron-rich mineral grains) in some rocks align with the magnetic field as the rocks form and preserve information about the field. Igneous rocks, such as basalt, and mud-rich sedimentary rocks are commonly used.

Samples are collected and tested using a variety of techniques referred to as paleomagnetic analyses. The results of the tests provide information about the magnetic field, such as its north-south polarity and orientation.

Polarity Reversals

In the 1950s, researchers measuring the magnetic properties of the igneous rock of the seafloor detected bands of rocks with alternating normal and reverse polarity preserved in them on either side of the Mid-Atlantic Ridge. This discovery confirmed the existence of seafloor spreading but also led scientists to the realization that the earth's magnetic field reversed its polarity rather frequently.

Why and When

There have been 19 reversals in the polarity of the magnetic field in the last five million years. Source: U.S. Geological Survey
magnetic reversals

The question of why the earth's magnetic field reverses polarity is still an open one. Presumably it has to do with changes in the circulation patterns in the outer core. But precisely how that happens and what causes it to happen is not known.

The previous changes do not happen at regular intervals. We do not even know what we should be looking for to tell us that a change is imminent. In fact, we may not have the answer until we live through the next one.

We do have a good idea of when the polarity has changed in the past, however. Studies of the seafloor basalt as well as igneous and sedimentary rocks on land show that the polarity of our planet's magnetic field has changed several hundred times in the past few hundred million years. At least 19 reversals have occurred in last five million years, the most recent one about 790,000 years ago.

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