Paleomagnetism and Polarity Reversals

by

Michael Newton

Spring 2007

Earth's Magnetic Field
Earth's Magnetic Field
(Image taken from http://www.physics.pacificu.edu/hall/astronomy/lecture0312a.html. Used by permission.)
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Magnetism and Earth’s Magnetic Field

The compass has been used to find direction for thousands of years. It is believed to have originated in China in the 13th century B.C. The idea, of course, is that a magnetic needle will align itself with the Earth’s magnetic field and point to one of the two magnetic poles. Magnetic fields are created by electrically charged particles in motion. Electric particles in motion, known as current, were first discovered to create a magnetic field in 1819. Hans Christian Oersted discovered that electric current in a wire deflected a nearby compass.

Hans Oersted
Hans Oersted
(Image taken from http://www-spof.gsfc.nasa.gov/Education/whmfield.html)

Prior to this, in 1600, William Gilbert was the first to suggest that the Earth exerted a magnetic field. Combining these two ideas is what led many to believe that somewhere near the Earth’s core existed large quantities of charged particles. These ideas were combined and, by the 1940s, the specific conditions necessary to create the Earth’s magnetic field were realized.

The Core

The inner core is composed of mostly solid iron, whereas the outer core is composed of liquid or molten iron. The liquid iron provides the first requirement for producing a magnetic field, charged particles. The motion requirement is believed to be driven by a combination of thermal energy, chemical energy, and the rotation of the Earth. This causes the molten core to rotate in multiple spirals or corkscrews. The spin direction of these corkscrews determines the polarity of the generated magnetic field. Physicists know this as the right-hand rule. If you curl the fingers of your right hand in the direction of the current, your thumb points in the direction of the magnetic north pole. Currently, the Earth’s magnetic north pole is located in the direction of the Earth’s geographic south pole. This is referred to as normal polarity, the opposite being reversed polarity.

Prior Reversals

Ocean Ridge
Polarity Strips at an Ocean Ridge
(Image taken from http://pubs.usgs.gov/gip/dynamic/developing.html#anchor10912731)

The Earth’s magnetic poles have reversed multiple times over geolgic history. This is evidenced by magnetic minerals in the rock layers. Mid-Ocean ridges provide a common example of the reversals. After heated material is produced at the ridges, magnetic minerals align themselves with the Earth’s magnetic field. Once the rock cools, the minerals are preserved in their aligned state. Samples gathered from either side of ridges show parallel strips normal and reversed polarities. Prior reversals appear to have no determined length or frequency. The present normal polarity has lasted for approximately 780,000 years.

Future Predictions

The irregularity of reversals makes predicting the next one difficult. All signs point to a fairly complicated system at work, with multiple variables influencing the outcome. It should be noted that the previously stated “corkscrew model” is not perfect. It does not answer all the questions needed to predict future reversals. Some details, previously overlooked here, could be very important in determining a polarity reversal. For example, the changes in boundary conditions between the inner and outer core, as well as the core/mantle boundary could contribute greatly to determining a change in polarity. Because of this, computer models in the past have been fairly inaccurate. Computers simply did not have the calculation power necessary to process all the variables needed. Temperature, for example, was assumed to be a uniform value at different depth levels. This was known not to be true, but since the computer was already factoring in things like density, velocity, and magnetic field intensity, temperature had to be simplified. Recent computer models have shown a more accurate representation of what actually happens.

Computer Model
Computer Model of Field Reverals
(Image taken from http://www.nasa.gov/vision/earth/lookingatearth/29dec_magneticfield.html)

Theories about convection in the core have been improved because of computer modeling. New ideas have emerged as well such as the core rotating faster than the Earth’s surface. A precise prediction of when a reversal will happen next is still uncertain. What is certain is that the overall magnetic field of the Earth has decreased in the last 150 years, and areas of opposite polarity have increased in the last 30 years.

Magnetic Flux
Magnetic Flux
(Image taken from www.bushwalking.org.au/FAQ/FAQ_Navigation.htm
Used by permission - original source Scientific American. April 2005, p 35.)

Also, when compared to the average time between reversals, 780,000 years is longer than average. This all suggests that a polarity reversal is due relatively soon, and no one really knows what will happen when it does.

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