Lava Flows Give Clues About Magnetic Field Reversals

Scientists know that Earth’s magnetic field reversed it’s direction hundreds of times before, but so far they did not know why. But a recent study made on ancient lava flows may give us a better understanding of what happens and why.

First of all, what is the Earth’s magnetic field?

Earth’s magnetic field (and the surface magnetic field) is approximately a magnetic dipole, with one pole near the geographic North Pole and the other near the geographic South Pole. The earth’s outer liquid core is mostly made up of iron that convects very rapidly, acting as a “dynamo” that generates a magnetic field. An imaginary line joining the magnetic poles would be inclined by approximately 11.3° from the planet’s axis of rotation. The locations of the magnetic poles are not static but they wander as much as 15 km every year.

This magnetic field reversed at intervals ranging from tens of thousands to many millions of years, with an average interval of approximately 250 000 years. The latest reversal of the magnetic poles is called the Brunhes-Matuyama reversal and has occurred some 780 000 years ago. If the magnetic field reverses, a compass needle will now point south rather than north.

In study made on ancient volcanic rocks, published in the Sept. 26 issue of the journal Science, Brad Singer (a geology professor at the University of Wisconsin-Madison) states that a second magnetic field that may originate in the shallow core just below the rocky mantle layer of the Earth may play a crucial role when the main north-south field weakens. This main field weakens prior to a reversing.

Singer, along with Paleomagnetist Kenneth Hoffman, have analyzed ancient lava flows from Tahiti and western Germany in order to study past patterns of the Earth’s magnetic field. The magnetism of iron-rich minerals in molten lava orients along the prevailing field, then becomes locked into place as the lava cools and hardens. Hoffman has been researching Earth’s magnetic field reversals for over 30 years and is a professor at both California Polytechnic State University at San Luis Obispo and UW-Madison.

They have shown that some rock’s ages show that the main field has weakened multiple times during the past million years and magnetic field has attempted to reverse its direction. Singer states that during those attempts “If you were on Tahiti when those eruptions were taking place, your compass needle would point to not the North Pole, not the South Pole, but Australia”.

“When the lava flows erupt and cool in the Earth’s magnetic field, they acquire a memory of the magnetic field at that time,” says Singer. “It’s very difficult to destroy that in a lava flow once it’s formed. You then have a recording of what the paleofield direction was like on Earth.”

The scientists believe that the shallow core can help them determine if the magnetic field weakens only prior to a reversing, or if it is able to recover its strength without a reversal. Mapping this field during transitional states may hold the key to understanding what happens in Earth’s core when the field weakens to a point where it can actually reverse,” Hoffman says.

“Right now, historic records show that the strength of the magnetic field is declining very rapidly. From a quick back-of-the-envelope prediction, in 1,500 years the field will be as weak as it’s ever been and we could go into a state of polarity reversal,” says Singer. “One broad goal of our research is to provide some predictive capability for what could happen and what could be the signs of the next reversal.”

Current evidence shows that the magnetic field will be reversing its direction in the next couple millenniums, but further studies are to be taken in order to provide a correct prediction.