GEOPHYSICS

Intro: One of the properties of our planet that we have struggled to explain is the fact that its magnetic field occasionally reverses. If this happened today, it would mean that our compasses would point to the south, rather than to the north.

A reversal in the magnetic field is probably not something we need to get overly worried about because the last full one occurred 780,000 years ago, while a partial reversal, known as a geomagnetic excursion, last happened 41,000 years ago – when the poles reversed for a few hundred years before flipping back again. The generally accepted theory of how the Earth’s magnetic field is generated states that heat from the solid inner core of the planet causes chaotic and swirling convection currents in the liquid outer core, and, as it is predominantly composed of magnetised iron, this rotational movement works like a giant dynamo, inducing a moving electric current, together with its accompanying magnetic field. The action of this geodynamo, as it is known, is thought to lead to the polarity of the planet and to maintain the magnetosphere, the region of space around the Earth to which the magnetic field extends.

The possibility of the magnetic field reversing was first proposed in 1906 by the French geologist Bernard Brunhes, after he had studied iron minerals in volcanic rocks from Auvergne, the region of central France well-known for its numerous extinct volcanoes. This was based on an anomaly he observed, in which some crystals of magnetic iron minerals in the volcanic rocks are orientated either to the north or south. Shortly afterwards, the Japanese geophysicist Motonori Matuyama carried out a systematic study of volcanic basalt rocks in different locations in Japan and China, which demonstrated that rocks in the same geological layers – ones that had been laid down at the same time – showed the same polarity, described as normal where iron minerals are oriented to the north, and reversed in those pointing south.

The volcanic landscape of the Massif Central in the Auvergne, France.

Matuyama’s work provided clear evidence to support the theory that the poles had reversed in the past, but it did not receive any great attention until the 1950s, when radiometric methods of dating rocks based on the decay of radioactive isotopes were developed, which allowed a chronology to be worked out. The pioneers of this field were later recognised, with their names being assigned to the periods, known as chrons, of normal or reversed polarity. We are currently in the Brunhes Normal Chron, which began 780,000 years ago, and this was preceded by the Matuyama Reversed Chron, beginning 2.59 million years ago, while the period during which the flip took place is called the Brunhes- Matuyama Transition. It used to be thought that this flip occurred over the course of thousands of years, but recent research, published in 2014, suggests that it could have been made quicker, perhaps taking as little as 100 years.

The Flipping Field

We don’t know what causes a reversal in polarity and may well have to wait until it happens again before we have the opportunity to study the phenomenon in enough detail to find out. We currently lack a clear enough understanding of what is happening in the outer core and mantle to generate the magnetic field in the first place, let alone know why it flips. Past reversals have occurred over an apparently random time frame, so it is impossible to predict when the next one will be. A gradual weakening of the magnetic field recorded over the course of the last century has led to some speculation that we are entering a transitional period, but, as we don’t know anything about the processes leading up to reversals, there is no way of knowing if this is really the case. A reversal may be beginning right at this moment, or it may not happen for hundreds of thousands of years.

One way of investigating reversals in the Earth’s polarity is to construct computer models of the way in which the Earth’s magnetic field is generated by the dynamo in the inner core, and then run simulations to see what happens. This involves attempting to recreate the interaction between the heat generated in the inner core and the convection currents thought to be the source of the magnetic field in the outer core, which, as we don’t fully understand what is going on in either region, is extremely difficult. One simulation developed by Gary Glatzmaier and Paul Roberts at University College, Los Angeles, in the 1990s, uses a complex set of equations, involving thermodynamics and fluid motion, to describe the physical properties of the geodynamo. It was found to provide an accurate model for the generation of known variations in the magnetic field, and, when run to simulate the changes occurring in polarity over hundreds of thousands of years, showed the process of reversal occurring on a number of occasions. The timing of the reversals was random, and apparently caused by the development of a particular set of circumstances, in which the thermodynamics and fluid motion evolved with the generation of the magnetic field in such a way as to weaken the strength of the poles. If the strength of the poles dropped below a certain point, this caused a reversal.

The Impact of Reversals

If computer-generated models accurately simulate what is happening in the outer core, and reversals are indeed caused by a weakening in the magnetic field, then this has implications for the ability of the magnetosphere to deflect potentially harmful high-energy particles found in cosmic radiation. If the magnetic field were to disappear completely, the planet could also be exposed to solar wind. This is thought by some scientists to have occurred on Mars, which does not have a magnetic field, and thus any atmosphere that may have existed would have effectively blown away. Needless to say, this would be disastrous for our planet, but as there have been numerous reversals in the past and the Earth still has an atmosphere, it is reasonably safe to assume that this scenario is not very likely to happen here.

Studies of transitional periods that lead to reversals and their impact on life on Earth have actually found nothing to suggest any harmful effects. There is, for instance, no correlation between the timing of reversals and extinction events or periods of increased seismic and volcanic activity. So it would appear that, beyond the disruption it would cause to our navigational systems, and the possibility of interference with some communications, we don’t have a great deal to worry about. There could be an impact on animals that make use of the magnetic field to navigate, though it would appear that reversals usually happen over the course of long enough periods to allow them to adapt. In the unlikely event of a flip suddenly happening tomorrow, aircraft may have to be grounded while we work out how to navigate, our phone services could be interrupted for a while, and homing pigeons might get rather confused.

Alternative Theories

In recent years, seismic images of the Earth’s inner core have been interpreted as showing it to be composed of slightly differing eastern and western hemispheres. One theory, known as translational instability, suggests that this difference is due to the growth of the core, caused by its cooling, being lopsided – with more iron crystallising out on the surface of the western side than on the eastern one.

In research published in 2012, Peter Olson and Renaud Deguen of Johns Hopkins University in Baltimore, Ohio, set-out to test this theory, by modelling what would happen to the magnetic field if the inner core were lopsided. They found that the axis of the magnetic field in the model shifted to the side that was growing, which led them to speculate that this change in the axis in the inner core may cause irregular convection patterns in the outer core, which could be responsible for reversals in the magnetic field. They also thought that the position of the axis in the inner core could be the reason why magnetic north is not the same true north – the Magnetic North Pole currently being off the coast of Canada, about 480 km (300 miles) from the Geographic North Pole. If this is correct, then tracing the movement of the Magnetic North Pole over time would give an indication of the way in which the inner core was growing, and perhaps would even show if a reversal in the magnetic field were likely to occur.