Up and down! What?

What happens when magnetic reversals occur? This is what I asked my professor during my first year at university when he mentioned the “reversals of the Earth’s magnetic field” and I remember that joking he answered: we will be “up and down”!

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Figure 1: Cartoon of a man being up and down.

Well, this would be an interesting experience but instead of us, our compass needle is reversing! This may sound boring but there is a lot more around this phenomenon. No human on Earth has experienced a magnetic reversal as we are too young for that (the latest one occurred 780,000 years ago); however there is evidence that it happened many times in the Earth’s history. Its discovery has played a major role in the development of Earth Science.

What are these magnetic reversals?

Have you ever heard the term “animal magnetism”? Sometimes we use it in a figurative way to mean the attraction between people. Actually, the word magnetism refers to physical phenomena arising from the force caused by magnets, objects that produce magnetic fields that attract or repel other magnetic objects.

Our Earth, like a magnet, has a magnetic field, a region of space where it exhibits a magnetic influence, which is defined by a series of lines converging at the so called magnetic poles (Figure 2). This field, according to the dynamo theory, is generated by the motion of molten iron-nickel alloys in the Earth’s outer core. At the Earth’s surface the geomagnetic field can be roughly approximated by the field of a magnetic dipole positioned at the center of the Earth, tilted at an angle of about 11.5° with respect to the rotational axis of the Earth. Like a dipole field it has a North and a South Pole which can flip during what is called a “magnetic reversal”. During geological times, the Earth’s field has alternated between periods of normal polarity, in which the direction of the field was the same as the present direction, and reverse polarity, in which the field was the opposite. South at North, North at South!

How does it happen?

First of all, when we think of the Earth’s magnetic field as a dipole field we have to consider that this is an approximation, valid at the Earth’s surface, but if we could make a “journey to the center of the Earth” with special glasses to see the invisible magnetic field lines, it would look much more complicated. Although this could sound strange, the magnetic poles are not diametrically opposed to one another and they move almost all the time, even though we do not feel it. Recent surveys on the wandering of the Geomagnetic poles suggest that the North magnetic Pole is moving north-northwest at c.a. 50 km per year. Analyses on past magnetic field behavior during a polarity transition (when the direction of the field change) suggest that geomagnetic poles move systematically relative to the Earth’s rotation axis, defining a path from one polar region to the opposite one.

Several models and simulations have been proposed to understand the reversal mechanism and in 1995 a successful simulation reproduced a complete reversal. However, more studies are needed to explain the complexity of the reversal mechanism. As you may notice, the middle figure below (Figure 2b) shows that during the transitional phase, the orange and the blue field lines are mixed and it is really difficult to define two distinct poles where the lines converge as before (Figure 2a) and after (Figure 2c) the transition.  

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Figure 2: Magnetic field lines in the Earth’s iron core, during the main phases of a simulated reversal. (Figure from Gary A. Glatzmaier & Paul H. Roberts, “A three-dimensional self-consistent computer simulation of a geomagnetic field reversal,” Nature, 377, 203-209, 1995).

Where is the evidence?

The phenomenon of magnetic reversal has become widely accepted since the discovery of the magnetic stripe pattern, the so-called zebra skin, on the ocean floors (Figure 3). In 1963 these linear magnetic anomalies parallel to the ridges in the oceans, have been interpreted as alternating linear bands of normally and reversely magnetized sections affecting the oceanic crust all along its life from oceanic ridges to subduction zones and they are now recognized as direct evidence of the creation and spreading of oceanic crust from the mid-oceanic ridges.

magnetic stripes = geomagnetic field reversals + spreading of oceanic crust

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Figure 3: Magnetic stripes on the ocean floor and spreading: a) The spreading ridge about 5 million years ago; b) About 2 to 3 million years ago; c) Present-day (from Wikipedia)

When magma from the ridge starts to harden into rock, iron-rich minerals such as magnetite solidify and record the direction of the existing geomagnetic field during formation, even after the magnetic reversal. This information is called “remanence magnetization” and can be measured. Plate divergence then moves the newly formed crust away from the ridge and new fresh volcanic material, hot and molten, comes up along the ridge crest and solidifies with the same mechanism as before. When the polarity changes, a new direction is recorded by the cooling rock. As long as the magnetic field remains constant, the polarity stripe widens. You may think that in this way by knowing the spreading rate, it is possible to define when these reversals happened in the past and you would be correct!

These reversals of the geomagnetic poles leave a record in rocks that is of value to paleomagnetists in calculating geomagnetic fields in the past. This is also helpful in studying the motions and ages of continents and ocean floors in the process of plate tectonics.

These magnetic reversals in terms of geologic time scales are unexceptional events that happen gradually over thousands of years. Records of these events in dated lava sequences and in deep-sea sediments indicate that the duration of the reversal is usually much shorter than the time spent at one polarity, which can last for hundreds of thousands or even millions of years. If the amount of time the world spent in either normal or reverse polarity is one hour, then the time for reversal would be the time it takes to snap your fingers. However, the mechanism that regulates these magnetic reversals is still unclear and this makes them so unpredictable.

Which effects could these reversals have on us?

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Figure 4: A volcanic eruption and Northern Lights in Iceland (Photograph by James Appleton).

There are conflicting studies on the effects that these reversals could have on life. It is unclear what would happen precisely. Many scientists believe that this phenomenon can be associated with an important weakening of the geomagnetic field. Normally the geomagnetic field acts as an obstacle to the solar wind. Some scientists believe that reduced protection from the solar wind could damage the communications systems, have impact on our health and on the climate. Other scientists believe that these reversals happened many times in the past without being lethal for our friend “Tyrannosaurus rex” and that reduced protection could mean a major number of Northern and Southern lights at unexpected latitude. In fact, these colorful and spectacular events are typical for polar regions where the field lines converge favoring the capture of the solar wind particles. And…. why not? During reversal we may observe a wonderful aurora in Sicily while drinking good lemonade with the Mt. Etna in the background.

It would be something similar to what we can observe today in Iceland (Figure 4).

I am sure there are and there will be many inspired scientists working on this topic that will find out some answers, as Einstein said: “The joy of looking and comprehending is Nature’s most beautiful gift”.

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