Earth's Magnetic Pole Reversals

Podcast transcript

Once every 450,000 years or so, the Earth undergoes a radical transformation.

The planet’s magnetic field will literally flip. The north pole becomes the south pole and vice versa.

Despite the fact that we know this has happened many times in the Earth’s history, we really don’t know what would happen if the poles were to reverse today.

Learn more about when the Earth’s magnetic poles reverse on this episode of Everything Everywhere Daily.

Before I get into the details of how and why the Earth’s magnetic poles will reverse, it is important to understand how geologists know that the Earth’s magnetic poles have reversed in the past.

Many rocks are made up of magnetic minerals, usually magnetite or the more weakly magnetic hematite, both of which are based on iron.

When a rock first forms from liquid magma, the individual magnetic molecules will align themselves with the Earth’s magnetic field. As the rock solidifies, the individual mineral molecules are then locked into place with this particular magnetic orientation.

In the late 19th and early 20th centuries, geologists began to notice that some rocks were magnetized, opposite to the Earth’s magnetic field. The first person to notice this was a French geologist by the name of Bernard Brunhes.

In the 1920s, the Japanese geologist Motonori Matuyama suggested that the cause of the oppositely orientated rocks was due to the Earth’s magnetic poles having been reversed.

While Matuyama and Brunhes suspected that the Earth’s poles had switched, they didn’t know when it happened or how many times it had happened. The best guess was that it happened millions of years ago.

In the 1950s and 60s, there were two developments that helped solve this puzzle.

The first of which was the development of radiometric techniques for dating rocks, which I covered in a previous episode. With these techniques, it became possible to date when most rocks were formed to a reasonable degree of accuracy.

The second development was the magnetic mapping of the sea floor. This was the real breakthrough.

What was discovered is that the on sea floor on either side of a mid-ocean rift are parallel and symmetrical bands where the magnetic orientation of the rock would be one way and then another.

For example, in the mid-Atlantic Rift, which runs the length of the Atlantic Ocean, immediately on either side of the rift, the rocks are aligned with the current magnetic field of the Earth.

Then, just beyond that, there is a stripe of rocks that are magnetized the other way, then there is another stripe beyond that, etc., etc. The width of each strip is the same on either side of the rift.

The reason for this is that a rift is where the tectonic plates are slowly pulling apart from each other. Liquid magma from deep in the Earth comes up in the rift and cools to form new seafloor.

As this new rock solidifies, the magnetic minerals inside the rock orientate themselves to the Earth’s magnetic field.

This process is continuous, and over millions of years, as the plates move apart, the rock gets further and further away from the rift as new rock keeps forming.

The magnetic stripes on the seafloor are the frozen magnetic record of what the Earth’s magnetic field was like when the rocks formed.

The current estimate is that there have been 183 magnetic pole reversals that have taken place over the last 83 million years. This means that they occur on average once every 450,000 years.

As the Earth is much older than 83 million years, there have been many more pole reversals than just 183. However, we don’t have the evidence to identify them.

While the average amount of time between pole reversals is 450,000 years, that doesn’t mean there is some sort of geomagnetic clock inside the Earth that flips the poles on some sort of schedule.

It is believed to be a fundamentally random occurrence. There is evidence of some pole reversals taking over ten million years and some which occurred after only a few centuries.

In addition to permanent long-term changes in the Earth’s magnetic field, there are also short-term changes that have been identified, known as Geomagnetic excursions.

Geomagnetic excursions only last about a few thousand years and are associated with a weakening of the Earth’s magnetic field by up to 20% and a significant change in the location of the magnetic poles by up to 45°.

Due to the brief geologic timescales that geomagnetic excursions occur, they are difficult to track in the deep past. However, since the last major magnetic reversal, which occurred 780,000 years ago, there are believed to have been 12 geomagnetic excursions that have taken place. The most recent, having occurred about 42,000 years ago.

So, if we have evidence that the Earth’s magnetic poles have reversed, why does this happen?

It has to do with the outer core of the Earth, which is a liquid consisting mainly of iron and nickel. The movement of the liquid due to convection creates a dynamo which creates electric currents and a magnetic field.

Fluid dynamics are inherently complex. So complex that when Albert Einsteins’ son, Hans, told his father he wanted to go into hydraulic engineering, he supposedly told him not to because it was too difficult.

There have been computer simulations of the Earth’s core which have been run for tens of thousands of simulated years, and the simulations have produced field reversals.

Likewise, experiments have been run with liquid metals, and they, too, have shown random reversals in magnetic polarity.

Basically, a liquid dynamo producing a magnetic field is much more unstable than a simple bar magnet that we might be used to.

Others have theorized that pole reversals are not spontaneous events, but rather they are caused by something which disrupts the liquid in the core. This could be anything from deep subduction of tectonic plates to a large meteor impact on the surface.

While the evidence for magnetic pole reversals is very strong, there are still some questions that remain.

The biggest one is how long does it take for the poles to switch. There are some estimates that claim a reversal takes as long as one thousand to ten thousand years.

Other estimates claim that a reversal can be completed in as little as a few decades.

The fact is, we really don’t know because modern humans have never experienced a pole reversal before.

The other really big question is what it would be like to live through a polar reversal and how it would affect life on Earth.

The Earth’s magnetic field is what protects the planet from the solar wind and most cosmic rays. As these high-energy particles approach the Earth, they are deflected by the magnetic field, protecting the planet from radiation.

There are some who argue that because there have been so many pole reversals throughout history the impact to the Earth’s biosphere can’t possibly be that great. The major extinction events in Earth’s history don’t align with magnetic pole reversals.

However, you will see some who make the exact opposite argument. In particular, the demise of the Neaderthals coincided with the last geomagnetic excursions about 42,000 years ago.

However, there hasn’t been 183 mass extinction events in the last 83 million years, so at best, such events could only align with a few pole reversals. Moreover, even if the last geomagnetic excursions had something to do with the extinction of the Neaderthals, it clearly didn’t wipe out humans.

We do know that the process of a pole reversal would certainly cause disruptions.

For starters, many animals, in particular birds, rely on the magnetic field of the Earth to navigate when they migrate. We aren’t sure how this would affect them or how quickly they could adapt.

If the magnetic field was weakened, it could potentially be devastating to the ozone layer. If the ozone layer were weakened, it would result in a significant increase in ultraviolet rays reaching the surface.

A weakening of the Earth’s magnetic field would also cause havoc with electronics in something like a widescale Carrington Event, which I covered in a previous episode. Many satellites, which are also protected by the Earth’s magnetic field, would also be rendered inoperable.

Some geologists have also speculated that after a pole reversal takes place,, it could result in increased vulcansim across the planet.

The really big question is when the next magnetic pole reversal will happen.

It certainly will happen, but we have no clue when it will happen. You can find people saying that because they happen on average every 450,000 years, and because it has been 780,000 since the last one, we are due.

However, that is basically the gambler’s fallacy in action. We aren’t “due” for a pole reversal any more than a roulette wheel is “due” to land on red after it landed on green three times in a row.

That being said, things are happening with the Earth’s magnetic field.

Since the invention of the magnetometer in the 1830s, the strength of the Earth’s magnetic field has decreased by about 10 percent.

In particular, there has been a large reduction in the strength of the magnetic field in an area known as the South Atlantic Anomaly, which I covered in a previous episode.

Finally, the magnetic poles have been wandering at a dramatic rate. The magnetic north pole now moves at a speed of 55 kilometers per year.

So, something is happening, but due to the chaotic nature of the magnetic dynamo in the core, it is impossible to tell if this is part of a geomagnetic excursion, the prelude to a pole reversal, or just a part of the natural fluctuations of the magnetic field.

Ultimately, whatever happens, whenever it happens, how it affects the planet will depend on how long it takes and how weak the magnetic field gets during the transition.

A magnetic pole reversal isn’t something I would stay up at night worrying about. The Earth has been through this many times before, and even if we are in the middle of something right now, it will take decades, if not millennia, to play out.