Here’s something you might not have thought about: The magnetic north pole that compasses point to, near the geographic north pole, is actually the south pole of the Earth’s magnetic field.

That’s why it attracts the north poles of magnets. But that hasn’t always been true.

Today’s compasses would’ve pointed south about 1.1 million years ago, when the geomagnetic north was also a magnetic north pole. But then, the magnetic poles switched places: the magnetic south pole went north, and the magnetic north pole went south. They swapped again a little less than a million years ago, and again about 780,000 years ago, giving us today’s orientation.

Now, if you like doomsday scenarios, you might’ve heard that we’re due for another switch that’ll make electronics useless and kill us all. But that’s a little extreme. And saying that we’re “due” implies that we know a lot about when and why these swaps happen, which we really don’t.

The Earth’s magnetic field comes from electromagnetic induction, where iron moving in the outer core generates an electric current, with the same physics that power plants use to generate electricity. And that electric current creates a magnetic field. As molten rock reaches the surface of the Earth and cools, any iron in it tends to line up with the Earth’s magnetic field.

So the iron in solidified lava acts like a time capsule for what Earth’s magnetic field looked like when it solidified. Some places, like where two tectonic plates have been consistently moving away from each other, have had molten rock coming out and solidifying for millions of years. The farther you get from these divergent plate boundaries, the older the iron-filled rocks are, and the older the trapped magnetic field is.

Many of these boundaries are on the ocean floor. So scientists can measure how the Earth’s magnetic field has changed over time by bringing sensitive magnetic measuring equipment with them on ships across the ocean. They first tried it back in the 1950s and ‘60s, and every measurement since has confirmed the same story: there has been a lot of flip-flopping.

Rocks of one age will have magnetic north pointing south, just like Earth’s magnetic field is today. But other rocks are backwards: magnetic north points north. The rocks themselves and the iron trapped inside haven’t flipped over or anything, so the Earth’s magnetic field must have reversed over time.

And it wasn’t just once or twice, either. There have been at least a hundred of these so-called magnetic field reversals. In fact, there’s evidence for reversals going all the way back before the Cambrian period, more than half a billion years ago.

Despite these and other measurements, like radioactive elements in rocks, scientists haven’t found much of a pattern in the timing of magnetic field reversals. Sometimes they happen back-to-back, geologically speaking. But other times, there are millions of years between them.

That’s one reason it’s a little silly to say that we’re “due” for another just because it hasn’t happened for around 780,000 years. The next one might be soon, or we could be at the beginning of something like the Cretaceous Normal Superchron, a period of about 40 million years without a single reversal. Or a reversal might’ve happened more recently than we thought.

Some scientists studying rocks and sediments from the Black Sea recently found magnetic evidence for a pair of reversals around 41,000 years ago, with only a couple hundred years between them. But that would be a pretty extreme pair of reversals, since we think that a single flip generally takes a few thousand years from beginning to end. Although there’s a ton of variation here, too: reversals could take hundreds, or thousands, or tens of thousands of years.

During a reversal, we think that the existing magnetic field generally weakens until it’s almost completely gone. And then the field gradually builds up in the opposite orientation. Since the Earth’s magnetic field helps protect us from some of the most extreme forms of interstellar and solar radiation, a reversal would be pretty bad news if it started tomorrow.

Because of all the extra radiation, satellites in low-Earth orbit and electronics down here on the surface might get fried over time and stop working. If the power grids go down, that could lead to a lot of problems. Not to mention, those kinds of radiation can damage cells, so it might cause extra bad sunburns or increased rates of cancers in humans and other living things.

But it still probably wouldn’t be apocalyptic. Now, despite all this research about when reversals happened, scientists still don’t know what causes them. They’re not sure what has enough energy to affect the Earth’s core so dramatically, changing the way a ton of iron and molten rock are sloshing around.

In the past, it could’ve been asteroids or comets smacking into Earth and sending powerful shockwaves through the planet, or triggering climate events that changed how its mass is distributed. Or it could have to do with big chunks of tectonic plates descending into Earth’s mantle, or blobs of magma moving up from the deep mantle and throwing things out of balance. But it’s hard to find evidence of exactly what’s happened in Earth’s core for the last billion years, and our computer simulations can only predict so much with incomplete datasets.

So geophysicists are still at the guessing stage of science, at least for now. Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who help make this show possible. If you want to help us keep making videos, you can go to patreon.com/scishow to learn more.

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