Polar Wandering

No, it’s not about poor explorers wandering across a featurless plain of snow, searching for the South Pole so they can have bragging rights to go with their frostbite when they get home.

I’ve been wanting to write about this for a while, just because it’s so cool!

It starts out very simply. There’s a mineral, called Magnetite. As the name indicates, it is magnetic, and naturally so. In fact it’s the most magnetic mineral on the planet. If you’ve ever heard of lodestones – which were used to make the earliest compasses – those are actually pieces of magnetite. Magnetite is an important source of iron ore, and is also used to “blue” steel to prevent it from rusting.

Magnetite can form in significant amounts, but more importantly, it’s present in at least small amounts in almost all igneous rocks. Igneous rocks (in case you don’t remember from grade school, which is the last time most of us had geology) are the ones that form directly from molten rock. They flow and explode from volcanoes, but also cool slowly beneath the surface of the earth in upwellings of magma from the mantle. There are a lot of igneous rocks in the world, and they’re pretty durable. This means that there are a lot of igneous rocks around for us to look at, and some of them are billions of years old.

Igneous rock forms when magma (or lava, which is what we call magma when it’s on the surface) cools completely. As the melted rock cools, all of the different minerals that have been mixed together in it start to group up and form crystals. If you’ve ever seen sugar crystals “grown” when you put a stick in a glass of sugary water, it’s exactly the same idea. At that point, they’re fixed in place and don’t change position until the rock is destroyed by either being melted again (geologic recycling!) or eroded away.

Okay, so what does any of this have to do with Polar Wandering… whatever that is.

Because Magnetite is magnetic, when it crystalizes, it points toward the magnetic north pole. And once it is crystalized, it can’t move. So you can look at it as a record, a photo of a little finger pointing north, indellibly etched with the date and time. At this moment in history, north was this way.

All very well and good, but north has always been north, right? So who cares?

Well, what if I told you that north hasn’t always been north? Oho!

Once we (meaning geologists) were able to examine the little crystals of magnetite to see what direction they were pointing, we discovered something very strange. In the different layers of igneous rock, the little bits of magnetite had a serious disagreement over which direction was north. Now, keep in mind that when you have different layers of rock, that means different ages of rock. The oldest ones are at the bottom, and then they get younger as you move toward the surface.

So, for example:

Rock layer #1 (the youngest) said that north was this way: /
Rock layer #2 (the middle) said that north was this way: |
Rock layer #3 (the oldest) said that north was this way: —
Well, how does that work? Magnetite doesn’t lie. When those crystals formed, they really did point toward north. After a lot of head scratching, the first thought was that maybe the magnetic north pole has moved over time, wandering across the surface of the Earth. (Whence, polar wandering, which can be plotted as a curved line across the planet.) It sounds kind of weird, but there had to be some explanation for what we’d seen. And science is very much about observing strange things in nature and then figuring out what might have caused them.

But then things got even weirder. We looked at magnetite from a different place – and entirely different continent. If the magnetic north pole had really gone meandering across the Earth, then the magnetite on that continent would agree.

Only… it didn’t.

We found rocks that were the same age as the first ones we looked at, and the little bits of magnetite helpfully pointed out that north was in a completely different direction from what the other ones said. And then we went to another continent and got a third set of answers that were different from the first two. And so on, and so on.

We sat back and scratched out heads. Every continent had a different path for the north pole to have wandered down throughout history. Did that mean that in the past, every continent had its very own north pole? That sounds pretty silly to begin with, and, well, right now we’ve obviously got only one north pole. The way the world works hasn’t suddenly changed just because humans discovered how to make compasses.

But… what if the continents themselves had moved? North had stayed the same (except of course when it switched places with the south pole, but that’s a subject for a different post) but the continents had wandered across the face of the planet, drifting and rotating.

That was the only explanation we had that fit the facts and followed the main principle of modern geology – that the geological forces and events that we see today are the same ones that shaped the world in the past.

Polar wandering is one of the strongest pieces of evidence for the theory of continental drift – the theory of plate tectonics. And since the time that scientists decided that having a multitude of wandering north poles was just plain silly, more evidence has been found to support the idea of continental drift. Today, we have satellites and other machines that can measure things so precisely that we can see the continents move. Some of them move as fast as six centimeters in a single year. To us, that sounds horribly slow, but think how much movement that adds up to in a billion years.

The ground beneath your feet is moving. We drift slowly across a sea of molten rock, even as the Earth spins out its days and rockets around the sun.

Now, how cool is that?

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