Scientists Solve Mystery of Earth's Shifting Poles
FLORA LICHTMAN, HOST:
You may have heard of Earth's tectonic plates, you know, the pieces of mantle and crust that slide around, breaking continents and kind of - and smooshing them together. But did you know that Earth's entire solid exterior can move, too?
OK, imagine this. Imagine the globe, and now take the surface of the planet and rotate it in your mind so that Boston is at the equator. Whoa. Some scientists think that a shift of this actually happened about 800 million years ago. So should we expect tropical water in Boston again anytime soon? Don't get rid of that parka yet.
A new study published in the journal Nature, though, may help explain what causes this colossal slip and slide, and that's what we're talking about next. If you have questions about this, give us a call. Our number is 1-800-989-TALK, 1-800-989-8255. Let me introduce my guest.
Adam Maloof is an associate professor of geosciences at Princeton University in New Jersey. Welcome to the show.
ADAM MALOOF: Thanks. Thanks for having me.
LICHTMAN: Let's talk about the terminology first. We're not talking about the magnetic poles, right?
MALOOF: Right. The magnetic pole is not moving, here. The magnetic pole stays aligned with the spin axis, and they should be unchanging as viewed from space.
LICHTMAN: OK. So what are we talking about?
MALOOF: We're talking about the rest of the Earth: the crust, the rest of the lithosphere and the entire mantle sliding over the outer core. So the way you imagine this is the core of the Earth, the outer part, is actually fluid iron, and it has about the viscosity of water. So we're literally sliding, you know, 2,700 kilometers of mantle over this so that, as perceived from space, what you'd see is the spin axis is staying the same, but all the continents are moving together to a new location.
LICHTMAN: Is this happening now?
MALOOF: This is happening now, in fact. It's happening at about 10 centimeters per year, which is slightly faster than that tectonic mashing of plates that you describe, maybe a little faster than your fingernail grows.
LICHTMAN: That's a good way to put it. So you've got this sliding going on, and then on top of that, different sliding of the tectonic plates.
MALOOF: Yup, exactly.
LICHTMAN: OK. OK. And what's driving the movement?
MALOOF: Well, the movement that's happening today - and actually any kind of true polar wander, or this motion of whole, solid Earth - is driven by redistributions of mass. So the way to think about it is you have a rotating body, and any rotating body will want to adjust to maintain equilibrium, so that any excess mass is located in the equator, and any mass deficiencies are aligned with the spin axis.
So, for example, today, as glaciers melt and atmosphere moves, some places get extra mass. Some places get less, and the Earth will always be adjusting so that any mass excesses get pushed towards the equator.
LICHTMAN: We've got a bulge in our belly region of the Earth.
MALOOF: Yeah. So that bulge, that's there just because the Earth rotates. The fact that Earth deforms and rotates means that - it raises a bulge called the equatorial bulge about 20 kilometers in amplitude. It's actually quite large.
LICHTMAN: And, I mean, this is partly what keeps us stable, too, right?
MALOOF: Exactly. It's the main stabilizing effect, certainly on short timescales. It's also what sticks out into the solar system and is torqued by other planets, like the moon and stuff, and cause the Earth to wobble.
LICHTMAN: Why doesn't that - why does that move the whole surface of the planet?
MALOOF: Well, what you should imagine is this, is that on long timescales, that bulge will actually deform, OK. We can actually observe this deformation because, for example, as the glaciers melt, the solid Earth rebounds beneath them. And we can measure how fast the Earth is rising, and that's the same kind of deformation.
So when true polar wander occurs, this wholesale motion of all the continents, what literally has to happen is you have to push the full thickness of mantle through a standing wave, through this 20-kilometer bulge. So the time it takes to push yourself through this big wave is about how fast you can move all the continents around.
LICHTMAN: I feel like - let me just make sure I understand. What's causing the Earth's surface to move in one direction and not the other?
MALOOF: Really just where you end up with mass excesses and deficiencies.
MALOOF: So, for example, on a large scale today, let's say you were to remove a ton of mass in the form of ice and place it into the oceans as water.
MALOOF: And generally, you're moving masses away from poles and towards the equator, then the Earth rebounds. So the Earth starts to move back towards the poles to replace that excess mass. This is a kind of mass redistribution.
Now, what's important 800 million years ago, and what was described in this recent paper by J.C. Creveling, et al, is that there, we're talking about much, much larger masses. We're talking probably about things moving around in the mantle, such as subducting plates of oceanic lithosphere or rising plumes. And these very, very large-scale changes in the distribution of mass would be driving these much larger-scale true polar wander events.
LICHTMAN: We're going to talk about that more when we come back from this break. Adam Maloof is the associate professor of geosciences at Princeton University. And if you have questions about this bizarre phenomenon - I had never heard of it before - call us: 1-800-989-8255, 1-800-989-TALK is our number. More on true polar wander when we come back.
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LICHTMAN: This is SCIENCE FRIDAY, from NPR.
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LICHTMAN: This is SCIENCE FRIDAY, and I'm Flora Lichtman. We're talking this hour about Earth's wandering poles. Apparently, they don't stay in the same place. My guest is Adam Maloof. He's an associate professor of geosciences at Princeton University.
And before the break, you were telling us that - about this huge, colossal slip and slide that happened 800 million years ago.
MALOOF: That's right. About 800 million years ago, we were actually looking at sedimentary rocks in Svalbard and Australia, two - today - opposite sides of the Earth, where we saw evidence that Earth seemed to have a shift in the poles relative to the continents on the order of 40 to 50 degrees.
And what was particularly bizarre about this shift is that it was a there-and-back-again motion. It seemed to rotate one way, and then rotate back.
LICHTMAN: And where did it rotate? Give us a sense. I mean, I know that the continents didn't look like they do now. But where would we be?
MALOOF: Yeah, well, if you were to imagine - so today, Earth's shape is not quite right to undergo this kind of true polar wander. But for the sake of a thought experiment, if it were, what you could imagine is if you were far away from the true polar wander axis, you'd essentially change 50 degrees in latitude. So, like, you open the show, you'd say Boston would end up on the equator.
If, on the other hand, you were very close to the true polar wander axis - in other words, the axis around which all this rotation is going on - you'd end up just spinning around. So if that was - if, for example, you were in, say, the - I don't know, somewhere in the tropics, say, the Bahamas, and this happened, you would literally - your shoreline would just rotate around 50 degrees. You might be facing north instead of east.
LICHTMAN: How fast did this happen?
MALOOF: Well, our time constraints are not very good, but based on what we can say, we're guessing somewhere between 10 and 20 million years.
LICHTMAN: How much is that a day?
MALOOF: Yeah. Per day, on the order of, say, 50 centimeters. So, for a geologist, this is extremely fast, believe it or not. Right?
MALOOF: And, you know, when we talk about plate tectonics, we talk about the fastest plates moving on the order of five centimeters today. So it's almost an order of magnitude faster, which is a big deal for geologists.
LICHTMAN: You talked about the Earth being ripe for that kind of movement at that time. What gives you that condition?
MALOOF: Well, one way to ripen the Earth would be to....
LICHTMAN: So to speak.
MALOOF: ...to change its shape. And so you talked earlier in the show about today the shape, as seen from space, is completely dominated by this rotational bulge, the 20-kilometer thing around the equator. If you assume that on long timescales that bulge isn't too important for stabilizing the Earth and just look at what's called the non-hydrostatic geode, the shape the Earth would have if it weren't rotating, today it's what we'd say is triaxial.
You could make three axes, all of which are slightly different in length. If, on the other hand, the Earth were more football-shaped, such that it had one long axis in the equatorial plane, but the other two axes were similar, then the Earth might have a propensity to spiral, just like a football, and that spiraling action could achieve these 30, 40, 50-degree rotations.
LICHTMAN: Well, what caused it to move back?
MALOOF: Yeah. So that's the really elegant innovation of this new paper that came out in Nature by J.C. Creveling, et al. And what they argued is that - so, in addition to all these forces we've described, Earth's lithosphere, this part of the Earth that takes part in plate tectonics and divides the Earth into all these different plates, has some elasticity to it. It doesn't just behave like a fluid.
And because of that, it basically records or sets in the aspect of Earth's rotational bulge so that if forces within the Earth, or redistributions of mass were to cause a 50-degree rotation, and then those loads would relax, the original rotational bulge would still be kind of in memory within this elastic lithosphere, and that would literally cause the Earth to rotate back where it came from.
LICHTMAN: I'm imagining rubber bands.
MALOOF: Yeah. You should think of it just like rubber bands. It's a little bit tricky, right, because Earth's surface, while individual plates are clearly elastic, they're broken. And so a lot of people originally had the intuition that these broken plates would all just kind of move up and down and not behave too elastically.
But it turns out, as this paper shows, even the tiniest bit of elastic strength or elastic thickness will cause the Earth to behave this way and have a memory of its earlier rotational bulge.
LICHTMAN: Let's go to the phones. Will in Pittsburgh, do you have a question?
WILL: Yeah, I do have a question. My question revolves around raw resources and the movement of raw resources from continent to continent and all the disparity between resources between Asia and America. So how would that affect the rotation of the Earth with this shift you're talking about?
LICHTMAN: The raw materials.
MALOOF: OK. I don't know exactly how to answer this question, but I can say that - and I'm not sure exactly what raw materials you're referring to, but, for example, one thing that does happen - if the poles are to shift - is that you move landscapes into very different climate zones.
For example, just say you were to imagine what would happen today, you'd move continents like Antarctica towards the equator. So suddenly, large ice sheets would melt and expose previously unknown continents to view. And so perhaps resources that were otherwise unknown would be there.
LICHTMAN: I think we have a caller with sort of a question on this exact topic, Allison from Wichita. Do you have a question?
ALLISON: Hi. I just wondered: Do the poles' movement - excuse me. Do the poles' movement around - on the Earth, do they affect climate? And is this anything like climate change, what we're experiencing today? Is there any reason for that?
MALOOF: Excellent question. So first, let me just get it straight, that remember, this process is slow. So it - pole shifting would definitely have an impact on climate in two ways, which I'll explain in a second. But today, their impact is so slow on a human timescale, it would be imperceptible. But if you turn on your geologist eyes and imagine a timescale of millions of years, it has two very important effects on climate.
Regionally, as you might imagine, if you moved Boston to the equator, Boston would become warmer. So you'd have a local climatic change. But in some ways, more importantly, pole shifting can actually cause global climate change. And here's just one example of how it does so.
By redistributing the continents on the surface of the Earth, you change the global albedo - in other words you change how reflective the Earth is, because you change the percent of continental land masses in different equatorial zones. For example, the more equatorial continents you have, the more reflective the Earth is and the cooler it will get.
Likewise, you redirect ocean currents and completely change the way the ocean circulates and where is warm and where is cold. So pole shifting definitely has impacts both on local and global climate. It's just that the timescale is much beyond the human timescale.
LICHTMAN: Thanks, Allison.
ALLISON: Thank you.
LICHTMAN: I'm going to look through my geologist glasses into the far future. Will the Earth ever be ripened up for this again?
MALOOF: OK, that's an excellent question, and I would say there's two aspects to consider. One, strictly speaking, we think that the ripening and un-ripening essentially is a process driven by plate tectonics itself. So it's kind of a big, circular thing. But, basically, where you have subduction zones and where you have continents - say, supercontinents versus fragmented continents - that really sets the geometry for convection in the mantle, which itself sets the shape of the Earth.
So, presumably, when the continents come back together into the next great supercontinent, we may very well have the right geometry for another set of true polar wander events. That said, our mantle and our Earth itself is always evolving, and one of the most important terms that's evolving is its temperature.
Ever since Earth has formed, the Earth has been cooling, and as it cools, that changes how fluid the mantle is. And this will affect how easily a true polar wander event could occur and how large it might be.
LICHTMAN: Always hard to predict the future.
MALOOF: Yeah, no kidding.
LICHTMAN: Adam, thanks for joining us today.
MALOOF: Oh, thanks for having me. That was fun.
LICHTMAN: Adam Maloof is associate professor of geosciences at Princeton University in New Jersey.
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