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Paleomagnetism, Polar Wander, and Plate Tectonics

The study of the Earth's magnetic field as recorded in the rock record was an important key in reconstructing the history of plate motions. We have already seen how the recording of magnetic reversals led to the confirmation of the seafloor spreading hypothesis. The concept of apparent polar wander paths was helpful in determining the speed, direction, and rotation of continents.

Apparent Polar Wander

To illustrate the idea of polar wander, imagine you have a composite volcano on a continent like the one in the sketch below. I assure you that the sketch will be better understood if you also watch the screencast in which I talk while I draw it.

Apparent polar wander sketch

Click here for transcript

In order to illustrate an apparent polar wander path, let’s say we’ve got the Earth here, and it’s got its poles like so, just the way they are today. The magnetic field lines are going like that. And let’s say we’ve got a continent sitting here. It looks like this. There’s a volcano on this continent and it’s a composite volcano. A composite volcano spews out lava and it gradually builds up the mountainside with its lava flows like this. Here’s the lava coming down this side. Let’s pretend we are a geologist and we’re going to go to this volcano and we’re going to take some samples of these lava flows. We’ll zoom in on these lava flows here. The uppermost sample of the lava flow, we’ll call that this green one here. Underneath that green one there’s a more orange-yellow lava flow and then under that there’s this oldest one here. We have a magnetometer and so we can try to figure out which way all these lava flows thought north was when they formed and cooled. Let’s say that the red one points sort of in this direction and the yellowish one looks like this. The green one was formed during the field like it is today so its north is like that. There are two possible explanations for how this could have occurred. We’ll draw those right here. Explanation 1 is that the poles moved around and the continent stayed in the same place. In that case, we’ve got a continent sitting here. When the most recent lava formed, this green stuff, the pole was right up here, where it is today. But back when this volcano was making the yellow lava, the pole was in a slightly different place. It was more like over here. The oldest lava flow is recording a pole that was more like in that direction. In this case we end up with what we call an apparent polar wander path. Over time from back when to the present time the pole moved in that direction. The other possibility is that the continent moved and the pole stayed in the same place. In that case, the green continent of today would be here. When this lava froze, it was pointing north toward the north pole. Back when this yellow lava formed, if the pole was in the same place then the continent would have to have been over here somewhere like this because its lava froze pointing north, but then over time when this continent moved to its present position with the lava still frozen in place it is now pointing in a different direction that isn’t where north is anymore. If we go back even farther in time toward the red lava, then the continent must have been sitting in a position sort of like this. When its lava formed, it was pointing north, then when this continent went through this rotation, this lava was already frozen in place, so the direction it’s pointing isn’t in the same place that north is now. We can construct a path — an apparent wander path if you will — of the continent. We can see that the continent must have gone sort of like this. This is in the opposite direction of the one we constructed before.

This volcano erupts from time to time, and when its lava solidifies and cools, it records the direction of the Earth's magnetic field. A geologist armed with a magnetometer could sample down through the layers of solidified lava and thus track the direction and intensity of the field over the span of geologic time recorded by that volcano. In fact, geologists did do this, and they discovered that the direction of the north pole was not stationary over time, but instead had apparently moved around quite a bit. There were two possible explanations for this:

• Either the pole was stationary and the continent had moved over time, or
• The continent was stationary and the pole had moved over time.

Seafloor Spreading Saves the Day!

Before plate tectonics was accepted, most geologists thought that the pole must have moved. However, once more and more measurements were made on different continents, it turned out that all the different polar wander paths could not be reconciled. The pole could not be in two places at once, and furthermore, the ocean floors all recorded either north or south, but not directions in between. So how could lavas of the same age on different land masses show historic directions of the north pole differently from each other? Once seafloor spreading was recognized as a viable mechanism for moving the lithosphere, geologists realized that these "apparent polar wander paths" could be used to reconstruct the past motions of the continents, using the assumption that the pole was always in about the same place (except during reversals).

Calculating a Paleomagnetic Latitude

The example in my fabulous drawing gives a rather vague description of the idea behind using paleomagnetic data to reconstruct the former positions of the continents, but how is it actually done? We use magnetometers.

The angle between the Earth's magnetic field and horizontal is called the magnetic inclination . Because the Earth is a round body in a dipole field, the inclination is directly dependent on latitude. In fact, the tangent of the angle of inclination is equal to twice the tangent of the magnetic latitude, which is the latitude at which the permanently magnetized rock was sitting when it became magnetized. Therefore, given knowledge of your present location and a magnetometer reading of the inclination of your geologic item of interest, such as a basalt flow, you can calculate the magnetic latitude at the time of its formation, compare it to your present location, and determine how many degrees of latitude your present location has moved since that rock cooled.

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Scientists Solve Mystery of Earth's Shifting Poles

Did you know that Earth's solid exterior can move around over its core, causing the planet's poles to wander back and forth? Adam Maloof, associate professor of geosciences at Princeton University, discusses the consequences of these shifts, and what may be causing them.

Drilling To The Mantle Of The Earth

Japan earthquake may have shifted earth's axis, next supercontinent could form at the north pole.

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.

LICHTMAN: OK.

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.

(SOUNDBITE OF MUSIC)

LICHTMAN: This is SCIENCE FRIDAY, from NPR.

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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• Published: 26 December 2022

Ordovician–Silurian true polar wander as a mechanism for severe glaciation and mass extinction

• Xianqing Jing 1 ,
• Zhenyu Yang   ORCID: orcid.org/0000-0003-1388-2519 1 ,
• Ross N. Mitchell   ORCID: orcid.org/0000-0002-5349-7909 2 ,
• Yabo Tong 3 ,
• Min Zhu   ORCID: orcid.org/0000-0002-4786-0898 4 &
• Bo Wan   ORCID: orcid.org/0000-0002-5896-9485 2  

Nature Communications volume  13 , Article number:  7941 ( 2022 ) Cite this article

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• Geodynamics
• Palaeomagnetism
• Palaeontology

The Ordovician–Silurian transition experienced severe, but enigmatic, glaciation, as well as a paradoxical combination of mass extinction and species origination. Here we report a large and fast true polar wander (TPW) event that occurred 450–440 million years ago based on palaeomagnetic data from South China and compiled reliable palaeopoles from all major continents. Collectively, a ~50˚ wholesale rotation with maximum continental speeds of ~55 cm yr −1 is demonstrated. Multiple isolated continents moving rapidly, synchronously, and unidirectionally is less consistent with and plausible for relative plate motions than TPW. Palaeogeographic reconstructions constrained by TPW controlling for palaeolongitude explain the timing and migration of glacial centers across Gondwana, as well as the protracted end-Ordovician mass extinction. The global quadrature pattern of latitude change during TPW further explains why the extinction was accompanied by elevated levels of origination as some continents migrated into or remained in the amenable tropics.

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Introduction

The Earth system underwent critical changes during the Ordovician–Silurian (O–S) transition 460–435 million years (Ma) ago. The end-Ordovician mass extinction, which can be regarded as the second most lethal of the “Big Five” mass extinctions, replaced much of the Cambrian marine fauna with later Paleozoic fauna 1 . Accompanying the O–S mass extinction was the first of three occurrences of significant glaciation during the Phanerozoic Eon (ca. 541 Ma to present), of which only ~25% represented glacial intervals 2 . The O–S glacial interval is unusual both because it was short-lived and occurred at a higher atmospheric partial pressure of CO 2 , perhaps 8–16 times higher than today 3 . There are multiple hypothesized causes for the Ordovician extinction, including intense volcanic eruptions and/or large igneous provinces 4 , 5 , 6 , 7 , oceanic anoxia 4 , 6 , special paleogeography 8 , large and short-lived glaciation 8 , 9 , 10 , and even the evolution of land plants 9 , 10 . The diversity of proposed mechanisms thus reflects the myriad changes in the atmosphere, biosphere, hydrosphere, lithosphere, as well as in the mantle at that time.

Among the candidate mechanisms behind the widespread O–S global change, intense volcanism and paleogeography are generally regarded as the basic causes for the other changes 4 , 6 , 7 , 8 . However, there is still debate over how exactly volcanism impacted the environment 4 , with some arguing that it resulted in global warming, while others claiming it caused glaciation. The volcanism theory has also been used to explain the traditional two-pulse extinction model 5 , but recently reported high-resolution biodiversity curves 1 , 7 , 11 suggest instead a protracted extinction rather than the simple traditional two-pulse model. Therefore, the mechanisms once fit to a two-pulse extinction model may no longer be applicable, or at least require modification.

Paleogeography is another critical boundary condition for understanding such marked transitions in Earth’s surface environment, but the prevalent palaeogeographic models used 5 , 6 , 8 , 12 are imprecise, lacking palaeolongitude control and high temporal resolution. Constraining palaeolongitude is particularly important when continents are dispersed as they were in the early Paleozoic during the transition between supercontinents Rodinia and Pangaea. Temporal resolution is critical when continents are moving fast and multiple kinematic models suggest some of the highest continental motions of the Phanerozoic Eon occurred during this time 13 . It is therefore difficult to evaluate the exact impact palaeogeographic changes may have had on the end-Ordovician environmental changes. For example, employing prevalent palaeogeographic models, biogeochemical models 14 fail to both recreate the environmental changes during the critical Hirnantian stage as well as to explain the migration of the glacial centers 15 , 16 .

Similar extreme transitions in Earth’s surface conditions occurred during the preceding Ediacaran–Cambrian periods, and this interval has been proposed to have experienced large-scale (60–90˚) true polar wander 17 , 18 , 19 , 20 , 21 , 22 , 23 . True polar wander (TPW) is the movement of the entire solid Earth (mantle and crust) relative to Earth’s spin axis in order to stabilize Earth rotation. It is different from the plate motion of plate tectonic theory, which proposes that tectonic plates, including continents or not, move over the asthenosphere relative to the underlying convecting mantle. Tectonic plates move in different directions and with different velocities (even in the case symmetric seafloor spreading, the directions of motion are opposite of each other). In contrast, TPW can induce wholesale polar motion of the plates unidirectionally and synchronously, thus changing paleogeography rapidly and globally. Therefore, TPW potentially impacts much of Earth system evolution including changing ocean currents, air circulation, relative sea level, and depocenters of the carbon cycle 18 , 21 , 24 , 25 , 26 .

Van der Voo 27 first proposed a round-trip TPW oscillation (two sequential back-and-forth TPW events) during the Late Ordovician to Late Devonian based on sparse and roughly-dated palaeomagnetic poles from three continents exhibiting similar large and rapid segments of apparent polar wander (APW) 27 . The proposed ~40˚ amplitude of the putative O–S TPW event (the first event of the pair in the oscillation), if shown to be valid, would represent the largest TPW event in the past 500 million years 28 . Although Piper et al. 29 revisited this TPW interval, they only studied the late Silurian–Early Devonian part. No follow-up research has reexamined the Ordovician–Silurian TPW event, which, as proposed, is too crude to assess its validity nor its potential impact on the environmental and ecological changes occurring at that time. Many new palaeomagnetic results during this purported interval of large-scale TPW have been reported since 30 , 31 , 32 , 33 , 34 , justifying a reexamination. Furthermore, although putative O–S TPW was proposed 27 prior to most of the numerous hypotheses attempting to explain the variegated aspects of global change during this transition, O–S TPW has never before been taken into account for its potential environmental effects. Even for those models involving palaeogeography as a critical aspect for the changing surface conditions during this dramatic environmental transition, TPW has been neglected.

In this work, we present palaeomagnetic data from South China as well as compile data globally from 6 continents to provide a rigorous and high-resolution palaeogeographic reconstruction of the O–S transition. Our results demonstrate the occurrence of a large and rapid TPW rotation synchronous with the environmental changes across the O–S boundary. The heretofore enigmatic features of global change during this time interval can collectively be reconciled by this refined TPW-based palaeogeographic model, explaining both glacial and extinction dynamics.

New palaeomagnetic data from South China

Previous palaeomagnetic data from South China tentatively suggest there may have been a rapid continental movement during the Late Ordovician to early Silurian 30 , 32 , 35 . However, data from the Silurian have been calculated as a mean pole for the whole Period (443.8–419 Ma) 32 , 35 (Supplementary Fig.  1 ), which precludes detailed evaluation of maximum rates of continental motion during the O–S transition. Due to its importance for palaeogeographic comparison before and after the O–S boundary, the upper Telychian strata of the Huixingshao Formation (ca. 436–435 Ma) in Xiushan county, Chongqing, South China (Supplementary Figs.  1 and 2 ) were selected for detailed palaeomagnetic study. Standard palaeomagnetic methods were employed and are detailed in the Methods. Stepwise thermal demagnetization revealed a stable component with high unblocking temperature suggestive of a remanence carried by hematite, which is also supported by rock magnetic experiments (Fig.  1 and Supplementary Figs.  3 , 4 , and 6 ). Detailed description of the palaeomagnetic results is provided in the Supplementary information. The magnetostratigraphic record reveals at least four coherent polarity zones (Fig.  2 ) strongly suggesting that the high-temperature component from section Yongdong (SY) is primary and can be used for palaeogeographic reconstructions. However, the K- value of dispersion of the virtual geomagnetic poles (VGPs) of these six sites is 90.3 (Supplementary Table  1 ), exceeding 70, which suggests that these data may not average out the palaeosecular variation (PSV) 36 . To overcome this issue, we sought to combine our new data with the most reliable coeval previous data.

Zijderveld plots ( a , d , g ), equal area projections ( b , e , h ) and normalized stepwise thermal decay curves ( c , f , i ) of the thermal demagnetization of representative samples from the section at Yongdong (SY) in geographic coordinates. In the Zijderveld plots, black and white dots represent horizontal and vertical projections, respectively, while in the equal area projections, they represent directions plotted in the lower and upper hemispheres respectively. j Equal area stereographic projection of site mean directions of the high-temperature components of the Huixingshao (HXS) Fm from this study in stratigraphic coordinates. k Virtual geomagnetic poles (VGPs) of HXS Fm from the SY section from this study compared with VGPs from the HXS Fm and the Rongxi (RX) Fm from Opdyke et al. 35 and Huang et al. 32 . Resulting combined early Silurian pole (S 1 M) using all data from the RX and HXS Formations from this study and previous work is shown as red star with associated cone of 95% confidence. l The new recalculated early Silurian pole (S 1 M) is distinct from existing poles of South China 71 . All plots were generated with PaleoMac 72 .

Sampled section at Yongdong (SY). Directions with declinations >240° were interpreted as reversed polarity, and otherwise, as normal polarity.

We reassign the ages of existing Silurian palaeomagnetic results 32 , 35 according to a recently updated stratigraphic timescale 37 , 38 , 39 (Supplementary Fig.  1 ). A notable revision in these age reassignments is that the Rongxi Formation previously regarded as ca. 420 Ma in age is in fact early Telychian (ca. 438.5–437 Ma) (Supplementary Fig.  1 ). Again, data from these previous studies 32 , 35 seem not to average out PSV 36 (Supplementary Table  1 ; detailed analysis in Supplementary information). Nonetheless, after combining all data from the Rongxi and Huixingshao Formations (total 28 sites), a K -value of 48.4 is achieved, which is below 70 and suggests sufficient averaging of PSV. Furthermore, these data also pass a fold test 40 at 99% confidence (k in geographic coordinates is 7.64, in stratigraphic coordinates is 31.17). This new early Silurian pole (S 1 M) calculated by averaging the VGPs from the Rongxi and Huixingshao Formations plots far from all younger poles and earns a reliability index of 6 of 7 (ref. 36 , Supplementary information). Intriguingly, the new early Silurian pole (S 1 M) plots far from (≥50°) away a high-quality Late Ordovician (late Sandbian–middle Katian; 454–448 Ma, or ca. 451 Ma) pole of South China 30 (Figs.  1 l, 3a ).

Given only ~10 Ma between these two ages, the 54.4° ± 6.4° arc distance between these two poles indicates a rapid APW rate of 5.4 ± 0.6 Ma −1 for South China. During this time interval, South China experienced a region tectonic movement, however it was restricted to only its southeastern part (Cathaysia terrane) 41 . Our early Silurian data and the Late Ordovician data are from northwestern South China (upper Yangtze terrane), which was largely unaffected by this tectonism. In addition, the regional tectonism should have only induced large differences in the declination of these data (due to potential vertical-axis rotation), but cannot explain the large inclination difference that is observed corresponding to a ~28.5° change in palaeolatitude. Non-uniformitarian magnetic fields (e.g., quadrupolar or octupolar) may also result in apparent changes in latitude 33 . However, in order to explain the reduced inclination of the Late Ordovician data (35°) to our Silurian data (18°), one would have to claim a same-sign octupole that was stronger than 20%, which is more extreme than any previous claims in the Phanerozoic 42 , and an opposite-sign octupole would increase, not decrease, inclination. Furthermore, both non-dipole cases would only affect inclination and therefore cannot explain the even larger anomaly in terms of the ~59° declination change. Lastly, an oscillation between polar and equatorial dipoles (if possible on Earth) could affect declination 43 , but would predict a ~90˚ change that is not observed. Therefore, we argue that this large and rapid motion of South China corroborates from an additional continent the proposed O–S TPW event 27 , albeit with an even larger amplitude than once thought. Nevertheless, any reproducibility test of TPW should aim to be global in scope, so we must consider the palaeomagnetic records of the other major continents.

Late Ordovician-early Silurian true polar wander

Strikingly, in addition to the large-scale 54° ± 6° APW of South China, the Late Ordovician–early Silurian palaeopoles from Tarim, Siberia, Baltica, and Gondwana also all demonstrate large arc distances of APW: 54° ± 9°, 47° ± 17°, 55° ± 14°, and 58° ± 21°, respectively (Fig.  3a , Supplementary Table  2 ), with associated APW rates of 5.4° ± 0.9°, 4.7° ± 1.7°, 5.5° ± 1.4°, and 5.8° ± 2.1° Ma −1 , respectively. Data from Baltica and Gondwana represent recent synthetic APW paths, which consider the age error and the quality of the data 34 . For comparison, we also calculate the arc distances for Baltica and Gondwana from 450–430 Ma using the synthetic APW paths of Torsvik et al. 44 (Supplementary Table  3 ), which are 51.2° ± 8.2° and 24.5° ± 18°, respectively. While the results for Baltica agree with both methods, the large difference of the two synthetic APW paths for Gondwana reflect either the larger 20 Ma age bins of Torsvik et al. 44 oversmoothing the data and/or the lack of poles during this time interval which is non-ideal for synthetic methods. Nonetheless, at least four continents demonstrate similar large amplitude and synchronous polar motion. As discussed, regional tectonics and non-uniformitarian geomagnetic fields cannot explain this systematic global APW anomaly. Plate motion, driven by slab subduction and mantle convection, also cannot explain these synchronous and similar large amplitude movements of multiple isolated continents either, as it requires relative motion between different plates with different velocities (speeds and/or directions).

a Global palaeomagnetic poles for 460–430 Ma from South China, Tarim, Siberia, Baltica, Laurentia, and Gondwana. Apparent polar wander (APW) paths all exhibit (except Laurentia) large shifts between ca. 450–440 Ma. Poles shown in present-day coordinates. Pole information is listed in Supplementary Table  2 . b Poles rotated into the common reference frame of Gondwana illustrating the APW overlap. Black dot is a reference point in central Gondwana used for palaeolatitude estimates in Fig.  6e . c APW arc distances for all poles globally (in Gondwana reference frame of b ) relative to the 460 Ma Gondwana pole (as an arbitrary reference point before the hypothesized true polar wander event). See text for discussion of Laurentia. Vertical bars are intervals of 95% confidence. Plots in a and b were generated with GPlates 73 .

TPW could explain the large and synchronous dispersions of O–S palaeopoles globally. TPW is rate-limited by the ability of the viscous mantle to deform into a reoriented hydrostatic figure 45 , 46 . TPW can occur as fast as the fastest plate motion or even comparatively faster, particularly in more ancient times when the mantle was hotter, less viscous, and thus more deformable 46 , 47 . Numerical simulations suggest that a 40–50° amplitude TPW event can occur in ~10 Ma if the viscosity of lower mantle is 10 22  Pa s 46 . Presently lower mantle viscosity is about 3 × 10 22  Pa s 48 , while it may be 3 times lower at 450 Ma 47 . Hence, considering almost all continents sped up synchronously, we propose that during Late Ordovician, most likely after the middle Katian Stage but before the Silurian early Telychian Stage, a TPW event occurred. Furthermore, the fact that all the ~50° arc distances of APW are within statistical uncertainty of each other means that the data pass the global reproducibility test of TPW.

We note that palaeomagnetic poles from Laurentia during this time are characterized, in contrast, by much less APW, and almost essentially a stillstand (Fig.  3 ). At face value, one continent with a statistically different arc distance of APW compared with those of other continents does not invalidate the TPW hypothesis 49 . This point of caution is particularly relevant here because during this time Laurentia was an isolated plate with its own tectonic motion vector. In the Paleozoic, the Iapetus and Rheic oceans that existed in between Laurentia and West Gondwana rapidly expanded and vanished 12 , 34 , which certainly would have resulted in fast tectonic movements of Laurentia and may seem at odds with its small amount of APW. As the tectonic motion of Laurentia during the closure of the Iapetus would have been mostly opposite to its sense of motion due to TPW, the effect of TPW would be partially offset and thus should appear as a relative stillstand, where APW = plate motion + TPW. In this sense, as Laurentia would have undergone large tectonic motion during this time, its palaeomagnetic stillstand can only be reconciled if TPW in the opposite direction is invoked. Thus, the TPW event inferred from all other continents provides a convenient way to explain the prior paradox of a Laurentian APW stillstand during the closure of the Iapetus Ocean.

We should also note that, strictly speaking, Laurentia may not exhibit a total stillstand. The circles of 95% confidence of the youngest and oldest Laurentian poles (460 and 430 Ma) only very slightly overlap, and the results of an F test 50 demonstrate that the poles are distinct from each other at the 99% confidence level ( F  = 10.6). This test indicates that the 18.9° ± 19.3° arc distance between the two O–S poles is statistically significant. Therefore, while the presumably considerable tectonic motion of Laurentia partially masks the ~50° TPW event, Laurentia nonetheless does indeed record a statistically significant portion of the TPW amplitude that, in reconstructed coordinates, is consistent with the sense of TPW rotation more clearly recorded on the other continents. Otherwise, this relative stillstand may be an artifact of the large age errors of these poles used for APW comparison 34 .

As defined as the migration of the maximum moment of inertia ( I max ) to align with Earth’s spin axis, TPW occurs as a rotation about an Euler pole controlled by the minimum moment of inertia ( I min ) that is equatorial and is therefore predicted to circumscribe a great-circle APW path. Identifying TPW as a great-circle APW path also assumes that plate motion of the continent relative to the mantle is negligible, the change in the orientation of the principal axes of non-hydrostatic moment of inertia is instantaneous, and those subsequently do not change at all. The similar amplitude and synchronicity of these five continents indicate their individual plate motions are negligible relative to the shared TPW motion. Numerical simulations indicate such a change in the orientation of the principal axes of non-hydrostatic moment of inertia can be completed within 10 Ma 46 . There is also a notable absence of poles in between the before/after poles recording the TPW shift (Fig.  3 ). These systematic gaps in the APW paths of all continents are consistent with the stroboscopic effect expected for TPW, which is a non-linear process that speeds up and slows down, thus rendering it less likely for rocks to form (making them available for palaeomagnetic sampling) during the peak rate of TPW in the middle of the event. A simple simulation (Methods) demonstrates that it is 20 times less likely to sample TPW “in action” than the endpoints largely before/after the TPW event (Fig.  4 ). This inherent bias can explain why the O–S TPW event is sampled exclusively by endpoints for all continents. We therefore confirm and refine the original proposal 27 of a large amplitude ~50° TPW event occurring across the O–S boundary.

a True polar wander (TPW) angle as a function of time with an initial condition of 25°. b TPW speed (black line) and probability function (shaded gray) as a function of time.

Given this was a time of major plate tectonic reorganization in between assembly of megacontinent Gondwana and its larger supercontinent Pangaea 51 , there is no shortage of potential sources of subduction-related mass anomalies that might have provided the excitation for the large-scale TPW event across the O–S boundary. The Australian Tasmanides, the Laurentian Appalachians, and the Baltic Caledonides were all active at this time; however, provided their positions relatively close the TPW axis ( I min ), their influence on Earth’s rotation would have been dampened compared to mass anomalies elsewhere. In contrast, both the Proto-Tethyan and Terra Australis subduction systems on either side of Gondwana were ~90° away from I min and thus in the plane of TPW containing I max and I int would have been ideally positioned relative to Earth’s prolate non-hydrostatic figure to have excited large-scale TPW.

In the Late Ordovician, the Proto-Tethyan system experienced a fundamental shift from subduction to collision 52 . Both the timing (pre-TPW) and the sense of this change in slab dynamics—with the foundering oceanic slab likely ponding at the mantle transition zone, thus causing a positive anomaly in the geoid kernel driving TPW for this region equatorward 53 —are consistent with the observed palaeogeographic shift of the Tethyan subduction zone from mid-latitudes into the tropics (Fig.  5 ). Also, in the Terra Australis system on the other side of Gondwana, an intriguing coincidence is that the new position of the South Pole (post-TPW) becomes centered on the Antarctica–South America segment of the subduction system (Fig.  5 ) that experienced a dramatic shift from negative to positive hafnium isotopes at this age 54 . Such a shift due to increased mantle-derived magmatism in the arc indicates slab retreat, which can occur before slab break-off as a slab meets resistance to subduction after impinging the mantle transition zone 55 . Because of the time lag between slab subduction in the upper mantle and its penetration into the lower mantle, a dramatic slab avalanche from the upper into the lower mantle after stagnation at the mantle transition zone could thus conveniently explain the new pole position assumed in the Silurian as the geodynamic change in the Terra Australis would have driven TPW for this region poleward 53 . Thus, the dramatic changes in slab dynamics of both subduction systems on either side of Gondwana could have contributed to the collective forcing behind the largest TPW event in the past 500 million years.

Reconstructions for: ( a ) 460–450 Ma, ( b ) 445 Ma, and ( c ) 440 Ma. Palaeomagnetic poles are color-coded as in Fig.  3a . I min , minimum moment of inertia (equatorial true polar wander axis of rotation). Palaeomagnetic poles (and associated continents) of each age are rotated to coincide with the South Pole, although a 5–10° range of flexibility is occasional adopted as is common praxis in ancient palaeomagnetic reconstructions. The latitudinal band between 15° north and south of the Equator is assigned as the humid tropical zone with intensive chemical weathering. Maps are shown in Mollweide projection. For better displaying their distribution, we fixed the continents and rotate the Mollweide projection to fit the Palaeo-south pole. All plots were generated with GPlates 73 .

It is also possible that the waxing and waning of ice sheets across Gondwana contributed to the mass anomalies driving O–S TPW, or there was some feedback between TPW and glaciation. In particular, there is a migration of glacial centers from northern Africa to southern Africa–South America, where glacial and periglacial strata in the former region are predominantly Ordovician and those in the latter neighboring regions are predominantly latest Ordovician or Silurian 15 . That is, the mass load associated with incipient Ordovician glaciation applied in northern Africa could have been driven to the equator by TPW, causing southern Africa–South America to move to the pole and thus moving the glacial center there in the earliest Silurian (Fig.  5 ). This hypothesis, by extension, would also predict ensuing oscillatory Silurian–Devonian TPW back in the direction of northern Africa (in order to drive the glacial center in southern Africa–South America equatorward), which has indeed been previously hypothesized 27 , but the assessment of which is beyond the scope of our study on O–S TPW. In the Cenozoic, however, glaciation is typically regarded more as an effect of TPW rather than a cause of it 56 , as the amplitude of glacially induced TPW is smaller than TPW driven by mass reorganizations in the mantle 56 . Nevertheless, given the larger size of the Paleozoic pan-Gondwanan ice sheet, and thus its presumably larger mass load, glacial loading deserves further investigation for potentially driving the O–S TPW event. If valid, such an interpretation—the incipient glacial load causing TPW, which then led to more severe glaciation as Gondwana became centered over the South Pole—presents a fascinating potential feedback between TPW and glaciation.

Palaeogeographic reconstructions based on true polar wander

Traditionally, the superposition of APW paths is used to reconstruct the configuration of different continents during time intervals of supercontinentality 57 . However, during times of plate tectonic reorganization in between supercontinents, this method cannot be used to reconstruct isolated continents that are in relative motion, which is most likely how the end-Ordovician world was kinematically configured 12 . Nonetheless, when APW is predominantly driven not by plate motion but by TPW, then the superposition of APW paths can be used to determine the relative positions of different continents whether they are united or isolated because the TPW motion is shared by all continents and thus provides a common global reference frame 26 , 58 . Such an APW comparison only requires a minimum of two poles from before and after the TPW event. Therefore, we can accurately reconstruct global paleogeography of the major continents across the O–S boundary by leveraging TPW.

To make our reconstructions, northwest Africa is fixed and all the other continents are rotated into northwest African coordinates (Euler rotation parameters listed in Supplementary Table  4 ). We first fitted a great circle to the palaeopoles from Gondwana, of which northwest Africa is a part (Fig.  5a ). Poles from all the other continents were then rotated to overlap the Gondwanan poles at their corresponding ages. The TPW-based reconstructions constrain the relative positions of all these continents during 460–440 Ma, not only including palaeolatitude constraints, but also commonly unconstrained relative palaeolongitude. As mentioned, the essentially opposite tectonic motion of Laurentia effectively cancels out some of the TPW rotation for Laurentia, therefore its position relative to other continents changes over time.

Three high-resolution, TPW-based palaeogeographic reconstructions are provided at 460–450, 445, and 440 Ma (Fig.  5 ). The 445 Ma reconstruction is an interpolated position between 450 and 440 Ma. Subduction zones and the evolution of Avalonia is simplified from Cocks and Torsvik 12 . A salient difference between our reconstructions and previous ones 5 , 6 , 8 , 12 , 59 is that Gondwana rapidly swept over the South Pole (Figs.  5 , 6e ). Meanwhile, during 460–450 Ma, the Niger–Chad zone was located at the South Pole rather than the Morocco–Algeria zone (Fig.  5a ). At 460–450 Ma, Gondwana was distributed from the South Pole to the Equator, with the majority of the landmass located at high-to-mid latitudes (Fig.  5a ). Laurentia straddled the equator, with its east coast (present coordinates) outside of the tropics. The positions of Baltica and Siberia are similar to previous reconstructions 5 , 6 , 12 , 59 . Constrained using the APW path of Tarim, Turan–Karakum–Tarim–North China 60 is constrained to a position between South China and Siberia. Most of South China was in the tropics, which is consistent with the palaeoequatorial setting suggested by the mega-nodular limestone, a time-specific carbonate facies 61 .

a Temporal distribution of the palaeomagnetic sampling horizons by Formation (Fm) from South China. Snowflake above the Hirnantian stage indicates the age of short, sharp glacial advance 3 . b δ 13 C record (black) and sea surface temperature variation (red) from Rasmussen et al. 7 c Biodiversity during the Sanbian–Telychian stages from Deng et al. 11 (purple line) and Rasmussen et al. 7 (blue line). CR capture–recapture modeling. d Rates of origination (blue) and extinction (magenta) with 1 million year age binning from Deng et al. 11 . e Palaeolatitudinal variation of a reference point in central Gondwana (12°S, 10°E) calculated by using 460–430 Ma Gondwana palaeopoles listed in Supplementary Table  2 . Note that the 430 Ma palaeolatitude is not displayed. Orange vertical bars are intervals of 95% confidence.

After 450 Ma, TPW initiated a dramatic change in palaeogeography. At 445 Ma, in the middle of the TPW event, northern Africa moved off the South Pole, where it was replaced by southern Africa and South America (Fig.  5b ). (In terms of tectonic motions, Laurentia moved closer to Baltica, but farther from Gondwana because of the fast opening of the Rheic Ocean.) During the TPW event, Baltica and Avalonia moved into low latitudes, and Siberia, Turan–Karakum–Tarim–North China, and South China ended up straddling the equator and were nearly all located within the tropics (Fig.  5b ). After the TPW event was over by 440 Ma (the Silurian), northern Africa and Arabia occupied low latitudes, while South America and southern Africa were located around the South Pole (Figs.  5 c, 6e ). Siberia and Turan–Karakum–Tarim–North China all moved out of the tropics, while Baltica moved into the tropics and South China moved into the tropics of the northern hemisphere. By the Silurian, more continents were positioned at mid-to-low latitudes (more than ~14,000,000 km 2 ; Fig.  5c ) than before (Fig.  5a, b ).

Having verified and refined the existence of a large-scale TPW rotation across the O–S boundary and reconstructed the associated rapid continental motions more precisely than ever before, we consider the potential impacts of such an extreme palaeogeographic disruption on the variegated changes to Earth’s surface conditions during this enigmatic time of transition. One unique puzzle of end-Ordovician global change is the occurrence of severe glaciation. Continental configuration is known to play an important role in setting the climate state on both long and short time scales 2 , 62 , so any changes in paleogeography due to TPW should also be critically considered. Phanerozoic glaciations have been generally correlated with the occurrences of arc-continent collisions distributed within the humid tropical zone, which serves to lower global temperatures by increasing silicate weathering and consuming CO 2 , an atmospheric greenhouse gas 2 , 62 .

Reliable evidence of O–S glacial deposits from Gondwana that was positioned near the South Pole at the time is mostly found during the Hirnantian stage, with only sporadic cases reported before and after 63 . The latest Katian and early Silurian glaciogenic sediments occurring before and after the Hirnantian, respectively, found in Niger and South America argue for a medium-to-large-scale glacial interval, if using previous paleogeography 5 , 6 , 8 , 12 , 59 . However, no other sectors of Gondwana record glaciogenic sediments during these time intervals. Furthermore, cyclostratigraphic analysis also indicates the main glaciation initiated in the early Hirnantian stage 64 . In our reconstruction of the latest Katian, Niger was close to the South Pole (Fig.  5a ), where glaciation is most likely to develop as it is the coldest place on Earth because incoming solar radiation is reduced by the high angle of incidence at high latitudes, as evidenced in the current polar ice caps of Antarctica and Greenland. Meanwhile, during the early Silurian, South America moved over the South Pole (Fig.  5c ) and glaciation, as expected, occurred there again. Therefore, our reconstructions match the migration of glacial centers across Gondwana quite well 15 , 16 , 63 , which in turn independently validates our proposal of O–S TPW. In particular, the fact that the short, sharp Hirnantian glaciation 3 is shown to have occurred precisely in the middle of the TPW event—as Gondwana swept over the South Pole (Figs.  5 , 6e ) and its ice sheet presumably should have expanded to its largest size for a very brief time interval—indicates that TPW is a previously unrecognized major factor in the total causal nexus explaining the Hirnantian glaciation, as for the Cenozoic northern hemisphere glaciation 56 , 65 .

We next consider how TPW interacted with other factors previously thought to control Late Ordovician early Silurian glaciation. Volcanic eruption 4 , 7 , plant and large phytoplankton evolution 9 , 10 , and silicate weathering 2 , 62 , 66 have all been proposed to explain the extreme climate change across the O–S boundary. However, previous palaeogeographic constraints limited the accuracy of such interpretations 6 , 14 , 59 . Our reconstructions demonstrate that after 450 Ma, TPW placed Siberia, Turan–Karakum–Tarim–North China, and South China entirely into the tropics; in addition, a large portion of Gondwana was moved down from high- to mid-latitudes (Fig.  5b ). All these palaeogeographic changes thus favor the observed intensification of silicate weathering helping drive cooling. These palaeogeographic conditions resulted in not only the Hirnantian glaciation, but also the marked positive Hirnantian carbon isotope excursion (Fig.  6b ) due to the increased fraction of organic carbon burial resulting from the preponderance of tropical continental margins (Fig.  5b ) analogous to the modern Amazon River. Previous studies suggest that arc-continent collisions within the humid tropical zone set Earth’s climate state to first order during the Phanerozoic Eon 2 . After the peak glaciation and positive carbon isotope excursion, a large portion of the arc-continent collisions of Laurentia, Siberia, and Turan–Karakum–Tarim–North China moved out of the tropics (Fig.  5c ), thereby silicate weathering plummeted causing deglaciation and carbon cycle recovery (Fig.  6 ).

Although there are myriad ways in which TPW may indirectly affect biodiversity through environmental change 26 , a direct link has also been proposed through a true polar wander–latitudinal diversity gradient (TPW–LDG) theory 18 , 21 . It suggests that continents shifting equatorward, i.e., moving into the LDG, would experience enhanced origination and hence diversity increase, while those shifting poleward, i.e., moving out of the LDG, would experience enhanced extinction and hence diversity decrease. This quadrature pattern of TPW effects on diversity is similar to those predicted for relative sea-level change during TPW 25 , which can further amplify the anticipated diversity changes through sea-level-related artifacts, e.g., continental margins moving equatorward should experience elevated origination (though TPW–LDG theory), and additionally, fossils of these new species are more likely to be preserved due to the concomitant transgression in sea level 21 . Recently reported high-resolution biodiversity records 1 , 7 , 11 suggest a protracted two- or three-phase extinction lasting from the Katian stage to the Hirnantian stage (Fig.  6c, d ), rather than a two-pulse extinction limited to the Hirnantian 5 . This modified extinction pattern may indicate that previous kill mechanisms proposed to fit the two-pulse extinction situation have become at least partially invalidated or weakened 59 .

Our reconstructions demonstrate that during 450–445 Ma, TPW moved southern Gondwana poleward and northern Gondwana equatorward (Fig.  5 ), with more area of Gondwana on average shifting poleward (Fig.  5a, b ). This observation, according to TPW–LDG theory, would cause both enhanced origination and extinction, but with extinction overwhelming origination. This palaeogeographic prediction appears to be supported by the fossil record 11 (Fig.  6d ). After that, a radiation phase (Fig.  6c, d ) should correspond with the continuing equatorward shift of Gondwana, Baltica, and Turan–Karakum–Tarim–North China until the early Hirnantian. Thus, during the fleeting Hirnantian stage, both the increased tropical weathering of arc-continent collisions triggered glaciation and the further poleward shifting of Siberia, Turan–Karakum–Tarim–North China, South China and south and east Gondwana, caused the second severe extinction (Fig.  6c, d ). Paleogeography after the TPW event also favored plant colonization as a majority of continents became located at low-to-mid latitudes (Fig.  5c ), which is supported by the positive carbon isotope signal (Fig.  6b ) and oxygen rise during this time 7 . Overall, the proposed TPW and the palaeogeographic reorganization resulting from the ~50˚ reorientation provide a simple and basic mechanism for the dramatic environmental changes of the end Ordovician early Silurian. These connections reflect the intimate coupling between the evolution of Earth’s spheres: TPW is induced by changes of subducting slabs in the mantle; in turn, TPW resulted in palaeogeographic changes that influenced Earth’s hydrosphere, cryosphere, and biosphere.

Our sampling sections are in Xiushan County, east Chongqing, China (Supplementary Figs.  1 , 2 ). Silurian strata in this area were folded during the middle Mesozoic (Jurassic–Cretaceous) 67 . Ascending in stratigraphic order, the Silurian strata consist of the Llandovery Longmaxi Formation (Fm) black shale, the Xiaoheba Fm green siltstone, the Rongxi Fm red beds, the Xiushan Fm siltstone, the Huixingshao Fm red beds, and the Ludlow-Pridoli Xiaoxi Fm siltstone with some red beds (Supplementary Figs.  1 , 2 ). There are disconformities between the Silurian strata and both its overlying Devonian and underlying Ordovician strata (Supplementary Figs.  1 , 2 ).

As previous studies 32 , 35 had intensively sampled the Rongxi Fm (Supplementary Fig.  1 ), we conducted our palaeomagnetic study on the previously sparsely sampled Huixingshao Fm at three sections (Supplementary Figs.  1 , 2 ). The section Yongdong (SY) is on the west limb of a steeply dipping syncline (GPS: 28.610°N, 109.157°E; Supplementary Fig.  2 ). Six sites about 104 samples are collected here. The sections Tianlu and Kapeng (ST and SK) are on the east limb of the same syncline (ST GPS: 28.551°N, 109.162°E; SK GPS: 28.631°N, 109.287°E; Supplementary Fig.  2 ). One site (10 samples) and four sites (43 samples) were collected from the ST and SK sections, respectively. All samples were collected with a portable gasoline-powered drill and oriented with a magnetic compass.

Palemagnetism and rock magnetism

All samples were cut into at least one standard specimen (height = 2.3 cm, diameter = 2.54 cm). Natural remanent magnetization (NRM) was firstly measured. Then, all specimens were subjected to stepwise thermal demagnetization using an ASC-TD48 oven, and remanent magnetizations were measured using a 2G-755 cryogenic superconducting magnetometer housed in a magnetically shielded room. Typically, demagnetization was applied in steps of 10–100 °C, starting at 100 °C and going up to 670 °C. Backfield demagnetization curves of representative specimens, with a 2 T saturation field followed by a progressively larger filed in opposite direction, were conducted on the VSM 8600 series (Lake Shore Cryotronics, Inc.). Susceptibility temperature ( κ-T ) experiments, heated and cooled between 30–700 °C, were measured in air by using the KLY4 Kappa bridge (AGICO) devices. All experiments were done at the Key Laboratory of Paleomagnetism and Reconstruction, Ministry of Natural Resources, Beijing.

Probability of sampling true polar wander

For any quantity, φ , that varies as a function of time, the probability density function (PDF) describes the relative amount of time spent at each value of φ or, alternatively, the likelihood that a particular value of φ will be sampled. For TPW, we let φ(t) be the angular separation of the rotation axis relative to a fixed geographic axis (TPW angle), or the angular distance from one estimate of the location of the spin axis to some later estimate. An un-normalized PDF for φ(t) is then given by

so that the probability of sampling φ within a given angular interval is

where α is a constant chosen so that the total probability P (0° <  φ  < 90°) equals 1. Equation ( 1 ) shows that the likelihood that φ is sampled is exactly inversely proportional to the TPW speed, | dφ/dt  | . This straightforward relationship between TPW sampling probability (PDF) and TPW speed shows that faster instantaneous TPW speeds are prone to be undersampled. Technically speaking, the uphill battle to observe rapid TPW is nothing more than the age-old stroboscopic effect, or aliasing: the difficulty of studying rotating planets, reciprocating blades, oscillating fans, or vibrating strings with discrete data; temporal undersampling is a major hurdle for understanding geologic processes 68 and TPW is not an exception.

We now demonstrate an example of the undersampling problem with a simple modeled TPW excursion. In the following example, we use the simple analytical framework of Tsai and Stevenson 69 to describe the TPW due to a chosen moment of inertia tensor variation. In this formulation, the Liouville equation for a viscoelastic planet is analytically solved for a given perturbation of the moment of inertia tensor, following Munk and MacDonald 70 , to obtain the TPW angle. For simplicity, we chose a moment of inertia variation that is sinusoidal with a period of 150 Ma, with an average viscosity of 3 × 10 22  Pa s, and an amplitude of 10 −5 C (where C is the Earth’s moment of inertia). This variation is chosen to very roughly approximate the observed TPW described in the next section. Equation (12) of Tsai and Stevenson 69 then yields the TPW angle as a function of time, φ(t) , which is plotted in Fig.  4a for an arbitrary chosen initial condition. The associated TPW speed (black line) and the PDF (shaded gray) for this TPW curve are plotted as a function of time in Fig.  4b . As shown, the maximum instantaneous TPW speed is about 2.2° Ma −1 (24 cm yr −1 ) and is associated with a minimum in the PDF, showing that it is the least probable value to be observed if one randomly samples the distribution. One can also compare the probability of sampling within a finite range by using Eq. ( 2 ), or by simply reading time intervals from Fig.  4a . For example, the values of φ in the range 4° <  φ  < 9° represent ~32% of all measurements whereas a similar 5° range 40° <  φ  < 45° represents only about 1.5% of all measurements. For this example, then, it is about 20 times more likely to sample within the first range of φ (low instantaneous TPW speed) compared with the second range (maximum instantaneous TPW speed).

Data availability

The palaeomagnetic data generated in this study, including the magnetometer measurements and backfield demagnetization data, have been deposited in the Open Science Framework database ( https://osf.io/z2cds/ ). Palaeomagnetic statistical data, palaeopoles and Euler parameters used in this study are provided in the Supplementary information.

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Acknowledgements

We thank Nie Lei and Chen Yang of Chongqing Key Laboratory of Exogenic Mineralization and Mine Environment, Chongqing Institute of Geology and Mineral Resources for information about the sampling sections and assistance to the drafting of Supplementary Fig.  2 . Victor Tsai provided Fig.  4 depicting the probability of sampling TPW. This study was supported by the Natural Science Foundation of China (grants 91855216, 41230208, 42002208). X.J. was supported by the China Scholarship Council (no. 201808110036). R.M. was supported by the Natural Science Foundation of China (grant 41888101) and the Key Research Program of the Institute of Geology & Geophysics, CAS (grant IGGCAS-201905).

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X.J., R.M., Z.Y. concept this research. X.J., Z.Y., M.Z., Y.T. conducted the field investigation and sampling. X.J. implemented all the thermal demagnetization and rock magnetic experiments. X.J., R.M., Z.Y., B.W. analyzed the data. Figures  1 l, 3 b, 5a–c , Supplementary Fig.  2 and Supplementary Fig.  5 were generated by X.J. Snowflake in Fig.  6a was generated by X.J. Map used in Supplementary Fig.  1 was generated by X.J. R.M. drew the Fig.  4 . Figures  3 , 5 , 6 were generated by X.J. and R.M. X.J., R.M., Z.Y. wrote the original draft. All authors contributed to the manuscript rewriting and editing.

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Jing, X., Yang, Z., Mitchell, R.N. et al. Ordovician–Silurian true polar wander as a mechanism for severe glaciation and mass extinction. Nat Commun 13 , 7941 (2022). https://doi.org/10.1038/s41467-022-35609-3

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4.2: Paleomagnetic Evidence for Plate Tectonics

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• Roger Williams University

Although Alfred Wegener would not live to see it, his theory of plate tectonics would gradually gain acceptance within the scientific community as more evidence began to accumulate. Some of the most important evidence came from the study of paleomagnetism , or changes in Earth’s magnetic field over millions of years.

Earth’s magnetic field is defined by the North and South Poles that align generally with the axis of rotation (Figure \(\PageIndex{1}\)). The lines of magnetic force flow into Earth in the Northern Hemisphere and out of Earth in the Southern Hemisphere. Because of the shape of the field lines, the magnetic force trends at different angles to the surface in different locations (red arrows of Figure \(\PageIndex{1}\)). At the North and South Poles, the force is vertical. Anywhere on the equator the force is horizontal, and everywhere in between, the magnetic force is at some intermediate angle to the surface.

In its fluid form, the minerals that make up magma are free to move in any direction and take on any orientation. But as the magma cools and solidifies, movement ceases and the mineral orientation and position become fixed. As the mineral magnetite (Fe 3 O 4 ) crystallizes from magma, it becomes magnetized with an orientation parallel to that of Earth’s magnetic field at that time, similar to the way a compass needle aligns with the magnetic field to point north. This magnetic record in the rock is called remnant magnetism . Rocks like basalt, which cool from a high temperature and commonly have relatively high levels of magnetite, are particularly susceptible to being magnetized in this way, but even sediments and sedimentary rocks, as long as they have small amounts of magnetite, will take on remnant magnetism because the magnetite grains gradually become reoriented following deposition. By studying both the horizontal and vertical components of the remnant magnetism, one can tell not only the direction to magnetic north at the time of the rock’s formation, but also the latitude where the rock formed relative to magnetic north.

In the early 1950s, a group of geologists from Cambridge University, including Keith Runcorn , Edward Irving and several others, started looking at the remnant magnetism of Phanerozoic British and European volcanic rocks, and collecting paleomagnetic data. They found that rocks of different ages sampled from generally the same area showed quite different apparent magnetic pole positions (green line, Figure \(\PageIndex{2}\)). They initially assumed that this meant that Earth’s magnetic field had, over time, departed significantly from its present position, which is close to the rotational pole.

The curve defined by the paleomagnetic data was called a polar wandering path because Runcorn and his colleagues initially thought that their data represented actual movement of the magnetic poles (since geophysical models of the time suggested that the magnetic poles did not need to be aligned with the rotational poles). We now know that the magnetic data define movement of continents, and not of the magnetic poles, so we call it an apparent polar wandering path (APWP). Runcorn and colleagues soon extended their work to North America, and this also showed apparent polar wandering, but the results were not consistent with those from Europe (Figure \(\PageIndex{2}\)). For example, the 200 Ma pole for North America placed somewhere in China, while the 200 Ma pole for Europe placed in the Pacific Ocean. Since there could only have been one pole position at 200 Ma, this evidence strongly supported the idea that North America and Europe had moved relative to each other since 200 Ma. Subsequent paleomagnetic work showed that South America, Africa, India, and Australia also have unique polar wandering curves. Rearranging the continents based on their positions in Pangaea caused these wandering curves to overlap, showing that the continents had moved over time.

Additional evidence for movement of the continents came from analysis of magnetic dip . Recall from Figure \(\PageIndex{1}\) that the angle of the magnetic field changes as a function of latitude, with the field directed vertically downwards at the north pole, upwards at the south pole, and horizontal at the equator. Every latitude between the equator and the poles will have a corresponding angle between horizontal and vertical (red arrows, Figure \(\PageIndex{1}\)). By looking at the dip angle in rocks, we can determine the latitude at which those rocks were formed. Combining that with the age of the rocks, we can trace the movements of the continents over time. For example, at around 500 Ma, what we now call Europe was south of the equator, and so European rocks formed then would have acquired an upward-pointing magnetic field orientation (Figure \(\PageIndex{3}\)). Between then and now, Europe gradually moved north, and the rocks forming at various times acquired steeper and steeper downward-pointing magnetic orientations.

This paleomagnetic work of the 1950s was the first new evidence in favor of continental drift, and it led a number of geologists to start thinking that the idea might have some merit.

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5.3: Magnetic Polarity Evidence for Continental Drift

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How did technology play a role in developing Wegener's idea?

After Wegener's death, the continental drift idea was pretty much dead. It would have remained that way except for the development of technology. Using technology, scientists would find more evidence that continents had drifted. They would also be able to find the mechanism. This type of magnetometer was one of their important tools.

Magnetic Polarity

Some important evidence for continental drift came after Wegener's death. The following is the magnetic evidence: Earth's magnetic field surrounds the planet from pole to pole. If you have ever been hiking or camping, you may have used a compass to help you find your way. A compass points to the magnetic North Pole. The compass needle aligns with Earth’s magnetic field.

Some rocks contain little compasses too! As lava cools, tiny iron-rich crystals line up with Earth’s magnetic field. These crystals are magnetite crystals. Anywhere lavas have cooled, these magnetite crystals point to the magnetic poles. The little magnets point to where the North Pole was when the lava cooled. A magnetometer is a device capable of measuring the magnetic field. A magnetometer can be used on land. A magnetometer also can be dragged behind a ship.

Evidence for Continental Drift

Geologists used magnetometers to look at rocks on land. They wanted to know which direction magnetite crystals pointed. This would tell them where magnetic north was at the time the rocks cooled. This is what they learned:

• Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole ( Figure below).

Earth’s current north magnetic pole is in northern Canada.

• Older rocks that are the same age and are on the same continent point to the same location. However, the location they point to is not the current north magnetic pole.
• Older rocks that are different ages and are on the same continent do not point to the same location. None of them point to the current magnetic north pole.
• Rocks on different continents that are the same age point to different locations. Only recent rocks point to the current north magnetic pole.

How did the geologists explain this? There is only one logical explanation. There was almost certainly only one north magnetic pole through Earth's history. The north magnetic pole is very likely in the same spot it has always been. If these two things are true, then the continents have moved.

Support for Wegener's Idea

Geologist tested the idea that the pole remained fixed but the continents moved. They fitted the continents together as Wegener had done. They moved them into the positions where they had been at the time the magnetite crystals cooled. The magnetite crystals pointed to the current north magnetic pole ( Figure below). The magnetic pole seemed to have moved, but had not. They named the phenomenon apparent polar wander .

On the left: The apparent North Pole for Europe and North America if the continents were always in their current locations. The two paths merge into one if the continents are allowed to drift.

Geologists were now more interested in continental drift. More than ever, they needed a mechanism.

• Magnetite is a magnetic mineral found in lava. The magnetite points to the magnetic north pole when it cools.
• Scientists used magnetometers to show where the north magnetic pole had been when magnetite crystals cooled.
• Magnetite crystals of different ages and on different continents pointed to different spots. The simplest explanation is that the continents have moved.
• Apparent polar wander is another line of evidence for drifting continents.
• What is apparent polar wander?
• Describe how magnetite indicates magnetic pole. Why does it sometimes point to a spot that is not where the pole is located?
• How did scientists use magnetic evidence to conclude that the continents moved?

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Chapter 10 Plate Tectonics

10.3 Geological Renaissance of the Mid-20th Century

As the mineral magnetite (Fe 3 O 4 ) crystallizes from magma, it becomes magnetized with an orientation parallel to that of Earth’s magnetic field at that time. This is called remnant magnetism . Rocks like basalt, which cool from a high temperature and commonly have relatively high levels of magnetite, are particularly susceptible to being magnetized in this way, but even sediments and sedimentary rocks, as long as they have small amounts of magnetite, will take on remnant magnetism because the magnetite grains gradually become reoriented following deposition. By studying both the horizontal and vertical components of the remnant magnetism, one can tell not only the direction to magnetic north at the time of the rock’s formation, but also the latitude where the rock formed relative to magnetic north.

In the early 1950s, a group of geologists from Cambridge University, including Keith Runcorn, Ted Irving, [1]  and several others, started looking at the remnant magnetism of Phanerozoic British and European volcanic rocks, and collecting paleomagnetic data. They found that rocks of different ages sampled from generally the same area showed quite different apparent magnetic pole positions (Figure 10.6). They initially assumed that this meant that Earth’s magnetic field had, over time, departed significantly from its present position — which is close to the rotational pole.

The curve defined by the paleomagnetic data was called a polar wandering path because Runcorn and his students initially thought that their data represented actual movement of the magnetic poles (since geophysical models of the time suggested that the magnetic poles did not need to be aligned with the rotational poles). We now know that the magnetic data define movement of continents, and not of the magnetic poles, so we call it an apparent polar wandering path (APWP).

What is a polar wandering path?

At around 500 Ma, what we now call Europe was south of the equator, and so European rocks formed then would have acquired an upward-pointing magnetic field orientation (see Figure 9.13 and the figure shown here). Between then and now, Europe gradually moved north, and the rocks forming at various times acquired steeper and steeper downward-pointing magnetic orientations.

When researchers evaluated magnetic data in this way in the 1950s, they plotted where the North Pole would have appeared to be based on the magnetic data and assumed that the continent was always where it is now. That means that the 500 Ma “apparent” north pole would have been somewhere in the South Pacific, and that over the following 500 million years it would have gradually moved north.

Of course we now know that the magnetic poles don’t move around much (although polarity reversals do take place) and that the reason Europe had a magnetic orientation characteristic of the southern hemisphere is that it was in the southern hemisphere at 500 Ma.

Runcorn and colleagues soon extended their work to North America, and this also showed apparent polar wandering, but the results were not consistent with those from Europe. For example, the 200 Ma pole for North America plotted somewhere in China, while the 200 Ma pole for Europe plotted in the Pacific Ocean. Since there could only have been one pole position at 200 Ma, this evidence strongly supported the idea that North America and Europe had moved relative to each other since 200 Ma. Subsequent paleomagnetic work showed that South America, Africa, India, and Australia also have unique polar wandering curves. In 1956, Runcorn changed his mind and became a proponent of continental drift.

This paleomagnetic work of the 1950s was the first new evidence in favour of continental drift, and it led a number of geologists to start thinking that the idea might have some merit. Nevertheless, for a majority of geologists working on global geology at the time, this type of evidence was not sufficiently convincing to get them to change their views.

During the 20th century, our knowledge and understanding of the ocean basins and their geology increased dramatically. Before 1900, we knew virtually nothing about the bathymetry and geology of the oceans. By the end of the 1960s, we had detailed maps of the topography of the ocean floors, a clear picture of the geology of ocean floor sediments and the solid rocks underneath them, and almost as much information about the geophysical nature of ocean rocks as of continental rocks.

Up until about the 1920s, ocean depths were measured using weighted lines dropped overboard. In deep water this is a painfully slow process and the number of soundings in the deep oceans was probably fewer than 1,000. That is roughly one depth sounding for every 350,000 square kilometres of the ocean. To put that in perspective, it would be like trying to describe the topography of British Columbia with elevation data for only a half a dozen points! The voyage of the Challenger in 1872 and the laying of trans-Atlantic cables had shown that there were mountains beneath the seas, but most geologists and oceanographers still believed that the oceans were essentially vast basins with flat bottoms, filled with thousands of metres of sediments.

Following development of acoustic depth sounders in the 1920s (Figure 10.7), the number of depth readings increased by many orders of magnitude, and by the 1930s, it had become apparent that there were major mountain chains in all of the world’s oceans. During and after World War II, there was a well-organized campaign to study the oceans, and by 1959, sufficient bathymetric data had been collected to produce detailed maps of all the oceans (Figure 10.8).

The important physical features of the ocean floor are:

• Extensive linear ridges (commonly in the central parts of the oceans) with water depths in the order of 2,000 to 3,000 m (Figure 10.8, inset a)
• Fracture zones perpendicular to the ridges (inset a)
• Deep-ocean plains at depths of 5,000 to 6,000 m (insets a and d)
• Relatively flat and shallow continental shelves with depths under 500 m (inset b)
• Deep trenches (up to 11,000 m deep), most near the continents (inset c)
• Seamounts and chains of seamounts (inset d)

Seismic reflection sounding involves transmitting high-energy sound bursts and then measuring the echos with a series of geophones towed behind a ship. The technique is related to acoustic sounding as described above; however, much more energy is transmitted and the sophistication of the data processing is much greater. As the technique evolved, and the amount of energy was increased, it became possible to see through the sea-floor sediments and map the bedrock topography and crustal thickness. Hence sediment thicknesses could be mapped, and it was soon discovered that although the sediments were up to several thousands of metres thick near the continents, they were relatively thin — or even non-existent — in the ocean ridge areas (Figure 10.9). The seismic studies also showed that the crust is relatively thin under the oceans (5 km to 6 km) compared to the continents (30 km to 60 km) and geologically very consistent, composed almost entirely of basalt.

In the early 1950s, Edward Bullard, who spent time at the University of Toronto but is mostly associated with Cambridge University, developed a probe for measuring the flow of heat from the ocean floor. Bullard and colleagues found the rate to be higher than average along the ridges, and lower than average in the trench areas. Although Bullard was a plate-tectonics sceptic, these features were interpreted to indicate that there is convection within the mantle — the areas of high heat flow being correlated with upward convection of hot mantle material, and the areas of low heat flow being correlated with downward convection.

With developments of networks of seismographic stations in the 1950s, it became possible to plot the locations and depths of both major and minor earthquakes with great accuracy. It was found that there is a remarkable correspondence between earthquakes and both the mid-ocean ridges and the deep ocean trenches. In 1954 Gutenberg and Richter showed that the ocean-ridge earthquakes were all relatively shallow, and confirmed what had first been shown by Benioff in the 1930s — that earthquakes in the vicinity of ocean trenches were both shallow and deep, but that the deeper ones were situated progressively farther inland from the trenches (Figure 10.10).

In the 1950s, scientists from the Scripps Oceanographic Institute in California persuaded the U.S. Coast Guard to include magnetometer readings on one of their expeditions to study ocean floor topography. The first comprehensive magnetic data set was compiled in 1958 for an area off the coast of B.C. and Washington State. This survey revealed a bewildering pattern of low and high magnetic intensity in sea-floor rocks (Figure 10.11). When the data were first plotted on a map in 1961, nobody understood them — not even the scientists who collected them. Although the patterns made even less sense than the stripes on a zebra, many thousands of kilometres of magnetic surveys were conducted over the next several years.

The wealth of new data from the oceans began to significantly influence geological thinking in the 1960s. In 1960, Harold Hess, a widely respected geologist from Princeton University, advanced a theory with many of the elements that we now accept as plate tectonics . He maintained some uncertainty about his proposal however, and in order to deflect criticism from mainstream geologists, he labelled it geopoetry . In fact, until 1962, Hess didn’t even put his ideas in writing — except internally to the U.S. Navy (which funded his research) — but presented them mostly in lectures and seminars. Hess proposed that new sea floor was generated from mantle material at the ocean ridges, and that old sea floor was dragged down at the ocean trenches and re-incorporated into the mantle. He suggested that the process was driven by mantle convection currents, rising at the ridges and descending at the trenches (Figure 10.12). He also suggested that the less-dense continental crust did not descend with oceanic crust into trenches, but that colliding land masses were thrust up to form mountains. Hess’s theory formed the basis for our ideas on sea-floor spreading and continental drift, but it did not deal with the concept that the crust is made up of specific plates . Although the Hess model was not roundly criticized, it was not widely accepted (especially in the U.S.), partly because it was not well supported by hard evidence.

Collection of magnetic data from the oceans continued in the early 1960s, but still nobody could explain the origin of the zebra-like patterns. Most assumed that they were related to variations in the composition of the rocks — such as variations in the amount of magnetite — as this is a common explanation for magnetic variations in rocks of the continental crust. The first real understanding of the significance of the striped anomalies was the interpretation by Fred Vine, a Cambridge graduate student. Vine was examining magnetic data from the Indian Ocean and, like others before, he noted the symmetry of the magnetic patterns with respect to the oceanic ridge.

At the same time, other researchers, led by groups in California and New Zealand, were studying the phenomenon of reversals in Earth’s magnetic field. They were trying to determine when such reversals had taken place over the past several million years by analyzing the magnetic characteristics of hundreds of samples from basaltic flows. As discussed in Chapter 9, it is evident that Earth’s magnetic field becomes weakened periodically and then virtually non-existent, before becoming re-established with the reverse polarity. During periods of reversed polarity, a compass would point south instead of north.

The time scale of magnetic reversals is irregular. For example, the present “normal” event, known as the Bruhnes magnetic chron, has persisted for about 780,000 years. This was preceded by a 190,000-year reversed event; a 50,000-year normal event known as Jaramillo; and then a 700,000-year reversed event (see Figure 9.15).

In a paper published in September 1963, Vine and his PhD supervisor Drummond Matthews proposed that the patterns associated with ridges were related to the magnetic reversals, and that oceanic crust created from cooling basalt during a normal event would have polarity aligned with the present magnetic field, and thus would produce a positive anomaly (a black stripe on the sea-floor magnetic map), whereas oceanic crust created during a reversed event would have polarity opposite to the present field and thus would produce a negative magnetic anomaly (a white stripe). The same idea had been put forward a few months earlier by Lawrence Morley, of the Geological Survey of Canada; however, his papers submitted earlier in 1963 to Nature and The Journal of Geophysical Research were rejected. Many people refer to the idea as the Vine-Matthews-Morley (VMM) hypothesis.

Vine, Matthews, and Morley were the first to show this type of correspondence between the relative widths of the stripes and the periods of the magnetic reversals. The VMM hypothesis was confirmed within a few years when magnetic data were compiled from spreading ridges around the world. It was shown that the same general magnetic patterns were present straddling each ridge, although the widths of the anomalies varied according to the spreading rates characteristic of the different ridges. It was also shown that the patterns corresponded with the chronology of Earth’s magnetic field reversals. This global consistency provided strong support for the VMM hypothesis and led to rejection of the other explanations for the magnetic anomalies.

In 1963, J. Tuzo Wilson of the University of Toronto proposed the idea of a mantle plume or hot spot — a place where hot mantle material rises in a stationary and semi-permanent plume, and affects the overlying crust. He based this hypothesis partly on the distribution of the Hawaiian and Emperor Seamount island chains in the Pacific Ocean (Figure 10.13). The volcanic rock making up these islands gets progressively younger toward the southeast, culminating with the island of Hawaii itself, which consists of rock that is almost all younger than 1 Ma. Wilson suggested that a stationary plume of hot upwelling mantle material is the source of the Hawaiian volcanism, and that the ocean crust of the Pacific Plate is moving toward the northwest over this hot spot. Near the Midway Islands, the chain takes a pronounced change in direction, from northwest-southeast for the Hawaiian Islands and to nearly north-south for the Emperor Seamounts. This change is widely ascribed to a change in direction of the Pacific Plate moving over the stationary mantle plume, but a more plausible explanation is that the Hawaiian mantle plume has not actually been stationary throughout its history, and in fact moved at least 2,000 km south over the period between 81 and 45 Ma. [2]

Exercise 10.2 Volcanoes and the Rate of Plate Motion

The Hawaiian and Emperor volcanoes shown in Figure 10.13 are listed in the table below along with their ages and their distances from the centre of the mantle plume under Hawaii (the Big Island).

Plot the data on the graph provided here, and use the numbers in the table to estimate the rates of plate motion for the Pacific Plate in cm/year. (The first two are plotted for you.)

There is evidence of many such mantle plumes around the world (Figure 10.14). Most are within the ocean basins — including places like Hawaii, Iceland, and the Galapagos Islands — but some are under continents. One example is the Yellowstone hot spot in the west-central United States, and another is the one responsible for the Anahim Volcanic Belt in central British Columbia. It is evident that mantle plumes are very long-lived phenomena, lasting for at least tens of millions of years, possibly for hundreds of millions of years in some cases.

Although oceanic spreading ridges appear to be curved features on Earth’s surface, in fact the ridges are composed of a series of straight-line segments, offset at intervals by faults perpendicular to the ridge (Figure 10.15). In a paper published in 1965, Tuzo Wilson termed these features transform faults . He described the nature of the motion along them, and showed why there are earthquakes only on the section of a transform fault between two adjacent ridge segments. The San Andreas Fault in California is a very long transform fault that links the southern end of the Juan de Fuca spreading ridge to the East Pacific Rise spreading ridges situated in the Gulf of California (see Figure 10.23). The Queen Charlotte Fault, which extends north from the northern end of the Juan de Fuca spreading ridge (near the northern end of Vancouver Island) toward Alaska, is also a transform fault.

In the same 1965 paper, Wilson introduced the idea that the crust can be divided into a series of rigid plates, and thus he is responsible for the term plate tectonics .

Exercise 10.3 Paper Transform Fault Model

Tuzo Wilson used a paper model, a little bit like the one shown here, to explain transform faults to his colleagues. To use this model print this page, cut around the outside, and then slice along the line A-B (the fracture zone) with a sharp knife. Fold down the top half where shown, and then pinch together in the middle. Do the same with the bottom half. When you’re done, you should have something like the example below, with two folds of paper extending underneath. Find someone else to pinch those folds with two fingers just below each ridge crest, and then gently pull apart where shown. As you do, the oceanic crust will emerge from the middle, and you’ll see that the parts of the fracture zone between the ridge crests will be moving in opposite directions (this is the transform fault) while the parts of the fracture zone outside of the ridge crests will be moving in the same direction. You’ll also see that the oceanic crust is being magnetized as it forms at the ridge. The magnetic patterns shown are accurate, and represent the last 2.5 Ma of geological time.

There are other versions of this model available at https://web.viu.ca/earle/transform-model/ . For more information see: Earle, S., 2004,   A simple paper model of a transform fault at a spreading ridge , J. Geosc. Educ. V. 52, p. 391-2.

• Ted Irving later set up a paleomagnetic lab at the Geological Survey of Canada in Sidney, B.C., and did a great deal of important work on understanding the geology of western North America. ↵
• J. A. Tarduno et al., 2003, The Emperor Seamounts: Southward Motion of the Hawaiian Hotspot Plume in Earth’s Mantle, Science 301 (5636): 1064–1069. ↵

Physical Geology Copyright © 2015 by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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After World War II, geologists developed the paleomagnetic dating technique to measure the movements of the magnetic north pole over geologic time. In the early to mid 1960s, Dr. Robert Dubois introduced this new absolute dating technique to archaeology as archaeomagnetic dating.

Magnetism occurs whenever electrically charged particles are in motion. The Earth's molten core has electric currents flowing through it. As the earth rotates, these electric currents produce a magnetic field that extends outward into space. This process, in which the rotation of a planet with an iron core produces a magnetic field, is called a dynamo effect.

The Earth's magnetic core is generally inclined at an 11 degree angle from the Earth's axis of rotation. Therefore, the magnetic north pole is at approximately an 11 degree angle from the geographic north pole. On the earth's surface, when you hold a compass and the needle points to north, it is actually pointing to magnetic north, not geographic (true) north.

The Earth's magnetic north pole can change in orientation (from north to south and south to north), and has many times over the millions of years that this planet has existed. The term that refers to changes in the Earth's magnetic field in the past is paleomagnetism. Any changes that occur in the magnetic field will occur all over the world; they can be used to correlate stratigraphic columns in different locations. This correlation process is called magnetostratigraphy.

Lava, clay, lake and ocean sediments all contain microscopic iron particles. When lava and clay are heated, or lake and ocean sediments settle through the water, they acquire a magnetization parallel to the Earth's magnetic field. After they cool or settle, they maintain this magnetization, unless they are reheated or disturbed. This process is called thermoremanent magnetization in the case of lava and clay, and depositional remanent magnetization in the case of lake and ocean sediments.

In addition to changing in orientation, the magnetic north pole also wanders around the geographic north pole. Archaeomagnetic dating measures the magnetic polar wander.

For example, in the process of making a fire pit, a person can use clay to create the desired shape of the firepit. In order to harden the clay permanently, one must heat it above a certain temperature (the Curie point) for a specified amount of time. This heating, or firing, process resets the iron particles in the clay. They now point to the location of magnetic north at the time the firepit is being heated. When the firepit cools the iron particles in the hardened clay keep this thermoremanent magnetization. However, each time the firepit is reheated above the Curie point while being used to cook something, or provide heat, the magnetization is reset. Therefore, you would use archaeomagnetic dating to date the last time the firepit was heated above the Curie point temperature.

Paleomagnetism and Archaeomagnetism rely on remnant magnetism,as was explained above. In general, when clay is heated, the microscopic iron particles within it acquire a remnant magnetism parallel to the earth's magnetic field. They also point toward the location around the geographic north pole where the magnetic north pole was at that moment in its wandering. Once the clay cools, the iron particles maintain that magnetism until the clay is reheated. By using another dating method (dendrochonology, radiocarbon dating) to obtain the absolute date of an archaeological feature (such as a hearth), and measuring the direction of magnetism and wander in the clay today, it is possible to determine the location of the magnetic north pole at the time this clay was last fired. This is called the virtual geomagnetic pole or VGP. Archaeologists assemble a large number of these ancient VGPs and construct a composite curve of polar wandering (a VGP curve). The VGP curve can then be used as a master record, against which the VGPs of samples of unknown age can be compared to and assigned a date.

Geologists collect paleomagnetic samples by drilling and removing a core from bedrock, a lava flow, or lake and ocean bottom sediments. They make a marking on the top of the core which indicates the location of the magnetic north pole at the time the core was collected. This core is taken back to a laboratory, and a magnetometer is used to measure the orientation of the iron particles in the core. This tells the geologist the orientation of the magnetic pole when the rock was hot.

Archaeologists collect archaeomagnetic samples by carefully removing samples of baked clay from a firepit using a saw. A nonmagnetic, cube-shaped mold (aluminum) is placed over the sample, and it is filled with plaster. The archaeologist then records the location of magnetic north on the cube, after the plaster hardens. The vertical and horizontal placement of the sample is also recorded. Eight to twelve samples are collected and sent to a laboratory for processing. A magnetometer is used to measure the orientation of the iron particles in the samples. The location of the magnetic pole and age are determined for that firepit by looking at the average direction of all samples collected.

Using this technique, a core or sample can be directly dated. There are a number of limitations, however.

Archaeometry Journal Home Page

Paleomagnetic Data at NOAA National Data Center

Centre for Environmental Magnetism and Palaeomagnetism (CEMP)

Fort Hoofddijk Paleomagnetic Laboratory, Utrecht University, Netherlands

Institute for Rock Magnetism, University of Minnesota

Rock-Magnetism & Paleomagnetism Lab, Geological Survey of Japan

Los Hornos: A Case Study in Chronology

Laboratory of Earth's Magnetism, Saint-Petersburg State University, Russia

CSU Archaeometric Laboratory

Eighmy, J.L. 1980. Archaeomagnetic Dating: A Handbook for Archaeologists .

Eighmy, J.L., and R.S. Sternberg, eds. 1990. Archaeomagnetic Dating .

Butler, R.F. 1992. Paleomagnetism: Magnetic Domains to Geologic Terrains .

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• Daily News Analysis

Polar wandering

UPSC GS PAPER I: Salient features of World’s Physical Geography.

Context: Melting glaciers due to climate change caused Earth’s axis to shift since mid-90s

• The drift happened as the melting glaciers redistributed water, which made the direction of the polar wander turn and accelerate.
• The latest research has that the Earth's North and South poles have moved since the mid-1900s.
• They have been affected due to the melting of glaciers and other factors caused by humans, namely climate change.
• The rapid melting of glaciers has also affected the rotation of Earth.
• This study was conducted by researchers from the University of the Chinese Academy of Sciences and the Technical University of Denmark (DTU).
• Melting glaciers due to climate change caused Earths axis to shift since mid90s.
• The drift happened in the mid-1990s as the melting glaciers redistributed water which made the direction of the polar wander turn and accelerate.
• The new study uses data from the Gravity Recovery and Climate Experiment (GRACE) and observations from the 2018 project GRACE-FO to explain the drift in the Earth’s axis due to glacier melting.
• It is the migration of the magnetic poles over Earth’s surface through geologic time.
• Polar wander is the motion of a pole in relation to some reference frame .
• It can be used, for example, to measure the degree to which Earth's magnetic poles have been observed to move relative to the Earth's rotation axis.
• It is also possible to use continents as reference and observe the relative motion of the magnetic pole relative to the different continents; by doing so, the relative motion of those two continents to each other can be observed over geologic time as

https://www.firstpost.com/science/melting-glaciers-due-to-climate-change-caused-earths-axis-to-shift-since-mid-90s-9573721.html

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