This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. In previous podcasts, we've talked about reconstructing the past motions of tectonic plates to piece together the formation and breakup of supercontinents, the building of mountain chains, and the rifting and seafloor spreading that created the oceans. Perhaps most dramatically, Dietmar Müller described his billion-year global reconstruction of plate motions. All of these discussions were based on data having a temporal resolution of about 5 million years. But what can we see when we examine the data recording plate motions at a temporal resolution of just 1 million years?

Chuck DeMets is emeritus professor of geosciences at the University of Wisconsin–Madison. In his research, he studies the record of magnetic anomalies in the seafloor at the highest possible resolution. The work has revealed completely unexpected changes in plate velocities on these hitherto unseen shorter timescales. Chuck DeMets, welcome to Geology Bites.

Chuck DeMets: Thank you, Oliver. I'm happy to participate.

Oliver Strimpel: I believe all your work starts with measurements of magnetic anomalies in the seafloor. Can you explain what these anomalies are and how they can enable us to reconstruct plate motions?

Chuck DeMets: So the basic principle is that rocks that contain the mineral magnetite can record the state of Earth's magnetic field at the time that the rock forms, from a lava, for example. During the last 780,000 years, all lava flows that have come from volcanoes, both on land and along Earth's seafloor spreading centers in the oceans, have recorded that Earth's magnetic north pole was, as it is presently, near our geographic North Pole. In fact, most of those tiny magnetite grains in the liquid lavas act like tiny compass needles, by aligning themselves to point toward Earth's magnetic north pole. So when a lava freezes to a solid, those tiny aligned magnetite grains leave behind a permanent, measurable magnetic field that a magnetometer can record very easily. So a rock's magnetic field therefore slightly increases, or can also decrease, the intensity and direction of Earth's overall, much larger magnetic field close to the rock. And so the localized change in Earth's magnetic field intensity is referred to as a magnetic anomaly, since the field's recorded value, the value recorded by the magnetometer, is slightly higher or lower than the expected Earth field value.

So it's very well established that Earth's magnetic field hasn't always pointed north. There are numerous examples all over Earth, throughout the rock column, that show that Earth's magnetic field has also pointed toward geographic south at some times in the past. For example, the most recent reversal of the field occurred 781,000 years ago, at which time Earth's field flipped to its present state after spending about 200,000 years in its reversed state, meaning that magnetic south at that time was at the geographic South Pole. So the time at which Earth's field changes its orientation is referred to as a magnetic reversal. And most notably, lavas that are constantly extruded along Earth's ocean ridges, where new seafloor is created, are often or always rich in minerals like magnetite that respond to our magnetic field. So Earth's seafloor spreading centers act like a gigantic tape recorder. They faithfully record every field reversal that has ever occurred. So the pattern of magnetic reversals that is preserved in the seafloor becomes a powerful and very clear means of reconstructing where the tectonic plates have been in the past, and how quickly the plates have moved.

Oliver Strimpel: Wow, that's a real gift of nature, but it does seem to rely on the fact that we have some independent way of knowing the age of these magnetic reversals in the record. How do we know that?

Chuck DeMets: Well, there are two different, independent means of estimating when reversals occurred. And the first is the well-known method of using radioactive element concentrations in a rock to estimate its age, which is called radiometric dating. The second is based on the well-understood principle that predictable, periodic changes in Earth's orbital characteristics are responsible for well-documented past variations in Earth's climate over millions of years. And these climatic variations, which largely involve Earth going into and coming out of ice ages, cause well-known variations in the chemistry of sediments that are being deposited in lakes and oceans at the time that the climate is changing. Astronomers have worked out these orbital variations with very high precision, and we refer to these as Milankovitch cycles. So one can use the known timing of these orbital variations and compare it to the well-documented variations in the chemistry of sediments in lakes and oceans to estimate magnetic field reversal ages, because those same sediments also are magnetized and hold within them all of the magnetic reversals.

So we refer to this as an astronomically based method for estimating magnetic reversal ages, and it has nothing to do with radioactivity. So it's a completely independent method for determining reversal ages. It's kind of analogous to using tree rings in order to count back in time to determine the age of a tree, or a given ring, but in this case, the tree rings are sedimentary rock layers, often, which have been extracted from the oceans with deep drilling rigs mounted on ships. So reversal ages have been estimated with radiometric dating and using Milankovitch cycles, and amazingly, those ages largely agree to the nearest one percent, which equates to an accuracy of about 10,000 to 100,000 years in the reversal age estimates.

Oliver Strimpel: You mentioned that this seafloor spreading is a bit like a tape recorder, and that reminds me of that: with the old audio reel-to-reel or cassette tape recorders, if you stored it on a shelf for long enough, you'd get a kind of degradation, and the field would kind of wear out. And I'm wondering if, over the millions of years since these rocks extruded from the mid-ocean ridges, whether the magnetic field imprinted in the, well, tape recorder kind of wears out.

Chuck DeMets: The good news is no. The magnetization in rocks is incredibly stable. And so what it takes to demagnetize a rock that has a permanent magnetization is to heat it to generally more than about 1,000 degrees Fahrenheit, and that only happens if a rock is very deeply buried. A magnetized rock may also lose some of its magnetization if the minerals in the rock go through enough metamorphism. But generally, what we see is that the hard part of rock magnetization, its original magnetization, does not change even after several hundred million years.

Oliver Strimpel: So these magnetic stripes in the seafloor: well, it's on the seafloor; it's under a bunch of water. So do we have to go down to the seafloor to tow a magnetometer close to the seafloor to measure these anomalies?

Chuck DeMets: No. There are three different ways that seafloor magnetization is measured, and interestingly, even an airplane that's flying 20,000 feet above the ocean, if it has a magnetometer, can sense the magnetic anomalies that are associated with those magnetic stripes. So the three means, then, are airplanes, ships and deep submersibles, like the Alvin craft.

Oliver Strimpel: I said in my introduction that for some decades now, we've used past plate positions measured to an accuracy of about 5 million years. But you just said that we can actually date these anomalies to much better than that. So what was it that set that 5-million-year limit on the temporal resolution in the past?

Chuck DeMets: Historically, reconstructions of the positions of tectonic plates and their motions were based on selecting the most prominently displayed magnetic anomalies. So some magnetic anomalies, for example anomaly 5, which has an age of about 11 million years, anomaly 6, which has an age of about 20 million years, and anomaly 2A, which is about 3 million years old, are very easy to see when a ship, for example, tows a magnetometer across a spreading center and records those anomalies. So those anomalies are easy to identify, and it so happens that they're spaced every five to 10 million years. So as a consequence, historically, the resolution in time of reconstructions of plate motions has had time steps of 5 to 10 million years. And although that's fine if you wish to reconstruct plate movements over 1 billion years, it's too coarse a resolution for geological studies that may be looking at processes that play out over only five or 10 million years.

Oliver Strimpel: So how were you able to improve the temporal resolution by a factor of five?

Chuck DeMets: Well, even though there are prominent magnetic reversals that are easy to pick, there are many other magnetic reversals that are identifiable in magnetic anomaly profiles. And so I simply started to identify these other, less prominently displayed magnetic reversals, sometimes more difficult to identify unambiguously. And because there are so many magnetic data that had been gathered by ships and planes during the past half-century, there's a lot of redundancy in the data, and one can identify these less prominent magnetic reversals with high confidence. I tried to identify reversals that were spaced roughly 1 million years apart. Sometimes they were only spaced a couple hundred thousand years apart, other times 2 million years apart. It just depends on when Earth's field reversed, which of course we have no control over. So fortunately, in the past 20 million years, Earth's field has reversed pretty frequently, frequently enough that I could select reversals on roughly a 1-million-year spacing.

Oliver Strimpel: In order to get those less clear timestamps, if you like, did you literally have to eyeball the magnetometer traces yourself, or could you use machine learning to pick out these signals?

Chuck DeMets: Nearly all interpretations of magnetic profiles and the seafloor spreading magnetic record for the last 50 years have been done by eye, by constructing what we call synthetic magnetic profiles, which are profiles of what the magnetic anomaly should look like if every reversal was perfectly recorded. And so these synthetic profiles become a basis for interpreting the noisier profiles that ships and airplanes collect. By comparing one against the other, and visually cross-correlating the two waveforms, we can identify all the little wiggles that are recording the magnetic reversals.

Oliver Strimpel: So it sounds like you really managed to improve the resolution by really analyzing existing magnetic data to extract the maximum information possible from it. But did you also have access to data from magnetic surveys of the seafloor that were not available to previous workers?

Chuck DeMets: Yes. Nearly all of this high-resolution plate motion research that we're talking about has been done in close collaboration with a Russian professor. Professor Merkouriev is perhaps the preeminent plate tectonicist in Russia, and as such, he has possession of some magnificent Russian magnetic surveys that were done by the Russian Navy back in the 1980s and 1990s. One of the surveys that he, but no one else, had was of a large area of seafloor in the northern and central Indian Ocean, which is a critical region for understanding the collision of India with Asia over the last 70 million years, and that particular Russian survey is the most extensive such magnetic survey that has ever been done by anyone anywhere on Earth.

Oliver Strimpel: What was the purpose of this survey?

Chuck DeMets: My best guess is that they were trying to determine what the background magnetic field associated with seafloor spreading anomalies looked like in the Indian Ocean, so in the future they could determine whether new magnetic surveys were seeing anomalies that might, for example, be associated with submarines. And so submarines, of course, are moving steel objects, and they can have enormous magnetic signatures and are detectable with magnetometers that are deployed in planes or on ships.

Oliver Strimpel: That sounds like pretty sensitive data. So I guess you must have been quite lucky to get your hands on it.

Chuck DeMets: Sergei Merkouriev was on the Soviet military ships that ran these surveys during a two-year period. His purpose on the ship was to guide the surveys, to collect the data for his PhD dissertation. So he spent two years of his life under miserable living conditions to gather these data. After he returned to the Soviet Union and began working on the data, the Soviet Union was going through its transition to Russia, so there was a lot of turmoil and loss of support for scientific research. So he published some early maps of the data around 1987, at which point I became aware of those data. And I then learned that Sergei had gone to Paris with the paper records of these surveys. All of the data were collected on paper rolls, not by computer, and he spent two years working with French investigators in Paris to digitize all these data, and they extracted low-resolution, coarse reconstructions. I contacted the French investigator who was leading that work with Sergei back in the late '90s, and I asked him whether I might get access to the data, and he just laughed and said, "You've got to be kidding. We just spent two years digitizing it. I'm not giving it to you." So I kind of gave up.

But then, around the late 1990s, I thought, why not just contact Sergei Merkouriev directly and ask him whether he might be willing to share the data? And so Sergei responded to me within about 20 minutes and sent me a hundred files full of data. I was absolutely stunned, because I had been waiting for five years hoping to get these data, and all of a sudden I just had them. So I quickly wrote a proposal to the National Science Foundation, informing them of this incredible opportunity to access this Soviet military data and do an international study with the Russian scientist, and they funded me very quickly, which then started a 15-year-long relationship between myself and Sergei, to use these and other data in order to study plate tectonics.

Oliver Strimpel: OK, well, let's talk about the Indian Ocean and the motions of the Indian plate. What did your kinematic studies reveal there?

Chuck DeMets: My work in the Indian Ocean basin focused on reconstructing at high resolution the seafloor spreading histories of three seafloor spreading centers. The first of these is the Carlsberg Ridge, and the Carlsberg Ridge is positioned at the southern edge of the northward-moving India plate. And so India, as it moves northward and collides with Asia, is moving away from the Carlsberg Ridge. I've also reconstructed plate motions along the Central Indian Ridge, which, like the Carlsberg Ridge, is also along the edge of the India plate. And then the third seafloor spreading center is the Southwest Indian Ridge, and it is positioned several thousand kilometers south of the Carlsberg Ridge, at the southern edge of the Africa, or if you will, the Somalia, plate.

Because the Carlsberg and Central Indian ridges are both positioned at the southern edge of the India plate, one might presume that they are passively recording any major geologic or tectonic events that continental India has experienced during its collision with Asia over the last 70 million years. Because the Southwest Indian Ridge doesn't border the India plate but borders the Africa plate, it may record major events associated with India, but is more likely passively recording major events that affected Africa, possibly including the opening of the East African Rift system during the last 20 million years, and Africa's rupture away from Eurasia and Arabia between about 30 and 20 million years ago. OK? So that's a little bit of background.

Along the Carlsberg Ridge, our high-resolution reconstructions confirm previous results that seafloor spreading rates between India and Africa slowed by about 90% between 60 million years and 46 million years ago. They slowed from what were some of the fastest ever known seafloor spreading rates on Earth, before 50 million years ago, to some of the slowest rates ever known on Earth, after 46 million years ago. So this enormous slowdown in seafloor spreading rates along the Carlsberg Ridge has been variously attributed to a response to the initial collision of continental India and Asia, or possibly affected or influenced by the arrival of the Réunion hotspot plume below India, and ultimately below Africa.

So our higher-resolution reconstructions revealed some surprising things that weren't known before. We found out that during this dramatic slowdown, there was an intervening several-million-year period during which seafloor spreading rates stopped going down and rose again, possibly increasing by as much as 100%, after which they finally then slowed rapidly until about 46 million years ago, at which point they remained very slow for at least 20 million years. We also found that at about 40 million years ago, there was a small pulse in spreading rates, a brief acceleration followed by a deceleration, and nobody knew about that before. It's a small feature, but we think it's robust. And between about 40 and 22 million years ago, seafloor spreading rates stayed very steady and very slow, possibly telling us that it was very difficult to jam continental India below continental Eurasia and build the Himalayas during that period. But surprisingly, then, at about 22 million years ago, seafloor spreading rates started increasing, and they had doubled by 18 million years, and after 18 million years, rates then declined slowly, and by 13 million years they became steady.

And interestingly, our plate reconstructions suggest that the seafloor spreading rates along the Carlsberg Ridge have not changed by even one percent during the last 13 million years, which is rather surprising, since your intuition might suggest that building up the Himalayas at India's northern edge might give rise to gradually slowing northward motion of the India plate, and by inference gradually slowing seafloor spreading rates along the Carlsberg Ridge. But that wasn't the case.

Oliver Strimpel: I'm reminded of a recent podcast that Douwe van Hinsbergen gave on the question of what drives plate motions, and he did allude to this rather counterintuitive speedup you just mentioned after the collision. And I just want to double-check: are you sure that that is really in the data, and not an artifact that will go away when you scrutinize it even more closely, perhaps?

Chuck DeMets: Well, I've invested a lot of effort in looking at the robustness of this speedup at about 53 million years, and it depends critically on how accurate our estimates are of the magnetic reversal ages that bracket that speedup. It's certainly conceivable that future revisions to those reversal age estimates could make the magnitude of that apparent speedup smaller, possibly make it go away altogether, although I'm skeptical that that could happen.

Oliver Strimpel: So of course, the immediate question that comes to mind is: what is going on here? What can make the plate accelerate while it continues to plow into Eurasia, as you mentioned, jacking up the Tibetan Plateau as it goes, and presumably encountering no less resistance as it goes, and perhaps more?

Chuck DeMets: So to first order, it seems quite clear that the strong downward pull on seafloor that is subducting along a deep-sea trench is what causes most plate motion. So if you want to understand why it is that India continues to collide northward with southern Asia, despite the obvious difficulty of driving continental crust together, one simple response is that the India plate also includes seafloor that is subducting along the Sumatra trench. So the India plate isn't just simply a continent. It's an amalgam of continental crust at its northern end, but also seafloor offshore from the Sumatra trench. And the forces that are pulling the Indian plate seafloor down the Sumatra trench are apparently enough to drive the larger Indian plate, including its continental portions, northward into Eurasia along the Himalayas.

So there's obviously a huge disparity between subducting seafloor, Indian plate seafloor, along the Sumatra trench, versus driving the Himalayan collision, a continent-continent collision. And because of that difference in the ease of subducting some part of the Indian plate versus colliding continental India with Asia, there are immense internal stresses in the Indian plate, and those stresses are playing out by faulting that is occurring within the Indian plate, and that is building a new mountain belt deep below the Indian Ocean, in the equatorial abyssal plain south of India. One of the effects of these huge internal stresses in the Indian plate is that in 2012, a magnitude 8.6 earthquake ruptured the internal part of the India plate south of continental India, and that was the largest such earthquake ever recorded within any tectonic plate.

Oliver Strimpel: Interesting. So the picture is really much more complicated than just the Indian plate hitting Eurasia and encountering resistance up there. So perhaps there was something that either increased the slab pull or reduced the resistance, or gave it an extra push, or somehow or other released the plate, that enables it temporarily to speed up, that has nothing to do with Himalayan collision.

Chuck DeMets: Yeah. Slab pull forces along the Sumatra trench might have evolved enough, or possibly changed suddenly enough, to alter the overall balance of forces that were driving India plate motion back at 53 million years ago, when it appears to have sped up. Another possibility, one that's hotly debated, is whether the rise of the Réunion hotspot plume below the India plate at 70 million years could have played a key role in altering the motion of the India plate. The emergence of that hotspot, the Réunion hotspot, at the base of the India plate was marked by the outpouring of the Deccan Traps basalts on India at that time. But besides the basalts pouring out onto the surface of continental India, the Réunion hotspot also imposed some really strong horizontal forces on the base of the India plate, and eventually on the base of the Africa plate, as plate motions shifted where the hotspot was located below the overlying plates. And so it's certainly conceivable that the Réunion hotspot played some role in causing this unexpected speedup in Carlsberg Ridge spreading rates at about 53 million years.

Oliver Strimpel: You said a moment ago that you measured the plate spreading rate at the Southwest Indian Ridge as well as the Carlsberg Ridge, and also the Central Indian Ridge, and both of those are much further south than the Carlsberg Ridge. Do your measurements there confirm this little bump speedup that you just mentioned after the collision?

Chuck DeMets: There are two bumps here. There's a small bump at 40 million years, and then, of course, there is this large speedup at about 53 million years.

Oliver Strimpel: By the way, could you just quantify for us and tell us how fast the plates were moving before, and then when they slow down, and then how much they sped up again?

Chuck DeMets: Yes. At 53 million years, prior to this speedup in Carlsberg Ridge spreading rates, the full seafloor spreading rate, new seafloor, was being created at a rate of about 80 millimeters per year. And during the speedup, the rates appear to have doubled, to possibly as high as 180 millimeters per year, for a relatively short period of time, one and at most 2 million years, before the rates then collapsed. Once the rates collapsed, they went from 180 millimeters per year at about 52 to 53 million years down to only 20 millimeters per year, so a 90% decline, by 46 million years ago. So within a 5-million-year period, the seafloor spreading rates dropped by roughly 90%. So there was roughly a 100-millimeter-per-year increase in the spreading rate at about 53 million years ago. For comparison, this small bump at 42 million years ago was only 10 millimeters per year, so only 10% of the magnitude of this much larger speedup at 53 million years ago.

Oliver Strimpel: OK, well, I interrupted. You were telling me about the difference or similarity between what you measure at these three different spreading ridges that you examined, right?

Chuck DeMets: Let's just briefly, again, review the geography of the Southwest Indian Ridge versus the Carlsberg Ridge. So let's forget about India for the moment and think about Africa. The Carlsberg Ridge sits at the northern edge of part of the Africa plate, and the Southwest Indian Ridge sits at its southern edge. So if you visualize a piece of paper and the top and bottom edges of the paper: if you put your hand on the paper and slide it toward the top, and there were a seafloor spreading center there, then the spreading rates would slow down at the top of the paper but speed up at the bottom. The changes in seafloor spreading, or opening, rates in those two locations are anticorrelated by dint of the geography of those seafloor spreading centers with respect to, in this case, the Africa plate.

So for example, if Africa started moving rapidly to the north, we'd expect seafloor spreading rates on the Carlsberg Ridge, at the northern edge of the Africa plate, to slow down, but seafloor spreading rates at the southern edge of the plate, the Southwest Indian Ridge, to speed up. And so changes in seafloor spreading rates along the Southwest Indian Ridge and the Carlsberg Ridge should be anticorrelated, opposite to each other, if the underlying cause of the changes is a change in the motion of that intervening plate, which is the Africa plate. So what we see is that between 43 million years ago and 17 million years ago, every change along the Carlsberg Ridge corresponds to an opposite change at the same time along the Southwest Indian Ridge. And this suggests, then, that those changes are attributable to tectonic or geological events that were acting on the Africa-Somalia plate, as opposed to India, for example. And so this becomes useful, because when a geodynamicist is trying to divine what geologic event caused the changes in plate motion, they might focus on geological events that impacted the Africa plate as opposed to India.

Oliver Strimpel: In general, though, when we see a change in the spreading rates at the edge of a given plate, can we always attribute that to something that is happening along some other margin of the same plate, for example where a collision is occurring, or perhaps a new subduction zone forms?

Chuck DeMets: So we start by looking at events that might occur close to where we observe a change. But in general, there is a domino effect between plates. All the tectonic plates are mechanically linked, or coupled, to each other by the structures, the faults, that form the boundaries between those plates. A push on one plate is going to propagate mechanically throughout the system of tectonic plates globally. And so there is, as I mentioned, this domino effect, in that a change or tectonic event in the Pacific might show up as a small change in plate velocities in the Atlantic basin, or possibly even in the Indian Ocean.

Oliver Strimpel: Have you seen motions correlated with motions in plates that are perhaps once removed, or on the opposite side of the globe, that point to this mechanical coupling that you just mentioned?

Chuck DeMets: Yes, and there is a good example of that. My high-resolution reconstructions throughout the entire Arctic and Atlantic basin, including the North and South Atlantic, and our reconstructions in the Indian Ocean, have revealed a distinct and previously unknown change in plate motion in all of those places at about 7 to 6 million years ago, and the source of the change is unknown. So identifying the cause of this change at 7 to 6 million years ago is pretty tricky, because, again, it could be that a tectonic event in the Pacific triggered it, and then it propagated tens of thousands of kilometers and in various ways affected plate motions at far distances.

We've often talked about plate velocity changes as being interesting, but the absence of changes can also be interesting. Our high-resolution reconstructions also tell us that since 6 million years ago, since that last major change, the rate of seafloor spreading in most places hasn't changed by more than 1%. Plate rates have remained remarkably steady. And if we use modern geodetic methods, like Global Positioning System measurements of sites on the various tectonic plates, those confirm that the long-term geological seafloor spreading rates estimated with research like mine agree to within 1% with what the geodetic rates are. And so even today, over the last decade, the rates of seafloor spreading are the same as they have been for the last 6 million years, and that itself may contain information that some future geodynamicist will extract.

Oliver Strimpel: What are you working on now?

Chuck DeMets: Right now, I'm trying to link the high-resolution reconstructions in the Atlantic basin to similarly high-resolution reconstructions in the southeastern Pacific, near South America and the Peru-Chile trench, and my goal with that work is to link the movement of the subducting Nazca plate and South America, those two tectonic plates, which would facilitate geoscientific studies of the history of the Andean mountain chain and volcanoes. Once I'm done with that, I'm going to go fly-fishing and hiking in western North America and Alaska, instead of modeling their tectonics.

Oliver Strimpel: Chuck DeMets, thank you very much.

Chuck DeMets: It was a pleasure, and thank you for asking me to do this.

Oliver Strimpel: For more about Geology Bites, as well as illustrations that support this podcast, you can go to geologybites.com.