This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. To a first approximation, the Earth is an ellipsoid, a sphere that is a bit flattened at the poles as a result of its spin. But the surface of the Earth deviates from that with the diverse topography we're so familiar with, such as its mountains and valleys, continents and oceans. Most of what causes this topography has been well understood for quite some time, but about 30 or 40 years ago, it became apparent that some significant features of the Earth's topography could not be explained in the traditional way. In fact, it became necessary to invoke the effect of the mantle on the overriding plates in order to understand these features. This gave rise to the name dynamic topography, which is a bit confusing, because in a sense all topography can be thought of as dynamic, in that it changes with time. But here, it is the dynamics of the mantle that is being referred to. Carolina Lithgow-Bertelloni is a professor of geosciences at the University of California, Los Angeles. She views plate tectonics and the Earth's topography as an expression of the interior processes of the Earth, and in particular of the horizontal and vertical currents in the mantle. Carolina Lithgow-Bertelloni, welcome to Geology Bites.

Carolina Lithgow-Bertelloni: Thank you so much for inviting me, Oliver.

Oliver Strimpel: We're going to talk about dynamic topography today. But before we do that, can you refresh us on what causes the prevailing topography on Earth that is not to do with the dynamics of the mantle?

Carolina Lithgow-Bertelloni: Most of the topography on Earth is due to variations in the thickness of the crust, the top layer of Earth's surface, on which we live. Variations in its thickness and variations in its density cause most of the topography that we see. Then there is another type of topography, which is flexural in nature, which is due simply to the fact that when you plop something heavy, like a mountain or a volcano, on top of Earth's crust, and the lithosphere as a whole, it bends, just like a steel beam would bend if you put something on it. So those two are the things that we usually think about, and the one that is responsible for most of the relief, such as the presence of the Himalayas or the Bolivian Altiplano, we call isostatic, because it's as if it's an iceberg or a piece of wood floating on water. And that isostatic topography is usually the result of plate motions acting on the thickness of the crust. For the Himalayas, it's the result of the thickening of the crust where the Indian and Asian continental plates are colliding.

Oliver Strimpel: Okay. So what is dynamic topography?

Carolina Lithgow-Bertelloni: Dynamic topography arises from currents in the interior of the Earth, in the part of the Earth that we call the mantle. The mantle, even though it's rock solid, no pun intended, so that if you were to hit it with a hammer it would actually break, if you just wait a really, really, really long time, it actually flows. It flows just like honey or syrup on top of the counter. And as it flows, it generates stresses, or forces, and these forces, the vertical ones, can actually deflect the surface of the Earth, and as they deflect the surface of the Earth, they generate topography. This topography tends to extend over much greater length scales than, say, the Himalayas, and it also tends to be smaller in amplitude. So while the Himalayas, Mount Everest, is what, 8 kilometers tall, dynamic topography will only be a few hundred meters to at most a kilometer, but spread over many thousands of kilometers, in general. And you can have cases in which you have much shorter and bigger expressions of dynamic topography. So it's really the topography that comes from the motions of the mantle, and as a result it is different from isostatic topography, because it does not depend on the density or mass of the material that is there, but actually on the forces elevating it. And these currents, which are generated just like in a boiling pot of water, by convection, are not the result of hot material or cold material that is right underneath the crust or the lithosphere. They can be all the way at 3,000 kilometers depth, but because the material might be light or might be dense, and it's going down or it's going up, it generates forces that transmit all the way through and can deflect the surface.

Oliver Strimpel: Can you describe what these motions of the mantle actually might look like? Are they like convection cells? You mentioned boiling water. Is it a bit like that?

Carolina Lithgow-Bertelloni: The motions in the mantle are not like in the boiling pot of water. We mentioned plate tectonics before, and they're not just simple convection cells. So it's not that the material is coming up at the mid-ocean ridge and then going down at subduction zones, in these simple convection cells. The patterns of motion are more complex than that. Even though they're not turbulent, they're very time-dependent; they change a lot with time, because even though the system is laminar, it's actually convecting pretty vigorously for something that is made of rock. And that means that over time, there are lots of changes in the pattern of convection.

Oliver Strimpel: What sort of timescales for imposing this dynamic topography are we talking about? So how long would a dynamic topographic feature last?

Carolina Lithgow-Bertelloni: So this is really interesting, because normally we speak of convection in terms of a steady-state regime: you reach a steady state, that is, there are no transient features. The time it takes to establish the dynamic topography, that transient part, might be on the order of 10,000 to 100,000 years. That is exactly the same time, what we call a relaxation time, that it takes for post-glacial rebound to act. So if you put an ice sheet on top of the crust, it takes about 10,000 to 100,000 years for it to depress the surface and for the material to flow. In post-glacial rebound, what we care about most is that transient time: the amount of time it takes to bend down, or, if you get rid of the ice sheet, to rebound back up. In the case of the mantle, we care about what happens after that time. So dynamic topography establishes itself very quickly, in the same amount of time, 10,000 to 100,000 years, which is fast on a geological timescale. But because it's driven by the mantle convection patterns, the change in dynamic topography is very slow. So once you've started a convection pattern, even though it changes over time, it changes very slowly, on the order of 10 to 100 million years. So there are these two different timescales: one that is transient, to establish the topography, and another for the topography to change.

Oliver Strimpel: So we would expect to see a dynamic topography feature last as long as a particular mantle flow pattern would persist under a particular part of the Earth's surface.

Carolina Lithgow-Bertelloni: That's absolutely correct. So here's an example. Suppose that you have a subduction zone, so material going down, say, under North America. Whenever that subduction zone started under North America, you would get the dynamic topography effect of that material going down, that convection cell downwelling and establishing itself. That will take a short amount of time to establish, but then it will last as long as that subduction zone is active. So in the case of North America, that has been a good 80 million years or more.

Oliver Strimpel: Okay, so how do we actually know that dynamic topography exists on the planet, in fact, and that it's not just a theoretical question?

Carolina Lithgow-Bertelloni: That's a great question. So it's by inference, from looking at the shape of the Earth. You mentioned at the beginning that to a first approximation, the Earth is an ellipsoid. And that initial shape is the shape that any fluid body takes as it rotates. You start rotating something, and if it's big enough, it will act as a fluid, and it becomes a sphere, but a slightly flattened sphere, therefore the ellipsoid. And that figure, which we call the hydrostatic figure of the Earth, is, to first order, the shape of the Earth. And that shape is actually modified, at much shorter length scales than the full ellipsoid, by things that are going on in the interior, by the convection currents. Think of a plum pudding, where the raisins might have a greater density than the rest of the pudding. But suppose now that the raisins could flow in the plum pudding; they could go up or down. Maybe some are less dense and some are more dense. They will generate flow, and that flow will change the shape, because that flow, which generates dynamic topography, affects the surface. So how do we know that dynamic topography is an issue? It's because for about 40 years, we have understood that to explain the shape of the Earth away from this hydrostatic figure, away from the first-order ellipsoid, we need to account for convection currents in the Earth and their dynamic effect on the topography.

Oliver Strimpel: Has the effect of this movement in the mantle, the downwellings and the upwellings, been modeled? And so do we actually know that motions of this kind can, in fact, bend the lithosphere by hundreds of meters or kilometers?

Carolina Lithgow-Bertelloni: Yes, we know that it has enough force to bend it, because after a certain amount of time, the lithosphere doesn't have any elastic strength, so you don't have to worry about its strength so much. You only worry about its viscous limit, so it just behaves as anything else that might be viscous rather than elastic. So it has lower strength at longer timescales. So we know that it can bend it on these long length scales, and that it can do so by hundreds of meters to a kilometer or two.

Oliver Strimpel: Can you give an example or two where the effects of dynamic topography have been identified in the geological record?

Carolina Lithgow-Bertelloni: Yes. Some of the first things that were identified as important for the geological record were in the subduction zones. The only way to explain the shape of the Earth over subduction zones was actually to account for the dynamic topography generated by past subduction. Then later on, I've done some work, not just I, obviously, on southern Africa. In southern Africa, the continental lithosphere, the continental crust, is very old, and it's so old and tectonically inactive that we call it a craton. There aren't very many cratons on Earth, maybe 10, and those tend to be quite close to sea level, at most maybe 500 meters above sea level. But the southern African craton is very unusual, because it's actually 1,500 meters above sea level. If you've ever been to Johannesburg, Cape Town or wherever, it will be obvious that it's very elevated. And there's nothing in the crustal thickness that explains this; it's a very normal, very average crustal thickness. There's nothing in lithospheric processes, such as an anomaly in the heat flow coming out of the Earth, that explains this elevated pattern. But instead, if you compute what the dynamic topography should be in this area, once you strip off all the other contributions, you find that it fits perfectly. And why is that? The reason is that, again, over the last 40 or 50 years, we have been developing techniques for imaging the interior of the Earth. This is a technique called seismic tomography, where you use the energy of the waves generated by an earthquake. These waves travel all throughout the Earth, and you pick them up at receivers all over, and much like a CAT scan of your brain, where you're passing electrical impulses through, you're passing seismic energy through. Things of different temperature, different density, different composition have different seismic velocities, and you can pick this up. So you make a CAT scan of the mantle, and you image the structure of the interior. And when we've done this, one of the main features that shows up is this region below almost the entire Atlantic basin, but especially under Africa, and southern Africa in particular, where there's a really low-velocity region. It extends over the entire Atlantic basin, and it also extends up in height to about 800 kilometers depth, and around 800 kilometers depth, it actually shifts a little bit towards the east and north and ends up being under the East African Plateau, so under Ethiopia and Kenya. And so this very low-velocity region that we have seen in this CAT scan, in the seismic tomography image of the Earth: if we interpret it as a region of low density, then we can calculate the flow, the forces that it induces, and the topography that it produces at the surface. And when we do that, it fits the pattern of the residual topography, that is, all the topography that is not isostatic, almost perfectly.

Oliver Strimpel: You mentioned that the low-velocity region we see in the seismic images of the mantle extends all the way up from southern Africa to under the East African Plateau, Ethiopia and Kenya. Does this play a role in initiating the rifting apart of the continent that we're seeing there now?

Carolina Lithgow-Bertelloni: I think it does. I think this region of low-density material, from 800 kilometers to about 100 kilometers depth, under East Africa, gives rise to dynamic topography that is strong enough to put the continent above under tension, and because it puts it under tension, it facilitates the rifting of Africa, by putting the system into a state of stress that will facilitate the breakup of Africa and the rifting of Africa. And in fact, we looked at the different forces available: the fact that you have high elevation, the fact that almost everything surrounding the African plate is ridges, and therefore there is spreading apart there. We looked at the topography, we looked at the gravity, and dynamic topography is one of the strongest forces available for the rifting of Africa.

Oliver Strimpel: So how does that work? Is it just because it's been flexed upwards?

Carolina Lithgow-Bertelloni: Yes, yes. It's not necessarily actively pulling it apart, but what it's doing is contributing to putting it under tension. It's actually really hard to break rocks. If you have a pristine continent, almost nothing will break it apart. You have to weaken it somehow. But all these areas, because of the history of tectonics, and continents, have all been weakened in some way, by the formation of mountains, et cetera. So once you have that, if you change the state of stress so that it's overall in tension, then any addition, say you have some magma coming up, or you have the building of a volcano or a mountain, all of it is going to act towards extending it further, as long as the state of stress continues.

Oliver Strimpel: When we talked earlier, you gave a couple of other fascinating examples of dynamic topography. One of them was the flooding of North America in the Cretaceous. Could you talk about that for a moment?

Carolina Lithgow-Bertelloni: Yes. I mentioned it before, as an example: I was talking about North America, if subduction starts under North America. And actually, subduction under western North America has been going on for a really long time, the subduction of the plates under North America. There were particularly two plates, around 80 million years or so ago, a little later: the Kula plate and the Farallon plate. And it's the subduction of the Farallon plate: when it starts, the material is going down, and it's inducing flow in the mantle, because this dense material that is going down is pulling the continent down, tilting it towards the subduction zone. So you could imagine that the east coast is tilted towards the west coast. And in the Cretaceous, when we had no ice sheets, the sea could encroach into the central part of North America, which was tilted downwards because of the presence of the subduction zone of the Farallon plate. Over time, some of that subducted slab has ended up under eastern North America, the eastern North American part of the Atlantic, and it's still sinking down, so it's still inducing dynamic topography there, so that now the continent is tilted the other way. But that flooding of North America during the Cretaceous is mostly explained by the presence of dynamic topography. And there is a famous paper by Mike Gurnis of Caltech, in 1990, where he took a model of where subduction zones were in the past, and then looked at the dynamic topography that they would generate, and looked at the sea level, and could explain most of the flooding of almost all the continents because of plates migrating on top of existing dynamic topography, or initiating dynamic topography.

Oliver Strimpel: Is a similar effect thought to be operating in South America, where the Nazca plate is subducting on the Pacific side of the continent?

Carolina Lithgow-Bertelloni: Yes, absolutely. And one of the really interesting things is that South America is a particularly brilliant place to go look for these things, because the subduction zone, where it was the Farallon plate and now it's the Nazca plate, has been going on for a long time, so dynamic topography has been established there for a long time. But along strike, going from Chile all the way to Colombia and Venezuela, the plate that is going down, the Nazca plate, today is tremendously variable. So instead of just going down as a slab of material all at the same angle, it has areas that go down at a 40-degree dip and areas that are completely flat, less than 10 degrees, because there's a lot of relief on that plate, and that means that there are regions that have very high crustal thickness, which is unusual for oceanic plates. So there are two sections, one under Peru and one under Chile and Argentina, that are flat, completely flat, less than 10 degrees. So the plate goes down and then flattens under the continent. And in these two areas, the thickness of the oceanic crust, which is on average 7 kilometers everywhere, is actually 20 kilometers, and that changes the density of the whole oceanic plate going down. It means that there are parts of that oceanic plate that don't want to go down at all. And therefore, if they don't want to go down, if they're not dense enough, they're not going to generate any flow, and if they don't generate any flow, they don't generate any dynamic topography. So if you take this into account, what you see is that today you explain the topography better over these regions if you have these two sections generating no dynamic topography. Now, the other thing we know about South America is that in the past, say 7 to 10 million years ago, maybe a little bit more, these flat sections did not exist. So the change from a totally typical subduction zone, with the plate all going down at the same angle, to sections that are now not going down, or are flat, changes the dynamic topography over time. So before the last 10 million years, the South American plate, much like the North American plate, was tilted towards the subduction zone; the dynamic topography was pulling down on the plate. And then once these very thick pieces of crust arrived in the subduction zone, that stopped, and so now it has reversed. And this change in the dynamic topography, from being tilted towards the subduction zone to being flat or tilted away from the subduction zone, means that in these two areas you get uplift that is driven by the change in dynamic topography. And the fascinating thing about that is that it has an impact on the drainage of the Amazon basin. We know that the Amazon River used to drain to the northwest into the Pacific, and now it drains, obviously, into the Atlantic. And so you have to somehow reverse that drainage, and one of the best ways to do that is basically to tilt the plate in the Peru area towards the Atlantic.

Oliver Strimpel: And you're currently thinking about the possible impact of dynamic topography on the origin of life. Can you tell us about that?

Carolina Lithgow-Bertelloni: Well, it's really highly speculative, but here it goes. The first thing is that evidence for some sort of life goes back a very long time, to shortly after the origin of the Earth. There's certainly evidence for organic compounds as far back as 3.8 or 3.9 billion years, and the Earth is 4.5 billion years old. So there's early evidence for life, prokaryotic life, very early on. Then you need oxygen to rise to the level that it is today in the atmosphere, and to do that, you likely need landmasses above sea level that are capable of burying organic carbon. I think that's one of the lines of evidence that people use. So how do you go from a planet that might have been completely submerged in water, an aqua world that had no continents, to one where you start having continents, and where those continents are actually above sea level? And that's actually not so easy. It requires the establishment of plate tectonics, but it also requires these continents to be the same composition as they are today, and that's difficult. But dynamic topography, because it acts all the time, because of convection in the mantle, and the Earth has been convecting from the beginning, might actually have been stronger and more impactful in the early Earth, when it was hotter, when the convection currents were faster, and as a result they would have deflected the surface more. And that might be a way to get a proto-continent, which might not have quite the composition of today, and therefore not be quite as light and capable of being above sea level, to get it a little bit above sea level. And that generates landmasses that can then be eroded; climatic systems can be established; sediments can be deposited; and then you can bury any organic carbon that might be there. And so there might be some sequence in which dynamic topography was crucial for land, and it looks also like land was critical for the biogeochemical cycles that you need to originate any life, and certainly to evolve it into the multicellular eukaryotic life that we have today.

Oliver Strimpel: That's fascinating. But there's one piece of the argument I didn't quite understand. You said that to start the process going, you really need to have land. Was that to generate the oxygen you need for life as we know it today?

Carolina Lithgow-Bertelloni: So, we talked about the Great Oxidation Event, maybe 2.5 to 3 billion years ago. But there is some evidence that suggests that to have the first Great Oxidation, which isn't enough, by the way, to get you to multicellular life, you need to bury organic carbon. People have had many other explanations, but I think one of the most accepted explanations today is that you need to bury organic carbon. And so you bury your organic carbon, and that allows you to oxygenate the atmosphere, to get to the levels that we have today, and to do that, you need sediments. And the only way to generate sediments is to have landmasses above sea level. So dynamic topography would act to push the landmass above sea level, and once you're there, you are able to generate a climatic cycle and a sedimentary cycle that generates sediments and deposits them in basins, where they can bury all sorts of interesting things.

Oliver Strimpel: Carolina Lithgow-Bertelloni, thank you very much.

Carolina Lithgow-Bertelloni: Thank you. This has been a great experience.

Oliver Strimpel: For more about Geology Bites, as well as pictures and diagrams that illustrate this podcast, you can go to geologybites.com.