This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Many of the observations that led to the adoption of plate tectonics in the 1960s and '70s focused on the oceanic plates. These included mid-ocean spreading ridges, the magnetic striping of the seafloor, and the trails of mid-ocean volcanoes caused by stationary mantle plumes as oceanic plates moved above them. They all supported the idea that plates moved as rigid caps floating on the asthenospheric mantle. But what about continental plates? Do these also behave like rigid plates? It turns out that the answer is sometimes yes and sometimes no.

Alex Copley studies continents, and especially how and why they deform in a manner that can be quite unlike oceanic plates. He is a professor in the Department of Earth Sciences at the University of Cambridge. Alex Copley, welcome to Geology Bites.

Alex Copley: Thank you. It's a pleasure to be here.

Oliver Strimpel: Before we talk about continental plates, I want to confirm that what I said about oceanic plates is indeed correct, namely that they behave rigidly.

Alex Copley: Yes, that's correct. In the oceanic plates, there are very narrow bands of deformation down the mid-ocean ridges. But once the plate has moved away from the mid-ocean ridge and has become old and cold, it's generally too strong to deform. So although it'll be littered with faults that used to be active when they were at the mid-ocean ridge, they're no longer active once they're in the middle of the plates. There are only two very special situations in which you can actually have deformation within the oceanic plates. One is as the plate is bending into a subduction zone, which generates very high stresses that can break the plate. And the other is if there are very unusually large forces being exerted on it. An example of that is the central Indian Ocean at the present day, where the very big forces related to the India-Eurasia collision are able to just slightly start breaking the plate.

Oliver Strimpel: OK, let's talk about the deformation in continental plates, then. Do they deform continuously throughout, or is it localized?

Alex Copley: Well, that very much depends on when and where you look. There are some parts of the continents that do indeed act like great big rigid plates, for example Siberia, or Arctic Canada, or the eastern sections of South America. These are large rigid areas which act like plates in very much the same way as the oceanic plates. But there are also big, pervasively deforming deformation belts that run through some areas of the continents. For example, there's the big Alpine-Himalayan deformation belt, which runs from Italy to China. There are the deformation belts that run all the way down the western sides of North America and South America. And in these areas, there are lots of faults spread over huge areas, thousands of kilometers in extent, which are all slipping in earthquakes. These are what make the big mountain belts that we associate with these kinds of places. So that huge area of earthquakes and mountain belts shows that some parts of the continents are actually deforming pervasively over very large areas, in stark contrast to these other areas that just act like big rigid plates.

Oliver Strimpel: So these areas that deform near the mountain belts, are they really deforming along many, many faults, or do they deform in their bulk, like molasses or treacle being heaped up under pressure?

Alex Copley: Well, yes, that's the next big question. Early in this subject of continental tectonics, the field kind of divided into people who were thinking in terms of what we'll call microplates, so thinking about it as plate tectonics in miniature, with lots of little plates separated by individual faults, and people who decided, OK, well, if there are so many faults, perhaps we should just not worry about each one individually and view it as a continuum that's describable by its kind of average properties, which take into account all these faults and also the ductile layer underneath them. For a long time, this was one of the big debates in continental tectonics: which of these is the most sensible way to think about how the continents deform?

Where the subject is now, and my personal view of this, is that it very much depends on where you're looking, what scale you're looking at, and what question you're trying to answer. On the very smallest scale, you know, in the brittle upper crust, the deformation is by two rigid chunks of rock moving relative to each other along an active fault zone that breaks in earthquakes. On that kind of scale, it certainly makes sense to think about rigid blocks separated by faults. But once you start thinking about areas that are thousands of kilometers wide, with many, many faults scattered all the way through them, then it becomes more useful to think about these as just a continuum that you can describe by its average properties. So think about things like honey deforming over a table, something like that. That's more how we could think about these large mountain belts.

Oliver Strimpel: But in fact, if you look at it on smaller and smaller scales, will you always wind up looking at faults or microfaults and fractures, or is there actually ductile deformation going on?

Alex Copley: Well, ductile deformation in the truest sense is generally happening deeper down in the Earth, below the cold, brittle layers breaking in earthquakes. We do see features like folds adjacent to faults, which are a spatially widespread form of deformation, but the motion on these folds is generally associated with earthquakes on the neighboring faults. So you can see this close association between shallow folding and the thrust faults on which the earthquakes are happening.

Oliver Strimpel: So you already mentioned a couple of deformation belts, the Alpine-Himalayan belt. Are there any other major deformation belts on continents that illustrate this point?

Alex Copley: Yeah, the big ones are the western sides of North America and South America, the Cordillera running all the way down that margin of the continents. There's the East African Rift System. And then, like I said, there's this Alpine-Himalayan belt that runs from the central Mediterranean through the Middle East and across to China. Between them, these encompass all of the main deformation belts that are currently affecting the continents.

Oliver Strimpel: That incredibly long belt that stretches from Italy, or even France really, all the way to Asia, is that really all on a single plate? I mean, do we think of Europe and Asia as one big plate?

Alex Copley: In terms of what we think of as plates, it very much depends on whether you're thinking about inside the deformation belt or what's happening on the outside of it. For example, Eurasia, say Europe and Asia, the northern parts of these areas, is one big rigid plate. Likewise, North Africa is itself a rigid plate, as is Arabia, as is India. So it's perfectly sensible to describe these bits around the edges of the deformation belt as rigid plates. But once you get into that deformation belt, you can't really start talking in terms of what plate something is on, you know, what plate Lhasa is on, or what plate Athens is on, simply because these areas are so pervasively faulted that you can't really go about defining big rigid plates as you can in the areas outside the deformation belt.

Oliver Strimpel: How do we know about the differences between the very hard areas that you talked about that do behave like rigid plates on continents and the softer rocks in the deformation belts? Do we actually observe these quakes from satellites, or can we actually witness this deformation happening in front of our eyes?

Alex Copley: Yes, yes we can. One of the main techniques we have available is analyzing the seismic waves produced by earthquakes. Every time there's an earthquake, there are elastic waves that travel through the Earth that we can record with instruments that are scattered all over the surface of the planet. By analyzing these waves, we can work out where the earthquake happened, how deep it was, and various things to do with the sense of motion on the fault in the earthquake. We can also use satellite data to map the surface motion caused by earthquakes, and we can get into the field and actually look at the surface ruptures that have been produced by both recent and very old, prehistoric earthquakes. So for looking at individual earthquakes, we've got quite a wealth of tools available to us.

And then if you want to zoom out and look at these big distributed deformation belts as a whole, we can also use satellite data to look at that. We can use sophisticated GPS receivers, or we can use satellite radar to map the motions over huge areas of the continents. So we can actually see which areas of the continents are acting as these rigid plates, moving as one coherent body with no earthquakes within them, and which areas are acting as these big, spatially pervasive deformation belts, with earthquakes all the way through them and surface motions, measured with the satellite data, that vary continuously across large areas.

Oliver Strimpel: So it seems as if we have two very different kinds of regimes on continents. We have these softer areas, if you like, that form these pervasive deformation belts, and adjacent to them, often we have these very hard areas, which I suppose may in some cases correspond to what we refer to as cratons. So what is it that makes this enormous difference between these two regions? Is it just happenstance that if you're on the edge of a plate undergoing collisions, you're going to deform, and if you're in the middle, you're sort of protected? Or is there something intrinsically different between the deforming belts and the rigid cratons?

Alex Copley: Yeah. It's very much something intrinsically different about the rocks in these places. How strong a rock is going to be depends on what minerals it's made of, what the compositions of those minerals are, and what kind of pressure and temperature conditions that rock is experiencing. What we see is that in places where we have these weak deforming belts, we generally have rocks that are made of minerals that contain water. That's water bound into the structure of the minerals as an integral part of the crystal, and also water incorporated into nominally water-free minerals as defects in the crystal lattice, which also acts to make the minerals much weaker. The areas that are acting in a very strong way, the areas that are behaving like plates, are generally totally water-absent. There are not even hundreds of parts per million of water around. So that's a big difference between the rocks you get in the rigid areas and the deforming areas. And this is all related to the geological history of the rocks: what rocks were they originally, and what's happened to them since? That's a big control on how weak or how strong they are at the present day.

Oliver Strimpel: That's fascinating. So is the history of these rocks, then, that they all used to have water and some have dried out and become hard? Or that none of them had water, and then water was later introduced to soften them up, if you like?

Alex Copley: Water is generally quite a pervasive thing around a lot of the Earth, obviously at the surface, but then also in the subduction zones, where a lot of the Earth's continents are formed, in the island arcs above subducting slabs. Those are also hydrous settings, because water has been released off the subducting slab into where the continental crust is being produced above it. So most rocks probably begin their life with a fair amount of water in them. The question then is, well, how do you go about dehydrating a rock to actually make it strong enough to act as one of these rigid pieces of continental craton?

We think the main situation in which that happens is actually during mountain-building events. If you thicken the crust to make a mountain belt, like, for example, the Tibetan Plateau at the present day, then as the crust gets thicker, it gets hotter. Eventually it gets hot enough that it starts to partially melt. Rocks don't just melt completely all at once. Various bits of them melt before various other bits, and volatiles such as water are more stable in the molten rock than in the solid that's left behind. So this is a very effective way to remove water from the rocks. It goes into the magma, which then rises upwards and gets emplaced somewhere above, and leaves behind this very strong, very dry residue. And that's what we think has happened in all of the areas that form these rigid cratons at the present day.

Oliver Strimpel: So all these large cratons that we see, for example in southern Africa or the center of the Indian continent, those are all leftovers, if you like, from earlier mountain-building events.

Alex Copley: Yes, exactly. Yeah. If we go and look at these places in the field, we can see that at the surface all the rocks are very high-grade metamorphic rocks that have been hot enough to have had melt extracted from them. And various detailed aspects of the chemistry of these rocks also tell us that they've had melt and volatiles extracted from them. So these really are what is left behind after you've created a mountain range, the motion has stopped, you've eroded away the upper crust, and you're just left with what was the lower crust of the mountain range, which has this very high-temperature history.

Oliver Strimpel: And once an area has been dehydrated in this way, does it remain dry for billions of years, as indeed these cratons seem to have existed for a very, very long time?

Alex Copley: There are ways in which water can subsequently be reintroduced, but it's difficult and slow. As you say, a lot of these cratons seem to have been rigid for billions of years, which is the first thing that tells us that it's not very easy to make them weak once they become strong. Some of my colleagues and I spent some time looking at rocks up in Canada to ask exactly this question: how easy is it to rehydrate a craton and make it weak again? We were looking at the remains of a roughly 2-billion-year-old mountain belt, where a bunch of very water-rich sedimentary rocks had been thrust over one of these anhydrous rigid cratons. And what we could see was the process of the water diffusing from these hydrous sedimentary rocks down into the underlying very dry old rocks. As this was happening, the water would react with the minerals in those rocks and make a different set of minerals that were weaker, and then it would start deforming.

So by dating how long this takes, and by looking at the distribution of water within these old rocks, we could actually work out how fast this process happens. And it turns out it's incredibly slow. This mountain range was around for about 30 million years, and this hydration and weakening only managed to make it about 3 kilometers into these old, anhydrous, rigid rocks. So this tells us that for a standard length of a mountain-building episode, which is tens of millions of years, it's not very easy to actually hydrate any kind of rigid cratonic material, which is why cratons stay dry and hard for so long.

Oliver Strimpel: So can we view all these soft deforming belts as essentially temporary phenomena that are all going to wind up one day as bits of cratons?

Alex Copley: Yes and no. "One day" depends on how long you're willing to wait. In order to actually get hot enough to partially melt it and remove all the water, you need quite a big mountain belt, because how hot a mountain belt gets depends on how thick it is. At the present day, something like the Tibetan Plateau is big enough and hot enough that this kind of process will be happening at depth underneath it, but smaller mountain belts won't be hot enough, so it won't be happening there. But earlier in Earth's history, this process was actually much easier, because there were more radioactive elements around. They were produced in the supernova that produced the material from which the Earth formed, and they have since been decaying away. And because there was more radioactive heating back in the early part of Earth's history, it was much easier to get the rocks hotter, which meant it was much easier to melt them and do this process of removing the volatiles and making the rock hard. So that's why a lot of these rigid cratonic areas formed quite early in Earth's history: it was easier to make things hotter back then, because there was more radiogenic heating around.

Oliver Strimpel: But the bottom line is that the amount of hard cratonic material, we think, is just increasing over time?

Alex Copley: Yeah, this whole process of mountain building and making rigid material is going on, but we also have this slow and inefficient, but still present, effect of weakening this material in places where it can get hydrated. I've talked about mountain belts, and also, if one of these rigid cratonic areas spends a lot of time sitting on top of a subduction zone, then the volatiles being input into it in that setting could also have a similar effect. So I wouldn't want to say whether the amount of cratonic material is necessarily increasing or decreasing at the present day, but what we can say is that the rate at which it changes is fairly slow.

Oliver Strimpel: And can we tell from the geological record how far back in time this kind of process has been operating?

Alex Copley: Well, yes. You could get a lot of different answers to that question, depending on who you ask. My personal view is that we've been back and looked at records of continental tectonics reaching as far back as there's a decent-sized rock record, and we don't think we can see anything that suggests that the behavior of the continents was any different, as far back as we have the information to be able to look at it. For the kind of techniques that I've been using, this involves needing decent-sized pieces of preserved crust in order to ask these questions. So this takes us back into the early Archean, between, say, 3.5 and 4 billion years ago. And since then, it looks to me as if continental tectonics has been operating in a fundamentally pretty similar way to the present day.

Oliver Strimpel: And can we not push it back further than that because we just don't have any rocks that are that old to look at?

Alex Copley: There are fragmentary pieces of information: some very old rocks that go back to just older than 4 billion years, and also some individual minerals that have been weathered out of now-vanished rocks and have been preserved in other sedimentary rocks. These give us tantalizing hints of what was going on back in the very early days of the Earth. We can measure their chemistry, and we can think about what that might mean, but it's very difficult to scale up from individual minerals or very small rock outcrops to what was happening on the very big scale. There are some people who have been very successful in doing that, but my preference is to work on places where I can actually make the observations at the large scale on which I'm trying to interpret them.

Oliver Strimpel: Can we see whether the relative position of these cratons, or their relative hardness with respect to the softer areas, has an effect on the mountain belts that we see? I mean, if you have a craton that's right next to a region that's currently deforming, will it behave differently from one that's, say, quite a bit further away, like perhaps the one that's in Africa, which doesn't seem to be close to any currently forming mountains?

Alex Copley: Well, yes. The behavior of mountain belts depends in a really crucial way on the strength of the stuff that's around their edges. If we use the Tibetan Plateau again as an example, India is one of these old, rigid and anhydrous cratonic areas, and it has been thrust underneath the southern edge of the Tibetan Plateau. What this means is that we have this big rigid chunk of rock that's been thrust underneath the mountain belt. So all the weaker rock that's forming the Tibetan Plateau is behaving effectively like honey spreading out over a table, since it's weak material sitting on top of this very strong stuff. That's why it develops this very characteristic shape: a flat top, a steeply sloping front, and a very gently curved southern edge in plan view. This is just a consequence of the fluid dynamics of how a fluid behaves when it's spreading out over a rigid base. Mountain belts that don't have this rigid material being thrust underneath them, for example the eastern edges of the Tibetan Plateau, look very different. They have long, very gentle gradients, where the same elevation difference occurs over roughly 10 times the horizontal distance that it does on the northern and southern edges. So yeah, mountain belts behave very differently depending on whether they have one of these rigid areas being underthrust at their edges or not.

It's hard to make these inferences for very old mountain belts, simply because we have a less complete knowledge of how they were behaving compared to the present-day ones. But what we can do is look at their elevations. If we look around at the present day, what we see is that the height of a mountain belt scales with how strong its surroundings are. This is simply because if you want to grow a big mountain belt, there needs to be a force there to hold it up, and the size of that force is governed by how much force you can transmit through its bounding plates, so by the strength of those bounding plates. For example, India is very hard, which is why the Tibetan Plateau is the biggest mountain range on Earth. The Arabian plate is weaker, which is why the mountains of Iran are lower. So what we can do, then, is look back into the geological past and use the metamorphic record to ask how big mountain ranges were, how thick the crust got, and then use that as a way to work out the strengths of the bounding plates back in the past. When we do that, we find that the range of plate strengths that we come up with was pretty much the same as the variation that we see at the present day.

Oliver Strimpel: And that's caused by different water content in the adjacent areas?

Alex Copley: Yeah. So things like different water content, and different thicknesses of the plate, which control the temperature. Hot rocks are weaker than cold rocks, so that temperature effect is also present. So yeah, it comes back to this idea that it's both the temperature and the composition of the rocks that control how strong they are.

Oliver Strimpel: In an earlier podcast episode, which this reminds me of, David Kohlstedt talked about his experiments on what he could do in terms of simulating mantle rock, and he showed that even a very, very small amount of water had a very, very big effect on the strength of the sample. So I guess that's what we're seeing here.

Alex Copley: Yes, yes, this is exactly the same effect. The same principles apply to the minerals that make up crustal rocks as to mantle rocks. So yes, just hundreds of parts per million of water incorporated into the crystal lattices as defects can have a very big effect on the strength of the minerals.

Oliver Strimpel: So if continental plates have regions that are really not rigid at all, does this affect the validity of all our plate movement reconstructions, in which both continental and oceanic plates are assumed to behave rigidly?

Alex Copley: Yes. This creates one of the big uncertainties when we're thinking about plate configurations back in the past. We've got these big rigid plate-like regions, and we can use things like paleomag to work out how they've been moving back through time.

Oliver Strimpel: Just to interject that paleomag refers to paleomagnetism, which is the magnetic field frozen into a rock when it formed, and from the direction of that field one can infer something about the location of that rock when it formed.

Alex Copley: The tricky thing comes with what you do with the stuff in between, these big, pervasively deforming deformation belts, which will be changing how much space they occupy as they deform. If you thicken something, it gets thicker, and it also takes up less horizontal space. So a lot of the debates about things like when India hit Asia come back to thinking about how we try to run backwards the deformation that has happened in the intervening region and work out how big these continents would have been before their edges got deformed. That's something that people have to try and do to work out when the two continents actually collided with each other.

Oliver Strimpel: But this doesn't necessarily significantly alter our understanding of paleogeography, like where and when Pangea and Rodinia assembled, and where certain parts of the world were when various rock suites were formed?

Alex Copley: No, the big large-scale features, which in many ways are dominated by these old, rigid, plate-like regions of the continents, are all quite robust. But you wouldn't want to hang your hat on what was happening in the soft, gooey bits in between the big rigid plates.

Oliver Strimpel: What are you working on at the moment?

Alex Copley: We're continuing to pursue this idea that you can do a lot of geophysics looking at present-day tectonics, and you can do a lot of geochemistry and petrology looking at the rocks that were affected by old tectonic episodes. By comparing the two, we've been able to make progress on various things we've been talking about today. What we really want to do is fully couple these two views of the world together and come up with a physically and chemically coupled view of how the continents behave. This means taking into account the feedbacks between deformation, changes in composition and changes in hydration state, which then change the material properties, which then control the deformation itself. So there are a lot of feedbacks going on that we don't yet fully understand. That's what we're working on at the moment, which involves making observations of active mountain ranges, and going to old exhumed mountain belts, and using these to try to come up with governing principles that we can then combine in numerical models to ask what's actually controlling the underlying dynamics of these mountain belts.

Oliver Strimpel: Are you doing your observations mainly in the Himalayas, or do you have other field sites as well?

Alex Copley: One thing we like to do is look at as many different places as possible, because it's by looking at a lot of different places in the world that you can work out what is a particular oddity unique to one place and what is an underlying principle that operates everywhere. So a hallmark of the kind of work we do in this topic is always to be looking at as many different places as we can, and to think about which systematic patterns are telling us something fundamental and which are just weird, esoteric local effects.

Oliver Strimpel: Alex Copley, thank you very much.

Alex Copley: Thank you.

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