This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Almost all erupted magma is initially generated in the Earth’s mantle. As the melt forces its way up through the many kilometers of lithosphere to the surface, it usually pauses in one or more magma chambers or partially melted mush zones for periods of up to a few thousand years before erupting. During these pauses, the magma can separate into fractions having different compositions, and also interact with the surrounding crust. But while we have seismic evidence and models that support this picture, we’ve not hitherto been able to watch how magma actually moves in the upper mantle and crust.

Bob White has set out to change that. Using a dense array of seismometers, he has been able to pinpoint thousands of tiny earthquakes that reveal the detailed movement of melt through the thick crust of Iceland just before it erupted. He combines this seismic data with geochemical analysis of the lava that can tell us about the depths at which the melt is formed. He is Emeritus Professor of Geophysics in the Department of Earth Sciences at the University of Cambridge. Bob White, welcome to Geology Bites.

Bob White: Well, it’s a pleasure to be with you. And of course, this work was done not just by myself, but by a whole team of graduate students and colleagues in Iceland.

Oliver Strimpel: Why did you pick Iceland for your detailed study of magma movement?

Bob White: The mid-ocean ridge, the Mid-Atlantic Ridge, cuts right through Iceland. So Iceland is a brilliant place to study the processes where the plates are spreading apart and generating a lot of volcanic rock.

Oliver Strimpel: And then how did you decide exactly where in Iceland to focus on?

Bob White: Well, we started working in the center of Iceland. It’s a place where absolutely nobody lives. You can only get there in the summer, and that has a huge advantage: it’s seismically very quiet. There are no cars and lorries and all the other noise that comes about where people live, so it’s very seismically quiet. It’s also the most active volcanic area in Iceland, because it happens to lie over a mantle hotspot. So it was a great place to watch volcanoes doing what volcanoes do.

Oliver Strimpel: So as I mentioned, you exploited two types of data in your study, seismic and geochemical. Let’s talk about how these work to give us detailed information on the movement of melt, starting with the seismic. By definition, melt is liquid, so we can’t be seeing earthquakes directly from the melt.

Bob White: Indeed you can’t. That’s correct. But the melt is formed very deep, in the upper part of the mantle. The melt under Iceland probably starts forming at depths of about 100 kilometers. And then it migrates up very quickly until it hits the brittle part at the top, the lithosphere. And once it gets there, it has to get its way through to the surface. It’s buoyant, so it wants to go upwards, but it has to create cracks to do that, to make a pathway for itself. So a crack is actually an earthquake, or rather, an earthquake is a very big crack. And the molten rock, as it pushes its way through the crust, creates many, many, many tiny earthquakes, tiny cracks. So by mapping where those tiny earthquakes are occurring, you’re actually mapping where the melt is moving. If it stalls, it tends not to generate any earthquakes, but when it’s on the move, it generates lots to open a pathway for itself.

Oliver Strimpel: So to be able to watch in detail where the melt is going, then, you need to track these microquakes, and you also need to know exactly where they are. So how do you pinpoint the location of these very faint earthquakes?

Bob White: Yes, it’s a technique called triangulation. We’re helped a little bit in seismology by the fact that there are two different types of seismic wave. One is called P waves, or compressional waves. The other is called shear waves, or S waves. And the S waves travel at about half the speed of the P waves, but they come from the same spot, of course. So actually, if you’ve only got one seismometer and you can measure the P wave and the S wave, you at least know how far away it is, because you know the speed at which they travel, but you don’t know where it is. So you still need a number of seismometers. And these are very, very sensitive instruments, recording the motion of the Earth in three dimensions: vertically and in two horizontal directions. So you can use that to work out how long it took for the earthquake wave to reach you, and also actually the direction of travel, because the wavefront hitting the seismometer can tell you where it’s coming from. So you have quite a lot of information. But the more the merrier: the more seismometers you can get, the better the location.

Oliver Strimpel: And then at the end of the day, roughly how precise a location can you get?

Bob White: Well, the thing you don’t know is exactly how fast the seismic waves travel through the Earth. That gives some uncertainty. And also, if there’s any noise on the arrival, that can perturb it. So in absolute terms, we probably know the location to 100 meters or so. But it turns out that if you’ve got a lot of earthquakes in essentially the same place, which is what you get at the tip of a crack as it’s propagating forward, as the melt moves forward, you can do rather better at calculating the relative locations of those different earthquakes. Because if you imagine it, the energy from each earthquake is traveling through the same heterogeneous crust to your receiver every time. So if you’ve got several earthquakes essentially in the same place, then you can cancel out all that heterogeneity and just look at the relative locations between them. And we can get that down to about 10 meters, which is remarkable really, because we can look at these tiny earthquakes down at six, eight, ten kilometers depth and know where they are to within 10 meters or so.

Oliver Strimpel: I imagine you have to get relatively close to these earthquakes in order to be able to get that level of accuracy. So when I asked you why you picked Iceland and where exactly in Iceland, you said, well, the center of Iceland is free of people and it’s more quiet and so on. But presumably you had to narrow it down way more than that in order to get close enough. So were you able to predict where you think the melt was going to move before a particular eruption, or did you just get lucky?

Bob White: We got lucky in one respect, but we also had an array out over quite a big area, so it was over 100 kilometers square, something like that. If the earthquake’s big enough and you can record it across the whole array, which might span 100 kilometers, then you can still locate it very accurately. If it’s a tiny earthquake, you need to be a bit closer to it, and ideally you want a receiver more or less directly above it, because that’s what controls the depth, which is probably the biggest uncertainty.

But we had a regional array out in the middle of Iceland. We’ve been operating one there for the last 15 years or so, because it is very active. And we’d actually caught a couple of other small injections of melt into the crust before the one I’m going to talk about, the big one in 2014. And on this occasion in 2014, we’d actually put out extra seismometers. We had more than we’d ever had before: more than 70 of these seismometers arrayed across an area, because what we were planning to do was to find out where the melt was residing under another active volcano called Askja. Now, that’s not the one that erupted, but it was adjacent to it, and we knew it had molten rock underneath it, and we were going to use tomography.

The way that works in medical systems is that you have a very high-frequency sound wave and you bounce it off your internal organs to look for tumors, or babies as they develop in pregnant women. And you can reconstruct an image of what’s going on inside your body with tomography. Well, we just pinched the same idea. We have a much longer wavelength. The wavelength of the seismic waves is about one kilometer, or at the very best several hundred meters. So our resolution is much poorer, and we can’t generate sound at the surface unless we let off explosives, which occasionally we do, but people are not very keen on you doing that these days. So mostly we use the earthquake itself as the energy source, and then we record how it travels through the Earth, and we can invert all that information to do tomography. And particularly, we can see where molten rock is, because of those two types of waves I explained, the shear waves don’t go through molten material, but the compressional waves do. So I’m talking to you now by compressional waves, which are going through the air. But shear waves would not go through the air. So it’s rather a good technique for seeing where molten rock is. And we were hoping to image molten rock underneath this volcano, Askja, which we have since done. Some of it is at about one kilometer depth.

But it meant we had a big array out when this big eruption in 2014 happened. So we were very fortunate, and we actually had about a dozen spare seismometers that we’d just brought off the glacier where we’d been doing another experiment. And we quickly deployed them where we thought the eruption would occur. And we thought the eruption would occur in the place where the weight of the rocks above it was least, and at any one place, that’s in a valley, not under a mountain, because the weight of the mountain creates greater pressure at a given depth. So we quickly put these out in the valley where we thought it would erupt. And indeed it did, so close, in fact, that we had to rescue two of the seismometers shortly after the eruption started, because they were in the path of the lava. So we had amazing data. Yeah, we were in the right place at the right time, but it was for another reason originally.

Oliver Strimpel: Was this eruption associated in any way with what’s going on in the Reykjanes Peninsula at the moment, near Reykjavik?

Bob White: The 2014 eruption wasn’t, no. It’s completely separate. There’s so much going on in Iceland at the moment. As you rightly say, Reykjanes is erupting as we speak. The last time it did, about 800 years ago, the eruptions went on intermittently for about 30 years, so I think it’s going to carry on for a long time. But the ones in the middle of Iceland erupt less often, but probably more voluminously. They’re bigger eruptions.

Oliver Strimpel: When you say you measure hundreds or thousands of microquakes, how micro are they, and how do they compare to the earthquakes that make the news, that create damage?

Bob White: They’re tiny, really tiny, and we record 50,000 to 100,000 microearthquakes a year in the center of Iceland. But take a magnitude 6: an earthquake that makes the news is generally 6 or bigger. These are magnitude 1. Now, the magnitude scale is not linear, it’s logarithmic. And in energy terms, every time you go down one number, that’s a decrease in energy of about a factor of 30. So a magnitude 1 is about 24 million times less energy than a magnitude 6. And the magnitude 6 that you would be worried about, with houses falling down and people getting hurt and so on, breaks the surface of the Earth for distances of 10 or tens of kilometers typically, and moves a few meters. That’s the sort of scale of it. The ones of magnitude 1 that we’re talking about are perhaps a meter long, and they move perhaps a centimeter or something like that. So a much, much smaller scale, but exactly the same process. It’s just the ground cracking, so very small. In fact, we get a lot smaller, because it’s a logarithmic scale. You go to negative numbers, oddly, so we get down to magnitude −1. We can measure those down at depths of 10 kilometers. So they are very sensitive instruments, and it’s a very quiet location.

Oliver Strimpel: Let’s talk a little about the geochemistry of the lava. What exactly can we measure, and what can that tell us about the depths at which the melt formed and fractionated on the way out?

Bob White: Yes, well, there are two different things. Where it forms, we can do quite well at, because we can look at the major element compositions. The deeper it forms in the Earth, the more magnesium there is in it, for example. And also there are things called rare earth elements, which don’t react with anything, but they come out of the solid into the molten rock with different affinities. So at different depths, different rare earths come out into the melt. And they have names like neodymium and samarium. But the beauty of them is that they don’t react with anything. And so once they’re in the melt, they just get carried along with it. And so once it’s erupted at the surface, you can look at the rare earth element composition and work out how deep it was generated in the mantle, which is a really powerful tool.

Whereas the other elements, the more common ones that we’re used to, like magnesium and iron and calcium, all these so-called major elements, as they come up, they react with the country rock, which pollutes them. They fractionate out, and different things fractionate out with different affinities. And so the difficulty with geochemistry is that you can only ever measure it on the rock that’s come out at the surface. And as you alluded to, it may have stalled on the way at several different depths. And that can be a problem, because it usually gets overprinted at the last depth it stopped at. So you can usually tell quite well the depth where it ponded just before it erupted. But even that can be difficult, because it reacts as it erupts. And if you haven’t got a very fresh sample, it’s reacted with the air when it comes out.

Now the rare earths, because they just fractionate along with everything else, they’re okay. They don’t change their relative concentration. So we can tell how deep it formed. It’s a bit harder to tell how deep it stalled on the way up. If you’re lucky, you can do it. And what you have to do is find some elements which are in equilibrium with each other at some given depth, and hope that they’ve come out at the surface quite quickly without reacting with other things. So then you can work out at what depth those three things would be in equilibrium. And so it can give you some idea of the depth, but not nearly so precise. If you can do it to within a few kilometers, you’re doing well for the depth, whereas we can tell much more accurately than that with the seismology.

Oliver Strimpel: So can you then describe what your study actually showed about the path taken by the melt in the instance that you just described in 2014?

Bob White: Yes, 2014 was a fascinating eruption, because it’s the biggest historic eruption in Iceland since 1783, I think it was. It was 1.8 cubic kilometers of rock, which is a huge amount of rock. It would cover most of our cities many hundreds of meters deep, and it lasted six months. But what was interesting about this one was that it came from a volcano which was underneath the big ice cap in Iceland called Vatnajökull, which is the biggest ice cap in the Northern Hemisphere, I think; biggest temperate ice cap, I should say. And it is precisely there because it’s over this mantle hotspot. So it’s actually the most elevated region of Iceland, which is why it’s got ice on top of it, but it’s also the hottest piece of mantle underneath, so it’s where more melt is generated than anywhere else. So it’s a funny juxtaposition: it’s the hottest mantle and the coldest at the surface.

So the volcano is called Bárðarbunga, and it’s been active for thousands of years. And what would normally happen is that the melt would come up straight through the glacier, through the cold area. It’s buried by about 800 meters of ice, and you’d expect the melt to come straight out the top of the volcano, as most volcanoes do. In this case, it didn’t do that. At about six kilometers depth, it started moving laterally, sideways, about six kilometers under the surface of the Earth, and it did so for nearly 50 kilometers before it erupted. And the way it moved was away from this high ice cap, and it erupted just at a low point before the topography started increasing up towards the next volcano along, which was Askja.

So it was a fascinating thing to happen, to go sideways 50 kilometers, and a bit scary, because if you live 50 kilometers from an active volcano, you might think, well, that’s fine, but maybe lava can pop up underneath your feet. Now actually, it’s not completely unusual, because in Hawaii that is exactly what happens. And there was a case a few years ago where the lava was flowing laterally from the high point, the Kilauea caldera on the Big Island of Hawaii, and then went laterally tens of kilometers before it erupted, and it erupted under a township on the coast. Literally a crack opened in the streets and molten rock started coming up, and they had to evacuate. And I don’t think they can go back now. So lava does flow sideways, and I think it would do so if it is easier for it to go sideways than to keep going up through another six kilometers of overburden of rock. And then it erupts, usually at a low point, where the pressure above it is the least.

And so that’s exactly what happened in our case. But interestingly, some of my students were out there when the first eruption started in 2014, putting out these seismometers, and they actually saw the eruption start. And they said it came up through the same craters as had been used in the previous big eruption there in the mid-19th century; it actually reoccupied them. So clearly there was a pathway down at six kilometers depth which the lava was reusing, flowing sideways. So in that respect it wasn’t surprising, because that’s probably an easier way for it to go. There was a lava route, which would have been blocked up with lava, but lava contracts a bit when it cools, so there’s probably some void space, and it’s easier to go there than to break through virgin rock above it. So yeah, it was a fascinating story, and it took about two weeks for the lava to propagate that distance of 50 kilometers.

Oliver Strimpel: About what speed does that correspond to?

Bob White: We found it went in bursts, and this was one of the things we discovered by seeing it actually in the act of propagating, because of course usually you just get geological exposures: you just get the rock exposed at the surface, and you can’t tell the history of it. It would propagate forward in bursts. It would propagate a few kilometers forward at a speed of between one and four kilometers per hour, so about the speed you can walk comfortably if it had been on the surface. And that’s very typical. But we found it would propagate sideways, and then it would stall for a few days up to a week, and then it would have a burst of propagating forward again.

And this is happening, we think, because it has a pressure head behind it of the molten rock in the reservoir in the volcano where it’s coming from. And as it propagates forward, it uses a lot of energy, of course, to crack its way forward, and it uses up that pressure head. And then it has to stop and wait for more melt to flow in and build up the pressure again before it can break its way forward the next bit. But once it’s broken its way forward, it can flow quite freely. And interestingly, you could see that the seismicity, the earthquakes, were concentrated in a few kilometers just at the front of the propagation. Once it opened up a passageway, no more earthquakes. And you can estimate how big the passage was, and we estimate it was roughly 10 meters in diameter, which is 30 feet. So it’s pretty reasonably large for molten rock to be flowing along.

And when this finally broke the surface and started erupting, you could see the reservoir dropping down in Bárðarbunga, the parent volcano, and it dropped something like 65 meters over the six months that it was erupting. You know the size of the caldera, so you know how far it dropped. So you can estimate the volume of how much was evacuated. And that all pretty well matched, one for one, how much we know was erupted at the other end, out of the final eruption site. So it’s a very simple system actually: you sort of evacuate one end and it spurts out the other, and it just flows laterally in between. But watching how it flowed was fascinating, and to be able to document that.

Oliver Strimpel: Just coming back to the question of what depth it either originated or where it last paused on its way up. So if it evacuated a caldera, was that just coming from one shallow location to another, or were you able to see that it had actually ponded at some greater depth?

Bob White: We don’t know for sure what depth it’s ponding at. We think it’s probably ponding at about six to eight kilometers under that volcano. But what we need to do is tomography on that volcano to see where the molten rock is. And it’s obviously a huge, great reservoir, so we should be able to see it quite easily, except it’s quite hard to work on a glacier. It’s a temperate glacier that accumulates several meters of snow every winter and melts back several meters every summer, so it’s not an easy place to work. But that’s precisely what one of my ex-students is doing. He’s going off to put an array of seismometers right over the top of this Bárðarbunga volcano to actually map what’s happening directly underneath and see if he can image this storage area.

Oliver Strimpel: Eruptions in Iceland come in various styles. There are explosive eruptions, most famously in recent times being the one that blasted out enough ash to bring most of European aviation to a standstill.

Bob White: You’re going to tell me the name of the volcano?

Oliver Strimpel: I’m waiting for you to tell me that.

Bob White: Eyjafjallajökull. Yes. It was a source of great amusement in Britain, with the newsreaders trying to read it.

Oliver Strimpel: But other eruptions, such as the one going on at the moment in the Reykjanes Peninsula, emerge as fountains of lava along fissures. What is it that causes the eruption style to vary so much from one place to another in Iceland, over just a few tens of kilometers?

Bob White: It’s whether it has a lot of ice above it, because that eruption in 2010 from Eyjafjallajökull was under an ice cap of several hundred meters at the top of the volcano. And so you can imagine that molten rock, which is at a temperature of 1,100°C, hits the ice as it comes out of the rock and melts it. It makes a big pond of water under the ice, and then subsequent melt coming in at the same temperature will promptly freeze very quickly in this water. And what happens when you freeze molten rock is you make glass. Actually, the way you make glass is to make molten silica and freeze it. So you make this massive glass, but there’s a lot of gas in the molten rock as well, and so it actually makes lots of tiny bubbles of glass. And then the glass is hot, so the gas expands, and these little bubbles expand and they burst. So you get millions and millions of tiny shards of glass, which then can erupt out the top, because it’s trying to expand. So it just blows its way out the top. And that’s basically what the ash was in the explosive eruptions. It was tiny, tiny shards of glass, coming up, in that case, to 35,000 feet and then drifting down over Europe. So it’s very nasty, carcinogenic stuff. You don’t really want to breathe it, and it’s not very good for jet engines either. I’ve seen a picture of a plane that flew through an ash cloud, and it just stripped off all the paint.

So that’s the difference, really. Most of the volcanoes start in a fissure, a crack which opens. They often then just concentrate into a crater, but if it’s under a glacier, it can be explosive. The other thing that can make it explosive is if there’s some melt hanging around under that glacier that’s been there for a few hundred years and it’s fractionated, changed its composition, and then some fresh melt comes in which is rich in volatiles. It can hit the old melt, and that can cause a very explosive eruption. And that happened under Eyjafjallajökull in 2010 as well. So yeah, there are a couple of reasons they can be explosive. But in general, they’re much calmer eruptions than the ones around subduction zones, where plates are pushing together. Those really are dangerous, because you don’t get much warning, and there are a lot of volatiles in the melt, from the rocks that have been carried down in the subduction zone. So it’s volatiles expanding that makes it so explosive in those cases. There are volatiles in the Icelandic ones, but not so much, so they’re generally not so explosive.

Oliver Strimpel: Let’s step back and talk a bit about what causes all the volcanism that we see in Iceland. And as you mentioned, it’s generally thought that there’s a hotspot or a plume of anomalously hot mantle below Iceland. And that in addition, you mentioned there’s the Mid-Atlantic spreading ridge, which runs right through it. So where does the location you studied in 2014 lie with respect to these structures?

Bob White: Well, it’s pretty well over the top of the mantle plume underneath, which probably has a diameter of about 100 kilometers. They’re not particularly big things as they rise up from deep. They get deflected into a big mushroom head when they hit the bottom of the lithosphere, the rigid part. But we were right over the central part of it, so the hottest part of it, if you like. And because the melts are generated so deep, they are, as you said, relatively low viscosity, so they can flow easily. They’re high-magnesium, high-iron, and they flow, compared to the subduction zone ones, which have much more silica in them, which makes them much more viscous, much more sticky. So they don’t flow very far; instead, they explode.

Oliver Strimpel: So as I mentioned in the introduction, almost all the erupted melt originates in the mantle, at least so we think. So what is it about this particular location that causes the mantle to melt there and not in other places?

Bob White: If you go back to what causes melting in the first place: the mantle of the Earth is under the rigid outer layer, which is the lithosphere. Typically that’s 100 kilometers thick on an old, stable continent. The mantle is actually solid, despite what some people think. They think it’s molten, but it’s basically solid, and it has a very high viscosity. It does flow, but pretty slowly on a human time scale. But if you take a piece of that mantle and bring it to the surface very quickly, it melts as it decompresses. Now, the reason for that is that it’s sitting quite close to the melting point of mantle rocks when it’s down at 100 kilometers depth. It’s called the solidus. It’s where you change from solid to liquid. But the pressure decreases as you get nearer the surface. Of course, there’s tremendous pressure at 100 kilometers depth, and as you come nearer the surface, the pressure decreases until you’re at atmospheric pressure at the surface. And at the same time, the melting point decreases by quite a significant amount. So if you take a parcel of mantle and move it up very quickly, it will cross this solidus, because it won’t have time to lose heat by conduction, which is quite slow. So it retains its heat, but it comes to an area where the melting point is much lower, and so it melts instantaneously. Actually, you don’t have to pump heat in; it just melts because the melting point is lower, so it’s crossed that phase change from solid to liquid.

So that’s exactly what happens under normal mid-ocean ridges. You’re pulling the plates apart, and so the thickness of the lithosphere is essentially going to zero at the rift in the middle. So the mantle can move very close to the surface, and it comes up very quickly, because the plates are spreading quite fast, between 20 and 150 millimeters a year. And on a geological time scale, that’s very fast. So the mantle doesn’t have time to cool down as it rises, so it retains its temperature, but it’s crossed the solidus. It’s now hotter than the melting point at that low pressure, so it melts. And at mid-ocean ridges, you generate about seven kilometers of melt, which is very buoyant and forms crust, and that’s the crust of the oceans, typically seven kilometers thick.

Now, you get mantle plumes essentially because the center of the Earth is very hot and is still trying to lose heat, and it can’t lose it fast enough by conduction. So you get blobs of material coming up and then entraining material behind them, which is very hot, a bit like those old lava lamps that were so popular in the ’60s. Mantle plumes are just like that. You get a big blob which rises up, and it pulls a tail up behind it of material which is hotter than the surrounding mantle, by not a huge amount, 150° or 200°, something like that. So when you stretch that and allow it to decompress, then it will generate a lot more melt, because you’re sitting so close to the melting point at depth. So instead of it melting when it gets up to about 30 or 40 kilometers depth, as you do under mid-ocean ridges, it’ll start melting at 80 or 100 kilometers instead.

And so that all accumulates, and under Iceland you generate about 40-kilometer-thick crust above the spreading center, even though it’s just a rift, just like the bit further south in the oceanic crust. So instead of seven kilometers, you get 40 kilometers, and that’s why you get lots of volcanoes there. Now, most of the melt doesn’t come out as a lava flow; most of it is just frozen in the crust. We only see a small percentage of it. So there is a great deal generated directly above the top of the mantle plume, and then that tails off. So as you move down towards the oceanic crust from the center of Iceland, it decreases from 40 kilometers to about 14 kilometers on the Reykjanes Peninsula. And then you keep going offshore and it gets thinner and thinner till you get back to the normal seven kilometers. But it takes about 1,000 kilometers before it gets back to that normal temperature.

Oliver Strimpel: Why is there such a huge variation in the amount of melt that you get in a place like Iceland, as opposed to, say, Hawaii, the mid-ocean hotspot volcano, where you’re not getting that kind of volume at all?

Bob White: That’s a very good question, because the difference is that Hawaii is in the middle of the Pacific plate, which is about 70 kilometers thick there. And so this rising material from deep in the Earth, which is very hot, actually probably hotter than Iceland, gets blocked by the rigid lithosphere above it at about 70 kilometers depth. So you can generate melt from when it crosses the solidus, at maybe 100 kilometers depth, up to that depth of about 70 kilometers. But after that it gets deflected sideways, and once it’s going sideways, there’s no more decompression, of course. It’s at the same pressure, and so you don’t generate any more melt. So you only generate it directly above the rising plume, because there’s no rift across the middle of Hawaii. And so the rate of melt generation is much decreased because of that thick lithospheric plate it’s sitting on.

And there are other volcanoes under continental crust as well. In North Africa, there are quite a number of hotspots under the continental crust, where the lithosphere is even thicker, and so you get less melting still. So it all depends on how thick the lithosphere is. And if there’s a mid-ocean ridge spreading above it, then it wants to stretch to zero, because it’s spreading apart about 20 millimeters a year in Iceland, and that’s why so much melt is generated there.

Oliver Strimpel: So it’s much more to do with the thickness of the lithosphere that happens to be encountered where the plume reaches the lithosphere, rather than variations in temperature of the plume from one plume to another.

Bob White: Well, it’s both things, of course. I think the temperatures probably do vary a bit, but they probably don’t vary hugely. The core of the Earth is very hot, the mantle is much cooler, and you get what’s called a boundary layer between them, where you go from the hot to the cooler. And to make that boundary layer go unstable, you need a certain temperature difference, and so probably most of the plumes are a pretty similar temperature. So the main control, yes, is the thickness of the lithosphere when the mantle plume impinges on it.

And it’s not quite happenstance that it’s impinging under the spreading center under Iceland, because the spreading center, where it’s pulling apart, likes to be in the lowest energy state. And if it’s above a mantle plume, because the plume is hot, it’s more buoyant, so it elevates the Earth’s surface a bit there, and so it’s easier for the rifting to occur above that point than it is off the side of the plume. And so actually, we can see that the spreading axis has been tracking the plume eastwards across Iceland. As you come from the Atlantic in the south, there’s a big dogleg to the east across the top of the plume, and then a dogleg to the west again to the spreading as you go up towards the Arctic. And so the spreading center is trying to keep itself above the plume, which is drifting eastwards with respect to the spreading center. Eventually it will give up, and it will jump back to just go straight through Iceland.

And we can see that’s happened in places like Tristan da Cunha in the South Atlantic. Tristan da Cunha is off on the east side of the Mid-Atlantic Ridge, but originally the hotspot was sitting right underneath the ridge, and it made a big underwater ridge, which you can still see, called the Walvis Ridge on one side and the Rio Grande Rise on the other. So you can actually see when the spreading center stopped tracking the mantle plume and gave up, and then Tristan da Cunha, above the mantle plume, is still carrying on moving eastward.

Oliver Strimpel: We know that continental rifting often accompanies the arrival of a mantle plume and the emplacement of a large igneous province, as Richard Ernst talked about in a recent Geology Bites podcast. Is it the thickness of the crust and the consequent volume of melt produced that determines whether rifting will occur at a given plume?

Bob White: It’s a really good chicken-and-egg question, isn’t it? Did the plume cause the continental breakup, or did the continental breakup allow the plume to rise? Personally, I think the answer is that the plume, which originates deep in the Earth, at least 650 kilometers but probably much deeper than that, at the core–mantle boundary, doesn’t know what’s going on with the plates zipping around on the surface of the Earth. It doesn’t really have any knowledge of that. There’s no direct connection between them, and so where it comes up is not really controlled by what’s above it. But you can imagine that if a plume does come up underneath a continent which is predisposed to be stretched, if it’s in tension for some reason, then the extra elevation from the plume material underneath the lithosphere, which will give it an elevation of two or three kilometers, quite a lot of potential energy in that elevation, might give it the impetus to finally break. And then when it does finally break, of course you’ve thinned the lithosphere, and so you’ve allowed that mantle plume material to rise right up near the surface and create lots of melt, as we were just discussing.

So it is slightly chicken and egg, but I think it’s likely that if a plume hits underneath a continental block which is predisposed to break up, it will give it the final push, and then it will generate, as you said, huge volumes of melt, maybe 10 million cubic kilometers in the space of a million years or so. So it’s a huge rate of production. But clearly we know sometimes plumes do arrive under continental crust and don’t cause breakup. As I said, in North Africa there are several plumes which we can identify which have not caused breakup. But the one that hit what is now the west coast of Africa caused breakup between South America and Africa. But again, that was predisposed to break. There was some rifting in the north from the North Atlantic and from the south. So the plume gave it the final push, and then it generated loads of melt.

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

Bob White: Well, you could guess, couldn’t you: the Reykjanes Peninsula. There are 20 or 30 seismometers deployed on the Reykjanes Peninsula now, and we’re tracking all the multiple dikes and melt intrusions that are happening there in unprecedented detail, actually, because it’s so easy to get to. It’s only an hour’s drive from Reykjavik. And so if something changes, you can nip out and fix it. The instruments in the interior of Iceland I was talking about before, you can only really get to in the summer, for two months, because it’s −20° in the winter and covered with ice. You literally can’t get your instruments out of the ground because they’re frozen in so hard. You’d have to use pickaxes.

So Reykjanes is going to produce all sorts of new information, but we are still working in the interior. We’ve still got an array there, because there’s a lot of benefit to having a long-term array, and we’re still doing that tomography we set out to do in 2014. In fact, just this week, as I speak to you, a paper has been published on tomography under Askja, where we can see melt at one kilometer depth and also at six kilometers depth, in at least two storage areas. And it is inflating. We know from GPS measurements it’s gone up getting on for a meter over the past couple of years. There’s always something going on in Iceland.

Oliver Strimpel: Bob White, thank you very much.

Bob White: It’s a pleasure.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you’ll also find transcripts and a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.