1 00:00:03,920 --> 00:00:06,920 This is Geology bites with Oliver Strimple. 2 00:00:08,680 --> 00:00:12,800 Almost all erupted magma is initially generated in the 3 00:00:12,800 --> 00:00:17,400 Earth's mantle as the melt forces its way up through the 4 00:00:17,400 --> 00:00:20,000 many kilometres of lithosphere to the surface. 5 00:00:20,400 --> 00:00:24,400 It usually pauses in one or more magma chambers or partially 6 00:00:24,400 --> 00:00:28,520 melted mosh zones for periods of up to a few thousand years 7 00:00:28,520 --> 00:00:32,520 before erupting. During these pauses, the magma 8 00:00:32,520 --> 00:00:35,760 can separate into fractions, having different compositions 9 00:00:36,080 --> 00:00:38,480 and also interact with the surrounding crust. 10 00:00:39,640 --> 00:00:43,000 But while we have seismic evidence and models that support 11 00:00:43,000 --> 00:00:46,880 this picture, we've not hitherto been able to watch how magma 12 00:00:46,880 --> 00:00:49,480 actually moves in the upper mantle and crust. 13 00:00:50,840 --> 00:00:53,000 Bob White has set out to change that. 14 00:00:53,760 --> 00:00:56,880 Using a dense array of seismometers, he has been able 15 00:00:56,880 --> 00:01:00,680 to pinpoint thousands of tiny earthquakes that reveal the 16 00:01:00,680 --> 00:01:03,480 detailed movement of melt through the thick crust of 17 00:01:03,480 --> 00:01:08,400 Iceland just before it erupted. He combines this seismic data 18 00:01:08,400 --> 00:01:12,280 with geochemical analysis of the lava that can tell us about the 19 00:01:12,280 --> 00:01:14,280 depths at which the melt is formed. 20 00:01:15,400 --> 00:01:18,360 He is Emeritus Professor of Geophysics in the Department of 21 00:01:18,360 --> 00:01:20,840 Earth Sciences at the University of Cambridge. 22 00:01:21,840 --> 00:01:24,080 Bob White, welcome to Geology Bites. 23 00:01:25,040 --> 00:01:26,440 Well, it's a pleasure to be with you. 24 00:01:26,960 --> 00:01:29,800 And of course, this work was done not just by myself, but by 25 00:01:29,800 --> 00:01:32,240 a whole team of graduate students and colleagues in 26 00:01:32,240 --> 00:01:34,400 Iceland. Why did you pick Iceland? 27 00:01:34,400 --> 00:01:36,680 For your detailed study of magma movement. 28 00:01:37,200 --> 00:01:39,960 The Mid Ocean Ridge The Mid-Atlantic Ridge cuts right 29 00:01:39,960 --> 00:01:42,240 through Iceland. So Iceland is a brilliant place 30 00:01:42,240 --> 00:01:45,800 to study the processes where the plates are spreading apart and 31 00:01:45,800 --> 00:01:47,280 generating a lot of volcanic rock. 32 00:01:47,560 --> 00:01:50,720 And then how did you decide exactly where in Iceland to 33 00:01:50,720 --> 00:01:53,880 focus on? Well, we started working in the 34 00:01:53,920 --> 00:01:56,400 centre of Iceland. It's a place where absolutely 35 00:01:56,400 --> 00:01:58,680 nobody lives. You can only get there in the 36 00:01:58,680 --> 00:02:02,000 summer and that has a huge advantage that it's seismically 37 00:02:02,000 --> 00:02:04,760 very quiet. There's no cars and lorries and 38 00:02:05,040 --> 00:02:08,440 all the other noise that comes about where people live, so it's 39 00:02:08,440 --> 00:02:11,280 very seismically quiet. It's also the most active 40 00:02:11,280 --> 00:02:15,680 volcanic area in Iceland because it happens to lie over a mantle 41 00:02:15,800 --> 00:02:18,840 hotspot underneath it. So it was a great place to watch 42 00:02:18,960 --> 00:02:21,200 volcanoes doing what volcanoes do. 43 00:02:21,760 --> 00:02:26,560 So as I mentioned, you exploited two types of data in your study, 44 00:02:26,880 --> 00:02:31,600 seismic and geochemical. Let's talk about how these work 45 00:02:31,600 --> 00:02:34,800 to give us detailed information on the movement of melt. 46 00:02:35,240 --> 00:02:40,040 Starting with the seismic. By definition, melt is liquid, 47 00:02:40,040 --> 00:02:43,120 so we can't be seeing earthquakes directly from the 48 00:02:43,120 --> 00:02:44,760 melt. Indeed you can't. 49 00:02:44,760 --> 00:02:47,600 That's correct. But the melt has formed very 50 00:02:47,600 --> 00:02:49,440 deep in the upper part of the mantle. 51 00:02:49,920 --> 00:02:52,880 The melt under Iceland probably starts forming at depths of 52 00:02:53,400 --> 00:02:56,080 about 100 kilometres. And then it migrates up very 53 00:02:56,080 --> 00:02:59,520 quickly until it hits the brittle part at the top or the 54 00:02:59,520 --> 00:03:03,120 lithosphere. And once it gets there, it has 55 00:03:03,120 --> 00:03:05,160 to get its way through to the surface. 56 00:03:05,160 --> 00:03:08,440 It's buoyant, so it wants to go upwards, but it has to create 57 00:03:08,440 --> 00:03:11,040 cracks to do that, to make a pathway for it. 58 00:03:11,400 --> 00:03:15,000 So a crack is actually an earthquake, or rather, an 59 00:03:15,000 --> 00:03:18,400 earthquake is a very big crack. And the molten rock, as it 60 00:03:18,400 --> 00:03:21,840 pushes its way through the crust, creates many, many, many 61 00:03:21,840 --> 00:03:26,720 tiny earthquakes, tiny cracks. So by mapping where those tiny 62 00:03:26,720 --> 00:03:29,400 earthquakes are occurring, you're actually mapping where 63 00:03:29,400 --> 00:03:32,400 the melt is moving. If it stalls, it tends not to 64 00:03:32,400 --> 00:03:34,680 generate any earthquakes, but when it's on the move, it 65 00:03:34,680 --> 00:03:37,400 generates lots to open a pathway for itself. 66 00:03:38,120 --> 00:03:42,640 So to be able to watch in detail where the melt is going then you 67 00:03:42,640 --> 00:03:46,800 need to track these micro quakes and you also need to know 68 00:03:46,800 --> 00:03:48,880 exactly where they are. So how do you pinpoint the 69 00:03:48,880 --> 00:03:51,560 location of these very faint earthquakes? 70 00:03:51,840 --> 00:03:54,480 Yes, it's a technique called triangulation. 71 00:03:54,680 --> 00:03:57,120 We helped a little bit in seismology about the fact there 72 00:03:57,120 --> 00:03:59,840 are two different types of seismic wave. 73 00:04:00,200 --> 00:04:03,200 One is called P waves or compression of waves. 74 00:04:03,200 --> 00:04:05,800 The other is called shear waves or S waves. 75 00:04:06,520 --> 00:04:10,520 And the S waves travel at about half the speed of the P waves, 76 00:04:10,840 --> 00:04:12,840 but they come from the same spot, of course. 77 00:04:13,240 --> 00:04:16,480 So actually, if you've only got 1 seismometer and you can 78 00:04:16,480 --> 00:04:19,320 measure the P wave and the S wave, you at least know how far 79 00:04:19,320 --> 00:04:23,280 away it is because you know the speed at which they travel, but 80 00:04:23,280 --> 00:04:25,840 you don't know where it is. So you still need a number of 81 00:04:25,920 --> 00:04:28,120 seismometers. And these are very, very 82 00:04:28,120 --> 00:04:31,640 sensitive instruments recording the motion of the earth in 3 83 00:04:31,640 --> 00:04:34,760 dimensions vertically and two horizontal directions. 84 00:04:34,760 --> 00:04:37,960 So you can use that to work out how long it took for the 85 00:04:38,160 --> 00:04:41,480 earthquake wave to reach you, and also actually the direction 86 00:04:41,480 --> 00:04:45,000 of travel, because the wavefront hitting the seismometer can tell 87 00:04:45,000 --> 00:04:47,440 you where it's coming from. So you have quite a lot of 88 00:04:47,440 --> 00:04:49,160 information. But the more the merrier, the 89 00:04:49,160 --> 00:04:51,520 more seismometers you can get, the better the location. 90 00:04:52,000 --> 00:04:56,240 And then at the end of the day, roughly how precise a location 91 00:04:56,280 --> 00:04:58,680 can you get? Well, the thing you don't know 92 00:04:58,680 --> 00:05:02,360 is exactly how fast the seismic waves travel through the Earth. 93 00:05:02,360 --> 00:05:05,560 That gives some uncertainty. And also if there's any noise on 94 00:05:05,560 --> 00:05:09,200 the arrival that can perturb it. So in absolute terms, we 95 00:05:09,200 --> 00:05:12,520 probably know the location to 100 meters or so. 96 00:05:13,160 --> 00:05:17,040 But it turns out that if you've got a lot of earthquakes in 97 00:05:17,200 --> 00:05:19,600 essentially the same place, which is what you get at the tip 98 00:05:19,600 --> 00:05:22,080 of a crack as it's propagating forward, as the melt moves 99 00:05:22,080 --> 00:05:25,480 forward, you can do rather better at calculating the 100 00:05:25,480 --> 00:05:28,600 relative locations of those different earthquakes, because 101 00:05:28,600 --> 00:05:31,080 if you imagine it, the energy from each earthquake is 102 00:05:31,080 --> 00:05:33,880 travelling through the same heterogeneous crust to your 103 00:05:33,880 --> 00:05:37,800 receiver every time. So if you've got several 104 00:05:37,800 --> 00:05:41,200 earthquakes essentially in the same place, then you can cancel 105 00:05:41,200 --> 00:05:43,920 out all that heterogeneity and just look at the relative 106 00:05:43,920 --> 00:05:46,920 locations between them. And we can get that down to 107 00:05:46,920 --> 00:05:50,240 about 10 metres, which is remarkable really because we can 108 00:05:50,320 --> 00:05:54,520 look at these tiny earthquakes down at six, 810 kilometres 109 00:05:54,520 --> 00:05:57,760 depth and know where they are to within 10 metres or so. 110 00:05:58,520 --> 00:06:01,120 I imagine you have to get relatively close to these 111 00:06:01,120 --> 00:06:04,200 earthquakes in order to be able to get that level of accuracy. 112 00:06:04,200 --> 00:06:08,160 So when I asked you why you picked Iceland and where exactly 113 00:06:08,160 --> 00:06:10,560 in Iceland, you said, well, the centre of Iceland is free of 114 00:06:10,560 --> 00:06:12,200 people and it's more secure and so on. 115 00:06:12,200 --> 00:06:15,800 But presumably you had to narrow it down way more than that in 116 00:06:15,800 --> 00:06:18,720 order to get close enough. So were you able to predict 117 00:06:18,720 --> 00:06:23,360 where you think the melt was going to move before a 118 00:06:23,360 --> 00:06:25,560 particular eruption, or did you just get lucky? 119 00:06:26,320 --> 00:06:30,360 We got lucky in one respect, but we also had an array out over 120 00:06:30,360 --> 00:06:34,080 quite a big area, so it was over 100 kilometres square, something 121 00:06:34,080 --> 00:06:35,880 like that. If the earthquake's big enough 122 00:06:35,880 --> 00:06:39,120 and you can record it across the whole array, which might span 123 00:06:39,120 --> 00:06:42,480 100 kilometres, then you can still locate it very accurately. 124 00:06:43,080 --> 00:06:45,720 If it's a tiny earthquake, you need to be a bit closer to it 125 00:06:46,080 --> 00:06:49,040 and ideally you want run receiver more or less directly 126 00:06:49,040 --> 00:06:51,520 above it because that's what controls the depth, which is 127 00:06:51,520 --> 00:06:53,840 probably the biggest uncertainty. 128 00:06:54,480 --> 00:06:57,160 But we had a regional array out in the middle of Iceland. 129 00:06:57,160 --> 00:07:01,440 We've been operating one there for the last 15 years or so 130 00:07:01,720 --> 00:07:04,480 because it is very active. And we've actually caught a 131 00:07:04,480 --> 00:07:08,480 couple of other small injections of melt into the crust before. 132 00:07:08,480 --> 00:07:12,800 The one I'm going to talk about, the big one in 2014, and on this 133 00:07:12,800 --> 00:07:16,080 occasion in 2014, we'd actually put our extra seismometers. 134 00:07:16,080 --> 00:07:18,080 We had more than we'd ever had before. 135 00:07:18,080 --> 00:07:21,440 We had more than 70 of these seismometers arrayed across an 136 00:07:21,440 --> 00:07:24,480 area because what we're planning to do was to find out where the 137 00:07:24,480 --> 00:07:27,720 melt was residing under another active volcano called ASCIA. 138 00:07:27,720 --> 00:07:31,080 Now that's not the one that erupted, but it was adjacent to 139 00:07:31,080 --> 00:07:35,680 it, and we knew it had molten rock underneath it, and we were 140 00:07:35,680 --> 00:07:38,480 going to use tomography. And the way that worked in 141 00:07:38,480 --> 00:07:42,800 medical systems is that you have a very high frequency sound wave 142 00:07:43,120 --> 00:07:46,440 and you bounce it off your internal organs to look for 143 00:07:46,440 --> 00:07:50,120 tumors or babies as they develop in pregnant women. 144 00:07:50,480 --> 00:07:54,160 And you can reconstruct an image of what's going on inside your 145 00:07:54,160 --> 00:07:57,440 body with tomography. Well, we just pinched the same 146 00:07:57,440 --> 00:07:59,120 idea. We have a much longer 147 00:07:59,120 --> 00:08:01,080 wavelength. The wavelength of the seismic 148 00:08:01,080 --> 00:08:05,040 waves is about one kilometre or at the very best several 100 149 00:08:05,200 --> 00:08:07,920 metres. So our resolution is much poorer 150 00:08:08,360 --> 00:08:11,640 and we can't generate sound at the surface unless we let off 151 00:08:11,640 --> 00:08:14,880 explosives, which occasionally we do, but people are not very 152 00:08:14,880 --> 00:08:16,480 keen on you doing that these days. 153 00:08:17,040 --> 00:08:20,760 So mostly we use the earthquake source itself, the energy from 154 00:08:20,760 --> 00:08:23,240 the earthquake to make the energy source, and then we 155 00:08:23,240 --> 00:08:26,480 record how it travels through the earth and we can invert all 156 00:08:26,480 --> 00:08:28,440 that information to do tomography. 157 00:08:28,440 --> 00:08:31,480 And particularly we can see where molten rock is because one 158 00:08:31,480 --> 00:08:34,280 of those two types of waves, I explained the shear waves don't 159 00:08:34,280 --> 00:08:37,640 go through molten material, the compression of waves do. 160 00:08:38,080 --> 00:08:40,760 So I'm talking to you now by compression of waves, which is 161 00:08:40,760 --> 00:08:43,120 going through the air. But shear waves would not go 162 00:08:43,120 --> 00:08:45,760 through the air. So it's rather a good technique 163 00:08:45,760 --> 00:08:49,280 for seeing where molten rock is. And we were hoping to image 164 00:08:49,280 --> 00:08:52,400 molten rock underneath this volcano, Askia, which we have 165 00:08:52,400 --> 00:08:54,560 since done. It's about 1 kilometre depth, 166 00:08:54,560 --> 00:08:57,160 some of it, but it meant we had a big array out when this big 167 00:08:57,160 --> 00:09:02,040 eruption in 2014 happened. So we were very fortunate and we 168 00:09:02,040 --> 00:09:06,040 actually had about a dozen spare seismometers that we just 169 00:09:06,040 --> 00:09:09,480 brought off the glacier where we're doing another experiment. 170 00:09:09,760 --> 00:09:12,360 And we quickly deployed them where we thought the eruption 171 00:09:12,360 --> 00:09:14,480 would occur. And we thought the eruption 172 00:09:14,480 --> 00:09:18,480 would occur in the place where the overweight of the rocks 173 00:09:18,560 --> 00:09:22,720 above it was leased now at any one place that's in a valley, 174 00:09:22,960 --> 00:09:25,080 not under a mountain, because the weight of the mountain 175 00:09:25,080 --> 00:09:26,880 creates greater pressure at a given depth. 176 00:09:27,320 --> 00:09:30,920 So we quickly put these out in the valley where we thought it 177 00:09:30,920 --> 00:09:32,160 would erupt. And indeed it did. 178 00:09:32,480 --> 00:09:35,080 So close, in fact, that we had to rescue two of the 179 00:09:35,080 --> 00:09:38,480 seismometers shortly after the eruption started because it 180 00:09:38,480 --> 00:09:41,360 arrived in the path of the lava. So we had amazing data. 181 00:09:41,360 --> 00:09:43,520 Yeah, we're in the right place at the right time, but it was 182 00:09:43,520 --> 00:09:47,400 for another reason originally. Was this eruption associated in 183 00:09:47,400 --> 00:09:50,320 any way with what's going on in the Rakhines peninsula at the 184 00:09:50,320 --> 00:09:53,800 moment near Reykjavik? The 2014 eruption wasn't no, 185 00:09:53,800 --> 00:09:56,840 it's completely separate. There's so much going on in 186 00:09:56,840 --> 00:09:59,360 Iceland at the moment. As you rightly say, Reykjavik is 187 00:09:59,360 --> 00:10:03,200 erupting as we speak. Last time it did it about 800 188 00:10:03,200 --> 00:10:05,160 years ago. The eruptions went on 189 00:10:05,160 --> 00:10:09,120 intermittently for about 30 years, so I think it's going to 190 00:10:09,120 --> 00:10:12,120 carry on for a long time. But the ones in the middle of 191 00:10:12,120 --> 00:10:16,360 Iceland erupt less often, but probably more voluminous. 192 00:10:16,360 --> 00:10:18,880 They're bigger eruptions. When you say you measure 193 00:10:19,160 --> 00:10:23,760 hundreds or thousands of micro quakes, how micro are they and 194 00:10:23,760 --> 00:10:26,760 how do they compare to the earthquakes that make the news 195 00:10:26,760 --> 00:10:30,880 that create damage? They're tiny, really tiny, and 196 00:10:31,280 --> 00:10:35,320 we record 50 to 100,000 micro earthquakes a year in the centre 197 00:10:35,320 --> 00:10:39,600 of Iceland. But if you take a magnitude 6 198 00:10:39,600 --> 00:10:42,800 earthquake is one that makes the news, generally 6 or bigger. 199 00:10:43,360 --> 00:10:46,200 These are magnitude one. Now the magnitude scale. 200 00:10:46,600 --> 00:10:49,040 It's not linear, it's logarithmic. 201 00:10:49,520 --> 00:10:52,800 And in energy terms, every time you go down one number, that's a 202 00:10:52,800 --> 00:10:55,880 decrease in energy of about a factor of 30. 203 00:10:56,400 --> 00:11:01,160 So a magnitude one is about 24,000,000 times less energy 204 00:11:01,520 --> 00:11:04,760 than a magnitude 6. And the magnitude 6 that you 205 00:11:04,760 --> 00:11:07,400 would be worried about, houses falling down and people getting 206 00:11:07,400 --> 00:11:10,120 hurt and so on, breaks the surface of the Earth for 207 00:11:10,120 --> 00:11:14,040 distances of 10 or 10s of kilometres typically, and moves 208 00:11:14,200 --> 00:11:17,160 a few metres. That's the sort of scale of it. 209 00:11:17,520 --> 00:11:22,280 The ones of magnitude one that we're talking about are perhaps 210 00:11:22,280 --> 00:11:25,760 a metre long and they move perhaps a centimetre or 211 00:11:25,760 --> 00:11:28,520 something like that. So much, much smaller scale, but 212 00:11:28,520 --> 00:11:31,120 exactly the same process. It's just the ground cracking, 213 00:11:31,600 --> 00:11:33,520 so very small. In fact we get a lot smaller 214 00:11:33,520 --> 00:11:35,120 because it's a logarithmic scale. 215 00:11:35,120 --> 00:11:38,520 You go to negative numbers, oddly, so we get down to 216 00:11:38,640 --> 00:11:41,840 magnitude -1. We can measure down at depths of 217 00:11:41,840 --> 00:11:44,400 10 kilometres. So they are very sensitive 218 00:11:44,400 --> 00:11:46,600 instruments and it's a very quiet location. 219 00:11:47,160 --> 00:11:50,240 Let's talk a little about the geochemistry of the lava. 220 00:11:50,800 --> 00:11:55,600 What exactly can we measure, and what can that tell us about the 221 00:11:55,600 --> 00:11:58,880 depths at which the melt formed and fractionated on the way out? 222 00:11:59,280 --> 00:12:02,400 Yes, well, there's two different things where it forms, We can do 223 00:12:02,400 --> 00:12:06,080 quite well at because we can look at the major element 224 00:12:06,080 --> 00:12:09,440 compositions. The deeper it forms in the 225 00:12:09,440 --> 00:12:12,240 earth, the more magnesium there is in it, for example. 226 00:12:12,800 --> 00:12:16,280 And also there are things called rare earth elements, which don't 227 00:12:16,280 --> 00:12:19,840 react with anything, but they come out of the solid into the 228 00:12:20,200 --> 00:12:22,240 molten rock with different affinities. 229 00:12:22,240 --> 00:12:25,320 So at different depths, different rare earths come out 230 00:12:25,320 --> 00:12:27,080 into the melt. And they have names like 231 00:12:27,080 --> 00:12:30,360 neodymium samarium. But the beauty of them is that 232 00:12:30,360 --> 00:12:33,520 they don't react with anything. And so once they're in the melt, 233 00:12:33,520 --> 00:12:35,000 they just get carried along with it. 234 00:12:35,280 --> 00:12:39,560 And so once it's erupted at the surface, you can look at the 235 00:12:39,560 --> 00:12:42,520 rare earth element composition, work out how deep it was 236 00:12:42,520 --> 00:12:45,680 generated in the mantle, which is a really powerful tool. 237 00:12:46,240 --> 00:12:49,040 Whereas the other elements, the more common ones that we're used 238 00:12:49,040 --> 00:12:53,360 to like magnesium and iron and calcium, all these so-called 239 00:12:53,360 --> 00:12:56,680 major elements, as they come up, they react with the country rock 240 00:12:56,680 --> 00:13:00,040 which pollutes them. They fractionate out and 241 00:13:00,040 --> 00:13:03,080 different things fractionate out with different affinities. 242 00:13:03,560 --> 00:13:06,600 And so the difficulty with geochemistry is that you can 243 00:13:06,600 --> 00:13:09,480 only ever measure it on the rock that's come out the surface. 244 00:13:10,000 --> 00:13:12,560 And as you alluded to, it may have stalled on the way at 245 00:13:12,560 --> 00:13:15,760 several different depths. And that can be a problem 246 00:13:15,760 --> 00:13:19,640 because it usually gets over printed at the last depth it 247 00:13:19,640 --> 00:13:21,560 stopped at. So you can usually tell quite 248 00:13:21,560 --> 00:13:25,760 well the depth where it ponded just before it erupted. 249 00:13:26,440 --> 00:13:29,400 But even that can be difficult because it reacts as it erupts. 250 00:13:30,000 --> 00:13:32,440 And if you haven't got a very fresh sample, it's reacted with 251 00:13:32,440 --> 00:13:35,200 the air when it comes out. Now the rare earths, because 252 00:13:35,200 --> 00:13:37,800 they just fractionate along with everything else, they're OK. 253 00:13:37,800 --> 00:13:40,280 They don't change their relative concentration. 254 00:13:40,280 --> 00:13:42,640 So we can tell how deep it formed. 255 00:13:42,640 --> 00:13:46,000 A bit harder to tell how deep it stalled on the way up. 256 00:13:46,200 --> 00:13:49,640 If you're lucky, you can do it. And what you have to do is find 257 00:13:49,640 --> 00:13:52,680 some elements which are in equilibrium with each other at 258 00:13:52,680 --> 00:13:56,640 some given depth and hope that they've come out at the surface 259 00:13:56,640 --> 00:13:58,880 quite quickly without reacting with other things. 260 00:13:59,200 --> 00:14:02,600 So then you can work out at what depth those three and things 261 00:14:02,600 --> 00:14:04,680 would be in equilibrium. And so I can give you some idea 262 00:14:04,680 --> 00:14:06,840 of the depth, but not nearly so precise. 263 00:14:06,840 --> 00:14:09,360 If you can do it with a few kilometres, you're doing well 264 00:14:09,480 --> 00:14:12,600 for the depth, whereas we can tell much more accurately than 265 00:14:12,600 --> 00:14:17,080 that with the seismology. So can you then describe what 266 00:14:17,080 --> 00:14:21,360 your study actually showed about the path taken by the melt in 267 00:14:21,360 --> 00:14:23,920 the instance that you just described in 2014? 268 00:14:24,480 --> 00:14:27,720 Yes, 2014 was a fascinating eruption because it's the 269 00:14:28,120 --> 00:14:31,760 biggest historic eruption in ice and since 1783 I think it was, 270 00:14:31,800 --> 00:14:36,120 it was 1.8 cubic kilometres of rock, which is a huge amount of 271 00:14:36,120 --> 00:14:37,840 rock. It would cover most of our 272 00:14:37,840 --> 00:14:41,240 city's many hundreds of meters deep and it lasted 6 months. 273 00:14:41,640 --> 00:14:44,240 But what was interesting about this one was that it came from a 274 00:14:44,240 --> 00:14:47,840 volcano which was underneath the big ice cap in Iceland called 275 00:14:47,840 --> 00:14:52,880 Vatniokol, which is the biggest ice cap in the Northern 276 00:14:52,880 --> 00:14:55,640 hemisphere, I think biggest temperate ice cap I should say. 277 00:14:56,000 --> 00:14:59,960 And it is precisely there because it's over this mantle 278 00:15:00,040 --> 00:15:02,080 hotspot underneath. So it's actually the most 279 00:15:02,080 --> 00:15:05,440 elevated region of Iceland, which is why it's got ice on top 280 00:15:05,440 --> 00:15:08,840 of it, but it's also the hottest piece of mantle underneath, so 281 00:15:08,840 --> 00:15:11,680 it's where more melt is generated than anywhere else. 282 00:15:11,680 --> 00:15:14,640 So it's a funny Juxta vision. It's the hottest mantle bit and 283 00:15:14,640 --> 00:15:18,520 the coldest at the surface. So the volcano is called Bada 284 00:15:18,520 --> 00:15:21,680 Bunga and it's been active for thousands of years. 285 00:15:21,960 --> 00:15:25,640 And what would normally happen is that the melt will come up 286 00:15:25,640 --> 00:15:28,840 straight through the glacier, through the cold area. 287 00:15:28,840 --> 00:15:32,080 It's buried by about 800 meters of ice, and you'd expect the 288 00:15:32,080 --> 00:15:34,480 melt to come straight out the top of the volcano, as most 289 00:15:34,480 --> 00:15:35,960 volcanoes do. Now. 290 00:15:36,080 --> 00:15:39,720 In this case, it didn't do that at about 6 kilometres depth. 291 00:15:39,720 --> 00:15:44,200 It started moving laterally sideways about 6 kilometres 292 00:15:44,200 --> 00:15:48,480 under the surface of the Earth, and it did so for nearly 50 293 00:15:48,480 --> 00:15:53,160 kilometres before it erupted. And the way it moved was it was 294 00:15:53,160 --> 00:15:57,560 moving away from this high ice cap and it erupted just at a low 295 00:15:57,560 --> 00:16:00,840 point before the topography started increasing up towards 296 00:16:00,840 --> 00:16:02,800 the next volcano along which was called Ascia. 297 00:16:03,320 --> 00:16:07,840 So it was a fascinating thing to happen to go sideways 50 298 00:16:07,840 --> 00:16:09,960 kilometres and a bit scary because, you know, if you live 299 00:16:10,360 --> 00:16:13,200 50 kilometres from an active volcano, you might think, well, 300 00:16:13,200 --> 00:16:17,160 that's fine, but maybe lava can pop up underneath your feet. 301 00:16:18,040 --> 00:16:21,160 Now actually, it's not completely unusual because in 302 00:16:21,160 --> 00:16:23,040 Hawaii that is exactly what happens. 303 00:16:23,040 --> 00:16:26,360 And there was a case a few years ago where the lava was flowing 304 00:16:26,360 --> 00:16:31,240 laterally from the high point, the Kilauea Caldera on Big 305 00:16:31,240 --> 00:16:34,720 Island of Hawaii, and then went laterally 10s of kilometers 306 00:16:34,720 --> 00:16:37,760 before it erupted and it erupted under a Township on the coast. 307 00:16:37,760 --> 00:16:41,040 Literally a crack opened in the streets and molten rock started 308 00:16:41,040 --> 00:16:43,680 coming up and actually they had to evacuate. 309 00:16:43,680 --> 00:16:45,160 And I don't think they can go back now. 310 00:16:45,200 --> 00:16:48,360 Even so, lava does flow sideways, and I think it would 311 00:16:48,360 --> 00:16:51,240 do it if the pressure gradient is easier for it to go sideways 312 00:16:51,240 --> 00:16:54,400 than to keep going up another 6 kilometers of overburden of 313 00:16:54,400 --> 00:16:56,800 rock. And then it erupts, usually in a 314 00:16:56,800 --> 00:16:59,320 low point where the pressure above it is the least. 315 00:16:59,720 --> 00:17:02,000 And so that's exactly what happened in our case. 316 00:17:02,440 --> 00:17:06,400 But interestingly, some of my students were out there when the 317 00:17:06,400 --> 00:17:10,160 first eruption started in 2014 and they've been putting out 318 00:17:10,160 --> 00:17:13,720 these seismometers and they actually saw the eruption start 319 00:17:14,040 --> 00:17:16,680 and they said it came up through the same craters as have been 320 00:17:16,680 --> 00:17:20,119 used in the previous big eruption there in the mid 19th 321 00:17:20,119 --> 00:17:22,720 century, actually reoccupied them. 322 00:17:22,720 --> 00:17:26,520 So clearly there was a pathway down at six kilometers depth 323 00:17:26,520 --> 00:17:29,960 which the lava was reusing, flowing sideways. 324 00:17:30,320 --> 00:17:32,480 So in that respect it wasn't surprising because that's 325 00:17:32,480 --> 00:17:34,280 probably an easier way for it to go. 326 00:17:34,280 --> 00:17:38,080 There was lava route which would have been blocked up with lava, 327 00:17:38,080 --> 00:17:40,320 but lava contracts a bit when it cools, so there's there's 328 00:17:40,320 --> 00:17:43,000 probably some void space and it's easier to go there than to 329 00:17:43,000 --> 00:17:44,680 breakthrough virgin rock above it. 330 00:17:44,680 --> 00:17:48,280 So yeah, it was a fascinating story, and it took about two 331 00:17:48,280 --> 00:17:51,760 weeks for the lava to propagate that distance of 50 kilometers. 332 00:17:52,240 --> 00:17:54,600 About what speed does that correspond to? 333 00:17:54,880 --> 00:17:57,560 We found it went in burst, and this was one of the things we've 334 00:17:57,560 --> 00:18:01,440 discovered by seeing it actually in the act of propagating, 335 00:18:01,440 --> 00:18:04,480 because of course, usually you just get geological exposures, 336 00:18:04,480 --> 00:18:07,800 you just get the rock exposed that service and you can't tell 337 00:18:07,800 --> 00:18:11,120 the history of it. This would propagate forward in 338 00:18:11,120 --> 00:18:14,880 bursts that would propagate a few kilometers forward at a 339 00:18:14,880 --> 00:18:19,000 speed of between 1:00 and 4:00 kilometers per hour. 340 00:18:19,120 --> 00:18:22,640 So about the speed, you can walk comfortably if it had been on 341 00:18:22,640 --> 00:18:26,360 the surface and that's very typical, but we found it would 342 00:18:26,360 --> 00:18:30,640 propagate sideways and then it would stall for a few days up to 343 00:18:30,680 --> 00:18:33,240 a week and then it would have a burst of propagating forward 344 00:18:33,240 --> 00:18:34,600 again. And this is happening and we 345 00:18:34,600 --> 00:18:38,480 think because it has a pressure head behind it of the molten 346 00:18:38,480 --> 00:18:41,000 rock in the reservoir and the the volcano where it's coming 347 00:18:41,000 --> 00:18:43,240 from. And as it propagates forward, it 348 00:18:43,240 --> 00:18:46,400 uses a lot of energy of course to crack its way forward and it 349 00:18:46,400 --> 00:18:49,880 uses up that pressure head. And then you have to stop and 350 00:18:49,880 --> 00:18:52,640 wait for more melt to flow in and build up the pressure again 351 00:18:52,640 --> 00:18:55,240 before it could break its way forward the next bit. 352 00:18:55,800 --> 00:18:58,480 But once it's broken its way forward, it can flow quite 353 00:18:58,480 --> 00:19:01,480 freely. And interestingly, you could see 354 00:19:01,480 --> 00:19:04,480 that the seismicity, the earthquakes, were concentrated 355 00:19:04,480 --> 00:19:06,960 in a few kilometers just to the front of the propagation. 356 00:19:07,360 --> 00:19:09,960 Once it opened up a passageway, no more earthquakes. 357 00:19:10,280 --> 00:19:13,720 And you can estimate how big the passage was, and we estimate it 358 00:19:13,720 --> 00:19:16,600 was roughly 10 meters diameter, which is 30 foot. 359 00:19:16,600 --> 00:19:19,520 You know, it's pretty reasonably large for molten rock to be 360 00:19:19,520 --> 00:19:22,320 flowing along. And when this finally broke 361 00:19:22,320 --> 00:19:25,240 surface started erupting out the surface, you could see the 362 00:19:25,240 --> 00:19:29,240 reservoir dropping down in Bardabunga, the parent volcano, 363 00:19:29,720 --> 00:19:33,520 and it dropped something like 65 meters over that six months that 364 00:19:33,520 --> 00:19:35,800 it was erupting. You know the size of the 365 00:19:35,800 --> 00:19:37,560 caldera, so you know how far it dropped. 366 00:19:37,560 --> 00:19:40,600 So you can estimate the volume of how much was evacuated. 367 00:19:41,120 --> 00:19:43,840 And that all pretty well matched, one for one, how much 368 00:19:43,840 --> 00:19:47,080 we know was erupted at the other end out of the final eruption 369 00:19:47,080 --> 00:19:49,480 site. So it's a very simple system 370 00:19:49,480 --> 00:19:52,480 actually, you know, you sort of evacuate one end and it spurts 371 00:19:52,480 --> 00:19:54,960 out the other, just flows laterally in between. 372 00:19:54,960 --> 00:19:56,840 But watching how it flowed was fascinating. 373 00:19:56,840 --> 00:19:59,920 And to be able to document that. Just coming back to the question 374 00:19:59,920 --> 00:20:04,080 of what depth it either originated or where it last 375 00:20:04,240 --> 00:20:07,600 paused on its way up. So if it evacuated A caldera, 376 00:20:07,600 --> 00:20:10,360 was that just coming from 1 shallow location to another or 377 00:20:10,360 --> 00:20:13,280 were you able to see that it had actually pondered at some 378 00:20:13,280 --> 00:20:15,960 greater depth? We don't know for sure what 379 00:20:16,080 --> 00:20:18,200 depth it's ponding at. We think it's probably ponding 380 00:20:18,200 --> 00:20:21,880 at about 6 to 8 kilometers under that volcano. 381 00:20:22,400 --> 00:20:27,680 But what we need to do is to do tomography on that volcano to 382 00:20:27,680 --> 00:20:30,200 see where the molten rock is. And it's obviously a huge, great 383 00:20:30,200 --> 00:20:32,480 reservoir, so we should be able to see it quite easily. 384 00:20:32,920 --> 00:20:35,960 With the exception it's quite hard to work on a glacier. 385 00:20:35,960 --> 00:20:38,920 It's a temperate glacier that accumulates several meters of 386 00:20:38,920 --> 00:20:42,360 snow every winter and melts back several meters every summer, so 387 00:20:42,360 --> 00:20:46,160 it's not an easy place to work. But that's precisely what one of 388 00:20:46,160 --> 00:20:48,760 my ex students is doing. He's going off to put an array 389 00:20:48,760 --> 00:20:53,440 of seismometers right over the top of this Badamunga volcano to 390 00:20:53,440 --> 00:20:56,640 actually map what's happening directly underneath and see if 391 00:20:56,640 --> 00:21:01,080 he can image this storage area. Eruptions in Iceland come in 392 00:21:01,080 --> 00:21:04,520 various styles. They're explosive eruptions, 393 00:21:05,040 --> 00:21:08,640 most famously in recent times being the one that blasted out 394 00:21:08,640 --> 00:21:11,480 enough ash to bring most of European aviation to a 395 00:21:11,480 --> 00:21:13,280 standstill. You're going to tell me the name 396 00:21:13,280 --> 00:21:15,600 of the volcano? I'm waiting for you to tell me 397 00:21:15,600 --> 00:21:18,880 that. AF Yachtly Yes. 398 00:21:18,880 --> 00:21:21,120 It was a source of great amusement in Britain with the 399 00:21:21,120 --> 00:21:25,400 news readers trying to read it. But other eruptions, such as the 400 00:21:25,400 --> 00:21:28,280 one going on at the moment in the Rakhiness Peninsula, emerge 401 00:21:28,280 --> 00:21:30,680 as fountains of lava along fissures. 402 00:21:31,280 --> 00:21:34,640 What is it that causes the eruption style to vary so much 403 00:21:34,640 --> 00:21:37,600 from one place to another in Iceland over just a few 10s of 404 00:21:37,600 --> 00:21:40,040 kilometres? It's whether it has a lot of ice 405 00:21:40,040 --> 00:21:44,600 above it, because that eruption in 2010 from Ayef Yerkla Yoko 406 00:21:45,280 --> 00:21:49,320 was under an ice cap of several 100 meters at the top of the 407 00:21:49,320 --> 00:21:51,680 volcano. And so you can imagine that 408 00:21:51,680 --> 00:21:55,880 molten rock, which is at a temperature of 1100°C. 409 00:21:56,040 --> 00:21:59,880 It hits the ice as it comes out of the rock and melts. 410 00:21:59,880 --> 00:22:03,200 It makes a big pond of water under the ice, and then 411 00:22:03,200 --> 00:22:06,520 subsequent melt coming in at the same temperature will promptly 412 00:22:06,800 --> 00:22:09,200 freeze very quickly in this water. 413 00:22:09,720 --> 00:22:12,640 And what happens when you freeze molten rock is you make glass. 414 00:22:12,800 --> 00:22:15,840 Actually, the way you make glass is to make molten silica and 415 00:22:16,080 --> 00:22:18,600 freeze it. So you make this massive glass, 416 00:22:18,600 --> 00:22:22,280 but there's a lot of gas in the molten rock as well, and so it 417 00:22:22,280 --> 00:22:25,320 actually makes lots of tiny bubbles of glass. 418 00:22:26,640 --> 00:22:32,120 And then the glass is hot, so the glass expands and these 419 00:22:32,120 --> 00:22:33,880 little bubbles expand and they burst. 420 00:22:33,880 --> 00:22:38,280 So you get millions and millions of tiny shards of glass, which 421 00:22:38,280 --> 00:22:41,720 then can erupt out the top because it's trying to expand. 422 00:22:41,720 --> 00:22:43,400 So it just blows its way out the top. 423 00:22:44,000 --> 00:22:46,840 And that's basically what the ash was of the explosive 424 00:22:46,840 --> 00:22:49,120 eruptions. It was tiny, tiny shards of 425 00:22:49,120 --> 00:22:53,440 glass coming up to, in that case, went up to 35,000 feet and 426 00:22:53,440 --> 00:22:57,400 then drifted down over Europe. So it's very nasty, carcinogenic 427 00:22:57,400 --> 00:22:58,760 stuff. You don't really want to breathe 428 00:22:58,760 --> 00:23:01,080 it, but it's not very good for jet engines either. 429 00:23:01,080 --> 00:23:03,840 I've seen a picture of a plane that flew through an ash cloud 430 00:23:03,840 --> 00:23:05,320 and it just stripped off all the paint. 431 00:23:05,920 --> 00:23:08,960 So that's the difference really. Most of the volcanoes start in a 432 00:23:08,960 --> 00:23:12,880 fissure, a crack which opens. They often then just concentrate 433 00:23:12,880 --> 00:23:16,320 into a crater, but if it's under a glacier, it can be explosive. 434 00:23:16,800 --> 00:23:19,840 The other thing that can make it explosive is that if there's 435 00:23:19,840 --> 00:23:22,840 some melt hanging around under that glacier that's been there 436 00:23:22,840 --> 00:23:25,960 for a few 100 years and it's fractionated, changed its 437 00:23:25,960 --> 00:23:29,760 composition, and then some fresh melt comes in which is rich in 438 00:23:29,760 --> 00:23:33,440 volatiles, it can hit the old melt and that can cause a very 439 00:23:33,440 --> 00:23:37,040 explosive eruption. And that happened under a FIACO 440 00:23:37,080 --> 00:23:40,360 in 2010 as well. So yeah, there's a couple of 441 00:23:40,360 --> 00:23:44,000 reasons they can be explosive, but in general they're much more 442 00:23:44,000 --> 00:23:47,800 calm eruptions than the ones around subduction zones where 443 00:23:47,800 --> 00:23:50,400 plates are pushing together, which really are dangerous 444 00:23:50,400 --> 00:23:52,160 because you don't get much warning, and there's a lot of 445 00:23:52,160 --> 00:23:55,640 volatiles from the melt, from the rocks that have been carried 446 00:23:55,640 --> 00:23:58,280 down in the subduction zone. So is volatiles expanding that 447 00:23:58,280 --> 00:24:01,680 makes it so explosive. In those cases, there are 448 00:24:01,680 --> 00:24:05,080 volatiles in the Icelandic ones, but not so much, so they're 449 00:24:05,080 --> 00:24:09,000 generally not so explosive. Let's step back and talk a bit 450 00:24:09,000 --> 00:24:11,920 about what causes all the volcanism that we see in 451 00:24:11,920 --> 00:24:14,160 Iceland. And as you mentioned, it's 452 00:24:14,160 --> 00:24:16,680 generally thought that there's a hot spot or a plume of 453 00:24:16,680 --> 00:24:19,880 anomalously hot mantle below Iceland. 454 00:24:20,080 --> 00:24:22,560 And that in addition, you mentioned there's the 455 00:24:22,560 --> 00:24:25,160 Mid-Atlantic spreading Ridge, which runs right through it. 456 00:24:25,720 --> 00:24:31,000 So where does the location you studied in 2014 lie with respect 457 00:24:31,000 --> 00:24:34,240 to these structures? Well, it's pretty well over the 458 00:24:34,240 --> 00:24:36,760 top of the mantle plume underneath, which probably has a 459 00:24:36,760 --> 00:24:39,200 diameter of about 100 kilometres. 460 00:24:39,200 --> 00:24:41,680 They're not particularly big things as they rise up from 461 00:24:41,680 --> 00:24:43,880 deep. They get deflected into a big 462 00:24:43,880 --> 00:24:46,480 mushroom head when they hit the bottom of the lithosphere, the 463 00:24:46,480 --> 00:24:50,160 rigid part, But we were right over the central part of it, so 464 00:24:50,160 --> 00:24:51,800 the hottest part of it if you like. 465 00:24:51,880 --> 00:24:54,640 And because the melts are generated so deep, they are, as 466 00:24:54,680 --> 00:24:57,280 you said, relatively low viscosity, so they can flow 467 00:24:57,280 --> 00:24:58,840 easily. They're high magnesium, high 468 00:24:58,840 --> 00:25:02,080 iron, and it flows compared to the subduction zone ones, which 469 00:25:02,080 --> 00:25:05,040 have much more silica in them, which makes them much more 470 00:25:05,040 --> 00:25:09,400 viscous, much more sticky. So they don't flow very far, but 471 00:25:09,400 --> 00:25:13,080 they instead they explode. So as I mentioned in the 472 00:25:13,080 --> 00:25:16,800 introduction, almost all the erupted melt originates in the 473 00:25:16,800 --> 00:25:20,800 mantle at least so we think. So what is it about this 474 00:25:20,800 --> 00:25:25,160 particular location that causes the mantle to melt there and not 475 00:25:25,160 --> 00:25:28,080 in other places? If you go back to what causes 476 00:25:28,080 --> 00:25:30,680 melting in the 1st place, The mantle of the Earth under the 477 00:25:30,680 --> 00:25:32,800 rigid outer layer, which is a lithosphere. 478 00:25:33,280 --> 00:25:36,200 Typically that's 100 kilometres thick on an old stable 479 00:25:36,200 --> 00:25:39,080 continent. The mantle is actually solid, 480 00:25:39,560 --> 00:25:42,520 despite some people think it's molten, but it's basically solid 481 00:25:43,120 --> 00:25:44,720 and it has a very high viscosity. 482 00:25:44,720 --> 00:25:47,880 It does flow, but pretty slowly on a human time scale. 483 00:25:48,880 --> 00:25:52,040 But if you take a piece of that mantle and bring it to the 484 00:25:52,040 --> 00:25:56,000 surface very quickly, it melts as it decompresses. 485 00:25:56,000 --> 00:25:59,120 Now the reason for that is it's sitting quite close to the 486 00:25:59,120 --> 00:26:04,040 melting point of mantle rocks when it's down at 100 kilometres 487 00:26:04,040 --> 00:26:06,080 depth. It's called the solidus. 488 00:26:06,080 --> 00:26:10,280 It's where you change from solid to liquid, but as the pressure 489 00:26:10,280 --> 00:26:12,680 decreases as you get nearer the surface, because there's 490 00:26:12,680 --> 00:26:14,720 tremendous pressure. Of course at 100 kilometres 491 00:26:14,720 --> 00:26:17,640 depth, as you come nearer the surface the pressure decreases 492 00:26:17,640 --> 00:26:21,520 until your atmospheric pressure at the surface and at the same 493 00:26:21,520 --> 00:26:24,560 time the melting point decreases by quite a significant amount. 494 00:26:24,840 --> 00:26:27,760 So if you take a parcel of mantle and move it up very 495 00:26:27,760 --> 00:26:31,840 quickly, it will cross this solidus because at its 496 00:26:31,840 --> 00:26:34,320 temperature it won't have time to lose heat by conduction 497 00:26:34,320 --> 00:26:37,400 because that's quite slow. So it retains its heat, but it 498 00:26:37,400 --> 00:26:41,200 comes to an area where the melting point is much lower and 499 00:26:41,200 --> 00:26:44,080 so it melts instantaneously. Actually, you don't have to pump 500 00:26:44,080 --> 00:26:46,960 heat in, it just melts because the melting point is lower, so 501 00:26:46,960 --> 00:26:49,200 it's crossed that phase change from solid to liquid. 502 00:26:49,640 --> 00:26:52,680 So that's exactly what happens under normal mid ocean ridges. 503 00:26:52,720 --> 00:26:55,600 You're pulling the plates apart and so the thickness is 504 00:26:55,600 --> 00:26:57,840 essentially going to zero at the rift in the middle. 505 00:26:58,120 --> 00:27:00,960 So the mantle can move very close to the surface and it 506 00:27:00,960 --> 00:27:03,600 comes up very quickly because the plates are spreading quite 507 00:27:03,600 --> 00:27:07,240 fast, between 20 and 150 millimetres a year. 508 00:27:07,800 --> 00:27:10,040 And on a geological time scale that's very fast. 509 00:27:10,280 --> 00:27:13,360 So the mantle doesn't have time to cool down as it rises, so it 510 00:27:13,360 --> 00:27:16,240 retains its temperature, but it's crossed the solid US. 511 00:27:16,280 --> 00:27:19,280 It's now hotter than the melting point at that low pressure, so 512 00:27:19,280 --> 00:27:23,680 it melts and mid ocean ridges. You generate about 7 kilometres 513 00:27:23,680 --> 00:27:27,240 of melt, which is very buoyant forms of crust, and that's the 514 00:27:27,240 --> 00:27:30,720 crust of the oceans, typically 7 kilometres thick. 515 00:27:31,200 --> 00:27:34,960 Now under a mantle plume you get mantle plumes essentially 516 00:27:34,960 --> 00:27:37,800 because the center of the Earth is very hot, still trying to 517 00:27:37,800 --> 00:27:40,920 lose heat, can't lose it fast enough by conduction. 518 00:27:40,920 --> 00:27:45,080 So you get blobs of material coming up and then in training 519 00:27:45,080 --> 00:27:47,320 material behind them, which is very hot. 520 00:27:47,760 --> 00:27:51,160 Bit like those old lava lamps that were so popular in the 60s, 521 00:27:51,600 --> 00:27:52,920 and mantle plumes just like that. 522 00:27:53,000 --> 00:27:56,000 You get a big BLOB which rises up and it pulls the tail up 523 00:27:56,040 --> 00:27:59,520 behind it of material which is hotter than the surrounding 524 00:27:59,560 --> 00:28:04,680 mantle by not a huge amount, 150° or 200°, something like 525 00:28:04,720 --> 00:28:07,560 that. So when you stretch that and 526 00:28:07,560 --> 00:28:11,120 allow it to decompress then it will generate a lot more melt 527 00:28:11,120 --> 00:28:13,760 because you're sitting so close to the melting point at depth. 528 00:28:13,760 --> 00:28:18,040 So instead of it melting when it gets up to about 30 or 40 529 00:28:18,040 --> 00:28:21,200 kilometres depth as you do under mid ocean ridges, it'll start 530 00:28:21,200 --> 00:28:23,800 melting at 80 or 100 kilometres instead. 531 00:28:23,800 --> 00:28:27,440 And so that all accumulates and under Iceland you generate about 532 00:28:27,440 --> 00:28:31,240 40 kilometres thick crust above the spreading centre, even 533 00:28:31,240 --> 00:28:34,680 though it's just a rift, just like the bit further South in 534 00:28:34,680 --> 00:28:37,000 the oceanic crust. So instead of 7 kilometres you 535 00:28:37,000 --> 00:28:40,680 get 40 kilometres and that's why you get lots of volcanoes there. 536 00:28:40,800 --> 00:28:43,760 Now most of the melt doesn't come out as a lava flow, most of 537 00:28:43,760 --> 00:28:47,240 it is just frozen in the crust. We only see a small percentage 538 00:28:47,240 --> 00:28:49,560 of it. So there is a great deal 539 00:28:49,560 --> 00:28:52,720 generated directly above the top of the mantle plume and then 540 00:28:52,720 --> 00:28:55,320 that tails off. So as you move down towards the 541 00:28:55,320 --> 00:28:58,760 oceanic crust from the centre of Iceland, it decreases from 40 542 00:28:58,760 --> 00:29:01,960 kilometres to about 14 kilometres just on the 543 00:29:01,960 --> 00:29:04,960 Raikkonen's peninsula. And then you keep going offshore 544 00:29:04,960 --> 00:29:07,240 and it gets thinner and thinner till you get back to the normal 545 00:29:07,240 --> 00:29:09,680 7 kilometres. But it takes about 1000 546 00:29:09,680 --> 00:29:11,920 kilometres before it gets back to that normal temperature. 547 00:29:12,600 --> 00:29:16,400 Why is there such a huge variation in the amount of melt 548 00:29:16,400 --> 00:29:20,520 that you get in a place like Iceland, say, as opposed to say, 549 00:29:20,520 --> 00:29:25,120 Hawaii, the mid ocean hotspot volcano where you're not getting 550 00:29:25,120 --> 00:29:28,240 that kind of volume at all? That's a very good question 551 00:29:28,240 --> 00:29:31,280 because the difference is that Hawaii is in the middle of the 552 00:29:31,280 --> 00:29:35,520 Pacific plate, which is about 70 kilometers thick there. 553 00:29:35,840 --> 00:29:40,200 And so this rising material from deep in the Earth, which is very 554 00:29:40,200 --> 00:29:42,600 hot, actually probably hotter than Iceland. 555 00:29:42,840 --> 00:29:46,160 It gets blocked by the rigid lithosphere above it at about 70 556 00:29:46,160 --> 00:29:48,880 kilometers depth. So you can generate melt from 557 00:29:48,880 --> 00:29:51,720 when it crosses A solidus at maybe 100 kilometers depth, up 558 00:29:51,720 --> 00:29:53,880 to that depth of about 70 kilometers. 559 00:29:54,200 --> 00:29:57,320 But after that it gets deflected sideways, and once it's going 560 00:29:57,320 --> 00:29:59,320 sideways, then there's no more decompression. 561 00:29:59,320 --> 00:30:01,560 Of course, it's at the same pressure, and so you don't 562 00:30:01,560 --> 00:30:04,560 generate any more melt, so you only generate it directly above 563 00:30:04,560 --> 00:30:08,080 the rising plume because there's no rift across the middle of 564 00:30:08,080 --> 00:30:10,560 Hawaii. And so the rate of melt 565 00:30:10,560 --> 00:30:13,280 generation is much decreased because of that thick 566 00:30:13,280 --> 00:30:14,760 lithosphere plate it's sitting on. 567 00:30:15,240 --> 00:30:17,640 And there are other volcanoes under continental crust as well. 568 00:30:17,640 --> 00:30:21,200 In North Africa there's quite a number of hotspots which run the 569 00:30:21,520 --> 00:30:24,480 continental crust where the lithosphere is even thicker and 570 00:30:24,480 --> 00:30:28,080 so you get less melting still. So it all depends on how thick 571 00:30:28,080 --> 00:30:30,960 the lithosphere is and if there's a mid ocean Ridge 572 00:30:30,960 --> 00:30:34,760 spreading above it, then it wants to stretch to 0 because 573 00:30:34,760 --> 00:30:38,160 it's spreading apart about 20 millimetres a year in Iceland 574 00:30:38,640 --> 00:30:40,680 and that's why so much melt is generated there. 575 00:30:41,680 --> 00:30:44,480 So it's much more to do with the thickness of the lithosphere 576 00:30:44,480 --> 00:30:48,000 that happens to be encountered where the plume reaches the 577 00:30:48,000 --> 00:30:51,840 lithosphere, rather than variations in temperature of the 578 00:30:51,840 --> 00:30:54,760 plume from one plume to another. Well, it's both things. 579 00:30:54,760 --> 00:30:56,400 Of course. I think the temperatures 580 00:30:56,400 --> 00:31:00,680 probably do vary a bit, but they probably don't vary hugely, 581 00:31:01,000 --> 00:31:05,240 because in order to go unstable, the core of the earth is very 582 00:31:05,240 --> 00:31:09,160 hot, the mantle's much cooler, and you get what's called a 583 00:31:09,160 --> 00:31:11,320 boundary layer between it, where you go from the hot to the 584 00:31:11,320 --> 00:31:13,400 cooler. And to make that boundary layer 585 00:31:13,400 --> 00:31:16,640 go unstable, you need a certain temperature difference, and so 586 00:31:16,640 --> 00:31:20,600 probably most of the plumes are pretty similar temperature. 587 00:31:20,760 --> 00:31:23,920 So the main control, yes, is the thickness of the lithosphere 588 00:31:23,920 --> 00:31:27,280 when the mantle plume impinges on the lithosphere. 589 00:31:27,720 --> 00:31:30,920 And it's not quite happenstance that it's impinging under the 590 00:31:30,920 --> 00:31:33,840 spreading centre under Iceland, because the spreading centre 591 00:31:33,840 --> 00:31:36,880 where it's pulling apart likes to be in the lowest energy 592 00:31:36,880 --> 00:31:40,040 state. And if it's above an elevated 593 00:31:40,040 --> 00:31:42,080 mantle plume, because it's hot, it elevates. 594 00:31:42,080 --> 00:31:44,640 It's more buoyant, so it elevates the Earth's surface a 595 00:31:44,640 --> 00:31:48,160 bit there, and so it's easier for the rifting to occur above 596 00:31:48,160 --> 00:31:52,440 that point than it is off the side of the plume. 597 00:31:52,560 --> 00:31:55,400 And so actually, we can see that the spreading axis has actually 598 00:31:55,400 --> 00:31:57,840 been tracking the plume eastwards across Iceland. 599 00:31:58,360 --> 00:32:01,440 And there's a big dog leg as you come from the Atlantic in the 600 00:32:01,440 --> 00:32:03,320 South. There's a big dog leg to the 601 00:32:03,320 --> 00:32:06,400 east across the top of the plume, and then a dog leg to the 602 00:32:06,400 --> 00:32:09,640 West again to the spreading as you go up towards the Arctic. 603 00:32:10,120 --> 00:32:12,920 And so the spreading centre is trying to keep itself above the 604 00:32:12,920 --> 00:32:15,360 plume which is drifting eastwards with respect to the 605 00:32:15,360 --> 00:32:18,280 spreading centre. Eventually it will give up and 606 00:32:18,280 --> 00:32:21,760 it will jump back to just go straight through Iceland, and we 607 00:32:21,760 --> 00:32:24,560 can see that's happened in places like Tristandacuna in the 608 00:32:24,600 --> 00:32:27,400 South Atlantic. Tristandacuna is off on the east 609 00:32:27,400 --> 00:32:30,360 side of the Mid-Atlantic Ridge, but originally the hotspot was 610 00:32:30,360 --> 00:32:32,880 sitting right underneath it and it made a big underwater Ridge 611 00:32:32,880 --> 00:32:35,840 which you can still see underwater, called the Walvis 612 00:32:35,840 --> 00:32:38,320 Ridge on one side and the Rio Grande Rise on the other. 613 00:32:38,640 --> 00:32:41,040 So you can actually see when the spreading centre stopped 614 00:32:41,040 --> 00:32:44,440 tracking the mantle plume and then gave up, and then Tristan 615 00:32:44,440 --> 00:32:47,240 de Kuna above the mantle plume is still carrying on moving 616 00:32:47,240 --> 00:32:50,400 eastward. We know that continental rifting 617 00:32:50,520 --> 00:32:53,720 often accompanies the arrival of a mantle plume and the 618 00:32:53,720 --> 00:32:56,280 emplacement of a large igneous province. 619 00:32:56,840 --> 00:33:00,920 As Richard Ernst talked about in a recent Geology Bias Podcast, 620 00:33:01,240 --> 00:33:04,760 Is it the thickness of the crust and the consequent volume of 621 00:33:04,760 --> 00:33:07,680 milk produced that determines whether rifting will occur at a 622 00:33:07,680 --> 00:33:09,600 given plume? It's a really good chicken and 623 00:33:09,600 --> 00:33:11,880 egg question, isn't it? Did the plume cause the 624 00:33:12,240 --> 00:33:15,080 continental breakup, or did the continental breakup allow the 625 00:33:15,080 --> 00:33:18,000 plume to rise? Personally, I think the answer 626 00:33:18,000 --> 00:33:20,320 is that the plume, which originates deep in the Earth, at 627 00:33:20,320 --> 00:33:24,960 least 650 kilometers but probably much deeper than that 628 00:33:24,960 --> 00:33:27,880 at the core mantle boundary, doesn't know what's going on 629 00:33:27,880 --> 00:33:30,280 with the plates sipping around on the surface of the Earth. 630 00:33:30,280 --> 00:33:31,760 It doesn't really have any knowledge of that. 631 00:33:31,760 --> 00:33:35,200 There's no direct connection between them, and so where it 632 00:33:35,200 --> 00:33:38,200 comes up is not controlled really by what's above it. 633 00:33:38,680 --> 00:33:41,360 But you can imagine that if a plume does come up underneath a 634 00:33:41,360 --> 00:33:45,680 continent which is predisposed to be stretched of it, it's 635 00:33:45,680 --> 00:33:50,400 intention for some reason, then the extra elevation of the plume 636 00:33:50,400 --> 00:33:53,280 material underneath the lithosphere, which will give it 637 00:33:53,280 --> 00:33:55,640 an elevation of two or three kilometres. 638 00:33:55,640 --> 00:33:59,400 Quite a lot of potential energy in that elevation might give it 639 00:33:59,400 --> 00:34:03,040 the impetus to finally break. And then when it does finally 640 00:34:03,040 --> 00:34:06,120 break, then of course you've thinned the lithosphere and so 641 00:34:06,120 --> 00:34:09,400 you allowed that mantle plume material to rise right up near 642 00:34:09,400 --> 00:34:11,360 the surface and create lots of melt, as we were just 643 00:34:11,360 --> 00:34:14,520 discussing. So it is slightly chicken and 644 00:34:14,520 --> 00:34:19,560 egg, but I think it's likely that if a plume hits underneath 645 00:34:19,560 --> 00:34:23,800 a continental block which is predisposed to break up, it will 646 00:34:23,800 --> 00:34:26,760 give it the finest push and then it will generate as you said 647 00:34:27,080 --> 00:34:31,159 huge volumes of melt, maybe 10 million cubic kilometers in the 648 00:34:31,159 --> 00:34:34,360 space of a million years or so. So it's a huge rate of 649 00:34:34,360 --> 00:34:37,159 production. But clearly we know sometimes 650 00:34:37,159 --> 00:34:40,520 plumes do arrive under continental crust and don't 651 00:34:40,520 --> 00:34:42,920 cause break up. So as I said in North Africa, 652 00:34:42,920 --> 00:34:45,960 there's several plumes which we can identify which have not 653 00:34:45,960 --> 00:34:49,800 caused breakup in North Africa. But the one that hits on what is 654 00:34:49,800 --> 00:34:52,960 now the West Coast of Africa caused breakup between South 655 00:34:52,960 --> 00:34:55,520 America and Africa. But again that was predisposed 656 00:34:55,520 --> 00:34:57,200 to break. There was there was some rifting 657 00:34:57,200 --> 00:34:59,600 in the north from the North Atlantic and from the South. 658 00:34:59,600 --> 00:35:02,760 So the plume gave it the final push and then it generated loads 659 00:35:02,760 --> 00:35:04,680 of melt. What are you working on at the 660 00:35:04,680 --> 00:35:06,320 moment? Well, you could guess, couldn't 661 00:35:06,320 --> 00:35:08,120 you, which on Reykjiani's peninsula. 662 00:35:08,200 --> 00:35:10,960 There's 20 or 30 seismometers deployed on Reykjiani's 663 00:35:10,960 --> 00:35:12,880 peninsula now. And we're tracking all the 664 00:35:12,880 --> 00:35:15,400 multiple dikes and melt intrusions that are happening 665 00:35:15,400 --> 00:35:18,880 there with unprecedented detail, actually, because it's so easy 666 00:35:18,880 --> 00:35:20,280 to get to. You know, it's only an hour's 667 00:35:20,280 --> 00:35:23,080 drive from Reykjavik. And so something changes. 668 00:35:23,080 --> 00:35:25,880 You can nip out and fix it. The instruments in the interior 669 00:35:25,880 --> 00:35:28,640 of Iceland I was talking about before, you can only really get 670 00:35:28,640 --> 00:35:32,600 there in the summer for two months because it's -20° in the 671 00:35:32,600 --> 00:35:34,720 winter covered with ice. You literally can't get your 672 00:35:34,720 --> 00:35:36,880 instruments out the ground because they're frozen in so 673 00:35:36,880 --> 00:35:38,720 hard. You'd have to use pickaxes. 674 00:35:39,320 --> 00:35:41,800 So Ray Connis is going to produce all sorts of new 675 00:35:41,800 --> 00:35:45,000 information, but we are still working in the interior. 676 00:35:45,000 --> 00:35:47,320 We've still got an array there because there's a lot of benefit 677 00:35:47,320 --> 00:35:49,960 to having a long term array and we're still doing that 678 00:35:49,960 --> 00:35:51,840 tomography we set off to do in 2014. 679 00:35:51,840 --> 00:35:56,160 In fact, just this week actually as I as I speak to you, a paper 680 00:35:56,160 --> 00:35:59,200 has been published on tomography under Askia where we can see 681 00:35:59,200 --> 00:36:02,280 melt at one kilometre depth and also at six kilometres depth in 682 00:36:02,280 --> 00:36:05,800 at least two storage areas. And it is inflating. 683 00:36:05,800 --> 00:36:08,480 We know from GPS measurements it's gone up getting on for a 684 00:36:08,480 --> 00:36:10,560 metre now over the past couple of years. 685 00:36:10,760 --> 00:36:12,440 There's always something going on in Iceland. 686 00:36:13,760 --> 00:36:16,600 Bob White, thank you very much. It's a pleasure. 687 00:36:17,960 --> 00:36:20,640 To see pictures and illustrations that support this 688 00:36:20,640 --> 00:36:26,200 podcast, go to geologybytes.com, where you'll also find 689 00:36:26,200 --> 00:36:30,560 transcripts and a subject matter index of all the episodes there. 690 00:36:30,560 --> 00:36:34,160 You can also give me feedback, which I welcome, as well as sign 691 00:36:34,160 --> 00:36:36,600 up to get my emails about new episodes.