1 00:00:04,000 --> 00:00:06,920 This is Geology Bytes with Oliver Strimple. 2 00:00:08,760 --> 00:00:12,400 Every half billion years or so, most of the Earth's continental 3 00:00:12,400 --> 00:00:15,120 lithosphere appears to have converged to form a 4 00:00:15,120 --> 00:00:19,640 supercontinent, A phenomenon known as a supercontinent cycle. 5 00:00:21,040 --> 00:00:25,440 The most recent supercontinent was Pangaea, which lasted from 6 00:00:25,440 --> 00:00:29,760 about 330 million years ago until about 170 million years 7 00:00:29,760 --> 00:00:34,000 ago. Until recently, the amalgamation 8 00:00:34,000 --> 00:00:37,480 and subsequent breakup of supercontinents was thought to 9 00:00:37,480 --> 00:00:41,480 result from processes happening on or near the Earth's surface. 10 00:00:42,280 --> 00:00:47,080 Specifically, the idea was the subduction of cold oceanic 11 00:00:47,080 --> 00:00:51,920 plates consumes the oceans lying between the continents, a 12 00:00:51,920 --> 00:00:55,360 process that only stops when continents bordering the 13 00:00:55,360 --> 00:00:59,400 subducting oceanic plates collide and a supercontinent is 14 00:00:59,400 --> 00:01:02,320 formed. Once assembled, the 15 00:01:02,320 --> 00:01:05,519 supercontinent was thought to behave like a thermal blanket 16 00:01:05,680 --> 00:01:09,000 that traps the heat generated in the mantle immediately beneath 17 00:01:09,000 --> 00:01:13,840 it, causing it to uplift thin and eventually break apart. 18 00:01:15,560 --> 00:01:19,320 But over the past decade or two we've learned a great deal about 19 00:01:19,320 --> 00:01:23,520 the mantle, mostly from seismic tomography and computer 20 00:01:23,520 --> 00:01:27,040 modelling. We now believe, for example, 21 00:01:27,240 --> 00:01:31,280 that subduction of cold oceanic plates can reach the base of the 22 00:01:31,280 --> 00:01:35,880 mantle and that subduction of the oceans that close up to form 23 00:01:35,880 --> 00:01:40,400 a supercontinent may interact with a deep mantle in such a way 24 00:01:40,920 --> 00:01:45,920 as to produce rising plumes of hot mantle material that impinge 25 00:01:45,920 --> 00:01:48,400 on the base of the supercontinent and foster it's 26 00:01:48,400 --> 00:01:51,800 break up. This has LED us to overhaul our 27 00:01:51,800 --> 00:01:55,800 ideas about what drives the long term motions and configurations 28 00:01:55,800 --> 00:01:59,440 of the continents and even whether we should be calling it 29 00:01:59,440 --> 00:02:04,760 the supercontinent cycle. Damien Nantz studies tectonic 30 00:02:04,760 --> 00:02:08,160 activity and the large scale dynamics of the lithosphere. 31 00:02:09,400 --> 00:02:13,720 Recently, he has concentrated his research on reconstructing 32 00:02:13,720 --> 00:02:17,800 past geographies of the continents, and particularly on 33 00:02:17,800 --> 00:02:22,400 whether or not a supercontinent called Panosha formed towards 34 00:02:22,400 --> 00:02:26,560 the end of the Proterozoic, before Pangaea, but after the 35 00:02:26,560 --> 00:02:28,880 previous supercontinent called Rodinia. 36 00:02:30,120 --> 00:02:33,600 He is Distinguished Professor Emeritus of Geological Sciences 37 00:02:33,600 --> 00:02:37,360 at Ohio University. Damien Nance. 38 00:02:37,520 --> 00:02:40,160 Welcome to Geology Bytes. Thank you, Oliver. 39 00:02:40,160 --> 00:02:41,960 It's a great pleasure to be here. 40 00:02:42,720 --> 00:02:46,560 Could you remind us as to how we reconstruct the past positions 41 00:02:46,560 --> 00:02:49,720 of the continents and thus infer the existence of 42 00:02:49,720 --> 00:02:53,120 supercontinents? Let's start with the most direct 43 00:02:53,120 --> 00:02:55,640 evidence. Well, the two most important 44 00:02:55,640 --> 00:03:00,520 sources of data come from paleomagnetism, which allows us 45 00:03:00,520 --> 00:03:04,280 to reconstruct the former position of continents at a 46 00:03:04,320 --> 00:03:09,400 particular time, at least with respect to latitude, and from 47 00:03:09,400 --> 00:03:13,280 absolute age constraints which allow us to determine when that 48 00:03:13,280 --> 00:03:17,320 time was. So those are the two key areas. 49 00:03:17,640 --> 00:03:20,920 But in reconstructing supercontinents, it's a bit like 50 00:03:20,920 --> 00:03:23,240 putting together a jigsaw puzzle. 51 00:03:23,800 --> 00:03:27,520 The geologic picture, if you like, on each continent, must 52 00:03:27,520 --> 00:03:31,240 also make sense in their reconstructed positions. 53 00:03:31,480 --> 00:03:35,880 There are also several indirect sources of evidence or proxies 54 00:03:35,880 --> 00:03:38,200 for the existence of supercontinents. 55 00:03:39,040 --> 00:03:41,440 Can you describe the most important of those? 56 00:03:41,720 --> 00:03:44,920 Yes, in fact there are a lot, but the two most important are 57 00:03:44,920 --> 00:03:48,280 the most obvious. Since the assembly of a 58 00:03:48,280 --> 00:03:52,640 supercontinent requires continents to collide, then 59 00:03:52,640 --> 00:03:56,920 their assembly should coincide with global orogeny and the 60 00:03:56,920 --> 00:04:01,200 things that go with it, like granitoid magnetism and zircon 61 00:04:01,200 --> 00:04:04,360 age peaks. Conversely, supercontinent 62 00:04:04,360 --> 00:04:09,280 breakup requires continents to rift, and so it should be marked 63 00:04:09,600 --> 00:04:14,160 by the things that indicate rifting, such as mafic dike 64 00:04:14,160 --> 00:04:19,680 swarms and the emplacement of usually hotspot related large 65 00:04:19,680 --> 00:04:22,520 igneous provinces, which are referred to as lips. 66 00:04:23,280 --> 00:04:26,440 But assembly and breakup influence a whole host of 67 00:04:26,440 --> 00:04:29,240 things. They should, for example, have a 68 00:04:29,240 --> 00:04:33,480 profound effect on sea level and climate, and therefore are 69 00:04:33,480 --> 00:04:37,480 likely to influence things like extinctions and major 70 00:04:37,480 --> 00:04:41,640 evolutionary radiations. They also have major impacts on 71 00:04:41,640 --> 00:04:44,840 things like the sedimentary record, the chemistry of 72 00:04:44,840 --> 00:04:49,360 seawater, the composition of the atmosphere, the list goes on and 73 00:04:49,360 --> 00:04:52,600 on. Do all these proxies then 74 00:04:52,760 --> 00:04:56,480 approximately line up and agree as to when a particular 75 00:04:56,480 --> 00:04:58,680 supercontinent formed and then broke up? 76 00:04:59,520 --> 00:05:07,240 They certainly do for Pangaea. They appear to for Panosha and 77 00:05:07,360 --> 00:05:11,280 those that we can use for earlier super continents appear 78 00:05:11,280 --> 00:05:13,880 to coincide. Some we lose. 79 00:05:13,880 --> 00:05:16,360 It's very difficult to judge sea level. 80 00:05:16,360 --> 00:05:20,440 As you get further back in time, things like major evolutionary 81 00:05:20,440 --> 00:05:24,080 radiations and extinctions become very difficult when all 82 00:05:24,080 --> 00:05:26,760 you're dealing with is little tiny creatures. 83 00:05:27,360 --> 00:05:31,960 But yes, they do. In my introduction, I briefly 84 00:05:31,960 --> 00:05:35,680 mentioned that until recently we explained the formation and 85 00:05:35,680 --> 00:05:39,280 breakup of supercontinents in terms of surface processes or 86 00:05:39,280 --> 00:05:42,480 near surface processes. Can you fill out that picture a 87 00:05:42,520 --> 00:05:44,360 bit more? Yes. 88 00:05:44,360 --> 00:05:46,520 Well, you have to remember that when the idea of the 89 00:05:46,840 --> 00:05:50,880 supercontinent cycle was first introduced, we knew precious 90 00:05:50,880 --> 00:05:56,000 little about the mantle. And we were of the view that 91 00:05:56,000 --> 00:05:58,880 supercontinents, as you mentioned, acted like thermal 92 00:05:58,880 --> 00:06:02,920 blankets on the underlying mantle, that this trapped mantle 93 00:06:02,920 --> 00:06:06,880 heat beneath them and that that caused the supercontinent to 94 00:06:06,880 --> 00:06:09,400 thermally uplift and eventually break up. 95 00:06:10,600 --> 00:06:14,560 That process may be part of the story, but I think today we 96 00:06:14,560 --> 00:06:17,040 would say it is only part of the story. 97 00:06:17,760 --> 00:06:21,360 Now, on the other hand, the oceans that opened when a 98 00:06:21,360 --> 00:06:25,640 supercontinent breaks up can be expected to eventually close. 99 00:06:26,320 --> 00:06:29,320 They're going to subduct and eventually close, and hence they 100 00:06:29,320 --> 00:06:31,840 may reunite another supercontinent. 101 00:06:32,480 --> 00:06:37,120 So we were sort of the view that supercontinents sowed the seeds 102 00:06:37,120 --> 00:06:42,040 of their own destruction, if you like, and would eventually break 103 00:06:42,040 --> 00:06:46,600 up. But in doing so, they would 104 00:06:46,600 --> 00:06:50,640 create new oceans that themselves had limited life 105 00:06:50,640 --> 00:06:54,160 expectancies and would ultimately close. 106 00:06:55,000 --> 00:06:59,240 But the cycle was very much viewed as a lithospheric upper 107 00:06:59,240 --> 00:07:02,120 mantle process. Everything we're talking about 108 00:07:02,280 --> 00:07:05,840 is relatively shallow, and we thought of it as a governing 109 00:07:05,840 --> 00:07:10,800 mechanism for plate tectonics. Things like what happened to 110 00:07:10,800 --> 00:07:16,440 subducting slabs after they subducted and what was going on 111 00:07:16,440 --> 00:07:20,200 in the deep mantle were completely unknown quantities at 112 00:07:20,200 --> 00:07:23,560 that time. OK, so we've learned a lot more 113 00:07:23,560 --> 00:07:27,360 about the mantle in the last few decades, and this has indeed 114 00:07:27,360 --> 00:07:29,200 caused us to rethink the picture. 115 00:07:29,960 --> 00:07:32,920 First of all, how did we actually gain this new knowledge 116 00:07:32,920 --> 00:07:35,520 of the Mantle? And then what did it reveal 117 00:07:35,520 --> 00:07:37,920 about the mantle? As you mentioned, the 118 00:07:37,920 --> 00:07:43,760 breakthroughs really came from 2 areas, from seismic tomography 119 00:07:44,320 --> 00:07:48,680 and from computer modelling. For example, seismic tomography, 120 00:07:48,680 --> 00:07:52,800 which is really just a means of X raying the Earth's deep 121 00:07:52,800 --> 00:07:58,160 interior, revealed that subducting oceanic slabs didn't 122 00:07:58,160 --> 00:08:02,280 simply sort of dissipate in the upper mantle as was generally 123 00:08:02,280 --> 00:08:05,760 presumed, but actually sink all the way down to the core mantle 124 00:08:05,760 --> 00:08:09,200 boundary. And the core mantle boundary 125 00:08:09,520 --> 00:08:15,320 seismic tomography revealed the existence of two Thermo chemical 126 00:08:15,320 --> 00:08:20,680 features that were antipodal to each other and which affected 127 00:08:20,680 --> 00:08:25,000 seismic velocities and were known as large low shear 128 00:08:25,000 --> 00:08:30,720 velocity provinces, or LLSVPS. I'll interject here that while 129 00:08:30,720 --> 00:08:34,480 seismic tomography reveals the enormous extent of these large, 130 00:08:34,480 --> 00:08:39,320 low shear velocity provinces, each of which extends several 131 00:08:39,320 --> 00:08:43,200 1000 kilometres across and rises about 1000 kilometres up from 132 00:08:43,200 --> 00:08:47,280 the core mantle boundary, we don't really know why the 133 00:08:47,280 --> 00:08:49,360 seismic shear wave slowed down there. 134 00:08:50,280 --> 00:08:53,680 It could be a higher temperature, a different denser 135 00:08:53,680 --> 00:08:59,240 composition, or some of both. And importantly, one of these 136 00:08:59,240 --> 00:09:05,080 LLSVPS was centred directly under the former position of 137 00:09:05,080 --> 00:09:10,520 Pangaea, So that in itself strongly suggested a link 138 00:09:10,520 --> 00:09:16,040 between supercontinents and the deep mantle, which was something 139 00:09:16,040 --> 00:09:19,760 we certainly never considered. And that connection has in fact 140 00:09:19,760 --> 00:09:24,520 been strengthened with the recognition that some of the 141 00:09:24,520 --> 00:09:30,120 hotspots, most of the hotspots that formed in the wake of 142 00:09:30,280 --> 00:09:35,200 Pangea breakup actually represented the surface 143 00:09:35,200 --> 00:09:40,000 manifestation of mantle plumes that emanated from the edges of 144 00:09:40,000 --> 00:09:44,360 these LLSVPS. Again, another connection 145 00:09:44,360 --> 00:09:49,400 between the Deep Mantle and Pangaea breakup, suggesting a 146 00:09:49,520 --> 00:09:52,200 Deep Mantle involvement, which we hadn't considered. 147 00:09:52,680 --> 00:09:58,400 How, then, do we use this knowledge to look at the 148 00:09:58,400 --> 00:10:02,640 mechanism of supercontinent formation, and particularly of 149 00:10:02,640 --> 00:10:08,080 the supercontinent cycle? It's hugely important because it 150 00:10:08,080 --> 00:10:12,080 demonstrates a link between supercontinence or the 151 00:10:12,080 --> 00:10:17,560 supercontinent cycle and the deep mantle And in doing that it 152 00:10:17,560 --> 00:10:22,120 actually elevates the importance of the cycle significantly from 153 00:10:22,120 --> 00:10:26,560 simply being a near surface phenomena that influences or 154 00:10:26,560 --> 00:10:30,320 controls plate tectonics to being a whole mantle process. 155 00:10:31,160 --> 00:10:36,680 And what we take from this is that the assembly of 156 00:10:36,680 --> 00:10:41,240 supercontinents somehow influences a mantle in a way 157 00:10:41,240 --> 00:10:46,800 that causes a major region of downwelling to become a major 158 00:10:46,800 --> 00:10:51,560 region of upwelling. So that the mantle downwelling, 159 00:10:51,760 --> 00:10:55,760 which we would expect to be associated with the closing 160 00:10:55,760 --> 00:11:01,400 oceans of the assembly of a supercontinent, somehow evolves 161 00:11:01,400 --> 00:11:07,120 into mantle upwelling, which is evident in the rise of these 162 00:11:07,120 --> 00:11:10,320 mantle plumes responsible for the breakup of the 163 00:11:10,320 --> 00:11:16,440 supercontinent. So somehow the cycle has taken 164 00:11:16,440 --> 00:11:20,000 an area of major mantle downwelling and caused it to 165 00:11:20,000 --> 00:11:25,800 evolve into one of upwelling. So the original idea of 166 00:11:26,240 --> 00:11:29,680 supercontinents acting as thermal blankets on the 167 00:11:29,680 --> 00:11:32,760 underlying mantle may still be part of the story, and maybe an 168 00:11:32,760 --> 00:11:37,160 important part of the story. But that would not involve the 169 00:11:37,160 --> 00:11:39,960 deep mantle. So it does not explain this 170 00:11:39,960 --> 00:11:46,480 relationship, and instead it is more likely that this change is 171 00:11:46,480 --> 00:11:53,600 linked to the seducting slabs of the oceans that close to form a 172 00:11:53,600 --> 00:11:57,000 supercontinent. And if you think about it, as 173 00:11:57,000 --> 00:11:59,280 you are bringing a supercontinent together, you've 174 00:11:59,280 --> 00:12:03,880 got seduction in the closing oceans and subducting slabs 175 00:12:04,000 --> 00:12:09,720 sinking down towards the core mantle boundary beneath what 176 00:12:09,720 --> 00:12:13,800 will be the future position of the supercontinent once it 177 00:12:13,800 --> 00:12:16,280 assembles. So that process clearly 178 00:12:16,280 --> 00:12:19,960 contributes to the downwelling beneath the supercontinent. 179 00:12:21,280 --> 00:12:25,320 But once these subducting slabs reach the core mantle boundary, 180 00:12:26,120 --> 00:12:30,280 they somehow managed to cause plumes to rise beneath the 181 00:12:30,280 --> 00:12:33,360 supercontinent that foster its breakup. 182 00:12:34,640 --> 00:12:37,680 Now, exactly how they do this is not yet known. 183 00:12:38,160 --> 00:12:41,200 There are a variety of possibilities, all or none of 184 00:12:41,200 --> 00:12:44,440 which may be correct. They may interact, for example, 185 00:12:44,440 --> 00:12:50,720 with these LLSVPS and existing LLSVP in such a way as to 186 00:12:50,720 --> 00:12:53,920 trigger the rise of mantle plumes at its edges. 187 00:12:55,040 --> 00:12:59,280 Or they may actually form an LLSVP at the core mantle 188 00:12:59,280 --> 00:13:03,840 boundary, and in doing so generate edge effect mantle 189 00:13:03,840 --> 00:13:07,680 plumes. Or they may simply act as 190 00:13:07,680 --> 00:13:12,480 thermal blankets themselves, but this time on the core ponding 191 00:13:12,480 --> 00:13:16,840 heat that way that then rises in the form of mantle plumes. 192 00:13:18,200 --> 00:13:24,320 Now, in addition to all that, once a supercontinent assembles, 193 00:13:24,800 --> 00:13:28,000 all of the seduction that was going on in the closing oceans 194 00:13:28,000 --> 00:13:33,760 beforehand, stops and must move. It has to relocate, and it will 195 00:13:33,760 --> 00:13:37,400 relocate to the edges of the supercontinent, to the ocean 196 00:13:37,400 --> 00:13:42,480 surrounding the supercontinent. And if you imagine that you now 197 00:13:42,480 --> 00:13:46,600 have a supercontinent with a sort of girdle of subduction all 198 00:13:46,600 --> 00:13:49,880 the way around it, or most of the way around it, and a bunch 199 00:13:49,880 --> 00:13:54,680 of new subduction slabs or subducting oceanic slabs now 200 00:13:54,680 --> 00:13:59,760 sinking down towards the core mantle boundary as a curtain 201 00:13:59,760 --> 00:14:04,400 around the Super Canton. And that downward motion alone 202 00:14:04,560 --> 00:14:09,080 might be sufficient to cause a return flow in the form of 203 00:14:09,080 --> 00:14:10,880 upwelling beneath the Super Canton. 204 00:14:11,840 --> 00:14:14,600 So that's another way you might be able to convert this 205 00:14:14,600 --> 00:14:18,720 downwelling to an upwelling. But whatever the path is, the 206 00:14:19,000 --> 00:14:23,360 effect is to cause the mantle downwelling associated with 207 00:14:23,360 --> 00:14:28,800 assembly to evolve into mantle upwelling responsible for break 208 00:14:28,800 --> 00:14:33,040 up. Why should the subduction when 209 00:14:33,040 --> 00:14:36,600 it shuts off underneath the downwelling that has brought the 210 00:14:36,600 --> 00:14:38,960 supercontinent together? What's the mechanism? 211 00:14:38,960 --> 00:14:43,120 Why should that flip into this curtain or girdled around the 212 00:14:43,120 --> 00:14:45,200 supercontinent? Why doesn't the subduction just 213 00:14:45,200 --> 00:14:47,920 shut off and happen in some random place somewhere else on 214 00:14:47,920 --> 00:14:50,480 the planet? This is a tendency of subduction 215 00:14:50,480 --> 00:14:55,920 systems to jump back towards the ocean continent margin. 216 00:14:56,280 --> 00:15:00,440 So there is a likelihood then that and certainly modeling 217 00:15:00,440 --> 00:15:05,680 shows this, that the subduction system would bounce out to the 218 00:15:05,680 --> 00:15:08,720 margins of the supercontinent. This is certainly what happened 219 00:15:08,720 --> 00:15:11,680 with Pangaea. It was almost entirely 220 00:15:11,800 --> 00:15:16,280 surrounded by subduction systems, and some of those still 221 00:15:16,280 --> 00:15:18,680 survive in the modern Pacific Ocean. 222 00:15:19,120 --> 00:15:23,640 So you said that we find that there's one of these large below 223 00:15:23,640 --> 00:15:27,400 shear velocity provinces sitting underneath the former location 224 00:15:27,400 --> 00:15:31,480 of Pangaea. I wasn't clear if that province 225 00:15:31,480 --> 00:15:35,400 was thought to have predated the formation of Pangaea and somehow 226 00:15:35,400 --> 00:15:39,400 rather caused it to form above it or the other way round, or 227 00:15:39,480 --> 00:15:42,200 what exactly is the connection thought to be? 228 00:15:42,920 --> 00:15:47,680 We're not certain about that. What we can see, however, is 229 00:15:47,680 --> 00:15:51,960 that there is an amazing coincidence between the position 230 00:15:51,960 --> 00:15:56,960 of one and the reconstructed position of Pangaea that sits 231 00:15:56,960 --> 00:16:00,680 slap dab underneath it and currently sits under Africa, 232 00:16:00,760 --> 00:16:05,120 which is the one piece of Pangaea that hasn't moved very 233 00:16:05,120 --> 00:16:09,320 much since it broke up. Everything is moved away from 234 00:16:09,320 --> 00:16:13,280 Africa and Africa hasn't done very much and the current LLSVP 235 00:16:13,280 --> 00:16:15,760 in that hemisphere sits right under Africa. 236 00:16:15,760 --> 00:16:19,480 So it's a kind of correlation that cries out for explanation, 237 00:16:19,480 --> 00:16:22,440 and we don't really have it yet. Exactly so. 238 00:16:22,440 --> 00:16:26,400 And when you couple that with the fact that all of these 239 00:16:26,400 --> 00:16:31,680 hotspots that are clearly linked to Pangaea breakup coincide with 240 00:16:31,680 --> 00:16:36,840 the edges of this thing, that likewise begs an explanation. 241 00:16:37,280 --> 00:16:40,440 We don't have one yet. We have ideas, but we certainly 242 00:16:40,440 --> 00:16:43,320 don't know yet. And I think that is probably 243 00:16:43,320 --> 00:16:47,720 where the future of this whole story will take us into trying 244 00:16:47,720 --> 00:16:52,040 to resolve those kind of issues in today's world rather than 245 00:16:52,040 --> 00:16:54,560 trying to do it in the distant past. 246 00:16:55,160 --> 00:16:58,440 What is the role of computer modelling being in helping us 247 00:16:58,440 --> 00:17:02,400 understand what's going on here? Computer modelling confirms the 248 00:17:02,400 --> 00:17:06,920 idea that supercontinents form over areas of mantle dam 249 00:17:06,920 --> 00:17:11,520 welding, but that they preferentially do so in what is 250 00:17:11,520 --> 00:17:14,400 known as a degree. One structure that is a 251 00:17:14,400 --> 00:17:19,440 supercontinent will form over an area of mantle dam welling that 252 00:17:19,560 --> 00:17:22,800 is antipodal to an area of upwelling. 253 00:17:23,119 --> 00:17:26,960 So you have a mantle with a relatively simple overall 254 00:17:26,960 --> 00:17:31,760 convection system, a single area of dam welling, and an antipodal 255 00:17:31,760 --> 00:17:37,640 area of upwelling. But as it sits there, the effect 256 00:17:37,640 --> 00:17:42,760 of the amalgamation is to start generating these plumes at the 257 00:17:42,760 --> 00:17:46,760 core mantle boundary underneath it, which of course rise and 258 00:17:46,760 --> 00:17:50,160 foster its break up. But in doing so they change the 259 00:17:50,160 --> 00:17:54,680 overall structure of the mantle from a degree one structure to 260 00:17:54,680 --> 00:17:59,640 what is known as a degree two. That is a mantle with antipodal 261 00:17:59,640 --> 00:18:04,480 areas of upwelling, because the original area of upwelling is 262 00:18:04,480 --> 00:18:07,920 still there and you've now created another one underneath 263 00:18:07,920 --> 00:18:10,200 the supercartinent, antipodal to it. 264 00:18:11,280 --> 00:18:15,760 And between these, what you see is a girdle of downwelling, 265 00:18:16,360 --> 00:18:19,880 which is where subduction is going to be focused, and that 266 00:18:19,880 --> 00:18:21,560 forms a girdle all the way around. 267 00:18:21,560 --> 00:18:24,360 It's referred to as the subduction girdle. 268 00:18:25,040 --> 00:18:29,920 And interestingly, that then bears on how supercontinents 269 00:18:29,920 --> 00:18:33,080 might assemble after they break up. 270 00:18:34,200 --> 00:18:38,200 Because in a world like that, once your supercontinent breaks 271 00:18:38,200 --> 00:18:43,760 up, the dispersing continents are going to move towards an 272 00:18:43,760 --> 00:18:47,440 area of mantle damwelling. And that area of mantle 273 00:18:47,440 --> 00:18:52,880 damwelling is now this girdle. And so the dispersing continents 274 00:18:53,080 --> 00:18:57,640 might be expected to collect over this girdle of mantle 275 00:18:57,640 --> 00:19:03,240 damwelling. And what they do then depends on 276 00:19:03,240 --> 00:19:06,240 what that girdle does in terms of movement. 277 00:19:07,360 --> 00:19:13,800 If it moves away from the former supercontinent by a process of 278 00:19:13,800 --> 00:19:18,200 rollback, for example, like we see in the Western Pacific, it 279 00:19:18,200 --> 00:19:24,240 may continue to move until it reaches the other pole antipodal 280 00:19:24,240 --> 00:19:27,920 to the former supercontinent. In which case the continents are 281 00:19:27,920 --> 00:19:33,240 going to coalesce there and you will create a new supercontinent 282 00:19:33,560 --> 00:19:38,680 antipodal to the former one. And in doing so you will have 283 00:19:38,680 --> 00:19:43,640 closed the ocean surrounding the supercontinent. 284 00:19:44,800 --> 00:19:49,680 So in the modern world, if we think about the breakup Pangaea, 285 00:19:49,960 --> 00:19:55,560 the effect of that would be to close the Pacific and reunite 286 00:19:55,560 --> 00:19:57,720 the continents as a result of doing that. 287 00:19:58,040 --> 00:20:02,720 And that's referred to as extroversion closure of the 288 00:20:02,760 --> 00:20:05,320 ocean surrounding the supercontinent. 289 00:20:06,640 --> 00:20:09,440 Conversely, it may collapse, this girdle may collapse on 290 00:20:09,440 --> 00:20:14,000 itself, and the continents will then come back to their former 291 00:20:14,000 --> 00:20:19,200 positions, more or less, and you will recreate a supercontinent 292 00:20:20,040 --> 00:20:23,240 in much the same position as the one you've just broken up. 293 00:20:24,160 --> 00:20:26,360 That process known as introversion. 294 00:20:26,720 --> 00:20:31,200 And in today's world, that would be like saying the next 295 00:20:31,200 --> 00:20:34,680 supercontinent forms as a result of the closure of the Atlantic 296 00:20:34,680 --> 00:20:39,640 Ocean, which would reunite the continents to form a 297 00:20:39,640 --> 00:20:43,320 supercontinent in much the same position that Pangaea was in. 298 00:20:44,800 --> 00:20:50,360 Now, 1/3 of possibility is a girdle doesn't move, and that 299 00:20:50,360 --> 00:20:54,640 the supercontinent effectively assembles on the girdle 300 00:20:54,640 --> 00:20:57,920 somewhere. And if that's the case, then the 301 00:20:57,920 --> 00:21:04,480 new supercontinent forms sort of about 90° to the former one, and 302 00:21:04,480 --> 00:21:07,800 that's referred to as author version. 303 00:21:08,920 --> 00:21:12,760 And in today's world, that would be akin to forming a 304 00:21:12,760 --> 00:21:17,880 supercontinent by closing up the Arctic Ocean and closing up the 305 00:21:17,880 --> 00:21:21,240 Caribbean, for example, and bringing the continents together 306 00:21:21,240 --> 00:21:25,360 that way. So those are the three methods 307 00:21:25,360 --> 00:21:29,120 of reuniting A supercontinent that are often banded around at 308 00:21:29,120 --> 00:21:31,600 the moment. Do we know enough about the 309 00:21:31,600 --> 00:21:34,560 actual positions of former supercontinents? 310 00:21:35,000 --> 00:21:39,120 Going back, maybe Pangaea, possibly Panosha, Radinia, maybe 311 00:21:39,280 --> 00:21:42,720 Canola? And before that to know if they 312 00:21:42,720 --> 00:21:47,880 did indeed reassemble in the same place, Antipodele, or at 313 00:21:47,880 --> 00:21:49,600 90°? In other words, which of these 314 00:21:49,600 --> 00:21:52,080 three theories is borne out by the data? 315 00:21:52,520 --> 00:21:56,240 There are certainly folks out there who have argued, yes, that 316 00:21:56,520 --> 00:22:00,400 the relationship has been one of 90° and that we see author 317 00:22:00,400 --> 00:22:05,240 version in the past. When we first advocated the 318 00:22:05,240 --> 00:22:09,800 cycle, the one we thought most likely was introversion. 319 00:22:10,440 --> 00:22:13,840 And the reason was not because we had any great idea of how 320 00:22:13,880 --> 00:22:19,280 these things happened, but we simply looked at Pangaea and 321 00:22:19,280 --> 00:22:24,400 Pangaea formed by the closure of oceans like Iapetus, the 322 00:22:24,400 --> 00:22:30,440 Tonquist, the Raik, et cetera. And these were all oceans that 323 00:22:30,440 --> 00:22:34,880 formed when what we thought was the previous supercontinent, 324 00:22:34,880 --> 00:22:41,200 Panosha, broke up. And so from that perspective, it 325 00:22:41,200 --> 00:22:48,000 looked as if Pangaea had formed by closing oceans produced by 326 00:22:48,000 --> 00:22:50,560 the breakup of the previous supercontinent. 327 00:22:51,160 --> 00:22:55,440 And that's introversion. But in truth, I don't think we 328 00:22:55,440 --> 00:22:59,960 can be specific about it. The relative positions of 329 00:22:59,960 --> 00:23:03,480 successive supercontinents seems to be quite a important 330 00:23:03,800 --> 00:23:08,000 observation to resolve these large scale mantle processes 331 00:23:08,000 --> 00:23:10,760 that we're talking about here, and I'm wondering whether either 332 00:23:11,200 --> 00:23:17,240 the global distribution of these relict or slab graveyards might 333 00:23:17,240 --> 00:23:20,320 give us some indication, or whether indeed it's the computer 334 00:23:20,320 --> 00:23:22,360 modeling that can come to the rescue here. 335 00:23:22,800 --> 00:23:25,160 I think it's more likely to be the computer modelling. 336 00:23:25,400 --> 00:23:27,840 You could argue, for example, that the reason we have 337 00:23:27,840 --> 00:23:32,680 antipodal slab graveyards or large low velocity provinces is 338 00:23:32,680 --> 00:23:37,800 because one formed under Pangaea and the other one formed under 339 00:23:37,800 --> 00:23:40,720 the previous supercontinent. It hasn't dissipated yet. 340 00:23:41,040 --> 00:23:43,000 And if that was the case, of course we're dealing with 341 00:23:43,000 --> 00:23:46,000 extroversion because they're on opposite poles. 342 00:23:46,640 --> 00:23:49,400 And our problem with establishing the precise 343 00:23:49,400 --> 00:23:55,120 existence of supercontinents is we don't have much longitudinal 344 00:23:55,120 --> 00:23:58,560 constraints. Paleomagnetism will get us 345 00:23:58,760 --> 00:24:03,520 latitudinal constraints, but it's often rather difficult to 346 00:24:03,520 --> 00:24:06,640 know whether we're on this side of the world or that side of the 347 00:24:06,640 --> 00:24:10,200 world. So exactly, positioning 1 348 00:24:10,200 --> 00:24:14,120 supercontinent with respect to another is not an easy process. 349 00:24:14,280 --> 00:24:18,440 And on top of that, we have this issue of true polar wonder, 350 00:24:19,120 --> 00:24:22,920 which may very well impact all supercontinents. 351 00:24:23,480 --> 00:24:28,960 And that is a feature of physics that requires the mass imbalance 352 00:24:29,560 --> 00:24:31,880 that is produced when you put all the continents together in 353 00:24:31,880 --> 00:24:37,440 one place to cause the Earth to rotate in such a fashion as to 354 00:24:37,440 --> 00:24:40,600 bring that mass imbalance to the equator. 355 00:24:41,440 --> 00:24:45,680 It's a whole Earth rotation of the mantle and the lithosphere. 356 00:24:46,160 --> 00:24:50,080 So to detach somewhere at the core mantle boundary, which 357 00:24:50,080 --> 00:24:55,280 brings the entire mass of a super continent, say one that 358 00:24:55,520 --> 00:24:58,760 assembled at the pole. This process of true polar 359 00:24:58,760 --> 00:25:03,880 wonder would slowly but surely bring that down to the equator, 360 00:25:04,240 --> 00:25:06,400 and in fact I think brings it down quite fast. 361 00:25:06,400 --> 00:25:08,800 I believe the rates of true polar wonder are quite a bit 362 00:25:08,800 --> 00:25:11,760 faster than the rates of plate motion, for example. 363 00:25:12,120 --> 00:25:15,080 So that means that all super continents, given enough time, 364 00:25:15,080 --> 00:25:18,240 will end up on the equator, regardless of where they formed. 365 00:25:18,960 --> 00:25:23,440 There is a lot of debate as to whether Panosia existed during a 366 00:25:23,440 --> 00:25:28,040 period between about 630 million years ago and 530 million years 367 00:25:28,040 --> 00:25:33,880 ago, pretty much midway between Rodinia and Pangaea, and you're 368 00:25:33,880 --> 00:25:37,600 an advocate of its existence. What brought you to this 369 00:25:37,600 --> 00:25:39,840 conclusion? Well, I would certainly like to 370 00:25:39,880 --> 00:25:42,200 think Panosia existed, but I may be wrong. 371 00:25:42,880 --> 00:25:49,000 And interestingly, whether or not it did, Penosha is central 372 00:25:49,280 --> 00:25:52,120 to some of the issues we've been talking about. 373 00:25:53,080 --> 00:25:58,160 It's also a bit ironic because poor old Penosha was the first 374 00:25:58,640 --> 00:26:04,600 pre Pangaean supercontinent to be proposed, and when we were 375 00:26:04,600 --> 00:26:08,720 putting the case of the supercontinent cycle together, 376 00:26:09,200 --> 00:26:13,360 it seemed to us the one for which the case was strongest. 377 00:26:13,840 --> 00:26:19,960 But the problem is that are two key tools for establishing 378 00:26:19,960 --> 00:26:23,960 supercontinents, namely paleomagnetism and absolute age 379 00:26:23,960 --> 00:26:27,880 dating, have failed to establish its existence. 380 00:26:29,000 --> 00:26:32,880 Paleomagnetic data for the IDIAC run, that is, the period during 381 00:26:32,880 --> 00:26:37,640 which this supercontinent would have existed if it did, is very 382 00:26:37,640 --> 00:26:40,280 inconclusive. The data are notoriously 383 00:26:40,280 --> 00:26:44,200 dispersed, and that may well mean that the Earth's magnetic 384 00:26:44,200 --> 00:26:47,040 field was misbehaving at that time. 385 00:26:47,400 --> 00:26:51,640 There may have been true polar wander going on, or it may not 386 00:26:51,640 --> 00:26:55,600 have been a dipole, but something odd was going on with 387 00:26:55,600 --> 00:26:59,120 the magnetic field, which means it's difficult to use 388 00:26:59,120 --> 00:27:03,320 paleomagnetic data to say yay or nay. 389 00:27:03,320 --> 00:27:07,640 If we look at the age dating, the problem there is age dating 390 00:27:07,640 --> 00:27:11,520 suggests that the supercontinent was breaking up before it ever 391 00:27:11,520 --> 00:27:14,880 got together. And if that's the case, then it 392 00:27:14,880 --> 00:27:18,760 never really existed. And even if it did, I think even 393 00:27:18,760 --> 00:27:22,480 I would concede that if it did exist, it was a very short lived 394 00:27:22,640 --> 00:27:25,600 supercontent. But why did we think it was a 395 00:27:25,600 --> 00:27:28,360 supercontent? The reason is because the proxy 396 00:27:28,360 --> 00:27:32,440 signals are unmistakable and these include all the ones that 397 00:27:32,440 --> 00:27:34,960 you'd expect. Things like global hierogeny, 398 00:27:35,280 --> 00:27:39,320 zircon age peak, evidence of widespread rifting and mayfic 399 00:27:39,320 --> 00:27:43,600 dyke, swarms and lips and changes in sea level and climate 400 00:27:43,600 --> 00:27:47,600 and extinctions and radiations, and just a host of geochemical 401 00:27:47,600 --> 00:27:51,160 and isotopic tracers, they're all there. 402 00:27:51,560 --> 00:27:55,520 The problem is, while they are collectively compelling, they 403 00:27:55,520 --> 00:28:01,160 are individually contestable and therefore they cannot be 404 00:28:01,160 --> 00:28:05,240 considered definitive. For example, you could argue 405 00:28:05,240 --> 00:28:09,720 that the orogenic activity and zircon age peak, which are very 406 00:28:09,720 --> 00:28:15,600 real, record the assembly of Gondwana, not Pernosa, and 407 00:28:15,600 --> 00:28:19,000 conversely the evidence of rifting and lips, et cetera. 408 00:28:19,560 --> 00:28:26,200 This was just a continuation of the break up of Rodinia and not 409 00:28:26,200 --> 00:28:28,480 the break up of a subsequent supercon. 410 00:28:29,560 --> 00:28:34,000 So there is an argument against each of them that can be made so 411 00:28:34,000 --> 00:28:39,200 that they're contestable. So although the proxy evidence, 412 00:28:39,200 --> 00:28:43,280 which was the only evidence we had at the time, was strong, it 413 00:28:43,280 --> 00:28:48,320 is not definitive. But interestingly, even if 414 00:28:48,320 --> 00:28:55,840 Penosha did not exist, what this proxy evidence suggests is there 415 00:28:55,840 --> 00:29:01,200 is actually a far more important question going on here, and that 416 00:29:01,200 --> 00:29:07,880 is, is a full blown supercontinent needed to make 417 00:29:07,880 --> 00:29:14,480 the supercontinent cycle work? Or might a close gathering of 418 00:29:14,480 --> 00:29:19,560 continents be sufficient to change mantle circulation in the 419 00:29:19,560 --> 00:29:25,360 way we've been talking about and cause the converging continents 420 00:29:25,480 --> 00:29:29,600 to reverse their convergence? So the key here is to convert 421 00:29:29,600 --> 00:29:32,800 that downwelling that's underneath where the newly 422 00:29:32,800 --> 00:29:37,560 forming supercontinent is forming into an upwelling. 423 00:29:37,560 --> 00:29:40,800 And that's why you're really saying this is perhaps more of a 424 00:29:41,160 --> 00:29:46,480 deep mantle cycle rather than a supercontinental, which implies 425 00:29:46,480 --> 00:29:48,840 a surface cycle. And the question then is, I 426 00:29:48,840 --> 00:29:52,080 suppose, what does it take to get that reversal? 427 00:29:52,080 --> 00:29:56,480 Do you actually have to close them up completely and produce 428 00:29:56,480 --> 00:30:01,040 that thermal blanket with no cracks or gaps in it, or is just 429 00:30:01,040 --> 00:30:03,920 proximity good enough? Exactly so. 430 00:30:04,160 --> 00:30:07,880 And if you don't need a supercontinent, then it isn't a 431 00:30:07,880 --> 00:30:12,080 supercontinent cycle. But although it involves the 432 00:30:12,080 --> 00:30:16,000 deep mantle as you just mentioned, it is not a mantle 433 00:30:16,000 --> 00:30:21,240 cycle either because the supercontinent or this near 434 00:30:21,240 --> 00:30:24,600 supercontinent obviously plays a vital role. 435 00:30:25,320 --> 00:30:30,280 So it is a mantle lithosphere cycle, but I think we're going 436 00:30:30,280 --> 00:30:33,880 to have to come up with a sexier title than that if we want it to 437 00:30:33,880 --> 00:30:35,960 catch on. Why are we so concerned as to 438 00:30:35,960 --> 00:30:39,400 whether Panosha actually happened or not? 439 00:30:39,680 --> 00:30:43,200 The existence of Panosha is critical for a number of 440 00:30:43,200 --> 00:30:45,280 reasons. If we don't actually need a 441 00:30:45,280 --> 00:30:49,800 supercontinent, and Panosha is an example of, well we didn't 442 00:30:49,800 --> 00:30:54,440 have one and yet we change mantle circulation, then the 443 00:30:54,440 --> 00:30:58,200 question is, are there other times when this has happened? 444 00:30:58,920 --> 00:31:02,400 Do we have other proximate supercontinents that have 445 00:31:02,400 --> 00:31:06,000 affected the cycle in the past that we haven't even thought 446 00:31:06,000 --> 00:31:08,400 about? There are other sort of key 447 00:31:08,640 --> 00:31:11,040 issues. The existence or non existence 448 00:31:11,040 --> 00:31:16,120 of Panosha obviously affects the interval between supercontinents 449 00:31:16,640 --> 00:31:22,320 and therefore it influences whether or not the cycle has 450 00:31:22,320 --> 00:31:25,680 experienced secular change through time. 451 00:31:26,680 --> 00:31:31,200 Without Panosha, the interval between supercontinents has 452 00:31:31,200 --> 00:31:35,760 remained pretty much constant since the end of the Archaean at 453 00:31:36,080 --> 00:31:41,840 around 606, fifty million years or so, suggesting A broadly 454 00:31:41,840 --> 00:31:46,800 steady state cycle. If we put Panosha in the picture 455 00:31:46,800 --> 00:31:50,880 and say it was a supercontinent, then the interval looks to be 456 00:31:50,960 --> 00:31:54,880 shortening towards perhaps 300 million years or so. 457 00:31:55,520 --> 00:31:58,120 And this would imply that the cycle is accelerating. 458 00:31:58,960 --> 00:32:02,080 And if that's the case, it's telling us something very 459 00:32:02,080 --> 00:32:04,960 important about mantle evolution. 460 00:32:05,520 --> 00:32:09,520 And the other thing that it influences is a question that 461 00:32:09,640 --> 00:32:12,920 geologists are tantalised by. And that is OK, we've got a 462 00:32:12,920 --> 00:32:16,360 cycle, when's the next one? So when does the next 463 00:32:16,360 --> 00:32:19,720 supercontinent assemble? Well that depends very much on 464 00:32:19,720 --> 00:32:25,800 the interval between them and without Kenosha we can expect 465 00:32:25,800 --> 00:32:30,160 another supercontinent in 300 million years or so, but with 466 00:32:30,160 --> 00:32:35,800 Kenosha it might form in as little as 100 million years and 467 00:32:35,800 --> 00:32:39,840 so a significant difference. So there are lots of quite 468 00:32:39,840 --> 00:32:45,760 important questions hinging on the existence or non existence 469 00:32:45,760 --> 00:32:52,720 of Kenosha, and I think perhaps more important than did it 470 00:32:52,720 --> 00:32:55,240 actually all come together to form a supercontinent? 471 00:32:55,560 --> 00:32:59,480 Is the answer to the question did Panosha, whether it formed 472 00:32:59,480 --> 00:33:04,920 or not, impact mantle circulation in such a way as to 473 00:33:04,920 --> 00:33:09,160 turn an area of downwelling into upwelling. 474 00:33:09,560 --> 00:33:13,520 If it did, then it is a supercontinent as far as a 475 00:33:13,520 --> 00:33:17,200 supercontinent cycle is concerned, regardless of whether 476 00:33:17,200 --> 00:33:20,080 or not it is a supercontinent by definition. 477 00:33:21,000 --> 00:33:26,360 I'm struck that these ideas are based on rather small number 478 00:33:26,360 --> 00:33:30,240 statistics and some of the sources of evidence we discussed 479 00:33:30,240 --> 00:33:33,320 have significant uncertainties associated with them. 480 00:33:33,920 --> 00:33:38,560 I'm wondering what observations or computer modelling or other 481 00:33:38,560 --> 00:33:43,480 line of research would you most like to see to help us get more 482 00:33:43,480 --> 00:33:46,400 confidence that we know what the long term dynamics of the 483 00:33:46,400 --> 00:33:48,720 continents are and what drives them. 484 00:33:49,920 --> 00:33:52,000 Well, you're absolutely right, I'm afraid. 485 00:33:52,000 --> 00:33:57,160 The topic is plagued by the fallacy of small numbers, but we 486 00:33:57,160 --> 00:33:58,600 have to work with what we've got. 487 00:33:59,160 --> 00:34:02,720 And as you mentioned, it's also plagued by uncertainties 488 00:34:02,760 --> 00:34:06,000 associated with the proxy record which bedeviled its 489 00:34:06,000 --> 00:34:08,639 interpretation. So there are a lot of issues 490 00:34:08,639 --> 00:34:13,000 here, but as to where we go from here, as I mentioned before, I 491 00:34:13,000 --> 00:34:19,040 think the answer lies not in scratching our heads over deep 492 00:34:19,040 --> 00:34:24,120 time, but it lies in the modern world and the improvements of 493 00:34:24,120 --> 00:34:27,679 the techniques we already have for studying the thermal 494 00:34:27,679 --> 00:34:31,440 structure of the mantle and modelling its evolution through 495 00:34:31,440 --> 00:34:34,920 time. And I'm fairly confident that 496 00:34:35,120 --> 00:34:39,080 within the next five years, certainly within the next 497 00:34:39,480 --> 00:34:44,719 decade, the increased resolution in seismic tomography and 498 00:34:44,719 --> 00:34:50,480 increased sophistication in modelling will allow us to 499 00:34:50,480 --> 00:34:54,239 understand the relationship between things like plate 500 00:34:54,239 --> 00:34:59,000 tectonics, mantle circulation, subducting slabs, mantle plumes. 501 00:34:59,640 --> 00:35:03,880 This will finally come together in a comprehensive picture. 502 00:35:04,840 --> 00:35:09,880 And once we've got that, it will provide us with a powerful new 503 00:35:10,080 --> 00:35:14,360 insight for interpreting the dynamics of the Earth in deep 504 00:35:14,360 --> 00:35:17,000 time. So once we've sorted out the 505 00:35:17,000 --> 00:35:20,080 present, then we can go back and have a look at all this 506 00:35:20,320 --> 00:35:23,840 information and see if we can't make much better sense of it. 507 00:35:24,720 --> 00:35:26,880 Damien Nance, thank you very much. 508 00:35:27,440 --> 00:35:29,520 Well, thank you, Oliver. It's been a real pleasure. 509 00:35:31,120 --> 00:35:33,800 To see pictures and illustrations that support this 510 00:35:33,800 --> 00:35:39,400 podcast, go to geologybytes.com, where you'll also find 511 00:35:39,400 --> 00:35:43,720 transcripts and a subject matter index of all the episodes there. 512 00:35:43,720 --> 00:35:47,320 You can also give me feedback, which I welcome, as well as sign 513 00:35:47,320 --> 00:35:49,720 up to get my emails about new episodes.