1 00:00:03,840 --> 00:00:06,840 This is Geology Bites with Oliver Strimpel. 2 00:00:08,800 --> 00:00:12,120 Since almost all of our modern understanding of the Earth has 3 00:00:12,120 --> 00:00:15,520 something to do with the theory of plate tectonics, it is 4 00:00:15,520 --> 00:00:18,560 natural to ask when plate tectonics started. 5 00:00:19,600 --> 00:00:22,840 This is a very challenging question, since so few rocks 6 00:00:22,840 --> 00:00:26,640 from the early Earth survived to the present day, and many of 7 00:00:26,640 --> 00:00:29,800 those that do have been heavily altered by intervening 8 00:00:29,800 --> 00:00:32,000 metamorphic or hydrothermal events. 9 00:00:33,400 --> 00:00:36,840 I discussed this topic in an earlier podcast episode with 10 00:00:36,840 --> 00:00:41,400 Peter Kaywood. One key manifestation of plate 11 00:00:41,400 --> 00:00:45,120 tectonics is the relative motion of tectonic plates. 12 00:00:46,200 --> 00:00:49,600 My guest today recently succeeded in detecting such 13 00:00:49,600 --> 00:00:54,200 motion occurring fully a billion years earlier than any previous 14 00:00:54,200 --> 00:00:59,040 such detection. Alec Brenner studies Archaean 15 00:00:59,040 --> 00:01:02,480 rocks using paleomagnetic and petrographic methods. 16 00:01:03,200 --> 00:01:06,360 He is a post doctoral associate at Yale University. 17 00:01:07,400 --> 00:01:09,760 Alec Brenner, welcome to geology bouts. 18 00:01:10,040 --> 00:01:14,080 Thanks for having me, Oliver. Let's start with your results. 19 00:01:14,800 --> 00:01:18,880 Exactly what motions did you measure and when did they occur? 20 00:01:19,200 --> 00:01:22,520 We looked at a piece of Earth's crust that's been around for 21 00:01:22,520 --> 00:01:25,640 over 3 1/2 billion years. Ancient pieces of crust like 22 00:01:25,640 --> 00:01:29,080 that are called cratons, and we specifically were looking at the 23 00:01:29,080 --> 00:01:32,640 Pilbara Craton. It's part of WA today, and by 24 00:01:32,640 --> 00:01:36,320 making magnetic measurements of some of its rocks that are 3 1/2 25 00:01:36,320 --> 00:01:39,480 billion years old, we measured the latitude where those rocks 26 00:01:39,480 --> 00:01:42,760 formed over time. And those measurements showed us 27 00:01:42,760 --> 00:01:46,640 that at the time, about 3.48 billion years ago, the Pilbara 28 00:01:46,640 --> 00:01:51,360 was at about 53° latitude, give or take somewhat. 29 00:01:51,400 --> 00:01:54,520 That's roughly the same place as Berlin, Germany is today. 30 00:01:54,600 --> 00:01:58,600 But when we then did the same measurements on rocks that were 31 00:01:58,600 --> 00:02:02,040 just a few million years younger than those, those slightly 32 00:02:02,040 --> 00:02:04,520 younger rocks showed that the Pilbara was at quite a bit 33 00:02:04,520 --> 00:02:07,640 higher of a latitude, about 77°, give or take. 34 00:02:07,640 --> 00:02:09,800 That's about where Svalbard is today. 35 00:02:10,199 --> 00:02:13,000 So a pretty big shift in just a few million years. 36 00:02:13,520 --> 00:02:17,640 And that means that the Pilbara had moved rapidly from 1 37 00:02:17,640 --> 00:02:19,760 latitude to another during that time, right? 38 00:02:19,800 --> 00:02:23,240 And the rate turned out to be about several 10s of centimeters 39 00:02:23,240 --> 00:02:27,360 a year, in line with the very fastest of modern plate motions. 40 00:02:27,880 --> 00:02:31,240 And what's especially important here is that our colleagues 41 00:02:31,520 --> 00:02:35,800 previously have measured rocks of the same ages in another 42 00:02:35,800 --> 00:02:38,440 ancient piece of Earth's crust that's called the Koppal craton 43 00:02:38,440 --> 00:02:41,880 in South Africa. And those measurements of those 44 00:02:41,880 --> 00:02:44,920 South African rocks showed that that craton, the Koppal, was at 45 00:02:44,920 --> 00:02:48,880 the equator at 3 1/2 billion years ago and was not moving at 46 00:02:48,880 --> 00:02:51,480 the same time. It that's a big contrast to the 47 00:02:51,480 --> 00:02:55,920 Pilbara part of Australia that was at higher latitudes and 48 00:02:55,920 --> 00:02:58,760 moving around. And that means that we've found 49 00:02:58,920 --> 00:03:03,040 two pieces of crust that were moving relative to each other 50 00:03:03,360 --> 00:03:07,880 3.48 billion years ago. And fundamentally speaking, that 51 00:03:07,880 --> 00:03:10,880 requires that there's a plate boundary in between those two 52 00:03:10,880 --> 00:03:13,560 pieces of crust. That's differential motion, and 53 00:03:13,560 --> 00:03:16,240 that's a hallmark of plate motions. 54 00:03:16,560 --> 00:03:21,320 And you know that that was actual translation motion, 55 00:03:21,320 --> 00:03:24,520 rather than just one giant rotation of a single plate. 56 00:03:25,040 --> 00:03:28,480 It's a little more complex than that in the sense that all plate 57 00:03:28,480 --> 00:03:32,200 motions are rotations, but yes, the fact that we see one piece 58 00:03:32,200 --> 00:03:36,120 of crust moving in the other isn't allows us to constrain 59 00:03:36,120 --> 00:03:40,960 that they aren't part of one global single plate, one global 60 00:03:40,960 --> 00:03:44,560 shell that doesn't have plate boundaries in it, as has been 61 00:03:44,560 --> 00:03:47,080 proposed pretty often for this time in Earth history. 62 00:03:47,760 --> 00:03:51,800 You said that the one plate that you actually worked on the Ilbro 63 00:03:51,800 --> 00:03:55,440 was moving fast, but the other plate was not really moving at 64 00:03:55,440 --> 00:03:58,440 all. Is that actually something 65 00:03:58,440 --> 00:04:01,080 different from what we see today, in the sense that we 66 00:04:01,080 --> 00:04:03,760 wouldn't really find any plates that are static over the time 67 00:04:03,760 --> 00:04:07,080 scale you described? It's quite similar today in that 68 00:04:07,080 --> 00:04:07,960 respect. Actually. 69 00:04:07,960 --> 00:04:11,080 There are plates that you can find that aren't really moving 70 00:04:11,080 --> 00:04:13,920 all that quickly. A good example is South America. 71 00:04:14,280 --> 00:04:18,519 It is moving in longitude somewhat, but paleomagnetically 72 00:04:18,760 --> 00:04:22,320 using the technique that I used to study this problem, you can't 73 00:04:22,320 --> 00:04:24,880 really see that it's moving because it's not moving in 74 00:04:24,880 --> 00:04:29,920 latitude or it's, it's also not rotating, it's just moving in 75 00:04:29,920 --> 00:04:33,200 longitude, if that makes sense. So that motion is invisible to 76 00:04:33,200 --> 00:04:35,280 us. And there are also a few other 77 00:04:35,280 --> 00:04:38,560 places on Earth where you can see no motion occurring. 78 00:04:39,200 --> 00:04:42,000 But yes, that's roughly in line with what you might expect. 79 00:04:42,600 --> 00:04:47,600 So as you said, you use paleomagnetic methods to measure 80 00:04:47,600 --> 00:04:52,000 this motion and paleomagnetism has actually come up in previous 81 00:04:52,000 --> 00:04:55,800 episodes of Geology Bites, in particular the one on super 82 00:04:55,800 --> 00:05:00,240 continents with David Evans. Can you refresh us on how 83 00:05:00,240 --> 00:05:04,360 paleomagnetic methods enable us to reconstruct past plate 84 00:05:04,360 --> 00:05:05,840 motions? Yeah, sure. 85 00:05:06,200 --> 00:05:10,160 Earth's got a liquid iron, nickel, metal outer core, and 86 00:05:10,160 --> 00:05:13,240 that's electrically conductive and also convecting. 87 00:05:13,360 --> 00:05:17,800 And so it generates a strong magnetic field that surrounds 88 00:05:17,800 --> 00:05:19,840 Earth. And it's in the shape of a 89 00:05:19,840 --> 00:05:21,880 dipole, right? The shape of a bar magnet with a 90 00:05:21,880 --> 00:05:26,880 North Pole and a South Pole. And what's handy is that most 91 00:05:26,880 --> 00:05:30,880 rocks forming on Earth's surface contain tiny grains of these 92 00:05:30,880 --> 00:05:34,640 magnetic minerals like magnetite and hematite and puritite. 93 00:05:35,160 --> 00:05:39,800 And those grains can act like microscopic compasses that when 94 00:05:39,800 --> 00:05:42,880 they form, they can freeze the magnetic field where they form 95 00:05:42,960 --> 00:05:46,640 in place and record it therefore for us to measure later. 96 00:05:46,640 --> 00:05:50,920 And so, much like you can use a compass to navigate your way 97 00:05:50,920 --> 00:05:54,720 around the world today, we can measure the directions of these 98 00:05:54,720 --> 00:05:58,280 magnetic signals preserved by tiny magnetic mineral grains and 99 00:05:58,280 --> 00:06:01,760 rocks to piece together how those rocks and the pieces of 100 00:06:01,760 --> 00:06:05,440 crust they sit on have shifted over Earth's surface over time, 101 00:06:05,960 --> 00:06:08,680 piercing together things like the supercontinent cycle and the 102 00:06:08,680 --> 00:06:12,840 formation and closure of oceans. And for this study that we're 103 00:06:12,840 --> 00:06:16,720 doing, we don't have very much context beyond just single 104 00:06:16,720 --> 00:06:19,560 points in time for individual plates. 105 00:06:19,880 --> 00:06:22,880 But what's important is that we can measure the latitude at 106 00:06:22,880 --> 00:06:26,640 which those rocks formed. And that, it turns out, is very 107 00:06:26,640 --> 00:06:30,520 straightforward to calculate from how steeply up or down the 108 00:06:30,520 --> 00:06:33,720 magnetic field is pointing on Earth's surface. 109 00:06:33,720 --> 00:06:37,480 If you're at the equator, the magnetic field is horizontal to 110 00:06:37,480 --> 00:06:38,800 the Earth's surface, pointing north. 111 00:06:39,240 --> 00:06:42,200 If you're at the North Pole, the magnetic field points straight 112 00:06:42,200 --> 00:06:44,240 down into the Earth, and if you're at the South Pole, it 113 00:06:44,280 --> 00:06:45,800 points straight up out of the Earth. 114 00:06:46,200 --> 00:06:47,960 And it smoothly varies in between. 115 00:06:48,280 --> 00:06:51,200 But how do you cope with the fact that the actual magnetic 116 00:06:51,200 --> 00:06:54,400 pole is usually wandering around, even if it's not 117 00:06:54,400 --> 00:06:56,760 reversing? Yeah, we cope with that with a 118 00:06:56,760 --> 00:06:59,920 lot of samples. Yes, the magnetic field does 119 00:06:59,920 --> 00:07:03,400 vary through what we would call in the business paleo secular 120 00:07:03,400 --> 00:07:07,120 variation. I've heard it analogized to a 121 00:07:07,120 --> 00:07:10,920 drunk person hanging on to a pole in a subway car, and they 122 00:07:10,920 --> 00:07:14,560 kind of dance around it as they're unstable, but on 123 00:07:14,560 --> 00:07:16,800 average, they're still standing up straight. 124 00:07:17,320 --> 00:07:20,240 And the same thing applies to Earth's magnetic field. 125 00:07:20,240 --> 00:07:24,400 It's chaotic and it varies quite a bit, but on average it's still 126 00:07:24,640 --> 00:07:27,520 oriented along Earth's rotation axis. 127 00:07:28,000 --> 00:07:32,600 And the reason that works is that Earth's rotation organizes 128 00:07:32,600 --> 00:07:37,000 all of the convecting motions inside the core along that 129 00:07:37,000 --> 00:07:41,160 vertical axis on average over 10s of thousands of years. 130 00:07:41,640 --> 00:07:44,840 And so if you take a lot of samples that are preserving the 131 00:07:44,840 --> 00:07:49,000 field at slightly different points in time over several 10s 132 00:07:49,000 --> 00:07:52,880 of thousands of years in the year's past, you can average out 133 00:07:52,880 --> 00:07:56,520 all of that variability and get at something a lot more 134 00:07:56,520 --> 00:08:00,400 coherent, an average result. So we sample a lot of rocks. 135 00:08:00,400 --> 00:08:04,000 In this case, we sampled several thousand samples for this study 136 00:08:04,400 --> 00:08:06,720 to be able to average all of that out properly. 137 00:08:06,880 --> 00:08:10,440 Wow, several thousand? And how much rock do you need to 138 00:08:10,640 --> 00:08:15,320 drill out for each sample? Each sample is, in our case, a 139 00:08:15,320 --> 00:08:20,960 core of rock that's about 2 1/2 centimeters an inch wide and a 140 00:08:20,960 --> 00:08:23,560 few inches few like 10 centimeters long. 141 00:08:24,120 --> 00:08:26,880 And we can also grab a block that we've oriented. 142 00:08:26,880 --> 00:08:29,560 But the main thing that we have to do when we're sampling is 143 00:08:29,720 --> 00:08:33,039 make sure that we're recording the orientation of the sample as 144 00:08:33,039 --> 00:08:36,679 we found it, because we're measuring the directions of the 145 00:08:36,679 --> 00:08:39,080 magnetization vector in these samples. 146 00:08:39,520 --> 00:08:43,600 And so we have all these fancy oriented devices in the field 147 00:08:43,600 --> 00:08:47,360 with us to measure that. But for any geologists or 148 00:08:47,360 --> 00:08:50,440 geology enthusiasts who have been out in the field and 149 00:08:50,440 --> 00:08:55,240 randomly seen, oh, that outcrop has a cluster of holes about 150 00:08:55,240 --> 00:08:57,480 that wide of it, that's people like us. 151 00:08:57,480 --> 00:09:00,960 That's paleomagnetists usually. And the number of samples there 152 00:09:01,080 --> 00:09:05,520 reflects again our effort to try and reproduce our data and make 153 00:09:05,520 --> 00:09:08,560 sure that we're getting that average field orientation. 154 00:09:09,200 --> 00:09:12,320 Let's talk a bit about the actual kind of rock it is that 155 00:09:12,320 --> 00:09:16,960 you're sampling. Are these rocks lava flows that 156 00:09:17,200 --> 00:09:22,200 took place on an ocean floor and left kind of phonetic stripes 157 00:09:22,200 --> 00:09:25,720 like we see today in either side of the Mid-Atlantic Ridge, for 158 00:09:25,720 --> 00:09:31,120 example, and then using uranium, lead and zircons inside that 159 00:09:31,120 --> 00:09:33,040 lava? Yes and no. 160 00:09:33,560 --> 00:09:37,160 They are, yes, lava flows that erupted onto the ocean floor 161 00:09:37,320 --> 00:09:41,560 about 3 1/2 billion years ago. They're mostly pillow lavas, so 162 00:09:41,560 --> 00:09:45,280 exactly the sort of thing you might see in say Hawaii today. 163 00:09:45,760 --> 00:09:49,400 They aren't necessarily in the same setting that would have 164 00:09:49,400 --> 00:09:52,840 produced magnetic stripes or magnetic lineations on the sea 165 00:09:52,840 --> 00:09:57,720 floor because these are forming in a setting that's not a mid 166 00:09:57,720 --> 00:09:59,400 ocean Ridge that's spreading apart. 167 00:09:59,800 --> 00:10:04,360 The setting would more look like maybe something like Hawaii or 168 00:10:04,400 --> 00:10:07,680 parts of Iceland today. We don't have very much context, 169 00:10:07,680 --> 00:10:09,560 but that's at least something that we do know. 170 00:10:10,080 --> 00:10:14,240 And yes, the way that we date these rocks is using uranium 171 00:10:14,240 --> 00:10:18,440 lead systematics and zircons. The lavas themselves are very 172 00:10:18,440 --> 00:10:21,480 poor in zircon. So these are mostly basalt 173 00:10:21,520 --> 00:10:25,280 lavas. And what's fortunate is that in 174 00:10:25,280 --> 00:10:29,440 between some of these lava flows, you get sedimentary 175 00:10:29,440 --> 00:10:34,520 horizons or Felzik volcanic horizons that are rich in 176 00:10:34,560 --> 00:10:37,280 zircons that can be dated using uranium lead methods. 177 00:10:37,800 --> 00:10:41,720 And so throughout this stack of lava flows that we've measured 178 00:10:41,720 --> 00:10:45,120 throughout this stratigraphy, there are different points that 179 00:10:45,120 --> 00:10:47,160 other of our colleagues have dated. 180 00:10:47,280 --> 00:10:50,120 And so we can interpolate what time all of those different 181 00:10:50,120 --> 00:10:54,160 rocks formed. So it seems like a huge question 182 00:10:54,160 --> 00:10:58,200 is how do we know that the magnetic signal you're measuring 183 00:10:58,200 --> 00:11:03,600 today was in fact printed so very long ago, and it's not the 184 00:11:03,600 --> 00:11:07,040 result of some intervening effects that overwrote or in 185 00:11:07,040 --> 00:11:10,280 some other way masked the original magnetic imprint. 186 00:11:11,040 --> 00:11:15,560 And I wonder how intense such effects would have needed to be 187 00:11:15,560 --> 00:11:17,400 in order to erase the original signal. 188 00:11:17,960 --> 00:11:20,360 Right, as you've alluded to, magnetizations that are 189 00:11:20,360 --> 00:11:24,120 preserved by rocks can get reset in a few different ways. 190 00:11:24,120 --> 00:11:27,880 The main one is thermally. So if you heat a magnetic 191 00:11:27,880 --> 00:11:31,520 material up to a high enough temperature, the magnetization 192 00:11:31,520 --> 00:11:34,600 that it's holding onto gets overwritten with a new one. 193 00:11:35,200 --> 00:11:38,920 You can actually try this at home by putting a fridge magnet 194 00:11:38,920 --> 00:11:42,760 in your oven and baking it. And that will actually we neuter 195 00:11:42,760 --> 00:11:44,360 the magnet. It will make it quite a bit 196 00:11:44,360 --> 00:11:48,440 weaker for the mineral that we're measuring in nature most 197 00:11:48,440 --> 00:11:51,560 often, that's magnetite. The temperature required to 198 00:11:51,560 --> 00:11:56,400 completely reset any magnetite grain is about 585 Celsius. 199 00:11:56,400 --> 00:12:02,040 That's almost 1100 Fahrenheit. And there's another issue also. 200 00:12:02,080 --> 00:12:05,480 Thermal processes are not the only thing that can reset a 201 00:12:05,520 --> 00:12:09,080 magnetization. You can also undergo chemical 202 00:12:09,080 --> 00:12:13,120 reactions between minerals and fluids, for example, that might 203 00:12:13,120 --> 00:12:16,240 dissolve or grow grains of magnetic minerals. 204 00:12:16,680 --> 00:12:20,480 And so we have to be really careful that the magnetizations 205 00:12:20,480 --> 00:12:25,120 that we are measuring date to when those rocks formed, or at 206 00:12:25,120 --> 00:12:27,680 the very least that we can date when those magnetizations 207 00:12:27,680 --> 00:12:31,240 happened in the first place. And that goes for anyone 208 00:12:31,240 --> 00:12:34,640 studying the very earlier, the oldest rock, as you pointed out, 209 00:12:34,960 --> 00:12:38,760 have all been heated and altered somewhat by billions of years of 210 00:12:38,760 --> 00:12:42,200 later geological events. And so it doesn't matter if 211 00:12:42,200 --> 00:12:45,800 you're a paleomagnetist like myself or a geochemist, you have 212 00:12:45,800 --> 00:12:48,400 to make sure that the signals that you're measuring actually 213 00:12:48,400 --> 00:12:50,160 date to when you think you're measuring them. 214 00:12:50,240 --> 00:12:52,040 And we really have our work cut out for us. 215 00:12:52,480 --> 00:12:56,600 We have developed a whole battery of tests in order to 216 00:12:56,680 --> 00:13:01,520 establish the authenticity of that magnetic signal. 217 00:13:01,600 --> 00:13:04,200 Can you describe what these tests are? 218 00:13:04,960 --> 00:13:07,200 Sure, yes. So they're called field tests in 219 00:13:07,200 --> 00:13:10,280 paleomagnetism. And as he said, they tell us 220 00:13:10,440 --> 00:13:14,680 whether magnetizations in a rock predate or post date some 221 00:13:14,680 --> 00:13:18,000 geological feature of interest and they take advantage of 222 00:13:18,120 --> 00:13:21,040 traditional cross cutting relationships that geologists 223 00:13:21,040 --> 00:13:24,200 use for mapping and relative dating of different units. 224 00:13:24,600 --> 00:13:27,880 So one of those is a fold test. So let's say you measured rocks 225 00:13:28,320 --> 00:13:31,360 in a bunch of different areas that are all the same age of 226 00:13:31,360 --> 00:13:34,600 rock, but they've been folded or tilted in different ways after 227 00:13:34,600 --> 00:13:37,960 they formed. And So what you should see if 228 00:13:37,960 --> 00:13:42,840 the magnetization is older than the folding event, is that, you 229 00:13:42,840 --> 00:13:46,240 know, two different rocks that originally had one coherent 230 00:13:46,240 --> 00:13:48,640 magnetization that then got folded. 231 00:13:49,040 --> 00:13:52,240 You should see that if you correct for that folding, their 232 00:13:52,240 --> 00:13:55,240 magnetization should line back up again, roughly speaking. 233 00:13:55,720 --> 00:13:58,480 And so therefore, again, the magnetization predates the 234 00:13:58,480 --> 00:14:02,360 folding, which again you can date using other mapping and 235 00:14:02,360 --> 00:14:05,720 uranium lead methods. Another one of these tests is a 236 00:14:05,720 --> 00:14:08,560 baked contact test. This is something you can try 237 00:14:08,560 --> 00:14:12,120 wherever there's a magma body, like a dike or a pluton that's 238 00:14:12,160 --> 00:14:16,280 intruded into some older rock. And the trick there is to 239 00:14:16,280 --> 00:14:21,080 measure the magnetization inside the intrusion, just outside of 240 00:14:21,080 --> 00:14:24,120 the intrusion, and then gradually further and further 241 00:14:24,120 --> 00:14:27,360 away from it in those surrounding older rocks. 242 00:14:27,880 --> 00:14:30,800 And what you should expect to see if nothing has been 243 00:14:30,800 --> 00:14:35,960 overprinted, is that the intrusion magnetization and the 244 00:14:36,000 --> 00:14:39,760 magnetization immediately outside of it is One Direction. 245 00:14:40,080 --> 00:14:42,680 And then as you work your way further and further away from 246 00:14:42,680 --> 00:14:46,680 it, you should expect to see it gradually give way to some other 247 00:14:46,680 --> 00:14:50,640 magnetization in the surrounding older rocks, which therefore 248 00:14:50,640 --> 00:14:53,720 means that that intrusion magnetization dates to that time 249 00:14:53,720 --> 00:14:56,840 of that intrusion. And the other magnetization in 250 00:14:56,840 --> 00:14:59,000 the country rocks around it is older. 251 00:14:59,720 --> 00:15:03,760 And there are more examples of tests than that, but those are 252 00:15:03,760 --> 00:15:07,520 the two tests that we've used in this study are rocks fortunately 253 00:15:07,520 --> 00:15:10,440 passed them with flying colors. That means that they're still 254 00:15:10,440 --> 00:15:13,840 holding on to magnetizations that are older than a folding 255 00:15:13,840 --> 00:15:18,080 event that's been dated to about 3.4 billion years ago and also 256 00:15:18,080 --> 00:15:21,160 older than magma intrusions dikes that have been dated to 257 00:15:21,360 --> 00:15:26,640 3.44 and 3.46 billion years ago. And again, just for reference, 258 00:15:26,880 --> 00:15:31,120 the rocks themselves that we're measuring are 3.48 to 3.45 259 00:15:31,120 --> 00:15:34,560 billion years old. So that's not much older in the 260 00:15:34,560 --> 00:15:37,080 grand scheme of things. And so we can have confidence 261 00:15:37,080 --> 00:15:40,720 that what we're looking at are truly ancient signals, rather 262 00:15:40,720 --> 00:15:44,160 than some later noise superimposed on top of it. 263 00:15:44,680 --> 00:15:50,000 And these tests rule out both sort of physical or metamorphic 264 00:15:50,000 --> 00:15:51,960 events as well as chemical overprinting. 265 00:15:52,520 --> 00:15:55,760 Yes and no actually. So yes, it doesn't matter where 266 00:15:55,760 --> 00:15:59,600 the magnetization came from. These tests can still apply. 267 00:15:59,720 --> 00:16:03,440 They're still useful for figuring out what the relative 268 00:16:03,440 --> 00:16:06,400 ages of the magnetization and geological events are. 269 00:16:07,120 --> 00:16:10,960 That said, we've actually done quite a bit of legwork on these 270 00:16:10,960 --> 00:16:14,760 rocks to determine how they got magnetized in the 1st place, and 271 00:16:14,760 --> 00:16:18,480 they're actually not holding on to the original thermal 272 00:16:18,480 --> 00:16:21,600 magnetizations that these lavas would have picked up when they 273 00:16:21,600 --> 00:16:24,320 cooled in the ocean floor. We've actually determined that 274 00:16:24,320 --> 00:16:28,240 they got remagnetized in sea floor hydrothermal systems, and 275 00:16:28,520 --> 00:16:31,800 this is where hot sea water percolates through the rocks, 276 00:16:32,120 --> 00:16:34,680 makes them undergo chemical reactions, And those reactions, 277 00:16:34,680 --> 00:16:38,400 as I mentioned earlier, both dissolve old magnetite grains 278 00:16:38,400 --> 00:16:42,600 and grow new ones. So that's a little disquieting, 279 00:16:42,600 --> 00:16:45,680 but what's fortunate here is that even though these 280 00:16:45,680 --> 00:16:49,560 hydrothermal overprint magnetizations are overprints, 281 00:16:49,840 --> 00:16:52,760 they're still incredibly old. And in fact, they actually 282 00:16:52,760 --> 00:16:55,600 happened just after the rocks formed within a few million 283 00:16:55,600 --> 00:16:58,920 years at most. And we've been able to determine 284 00:16:58,920 --> 00:17:02,720 that used by mapping the hydrothermal deposits that are 285 00:17:02,720 --> 00:17:06,760 formed by these systems with help from all sorts of different 286 00:17:06,760 --> 00:17:09,440 data sets. A major one are regional scale 287 00:17:09,440 --> 00:17:13,800 images, spectral images taken by planes and satellites of the 288 00:17:13,800 --> 00:17:16,760 field area that we're in. And that all allows us to 289 00:17:16,760 --> 00:17:20,200 pinpoint where in the stratigraphy, where in the stack 290 00:17:20,240 --> 00:17:24,319 of lava flows and therefore when in the, you know, relative 291 00:17:24,319 --> 00:17:26,880 timing of things. These hydrothermal circulating 292 00:17:26,880 --> 00:17:29,640 fluids percolated through each of the rock units that we 293 00:17:29,640 --> 00:17:32,920 sampled and reset them. So we're still able to use them. 294 00:17:33,080 --> 00:17:36,240 We're leveraging that alteration as signal itself. 295 00:17:37,000 --> 00:17:41,200 OK, let's get back to your results and what we can learn 296 00:17:41,200 --> 00:17:45,800 from them. You show relative motion between 297 00:17:45,800 --> 00:17:48,920 two plates at about 3 1/2 billion years ago. 298 00:17:49,840 --> 00:17:54,240 Does this require full blown modern style plate tectonics 299 00:17:54,240 --> 00:17:57,840 with subduction and so on? Or can we explain your 300 00:17:57,840 --> 00:17:59,760 observations with alternative models? 301 00:18:00,160 --> 00:18:03,320 No, it does not necessarily require modern style subduction. 302 00:18:03,320 --> 00:18:07,000 So fundamentally, stepping back and looking at our results, all 303 00:18:07,000 --> 00:18:10,320 that we can completely confidently say geodynamically 304 00:18:10,320 --> 00:18:14,280 speaking, is that we see one piece of crust, the Pilbara, 305 00:18:14,600 --> 00:18:18,280 moving relative to a different piece of crust, the Copvol. 306 00:18:18,960 --> 00:18:22,760 And this rules out a situation, as I've mentioned before, that 307 00:18:22,960 --> 00:18:25,360 has often been proposed for this time in Earth history where 308 00:18:25,360 --> 00:18:28,960 Earth's whole surface is 1 giant global shell that's rendered 309 00:18:29,120 --> 00:18:32,160 stagnant and unmoving by its lack of plate boundaries. 310 00:18:32,720 --> 00:18:35,040 And that's because our data require a plate boundary of some 311 00:18:35,040 --> 00:18:38,640 kind in between the two pieces of crust that we can see, again, 312 00:18:38,640 --> 00:18:42,240 moving relative to each other, each on a separate plate 3.48 313 00:18:42,240 --> 00:18:46,160 billion years ago. What we don't know is where or 314 00:18:46,160 --> 00:18:50,240 how many or what kind of plate boundary that might have been. 315 00:18:50,560 --> 00:18:53,000 There are different scenarios depending on what that might 316 00:18:53,000 --> 00:18:56,360 have looked like that feature moving plates that are not 317 00:18:56,440 --> 00:18:59,160 modern plate tectonics. So of course, modern plate 318 00:18:59,160 --> 00:19:01,120 tectonics, as you mentioned, is possible. 319 00:19:01,120 --> 00:19:05,760 So that's where plates actively move themselves as they're 320 00:19:05,880 --> 00:19:09,280 pulled into the mantle by sinking at subduction zones. 321 00:19:09,840 --> 00:19:13,760 But you can also maybe get plates moving as they're more 322 00:19:13,760 --> 00:19:18,080 passively, sluggishly pulled along by the mantle as it drags 323 00:19:18,080 --> 00:19:21,240 on their underside. And you can also envision an 324 00:19:21,240 --> 00:19:24,320 Earth that might be stagnant most of the time, but 325 00:19:24,320 --> 00:19:28,360 intermittently it might undergo these really brief but rapid 326 00:19:28,360 --> 00:19:31,880 episodes of plate motion that resurface big portions of 327 00:19:31,880 --> 00:19:35,080 Earth's crust. And our data can work with all 328 00:19:35,080 --> 00:19:37,160 of those situations that I just mentioned. 329 00:19:37,600 --> 00:19:40,120 So we can't necessarily with what we have in hand at the 330 00:19:40,120 --> 00:19:41,960 moment say anything more than that. 331 00:19:42,440 --> 00:19:46,160 We are working on additional data from a longer time span in 332 00:19:46,160 --> 00:19:49,240 the pill borough. And those data may suggest that 333 00:19:49,240 --> 00:19:53,160 what we're looking at is that last episodic intermittent case 334 00:19:53,160 --> 00:19:55,440 that I just described. So that again, that'd be quick 335 00:19:55,440 --> 00:19:59,920 bursts of plate motion punctuating these long 10s of 336 00:19:59,920 --> 00:20:03,960 millions of year long periods of quiescence and static with no 337 00:20:03,960 --> 00:20:06,800 motion periods. But just a work in progress. 338 00:20:07,040 --> 00:20:09,640 But you can say pretty confidently that it wasn't just 339 00:20:09,640 --> 00:20:14,360 a big stagnant lid all the way around the Earth, perhaps a bit 340 00:20:14,360 --> 00:20:16,880 like Mars or Venus today. Correct. 341 00:20:16,880 --> 00:20:21,160 Our data do require that there was some kind of plate boundary 342 00:20:21,160 --> 00:20:24,120 to accommodate that differential, that relative 343 00:20:24,120 --> 00:20:27,600 motion between two different pieces of Earth's lithosphere at 344 00:20:27,600 --> 00:20:30,720 the time. Do your results enable us to say 345 00:20:30,720 --> 00:20:34,680 anything about what else was happening in the Earth around 3 346 00:20:34,680 --> 00:20:37,840 1/2 billion years ago? Yeah, I guess I'll make 2 points 347 00:20:37,840 --> 00:20:40,320 there. One is that paleomagnetic data 348 00:20:40,320 --> 00:20:44,560 are direct observations of what Earth's core is doing because 349 00:20:44,560 --> 00:20:46,960 that's what makes the magnetic field in the first place. 350 00:20:47,560 --> 00:20:50,720 And there are things that we can look for in our data that keep 351 00:20:50,720 --> 00:20:52,760 tabs on that. And a great example of one would 352 00:20:52,760 --> 00:20:56,200 be a geomagnetic reversal. This is where Earth's North and 353 00:20:56,200 --> 00:20:58,240 South magnetic poles switch places. 354 00:20:58,240 --> 00:21:01,520 And this happens pretty frequently on the modern Earth 355 00:21:01,600 --> 00:21:07,600 and a few to a few 10s of times per million years or so, one to 356 00:21:07,600 --> 00:21:10,640 10 times per million years. And excitingly, we actually 357 00:21:10,640 --> 00:21:14,600 found a reversal in our data that happened about 3.46 billion 358 00:21:14,600 --> 00:21:17,040 years ago. You can see its signature in the 359 00:21:17,040 --> 00:21:20,960 Pilbara data and the data from the Kopfall Craton as well at 360 00:21:20,960 --> 00:21:23,520 the same time. And that's actually the oldest 361 00:21:23,520 --> 00:21:26,920 example of a reversal that's ever been identified. 362 00:21:27,320 --> 00:21:30,160 And so you wouldn't be wrong to conclude that Earth's score 363 00:21:30,160 --> 00:21:33,440 could circulate in a pattern that makes a modern like 364 00:21:33,440 --> 00:21:36,560 magnetic field. And also could conveniently 365 00:21:36,560 --> 00:21:40,760 enables us to do this geographic analysis that traces how things 366 00:21:40,760 --> 00:21:44,040 moved around on Earth's surface. The other thing that I'd point 367 00:21:44,040 --> 00:21:48,280 out here is that I'm working on another also unpublished data 368 00:21:48,280 --> 00:21:51,480 set here, also from the Pilbara that likely tells us actually 369 00:21:51,480 --> 00:21:55,040 about how the atmosphere circulated 3.4 billion years 370 00:21:55,040 --> 00:21:56,960 ago. So that's the age of a bunch of 371 00:21:56,960 --> 00:22:00,160 deposits that are in the Pilbara called evaporites. 372 00:22:00,440 --> 00:22:03,280 Evaporites form inside of sediments that are exposed to 373 00:22:03,480 --> 00:22:05,760 arid air and arid surface conditions. 374 00:22:05,760 --> 00:22:10,280 And what we're finding is that those evaporites formed at 375 00:22:10,280 --> 00:22:15,440 roughly the same subtropical latitudes, like 15 to 35° or so, 376 00:22:16,240 --> 00:22:18,920 that most of Earth's deserts form in today. 377 00:22:19,400 --> 00:22:23,120 And that latitude relationship is actually governed today by 378 00:22:23,120 --> 00:22:25,960 how the atmosphere circulates from equator to pole. 379 00:22:26,800 --> 00:22:29,680 And so it's a great illustration, I think, of how 380 00:22:29,680 --> 00:22:32,600 powerful paleomagnetism can be as a technique, because it can 381 00:22:32,600 --> 00:22:37,520 tell you about dynamical processes on Earth deep in the 382 00:22:37,520 --> 00:22:42,160 past, all the way from the core to the mantle and the crust and 383 00:22:42,160 --> 00:22:44,240 even the atmosphere. Wow, that's amazing. 384 00:22:44,240 --> 00:22:50,160 So the subtropical desert regions are the result of a 385 00:22:50,160 --> 00:22:54,320 certain kind of atmospheric circulation pattern that is of 386 00:22:54,320 --> 00:22:57,360 the type that we have today. That's what it appears to be. 387 00:22:57,560 --> 00:23:00,280 The type of circulation pattern here is called the Hadley 388 00:23:00,280 --> 00:23:04,000 circulation, after I believe George Hadley in the 1700s came 389 00:23:04,000 --> 00:23:07,560 up with it. But it's basically that warm air 390 00:23:07,560 --> 00:23:12,240 rises and drops all of its moisture at the equator and then 391 00:23:12,720 --> 00:23:17,560 makes its way northward at high altitude and eventually sinks 392 00:23:17,560 --> 00:23:22,400 back down to Earth's surface at roughly about 30° latitude. 393 00:23:22,920 --> 00:23:26,440 And that's responsible for the subtropical jet stream. 394 00:23:26,440 --> 00:23:29,320 It's responsible for the trade winds at those latitudes. 395 00:23:29,840 --> 00:23:32,920 And also, again, because all of that air had dropped its 396 00:23:32,920 --> 00:23:35,320 moisture at the equator, it's dry. 397 00:23:35,440 --> 00:23:38,840 And so when it gets back to Earth's surface, it dries out 398 00:23:38,840 --> 00:23:42,120 that part of the Earth again. Hence why if you look at a map 399 00:23:42,120 --> 00:23:45,240 of the Earth and look at where all the large deserts are, the 400 00:23:45,240 --> 00:23:50,040 Sahara, the Kalahari in Africa, Australia, a match of the Middle 401 00:23:50,040 --> 00:23:53,360 East, the desert southwest and and parts of northern Central 402 00:23:53,360 --> 00:23:56,480 America. These are all deserts in that 403 00:23:56,480 --> 00:24:00,360 latitude band for a reason. It's where that air is is coming 404 00:24:00,360 --> 00:24:02,080 from high altitude and drying things out. 405 00:24:02,080 --> 00:24:05,960 And it looks like we can see the same thing happening 3.4 billion 406 00:24:05,960 --> 00:24:09,520 years ago. It's striking also that the time 407 00:24:09,520 --> 00:24:12,440 period you're studying coincides with the time when we see 408 00:24:12,440 --> 00:24:16,320 evidence in the rock record of the very first life forms. 409 00:24:17,160 --> 00:24:20,160 I talked about this and about traumatolites in particular in 410 00:24:20,160 --> 00:24:23,440 an earlier podcast episode with Martin Van Kraenndonk. 411 00:24:24,120 --> 00:24:28,520 Have you seen any evidence for life associated with the the 412 00:24:28,520 --> 00:24:31,600 rocks that you've been studying? Yeah, it's it's kind of perfect 413 00:24:31,600 --> 00:24:34,440 actually that you mentioned Martin here, because it turns 414 00:24:34,440 --> 00:24:37,440 out the stromatolites he's worked on for the last few 415 00:24:37,440 --> 00:24:41,440 decades, among other things, are actually right in between the 416 00:24:41,440 --> 00:24:43,640 rocks that I'm studying for this this paper. 417 00:24:43,640 --> 00:24:47,280 So they're the oldest well established fossils on Earth, 418 00:24:47,280 --> 00:24:52,000 and some of them even grew in a hot Springfield that Martin and 419 00:24:52,000 --> 00:24:56,400 his colleagues were responsible for documenting that was fed by 420 00:24:56,400 --> 00:25:00,200 one of the hydrothermal systems that gave our lavas their 421 00:25:00,200 --> 00:25:03,840 magnetic signals. And so that means that now we 422 00:25:03,840 --> 00:25:07,440 have a little bit of direct tectonic context for those life 423 00:25:07,440 --> 00:25:10,000 forms. They grew on a tectonic plate 424 00:25:10,000 --> 00:25:12,120 either in motion already or about to be. 425 00:25:12,800 --> 00:25:15,920 And that's really intriguing because plate tectonics is 426 00:25:16,160 --> 00:25:19,400 intimately linked with all sorts of processes that foster 427 00:25:19,400 --> 00:25:22,080 habitable conditions. It plays a role in keeping 428 00:25:22,080 --> 00:25:26,000 surface temperature stable for billions of years by modulating 429 00:25:26,000 --> 00:25:27,560 them out of CO2 in the atmosphere. 430 00:25:27,920 --> 00:25:31,560 It creates environments with sources and gradients of 431 00:25:31,560 --> 00:25:35,680 chemical elements that living things can metabolize and build 432 00:25:35,680 --> 00:25:38,040 from, for instance, in hybrothermal systems. 433 00:25:38,560 --> 00:25:41,800 And to be clear, I should point out that the fossils that 434 00:25:41,800 --> 00:25:44,960 Martin's worked on, the plate motion I found, they're probably 435 00:25:44,960 --> 00:25:48,720 not the oldest examples of those things, full stop, right? 436 00:25:48,720 --> 00:25:51,640 There are other even older examples almost certainly came 437 00:25:51,640 --> 00:25:55,520 before them. But still, I find it profound 438 00:25:55,520 --> 00:25:59,880 that in the endless quest of geologists to look further back 439 00:25:59,880 --> 00:26:05,080 in geologic time, the oldest records of life, habitable 440 00:26:05,080 --> 00:26:08,720 energy rich environments for that life, and the tectonic 441 00:26:08,720 --> 00:26:12,200 plates that might have generated such environments all just 442 00:26:12,200 --> 00:26:15,800 coincide pretty much perfectly. I I can't help but imagine that 443 00:26:15,800 --> 00:26:18,840 early Earth might have dealt life a really great hand to get 444 00:26:18,840 --> 00:26:23,440 started and develop. You spent quite a bit of time in 445 00:26:23,440 --> 00:26:27,920 the Pilbara region of WA collecting all those thousands 446 00:26:27,920 --> 00:26:31,760 of samples you talked about. What's it like to spend time in 447 00:26:31,760 --> 00:26:35,000 the wilderness there? On a practical level, it's a 448 00:26:35,000 --> 00:26:38,520 place what you might call ruggedly beautiful, I guess so 449 00:26:38,520 --> 00:26:42,520 it's it's hot, it's dry. Pilbara comes from a local 450 00:26:42,520 --> 00:26:44,600 Aboriginal word meaning dry country. 451 00:26:45,160 --> 00:26:48,720 It's very hilly terrain that it's pockmarked by all these 452 00:26:48,720 --> 00:26:52,600 spiky bushes of grass everywhere, scraggly desert 453 00:26:52,600 --> 00:26:56,640 trees, the occasional kangaroo, sometimes an eagle if you're 454 00:26:56,640 --> 00:26:59,840 lucky. And under all that desert is 455 00:26:59,840 --> 00:27:02,840 like this deep sense of antiquity that you can't really 456 00:27:02,840 --> 00:27:04,520 escape from. You're always aware of it. 457 00:27:04,520 --> 00:27:07,280 The rocks, of course, are crazy old and they look like it too. 458 00:27:07,280 --> 00:27:09,960 They've been out in the weather for billions of years and that 459 00:27:09,960 --> 00:27:14,600 stained them all these weathered hues of red and orange, much 460 00:27:14,600 --> 00:27:17,040 like a lot of the continent of Australia. 461 00:27:17,720 --> 00:27:20,440 But you also get hints of a history that's a lot closer to 462 00:27:20,440 --> 00:27:23,160 home. So during my field work, I've 463 00:27:23,200 --> 00:27:26,560 had the great privilege of stumbling into prehistoric rock 464 00:27:26,560 --> 00:27:29,360 art sites. Petroglyphs and chippings in the 465 00:27:29,360 --> 00:27:33,480 rock, faces that depict people, human figures, interacting 466 00:27:33,800 --> 00:27:36,960 animals. You'll see dingoes and Kangaroos 467 00:27:36,960 --> 00:27:41,360 and emus all depicted there, parts of the landscape, maps of 468 00:27:41,360 --> 00:27:45,360 river systems. And having had a chance to talk 469 00:27:45,360 --> 00:27:47,440 with some of the modern descendants of those ancient 470 00:27:47,440 --> 00:27:51,720 artists, they were clearly all trying to understand their place 471 00:27:51,720 --> 00:27:54,200 in the world around them 10s of thousands of years ago. 472 00:27:54,960 --> 00:27:58,800 And in a weird way, that's kind of what my colleagues and I do 473 00:27:58,800 --> 00:28:00,760 out there too. We're trying to understand our 474 00:28:00,760 --> 00:28:04,160 place in the world around us and how we got here, just using 475 00:28:04,160 --> 00:28:06,440 different tools. And 10s of thousands of years 476 00:28:06,440 --> 00:28:10,800 later and walking around the Pilbara really gives you an 477 00:28:11,280 --> 00:28:15,160 immense sense of timelessness and heritage. 478 00:28:15,720 --> 00:28:19,240 Some of the oldest art made by human beings to the oldest known 479 00:28:19,240 --> 00:28:21,440 life forms that are right there next to each other. 480 00:28:21,960 --> 00:28:24,480 And it makes you feel small in all the right ways. 481 00:28:25,560 --> 00:28:27,480 Alec Brenner, thank you very much. 482 00:28:27,640 --> 00:28:28,960 Thanks, Oliver. It's been my pleasure. 483 00:28:30,240 --> 00:28:32,920 To see pictures and illustrations that support this 484 00:28:32,920 --> 00:28:38,200 podcast, go to geologybytes.com, where you'll also find a subject 485 00:28:38,200 --> 00:28:40,120 matter index of all the episodes. 486 00:28:40,560 --> 00:28:43,920 There you can also give me feedback which I welcome, as 487 00:28:43,920 --> 00:28:46,920 well as sign up to get my emails about new episodes.