1 00:00:04,760 --> 00:00:07,760 This is geology bytes with Oliver Strumple. 2 00:00:09,400 --> 00:00:12,920 The surface of the Earth is capped by a thin rocky layer 3 00:00:12,920 --> 00:00:17,360 called the lithosphere. The lithosphere is composed of 4 00:00:17,360 --> 00:00:21,400 discrete plates which float on the acinosphere, the ductile 5 00:00:21,400 --> 00:00:25,480 upper part of the mantle. Since the acinosphere is far too 6 00:00:25,480 --> 00:00:29,320 deep down for us to access it by drilling, our knowledge of it 7 00:00:29,320 --> 00:00:32,479 comes from mantle rock that finds its way to the surface, 8 00:00:32,960 --> 00:00:36,840 such as in ophulites or Kimber lights, or via geophysical 9 00:00:36,840 --> 00:00:41,240 phenomena that probe the mantle remotely, such as seismic waves, 10 00:00:41,360 --> 00:00:45,720 gravity and magnetic fields. But many of the properties of 11 00:00:45,720 --> 00:00:48,320 the mantle that are thought to determine the mechanisms of 12 00:00:48,320 --> 00:00:53,080 plate movements, subduction and magnetism are poorly understood, 13 00:00:53,280 --> 00:00:56,720 as they depend on the detailed physical and chemical behaviour 14 00:00:56,720 --> 00:00:59,480 of the mantle under the conditions that prevail at 15 00:00:59,480 --> 00:01:03,400 depth. David Colstead has sought to 16 00:01:03,400 --> 00:01:07,200 replicate these conditions in the lab so as to observe these 17 00:01:07,200 --> 00:01:11,640 critical mechanisms directly. He is professor emeritus at the 18 00:01:11,640 --> 00:01:13,920 School of Earth and Environmental Science at the 19 00:01:13,920 --> 00:01:18,080 University of Minnesota. David Colstead, Welcome to 20 00:01:18,080 --> 00:01:20,120 Geology Bites. Thank you. 21 00:01:20,200 --> 00:01:22,760 I am delighted that you have invited me. 22 00:01:23,040 --> 00:01:27,480 What exactly are the properties of the acinospheric mantle that 23 00:01:27,480 --> 00:01:29,320 you try and learn about in the lab? 24 00:01:30,000 --> 00:01:34,720 Our experiments are designed to study the viscosity of the 25 00:01:34,720 --> 00:01:38,400 acinosphere. Question that motivates us is, 26 00:01:38,400 --> 00:01:42,200 why is the acinosphere so weak? Is it simply that the 27 00:01:42,200 --> 00:01:46,360 temperature has reached a critical point and the 28 00:01:46,520 --> 00:01:50,000 stenosphere begins to flow much like warming molasses in 29 00:01:50,000 --> 00:01:53,120 January? Or is it the presence of melt 30 00:01:53,120 --> 00:01:56,440 and water that makes these stenosphere weak? 31 00:01:57,040 --> 00:02:00,000 To answer these questions, we have concentrated our research 32 00:02:00,000 --> 00:02:04,520 on quantifying the dependency of viscosity on the melt fraction 33 00:02:04,640 --> 00:02:08,360 and on water content. And though those two quantities 34 00:02:08,440 --> 00:02:11,560 affect each other, they're two separate things. 35 00:02:11,600 --> 00:02:16,080 And I understand that in your experiments you study the effect 36 00:02:16,080 --> 00:02:19,640 of each of these separately. Let's start with the melt 37 00:02:19,640 --> 00:02:22,560 fraction. Do you experiment on samples of 38 00:02:22,560 --> 00:02:25,400 mantle rock that we can find on the surface? 39 00:02:25,640 --> 00:02:28,720 No, we don't use rocks provided by Mother Nature. 40 00:02:29,280 --> 00:02:32,480 Those rocks that have come to the surface in Kimber Lights or 41 00:02:32,480 --> 00:02:35,760 ophiolites have been on the surface long enough that they 42 00:02:35,760 --> 00:02:39,240 begin to weather and introduce products that we'd rather not 43 00:02:39,240 --> 00:02:41,320 have in our samples during the experiments. 44 00:02:41,880 --> 00:02:46,440 So instead we take gem quality olivine, which is the dominant 45 00:02:46,440 --> 00:02:50,320 mineral in Earth's upper mantle, and we combine it with basalt. 46 00:02:50,480 --> 00:02:54,520 And we take the gem quality olivine, grind it into a powder, 47 00:02:55,120 --> 00:02:57,920 and then grind some of the basalt into powder and mix a 48 00:02:57,920 --> 00:03:01,160 specific amount of a salt in with the olivine. 49 00:03:01,680 --> 00:03:05,040 And the salt at high temperature will melt, but the amount of 50 00:03:05,040 --> 00:03:08,000 melt that you get from the basalt is independent of 51 00:03:08,000 --> 00:03:11,080 temperature, so that we can really isolate the effect of 52 00:03:11,080 --> 00:03:15,560 melt fraction on the deformation behavior of the material. 53 00:03:16,680 --> 00:03:20,640 We also use lurzolite sometimes for our samples. 54 00:03:20,760 --> 00:03:25,960 And lurzolite the melt is formed by melting the aloving plus 55 00:03:26,080 --> 00:03:29,520 orthopyrixine plus colinopyrixine, which react 56 00:03:29,520 --> 00:03:32,440 together to form the melt. But in that case the amount of 57 00:03:32,440 --> 00:03:34,680 melt is a function of temperature. 58 00:03:34,720 --> 00:03:37,040 So we use two different approaches, two different types 59 00:03:37,040 --> 00:03:42,280 of samples, and then the challenge is to demonstrate that 60 00:03:42,280 --> 00:03:45,280 you get essentially the same results taking either approach. 61 00:03:45,440 --> 00:03:48,640 That's interesting. So you actually control the 62 00:03:48,640 --> 00:03:53,280 amount of melt in your samples through really the composition, 63 00:03:53,280 --> 00:03:56,760 the mixture of materials that you put in to your sample to 64 00:03:56,760 --> 00:03:59,680 begin with. Yes, that's exactly right. 65 00:03:59,840 --> 00:04:03,360 And it's a way in which we can really design the experiments to 66 00:04:03,360 --> 00:04:05,480 probe. For example, what is the 67 00:04:05,480 --> 00:04:09,960 strength of material that has 1% melt versus having 10% melt? 68 00:04:10,200 --> 00:04:13,640 And presumably you keep those as dry as possible so as to take 69 00:04:13,640 --> 00:04:16,240 water out of the equation completely for those 70 00:04:16,240 --> 00:04:19,760 experiments. Yes, it's important for us to 71 00:04:20,399 --> 00:04:25,200 try to design experiments where we can vary 1 parameter at a 72 00:04:25,200 --> 00:04:27,240 time. It makes understanding the 73 00:04:27,240 --> 00:04:29,360 physics of what's going on in the problem much more 74 00:04:29,360 --> 00:04:31,920 straightforward. And there's not complications 75 00:04:31,920 --> 00:04:35,120 arise from having three or four different variables changing. 76 00:04:35,120 --> 00:04:38,080 Every time you change one variable, you change two or 77 00:04:38,080 --> 00:04:41,040 three others at the same time. So that's really kind of an 78 00:04:41,040 --> 00:04:45,080 important aspect of the way we try to design our experiments. 79 00:04:45,200 --> 00:04:47,240 Let's talk a bit about water then. 80 00:04:47,240 --> 00:04:51,720 How do you control for the amount of water in your sample? 81 00:04:52,320 --> 00:04:55,960 We work primarily with the mineral olivine and we fabricate 82 00:04:55,960 --> 00:05:00,240 samples again by powdering gym quality single crystals, hot 83 00:05:00,240 --> 00:05:03,520 pressing them and making them a back into a solid cylinder of 84 00:05:03,520 --> 00:05:07,080 material. In this case we put an allobene 85 00:05:07,080 --> 00:05:11,320 sample into a nickel capsule. Nickel helps keep the allobene 86 00:05:11,320 --> 00:05:14,880 in its stability field so that it doesn't oxidize or reduce. 87 00:05:15,240 --> 00:05:18,560 And to that capsule we add a couple of drops of water and 88 00:05:18,560 --> 00:05:22,320 then we seal the capsule. We Weld it shut and then we vary 89 00:05:22,320 --> 00:05:25,880 the water content in the sample by heating assembled a high 90 00:05:25,880 --> 00:05:29,840 temperature and high pressure and the water concentration or 91 00:05:29,840 --> 00:05:33,520 water fugacity in the capsule is a function of temperature and 92 00:05:33,520 --> 00:05:35,600 pressure. Once said high temperature and 93 00:05:35,600 --> 00:05:40,720 pressure, the water dissociates and produces hydrogen ions which 94 00:05:40,720 --> 00:05:43,440 diffuse fairly rapidly into the Aladdin. 95 00:05:43,880 --> 00:05:47,320 So you introduce a fixed amount of water into these experiments 96 00:05:47,320 --> 00:05:51,160 at the beginning, and then you control for the amount of water 97 00:05:51,160 --> 00:05:53,640 by varying the temperature and pressure. 98 00:05:53,640 --> 00:05:59,080 And somehow you know what amount of water, what the effective 99 00:05:59,320 --> 00:06:01,680 amount of water is inside the sample as a function of 100 00:06:01,680 --> 00:06:03,520 temperature and pressure. We do. 101 00:06:03,800 --> 00:06:07,040 We've done calibration experiments prior to doing the 102 00:06:07,040 --> 00:06:11,320 deformation experiments, in which we simply go through this 103 00:06:11,320 --> 00:06:15,520 routine sample to a certain temperature and certain pressure 104 00:06:16,080 --> 00:06:20,440 and then quench it, and then use infrared spectroscopy to measure 105 00:06:20,440 --> 00:06:23,080 the amount of water in the sample so that when we do a 106 00:06:23,080 --> 00:06:25,680 deformation experiment, we know what the temperature and 107 00:06:25,680 --> 00:06:28,200 pressure conditions are, we know what the water content in the 108 00:06:28,200 --> 00:06:32,000 sample will be. What are the size of the samples 109 00:06:32,000 --> 00:06:33,520 that you use in your experiments? 110 00:06:33,800 --> 00:06:39,040 Typically a sample size is about 1 centimeter in diameter and 2cm 111 00:06:39,040 --> 00:06:42,120 in length. So the conditions in the mantle 112 00:06:42,120 --> 00:06:46,200 are extreme, enormous pressures and high temperatures. 113 00:06:46,680 --> 00:06:50,880 What conditions do you aim to achieve in your samples during 114 00:06:50,880 --> 00:06:53,480 the experiments? We typically would go to a 115 00:06:53,480 --> 00:06:58,440 pressure of 300 mega pascals and a temperature of about 1200 116 00:06:58,440 --> 00:07:02,160 thirteen 100°C. Now that corresponds only to a 117 00:07:02,160 --> 00:07:05,920 relatively shallow depth in the Earth of 10 kilometers, so we 118 00:07:05,920 --> 00:07:10,120 certainly need to then be able to determine the flow laws that 119 00:07:10,120 --> 00:07:13,320 allow us to extrapolate to higher pressures and higher 120 00:07:13,320 --> 00:07:16,320 temperatures. Do you not try and simulate 121 00:07:16,320 --> 00:07:20,240 those greater depths and higher temperatures because it's just 122 00:07:20,440 --> 00:07:23,280 technically too difficult? It is technically difficult, 123 00:07:23,280 --> 00:07:26,520 that's for sure. And our limitation is we're 124 00:07:26,520 --> 00:07:30,480 doing experiments in a steel pressure vessel which has a hole 125 00:07:30,480 --> 00:07:34,440 drilled down the axis of the vessel, which limits our ability 126 00:07:34,440 --> 00:07:36,640 maybe 1 giga. Pascal would kind of be an upper 127 00:07:36,640 --> 00:07:41,000 limit of what would be possible, but there are other researchers 128 00:07:41,000 --> 00:07:44,120 who have been very clever and designed apparatuses that can go 129 00:07:44,120 --> 00:07:47,480 to significantly higher pressures, up to 10 gigapascals, 130 00:07:47,680 --> 00:07:51,200 which will get you down to roughly 300 kilometers depth in 131 00:07:51,200 --> 00:07:54,400 the Earth. And these pressure vessels are 132 00:07:54,480 --> 00:07:58,160 taken to the synchrotron, A synchrotron source for X-rays. 133 00:07:58,160 --> 00:08:02,280 And the high intensity beam of X-rays is showing on the samples 134 00:08:02,600 --> 00:08:04,320 during the deformation experiment. 135 00:08:04,360 --> 00:08:07,680 And that allows you to measure the length of the sample and 136 00:08:07,680 --> 00:08:10,720 therefore monitor what the rate of deformation of the sample is 137 00:08:11,240 --> 00:08:15,640 and from X-ray diffraction to measure the stress that's being 138 00:08:15,640 --> 00:08:19,200 applied at the same time. So if you have stress and strain 139 00:08:19,200 --> 00:08:22,760 rate, you have viscosity. But in those experiments that 140 00:08:22,760 --> 00:08:25,600 get to these much higher pressures and temperatures, 141 00:08:25,960 --> 00:08:28,960 presumably they don't do it in the sample size of a few 142 00:08:28,960 --> 00:08:31,720 centimeters like you're doing. That's a really good point. 143 00:08:32,159 --> 00:08:37,280 Oh, the sample size typically is a millimeter by a millimeter, so 144 00:08:37,280 --> 00:08:40,000 the samples get much smaller as the pressure goes up. 145 00:08:40,000 --> 00:08:42,919 Sample sizes tend to get smaller progressively. 146 00:08:43,679 --> 00:08:46,400 OK, so you have this centimeter scale sample. 147 00:08:46,480 --> 00:08:50,000 How do you actually sustain these high temperatures and also 148 00:08:50,000 --> 00:08:55,600 these incredibly high pressures? So in our lab, we focus on using 149 00:08:55,760 --> 00:08:59,880 gas as a confining medium for the sample and the pressure 150 00:08:59,880 --> 00:09:04,640 vessel that we use is about a foot across and about 3 feet 151 00:09:04,640 --> 00:09:07,320 long. And down the center of that 152 00:09:07,320 --> 00:09:12,080 vessel, there's a hole been drilled that's about 3 inches in 153 00:09:12,080 --> 00:09:14,360 diameter. So it's a relatively small hole 154 00:09:14,360 --> 00:09:17,920 in a relatively large vessel. And then the trick is that you 155 00:09:18,080 --> 00:09:21,600 put end caps on the vessel that seal the the vessel. 156 00:09:22,040 --> 00:09:25,160 So you can pump gas into the vessel and increase the pressure 157 00:09:25,360 --> 00:09:29,360 through a series of pumps that each stage increases the 158 00:09:29,360 --> 00:09:32,440 pressure of the gas. That comes basically out of a 159 00:09:32,440 --> 00:09:36,400 welder's argon tank at maybe 10 mega pascals. 160 00:09:36,400 --> 00:09:39,400 And you can increase the pressure with the series of 161 00:09:39,400 --> 00:09:43,000 intensifiers until you get up to 300 mega pascals. 162 00:09:43,480 --> 00:09:45,720 And inside that bore slides a furnace. 163 00:09:45,720 --> 00:09:49,240 And the furnace design in some ways is relatively simple. 164 00:09:49,240 --> 00:09:52,440 There's a lot of insulation that keeps the heat from the center 165 00:09:52,440 --> 00:09:55,720 of the furnace from going out and overheating the steel 166 00:09:55,720 --> 00:10:00,440 vessel, and the furnace is basically simply a wire wound 167 00:10:00,640 --> 00:10:04,160 aluminum oxide tube and it works much like your toaster works at 168 00:10:04,160 --> 00:10:06,640 home. The whole thing is cooled on the 169 00:10:06,640 --> 00:10:10,000 outside to keep the steel basically at room temperature 170 00:10:10,000 --> 00:10:13,080 while the center of the furnace gets up to high temperature. 171 00:10:13,520 --> 00:10:17,400 And you can run a piston through one of the plugs at the end of 172 00:10:17,400 --> 00:10:22,720 the pressure vessel and it moves by a mechanical motor on the 173 00:10:22,720 --> 00:10:26,080 outside of the vessel. And then all the instrumentation 174 00:10:26,360 --> 00:10:29,720 that allows us to measure the stress and displacement gauges 175 00:10:29,720 --> 00:10:33,920 are all inside the vessel. And that design allows us fairly 176 00:10:33,920 --> 00:10:37,440 high resolution experiments. You put all the instrumentation 177 00:10:37,440 --> 00:10:39,320 outside. You could do similar type of 178 00:10:39,320 --> 00:10:42,520 measurements, but you then have to deal with the fact that 179 00:10:42,520 --> 00:10:46,120 you're also deforming the piston at the same time, at least 180 00:10:46,120 --> 00:10:51,120 elastically if not plastically. And you have other distortions 181 00:10:51,120 --> 00:10:53,920 that affect the stress. There's friction on the piston, 182 00:10:53,920 --> 00:10:56,520 so you have to account for the friction with the 183 00:10:56,640 --> 00:10:58,920 instrumentation inside. You get around those 184 00:10:58,920 --> 00:11:02,840 complications. How do you make a sensitive 185 00:11:03,040 --> 00:11:07,400 instrument survive under those conditions inside your 186 00:11:07,400 --> 00:11:11,320 apparatus? This is a really good question, 187 00:11:11,360 --> 00:11:14,360 and it takes clever engineering to make it happen. 188 00:11:14,880 --> 00:11:18,880 If you just buy a commercial strain gauge off the shelf, 189 00:11:18,880 --> 00:11:22,600 they're typically attached to a sample via an epoxy. 190 00:11:22,920 --> 00:11:24,920 Put epoxy inside of a pressure vessel. 191 00:11:24,920 --> 00:11:29,040 When the gas gets up to a pressure of 300 megapascals, it 192 00:11:29,040 --> 00:11:32,200 permeates the epoxy really quickly, and that's not a 193 00:11:32,200 --> 00:11:35,320 problem at that point. But when you lower the pressure 194 00:11:35,320 --> 00:11:38,880 back down, all that argon gas trapped inside the epoxy wants 195 00:11:38,880 --> 00:11:41,720 to get out and it tries to diffuse out, and when it does, 196 00:11:41,840 --> 00:11:44,840 it forms bubbles. And when you now have a high 197 00:11:44,840 --> 00:11:47,240 pressure bubble and you take it down to one atmosphere, the 198 00:11:47,240 --> 00:11:50,400 bubbles will explode and your strained age is destroyed. 199 00:11:50,400 --> 00:11:54,320 So it's complicated, but people have worked on this problem to 200 00:11:54,320 --> 00:11:58,720 try to design displacement transducers that will work under 201 00:11:58,720 --> 00:12:03,080 very high pressure conditions. So how do you actually go about 202 00:12:03,080 --> 00:12:06,040 determining the viscosity of your sample under these 203 00:12:06,040 --> 00:12:10,040 conditions? Well, our focus has been in two 204 00:12:10,040 --> 00:12:13,720 types of experiments. Until about 2000 or the turn of 205 00:12:13,720 --> 00:12:18,120 the century, we did most of our experiments in uniaxial 206 00:12:18,120 --> 00:12:21,000 compression. So we would put a sample inside 207 00:12:21,000 --> 00:12:24,480 of the pressure vessel, confined it with high pressure gas and 208 00:12:24,480 --> 00:12:29,200 then use a piston coming through the lower seal in the vessel to 209 00:12:29,200 --> 00:12:32,360 push on the sample and compress it and make it grow shorter as a 210 00:12:32,360 --> 00:12:35,760 function of time. The other approach we've taken 211 00:12:35,760 --> 00:12:39,720 is to torsion experiments, where you can twist the sample to 212 00:12:39,720 --> 00:12:43,000 fairly high strain. You do one full revolution of a 213 00:12:43,000 --> 00:12:45,480 sample and torsion. You can get strains on the order 214 00:12:45,480 --> 00:12:47,800 of 10. Just to explain the units, 215 00:12:48,160 --> 00:12:52,600 strain is defined as the ratio of a change in a dimension to 216 00:12:52,600 --> 00:12:57,040 the original dimension, and strain rate is the strain change 217 00:12:57,040 --> 00:13:00,680 per second and. That's kind of fun to do because 218 00:13:00,680 --> 00:13:04,360 the Earth does deform rocks to fairly high strains and you can 219 00:13:04,360 --> 00:13:07,800 begin to reproduce that. And again, what we measure in 220 00:13:07,800 --> 00:13:11,280 these experiments typically is the length of the sample as a 221 00:13:11,280 --> 00:13:13,800 function of time. That is how long the stress has 222 00:13:13,800 --> 00:13:17,400 been on the sample, and you can watch the sample get shorter. 223 00:13:17,400 --> 00:13:20,000 Or if you're twisting, you can watch the deformation of the 224 00:13:20,000 --> 00:13:24,000 sample rotate through. And we also measure the stress 225 00:13:24,000 --> 00:13:27,480 on the sample at the same time. So we typically apply a constant 226 00:13:27,480 --> 00:13:30,480 rate of deformation and measure the stress that it requires to 227 00:13:30,480 --> 00:13:32,680 keep that rate of deformation going. 228 00:13:33,280 --> 00:13:36,360 And for us, we've really focused on the steady state part of the 229 00:13:36,360 --> 00:13:40,040 problem where you twist the sample at a constant rate and 230 00:13:40,040 --> 00:13:41,720 eventually the stress will level off. 231 00:13:41,840 --> 00:13:45,600 And when the rate of deformation is fixed by the stress level, 232 00:13:45,600 --> 00:13:48,600 you reached into a steady state condition and that's kind of 233 00:13:48,600 --> 00:13:50,920 where we've largely focused our effort. 234 00:13:51,120 --> 00:13:53,800 So what are the time scales of your experiments? 235 00:13:53,800 --> 00:13:56,560 I mean, I know on earth these things happen over millions of 236 00:13:56,560 --> 00:13:59,320 years. How long do you take to rotate 237 00:13:59,320 --> 00:14:04,880 your sample, say through 360 or two revolutions, and achieve 238 00:14:04,880 --> 00:14:07,280 that steady state that then you're trying to measure? 239 00:14:08,320 --> 00:14:11,600 For torsion experiments, we typically work at strain rates 240 00:14:11,600 --> 00:14:16,160 of about 10 to the -4, maybe 5 * 10 to the -5. 241 00:14:16,440 --> 00:14:21,360 But one of the points about gas medium deformation apparatuses 242 00:14:21,480 --> 00:14:24,280 is that they really do take some level of monitoring. 243 00:14:24,320 --> 00:14:27,360 So we typically would start the experiment early in the morning 244 00:14:27,360 --> 00:14:30,480 and run it for maybe a 12 hour period and that's what really 245 00:14:30,480 --> 00:14:34,120 dictates the strain rate. We could deform more slowly and 246 00:14:34,120 --> 00:14:37,040 in compression experiments we do, we might deform at 10 to the 247 00:14:37,040 --> 00:14:40,520 -6 per second, but the Earth is still out there at 10 to the 248 00:14:40,520 --> 00:14:42,600 -14. So the difference between 10 to 249 00:14:42,600 --> 00:14:47,480 the -410 to the -6 doesn't get you hugely closer to what's 250 00:14:47,480 --> 00:14:50,800 going on in the Earth. And so that again the element 251 00:14:50,800 --> 00:14:54,080 that's important is to try to design experiments where you 252 00:14:54,080 --> 00:14:57,800 really take advantage of what you can do well. 253 00:14:57,800 --> 00:15:02,000 And then again as we've talked a little bit about have equations 254 00:15:02,000 --> 00:15:05,720 or flow laws that allow you to extrapolate that with some level 255 00:15:05,720 --> 00:15:08,840 of confidence to the Earth. How long does it take roughly 256 00:15:08,840 --> 00:15:11,600 then to get up to that steady state? 257 00:15:12,200 --> 00:15:16,040 Within an hour or two you can get from ambient conditions to a 258 00:15:16,040 --> 00:15:19,600 steady state deformation. And the nice part about that is 259 00:15:19,600 --> 00:15:23,440 that then if you want to determine, for example how the 260 00:15:23,440 --> 00:15:26,800 viscosity of the rock depends upon stress, you can look at 261 00:15:26,800 --> 00:15:29,440 several different stresses during the experiment and then 262 00:15:29,440 --> 00:15:32,800 have the ability to go back, analyze that data and determine 263 00:15:32,800 --> 00:15:36,400 what is the constitutive relation that describes the 264 00:15:36,400 --> 00:15:38,960 viscosity dependence in this case on stress. 265 00:15:39,240 --> 00:15:41,560 Or you could do temperature stepping experiments and maybe 266 00:15:41,560 --> 00:15:45,520 look at three or four different temperatures on the same sample. 267 00:15:45,800 --> 00:15:48,720 So we try to extract more than just getting to study state. 268 00:15:48,720 --> 00:15:52,120 We try to kind of tickle the sample a little bit once we have 269 00:15:52,120 --> 00:15:56,440 it at conditions and get it to tell us exactly how it wants to 270 00:15:56,440 --> 00:15:59,640 behave. Let's talk about your results 271 00:15:59,640 --> 00:16:03,760 then. How does the viscosity at 272 00:16:03,760 --> 00:16:07,080 various temperatures and pressures depend on the melt 273 00:16:07,080 --> 00:16:12,360 fraction and the water content? So the viscosity of these Alvin 274 00:16:12,360 --> 00:16:16,000 rich rocks at a given pressure and temperature decreases 275 00:16:16,200 --> 00:16:19,720 approximately linearly with increasing water content in the 276 00:16:19,720 --> 00:16:23,160 sample. For melt fraction, the viscosity 277 00:16:23,360 --> 00:16:27,600 decreases approximately exponentially with increasing 278 00:16:27,600 --> 00:16:31,480 melt fraction in the sample. So it's very very sensitive to 279 00:16:31,480 --> 00:16:34,200 melt fraction. It is very sensitive to melt 280 00:16:34,200 --> 00:16:38,280 fraction. If you add 1% melt, you get a 281 00:16:38,880 --> 00:16:42,040 decrease in viscosity by about a factor of 2. 282 00:16:43,080 --> 00:16:46,720 If you go to 10% melt, decrease the viscosity very well over a 283 00:16:46,720 --> 00:16:49,800 factor of 10 O. If you can store much melt in 284 00:16:49,800 --> 00:16:53,080 the sample, it will be very effective at reducing its 285 00:16:53,080 --> 00:16:55,360 viscosity. Of course, you have to keep in 286 00:16:55,360 --> 00:16:59,160 mind that Melt has a lower density than the solid art 287 00:16:59,160 --> 00:17:02,120 around it, so it does want to rise buoyantly out of the 288 00:17:02,120 --> 00:17:04,280 mantle. So there is some competition 289 00:17:04,280 --> 00:17:07,400 between how much Melt you can maintain and its effect on the 290 00:17:07,400 --> 00:17:10,319 viscosity of the rock. But then the melt fraction then 291 00:17:10,319 --> 00:17:12,400 also as you just said, if it's more mobile. 292 00:17:12,400 --> 00:17:16,079 And so does that actually escape from your sample? 293 00:17:16,079 --> 00:17:18,760 And do you find it all kind of clustering around the edge of 294 00:17:18,760 --> 00:17:21,359 your capsule? Fortunately, gravity doesn't 295 00:17:21,359 --> 00:17:25,920 play a very big role because our sample sizes are so small, so in 296 00:17:25,920 --> 00:17:30,160 fact surface tension is more than adequate to keep the melt 297 00:17:30,160 --> 00:17:33,200 distributed essentially homogeneously throughout the 298 00:17:33,200 --> 00:17:35,480 sample. OK, so the melt stays within the 299 00:17:35,480 --> 00:17:38,320 sample, but how is it structured? 300 00:17:38,520 --> 00:17:42,920 Does it form in particular regions and does that give us 301 00:17:42,920 --> 00:17:46,600 any insight as what happens to melt when it occurs naturally 302 00:17:46,600 --> 00:17:49,960 100 kilometers down? It's a really interesting 303 00:17:49,960 --> 00:17:52,800 question. The first thing that happens if 304 00:17:52,800 --> 00:17:55,800 you start twisting A partially molten sample is the melt 305 00:17:55,800 --> 00:17:57,960 pockets. The melt, which sits in triple 306 00:17:57,960 --> 00:18:01,440 junctions between grains, begin to get themselves oriented, and 307 00:18:01,440 --> 00:18:04,440 they know about the stress field and about pressure gradients in 308 00:18:04,440 --> 00:18:07,280 the sample. If you start to twist the higher 309 00:18:07,280 --> 00:18:11,760 strains, typically a strain of one or higher, the melt actually 310 00:18:11,760 --> 00:18:14,760 begins to segregate. So at first the melt pockets at 311 00:18:14,760 --> 00:18:18,720 the grain scale Orient and then those melt pockets feel a 312 00:18:18,720 --> 00:18:22,040 pressure gradients develop throughout the sample and they 313 00:18:22,040 --> 00:18:25,280 melt will segregate and they'll form melt rich regions, which 314 00:18:25,280 --> 00:18:29,440 we've called melt rich bands or sheets of melt that are kind of 315 00:18:29,440 --> 00:18:32,720 separated by regions that are basically melt free. 316 00:18:32,720 --> 00:18:37,240 So you produce a band that are space maybe 100 microns apart 317 00:18:37,320 --> 00:18:41,480 and the amount of melt in those bands might reach 25% while the 318 00:18:41,480 --> 00:18:44,160 regions between the bands are actually drops way down. 319 00:18:44,160 --> 00:18:48,080 So you produce these really melt rich regions that can act as 320 00:18:48,080 --> 00:18:51,040 shear zones because they pine melt fraction regions have low 321 00:18:51,040 --> 00:18:54,440 viscosity so you can share on them easily, and they're also 322 00:18:54,440 --> 00:18:58,080 high permeability paths. And those paths may be important 323 00:18:58,280 --> 00:19:01,200 as one way in which you can remove melt from the melt source 324 00:19:01,200 --> 00:19:04,440 region and get it to volcanic structures sitting near the 325 00:19:04,440 --> 00:19:07,720 surface of the Earth. In the real Esteemosphere, we 326 00:19:07,720 --> 00:19:10,960 think there was both melt and water present. 327 00:19:11,960 --> 00:19:15,840 Can we use your results to predict what happens in those 328 00:19:15,840 --> 00:19:19,600 circumstances? We can certainly make some 329 00:19:20,040 --> 00:19:22,760 inferences from our experimental results. 330 00:19:22,760 --> 00:19:27,680 I think ultimately these results go into people who are doing 331 00:19:27,680 --> 00:19:30,720 numerical models of convection and subduction, but we can 332 00:19:30,720 --> 00:19:33,800 understand where these results might be important. 333 00:19:34,640 --> 00:19:39,120 One area that we've talked a little bit about is what happens 334 00:19:39,240 --> 00:19:43,840 beneath the mid ocean Ridge as upwelling mantle begins to melt, 335 00:19:43,840 --> 00:19:48,560 as it depressurizes and that melt moves upward, and as it 336 00:19:48,560 --> 00:19:53,080 does, the water that's in the mantle partitions into that melt 337 00:19:53,720 --> 00:19:57,080 and the melt then as it goes up buoyantly to the surface to form 338 00:19:57,080 --> 00:20:00,880 oceanic crust has to strip the mantle in that region of its 339 00:20:00,880 --> 00:20:04,880 water and therefore you should be strengthening that part of 340 00:20:04,880 --> 00:20:07,280 the mantle. It's one way in which you could 341 00:20:07,280 --> 00:20:10,880 think about forming A lithosauric plate is by having 342 00:20:10,880 --> 00:20:14,360 the melt move through the rock. Suck out all the water and leave 343 00:20:14,360 --> 00:20:17,240 behind dry rocks that are stronger than the wet rocks. 344 00:20:17,760 --> 00:20:20,160 So that's one way in which you can think about evolution of a 345 00:20:20,160 --> 00:20:23,240 lithosphere. And there's been a group in 346 00:20:23,240 --> 00:20:27,080 Japan who've looked at what happens in the oceanic 347 00:20:27,080 --> 00:20:30,080 lithosphere and seismically looking at the transition 348 00:20:30,080 --> 00:20:34,880 between the lithosphere and the athenosphere, and that interface 349 00:20:34,880 --> 00:20:38,040 is often called the lithosphere athenosphere boundary. 350 00:20:38,440 --> 00:20:40,560 And that transition is relatively sharp. 351 00:20:40,880 --> 00:20:44,480 It occurs over a distance of about 10 kilometers. 352 00:20:44,920 --> 00:20:47,680 And it's really hard to see how you can do that with just 353 00:20:47,680 --> 00:20:51,080 changing the temperature, which changes relatively gradually as 354 00:20:51,080 --> 00:20:54,440 you go down to greater depths. And they've argued that if you 355 00:20:54,440 --> 00:20:58,440 had even 1% melt in the acinosphere and you allowed it 356 00:20:58,440 --> 00:21:01,400 to segregate into these melt rich sheets that we talked 357 00:21:01,400 --> 00:21:06,760 about, that the net viscosity of that part of the mantle or the 358 00:21:06,760 --> 00:21:09,840 acinis or part of the mantle would be in fact fairly 359 00:21:09,840 --> 00:21:12,840 significantly lower than in the lithosphere above it. 360 00:21:13,120 --> 00:21:16,200 Again, just not with a large amount of melt, but with a small 361 00:21:16,200 --> 00:21:19,240 amount of melt, but where that melt is kind of localized or 362 00:21:19,240 --> 00:21:21,960 focused into these melt rich bands. 363 00:21:21,960 --> 00:21:25,080 Well, that's really interesting. So it suggests that really the 364 00:21:25,080 --> 00:21:28,920 mechanical contrast between the lithosphere and the acinosphere 365 00:21:28,920 --> 00:21:31,360 is fundamentally down to the melt fraction. 366 00:21:32,520 --> 00:21:34,480 Certainly you can make the case for that. 367 00:21:34,480 --> 00:21:36,920 There were experimental results from the lab would support that 368 00:21:36,920 --> 00:21:40,800 point of view. Again, it requires in that 369 00:21:40,800 --> 00:21:44,320 particular model that melt segregate into melt with sheets, 370 00:21:44,680 --> 00:21:47,520 which provides some areas that are really quite weak sitting 371 00:21:47,520 --> 00:21:50,240 beneath the lithosphere. I think there's still quite a 372 00:21:50,240 --> 00:21:52,680 bit of debate about exactly how this works. 373 00:21:52,880 --> 00:21:55,400 There have been a number of papers written more recently 374 00:21:55,400 --> 00:21:58,120 that kind of take a more detailed look at the rules of 375 00:21:58,120 --> 00:22:01,680 water and the rule of melt. We already alluded to the 376 00:22:02,280 --> 00:22:06,680 enormously different time scales and then of course there are 377 00:22:06,680 --> 00:22:12,040 also the space scales between your experiments as compared to 378 00:22:12,040 --> 00:22:15,400 those that prevail in the tectonic settings that we're 379 00:22:15,400 --> 00:22:19,080 trying to understand. How do we deal with that? 380 00:22:19,080 --> 00:22:24,440 And are you able to develop scaling laws that somehow rather 381 00:22:24,560 --> 00:22:29,960 enable us to get from the 24 hour centimetre scale of your 382 00:22:29,960 --> 00:22:33,960 experiments to the varsity slower millions of years and 383 00:22:33,960 --> 00:22:36,800 kilometer scale of the actual Earth? 384 00:22:37,920 --> 00:22:41,640 Well, fortunately we're not the first people to work on the 385 00:22:41,640 --> 00:22:45,000 strength of materials. There's whole fields of material 386 00:22:45,000 --> 00:22:47,920 science and engineering, for example, that has been very 387 00:22:48,320 --> 00:22:51,920 concerned with the strength of materials and not only with the 388 00:22:51,920 --> 00:22:54,440 strength of materials or the viscosity of materials. 389 00:22:55,000 --> 00:22:57,560 They also have engineering problems where you need to 390 00:22:57,560 --> 00:23:01,400 extrapolate from time scales. Not as extreme as we're 391 00:23:01,400 --> 00:23:05,160 interested in, but they might need to extrapolate from testing 392 00:23:05,320 --> 00:23:08,880 materials over a day or two, much like we do. 393 00:23:09,320 --> 00:23:12,800 And they have applications that might need to be extrapolated 394 00:23:12,800 --> 00:23:16,120 out to maybe 100 years. That might be the case in 395 00:23:16,120 --> 00:23:18,240 buildings or other kind of structures. 396 00:23:18,640 --> 00:23:22,480 Are you're testing jet engines that need to last for 30 or 40 397 00:23:22,480 --> 00:23:25,480 years. One really wants to do testing 398 00:23:25,480 --> 00:23:28,920 where you can have the scaling laws that allow you to go out to 399 00:23:28,920 --> 00:23:32,720 at least out to 30 or 40 years. So that helps a lot. 400 00:23:32,720 --> 00:23:35,960 And there's also a lot known about the details of physics of 401 00:23:35,960 --> 00:23:39,520 ductal deformation materials. We know that deformation occurs 402 00:23:39,520 --> 00:23:41,600 primarily by movement of defects. 403 00:23:42,040 --> 00:23:45,280 Those defects can be point defects like atoms and ions 404 00:23:45,280 --> 00:23:48,520 moving around, or they can be dislocations, which are line 405 00:23:48,520 --> 00:23:50,000 defects moving through the material. 406 00:23:50,560 --> 00:23:53,800 So we at least have the starting skeleton of what we're trying to 407 00:23:53,920 --> 00:23:56,920 work out in terms of constitutive relations or flow 408 00:23:56,920 --> 00:24:01,320 laws for extrapolation. And once we've done the 409 00:24:01,320 --> 00:24:04,680 extrapolation that we take our lab results and extrapolate it 410 00:24:04,720 --> 00:24:10,840 to a depth of 300 kilometers and a temperature of 1500°C, we can 411 00:24:10,840 --> 00:24:13,120 look in the Earth and find places where there are 412 00:24:13,120 --> 00:24:15,800 geophysical measurements that give us some constraints. 413 00:24:16,280 --> 00:24:19,280 People have used analysis, for example, of the geoid and 414 00:24:19,680 --> 00:24:22,920 analysis of the rate of post glacial rebound as ways of 415 00:24:22,920 --> 00:24:26,960 constraining the strain rate and calculating the stresses that 416 00:24:26,960 --> 00:24:30,320 should be applicable under those deep Earth conditions. 417 00:24:30,480 --> 00:24:34,600 And it turns out that the lab results do a pretty good job of 418 00:24:34,640 --> 00:24:37,880 predicting what, in fact, the Earth is telling us it's doing. 419 00:24:37,880 --> 00:24:41,240 Is it possible to say that we know what the viscosity is on 420 00:24:41,240 --> 00:24:44,000 geological time scales? Just say 300 kilometers depth 421 00:24:44,000 --> 00:24:51,680 and 1500°C to within a factor of 10% or a factor of 2 or just the 422 00:24:51,680 --> 00:24:54,520 right order of magnitude. Right. 423 00:24:54,520 --> 00:24:57,760 Order of magnitude might be roughly where we're at right 424 00:24:57,760 --> 00:25:00,280 now. I think these things can be 425 00:25:00,280 --> 00:25:04,480 tightened up with additional experimental work, but there are 426 00:25:04,560 --> 00:25:09,040 a lot of complications that haven't even been put into the 427 00:25:09,040 --> 00:25:12,120 experiments at this point. I've been talking about olivine, 428 00:25:12,120 --> 00:25:14,360 but the Earth is made-up of more than olivine. 429 00:25:15,080 --> 00:25:17,680 What happens if you add a little bit of parenting to that sample? 430 00:25:17,760 --> 00:25:20,320 Or what happens if you change the grain size? 431 00:25:20,320 --> 00:25:24,320 So it's a big parameter space and I would argue a real need 432 00:25:24,320 --> 00:25:27,720 for additional experiments and additional experimentalists to 433 00:25:27,720 --> 00:25:32,280 do those experiments in order to make the flow laws and 434 00:25:32,280 --> 00:25:35,400 extrapolation of those flow laws more robust than it is right 435 00:25:35,400 --> 00:25:37,240 now. I did an earlier session with 436 00:25:37,240 --> 00:25:40,560 Alan McNamara who models the deep mantle, does computer 437 00:25:40,560 --> 00:25:44,320 simulations and no doubt uses what he can from experiments 438 00:25:44,320 --> 00:25:46,200 such as yours as inputs to his model. 439 00:25:46,520 --> 00:25:50,440 And I asked him what the biggest uncertainty was in his models. 440 00:25:50,440 --> 00:25:53,280 And he said that he really thought that we didn't know the 441 00:25:53,280 --> 00:25:57,160 viscosity at depth to within several orders of magnitude 442 00:25:57,160 --> 00:26:00,520 because it's extremely non linear in the way it depends on 443 00:26:00,520 --> 00:26:03,160 these parameters. So getting down to one order of 444 00:26:03,160 --> 00:26:05,600 magnitude is something we should be extremely proud of. 445 00:26:07,520 --> 00:26:10,880 Well, I think it's quite amazing actually that we can do that. 446 00:26:10,880 --> 00:26:15,720 Well, if if you think about all the complications in the Earth 447 00:26:15,720 --> 00:26:19,040 in terms of knowing what the temperature is, the pressure, we 448 00:26:19,040 --> 00:26:23,040 can calculate quite well. But there's just a lot of trying 449 00:26:23,040 --> 00:26:26,280 to constrain what the physical parameters are in terms of rock 450 00:26:26,280 --> 00:26:29,960 composition and water content and melt fraction. 451 00:26:30,680 --> 00:26:33,680 There are a lot of variables in the system and then what's down 452 00:26:33,680 --> 00:26:36,840 there is a long way away. Let's talk a bit about some of 453 00:26:36,840 --> 00:26:39,840 the implications of your experimental results on our 454 00:26:39,840 --> 00:26:42,080 understanding of plate tectonics. 455 00:26:42,520 --> 00:26:47,120 For example, does your work help us understand whether the 456 00:26:47,120 --> 00:26:51,040 acinosphere might have the convection cells that have been 457 00:26:51,040 --> 00:26:55,200 theorized with lithospheric plates being carried along above 458 00:26:55,200 --> 00:26:59,000 them? The viscosity measurements from 459 00:26:59,080 --> 00:27:03,000 the lab do help constrain what's going on in the convection 460 00:27:03,000 --> 00:27:06,240 cells. For example, when I first 461 00:27:06,240 --> 00:27:09,720 started thinking about this problem, people were modeling 462 00:27:09,720 --> 00:27:14,440 convection as a linear viscosity and Newtonian viscosity, where 463 00:27:14,960 --> 00:27:18,000 the rate of deformation was linearly proportional to the 464 00:27:18,000 --> 00:27:20,920 stress. That has really changed due to 465 00:27:20,920 --> 00:27:24,280 the laboratory experiments which demonstrate quite clearly that 466 00:27:24,280 --> 00:27:27,560 the viscosity indeed is a function of the stress, not a 467 00:27:27,560 --> 00:27:32,040 Newtonian or linear viscosity. And I think that has entered 468 00:27:32,040 --> 00:27:36,560 into people's calculations of convection cells and had had 469 00:27:36,560 --> 00:27:40,760 some effect on how people think about modelling convection, kind 470 00:27:40,760 --> 00:27:43,240 of locally near the surface, but even at greater depths in the 471 00:27:43,240 --> 00:27:46,560 Earth. In an earlier episode, David 472 00:27:46,560 --> 00:27:50,760 Berkovici talked about how stressed mineral grains in 473 00:27:50,760 --> 00:27:55,400 oceanic lithosphere can radically weaken oceanic plates, 474 00:27:55,640 --> 00:27:59,440 which in turn could help explain how such plates can sag down 475 00:27:59,600 --> 00:28:01,840 into the mantle and start subducting. 476 00:28:02,400 --> 00:28:04,720 Does your work support this idea? 477 00:28:05,800 --> 00:28:10,040 So we've recently have undertaken experiments which we 478 00:28:10,160 --> 00:28:13,440 combined 2 minerals, still not all the complexity of the 479 00:28:13,440 --> 00:28:18,080 mandrel, but taking two minerals, olivine and paroxene 480 00:28:18,120 --> 00:28:21,560 and mixed the two together and then deformed the very large 481 00:28:21,560 --> 00:28:25,480 strain and watch the evolution of the microstructure and the 482 00:28:25,480 --> 00:28:29,120 strength of the rock as the strain increases. 483 00:28:29,160 --> 00:28:33,160 And much as was predicted, the two phases as they mixed 484 00:28:33,160 --> 00:28:37,680 together often form shear zones that are weak and these are the 485 00:28:37,680 --> 00:28:42,080 shear zones that extrapolated back to the subduction problem, 486 00:28:42,360 --> 00:28:47,120 allow the plate to bend and deform and to soften as it goes 487 00:28:47,200 --> 00:28:50,480 down deep into the mantle. So yes, there's a really direct 488 00:28:50,480 --> 00:28:53,720 connection between David Bergavici's work and what we've 489 00:28:53,720 --> 00:28:56,440 been doing. And our hope is that we're able 490 00:28:56,440 --> 00:28:59,800 to provide him with, for example, constitutive equations 491 00:28:59,800 --> 00:29:02,640 that describe the strength of the rock or the viscosity of the 492 00:29:02,640 --> 00:29:06,480 rock as a function of, for example, how much peroxine is 493 00:29:06,480 --> 00:29:10,400 present, what is the allavine to peroxine ratio, How quickly does 494 00:29:10,400 --> 00:29:12,840 the strength evolve? As you go to higher strains, 495 00:29:13,280 --> 00:29:16,120 these rocks tend to get weaker and weaker and weaker. 496 00:29:16,400 --> 00:29:20,520 Do your results on the melt fraction help us understand how 497 00:29:20,520 --> 00:29:23,520 the mantle melts and causes volcanism? 498 00:29:24,680 --> 00:29:28,160 Once the melting has occurred, our results really focus on 499 00:29:28,160 --> 00:29:31,720 telling you something about how strong the rock is and how 500 00:29:31,720 --> 00:29:35,160 rapidly you can transport melt from the source region to the 501 00:29:35,160 --> 00:29:37,400 surface. Again, if you form these melt 502 00:29:37,400 --> 00:29:41,440 rich bands, they really are high permeability bands and allow 503 00:29:41,440 --> 00:29:44,840 melt transport to occur much more quickly than it would. 504 00:29:44,840 --> 00:29:48,280 If the melt had to move through the Alabama grains like water 505 00:29:48,280 --> 00:29:50,720 moves through sands, it would move fairly slowly. 506 00:29:51,360 --> 00:29:54,360 But if you can concentrate the melt into certain regions, that 507 00:29:54,360 --> 00:29:57,360 permeability and rate of melt transport is really is much 508 00:29:57,360 --> 00:30:00,160 quicker through that was the region than through the water, 509 00:30:00,160 --> 00:30:03,560 through sand kind of analogue. Geo experimental results help us 510 00:30:03,560 --> 00:30:07,600 understand the conditions under which rocks give way suddenly 511 00:30:07,920 --> 00:30:10,440 under applied stress and trigger an earthquake. 512 00:30:11,520 --> 00:30:13,920 If you think about it in terms of depth in the Earth, we've 513 00:30:13,920 --> 00:30:18,200 mapped out the lower part of the deformation behavior and there 514 00:30:18,200 --> 00:30:20,680 have been studies that have focused totally on brittle 515 00:30:20,680 --> 00:30:24,480 behavior that have mapped the upper part near the surface as a 516 00:30:24,480 --> 00:30:27,840 function of depth. And if you take as a crude but 517 00:30:27,840 --> 00:30:31,760 first order approximation that the intersection of those two 518 00:30:31,800 --> 00:30:35,560 real logical laws, the one for brittle deformation up above and 519 00:30:35,560 --> 00:30:37,920 the one for ductal deformation down below, and look where they 520 00:30:37,920 --> 00:30:41,320 intersect, they kind of begins to get you into the right regime 521 00:30:41,320 --> 00:30:43,720 of where you have to have interaction between brittle 522 00:30:44,120 --> 00:30:46,920 inductal processes. If you look at the location 523 00:30:46,920 --> 00:30:50,320 where these two geological laws intersect each other, you're 524 00:30:50,320 --> 00:30:53,520 getting yourself into the earthquake forming region and 525 00:30:53,520 --> 00:30:55,640 then it's a matter of mapping out in detail. 526 00:30:56,000 --> 00:30:59,480 Now I get to have both brittle processes and ductal processes 527 00:30:59,480 --> 00:31:01,840 happening. And how do we interact? 528 00:31:01,840 --> 00:31:05,000 Does one stimulate the other? Does one enhance the other? 529 00:31:05,120 --> 00:31:08,240 Does one suppress the other? You begin to to know where the 530 00:31:08,240 --> 00:31:11,600 earthquakes are occurring, and the complication really is 531 00:31:11,600 --> 00:31:14,000 trying to understand those physical details at the 532 00:31:14,000 --> 00:31:19,400 microstructural level of how, in fact, does a crack forming cause 533 00:31:19,440 --> 00:31:22,680 dislocations to be activated, combining the brittle behaviour 534 00:31:22,680 --> 00:31:25,080 of cracks and the ductile behaviour produced by 535 00:31:25,080 --> 00:31:27,520 dislocations. David, Cole said. 536 00:31:27,600 --> 00:31:30,840 Thank you very much. It has been a pleasure chatting 537 00:31:30,840 --> 00:31:33,760 with you, Oliver. To see pictures and 538 00:31:33,760 --> 00:31:39,160 illustrations that support this podcast, go to geologybytes.com, 539 00:31:39,880 --> 00:31:43,040 where you'll also find transcripts and a subject matter 540 00:31:43,040 --> 00:31:47,040 index of all the episodes there. You can also give me feedback, 541 00:31:47,040 --> 00:31:50,800 which I welcome, as well as sign up to get my emails about new 542 00:31:50,800 --> 00:31:51,520 episodes.