1 00:00:05,100 --> 00:00:07,800 This is geology B with all of us trampled. 2 00:00:09,200 --> 00:00:12,600 Subduction zones are places where a slab of oceanic 3 00:00:12,600 --> 00:00:15,700 lithosphere plunges down into the mantle below. 4 00:00:16,400 --> 00:00:20,500 The slab, consists of sediments on top crustal, rocks, in the 5 00:00:20,508 --> 00:00:24,800 middle, and the lithospheric mantle on the bottom, all 6 00:00:24,800 --> 00:00:27,300 plunging down together as a kind of sandwich. 7 00:00:28,600 --> 00:00:32,299 In each of these layers is an ingredient that plays a key role 8 00:00:32,299 --> 00:00:36,600 in shaping the evolution of the earth over geological time and 9 00:00:36,600 --> 00:00:40,400 that is water. Jeff Abbas has conducted 10 00:00:40,400 --> 00:00:43,400 extensive research on water and subduction zones. 11 00:00:44,200 --> 00:00:47,800 He uses seismic observations to map, the distribution of water 12 00:00:47,800 --> 00:00:50,800 in subducting plates and in the overriding mantle. 13 00:00:51,600 --> 00:00:55,200 He couples these observations with computer-based models of 14 00:00:55,200 --> 00:00:58,100 the physics and chemistry of the subducting plates. 15 00:00:58,900 --> 00:01:01,300 He is Professor of geological scientists at Cornell 16 00:01:01,300 --> 00:01:04,099 University. Jeff abers. 17 00:01:04,099 --> 00:01:06,700 Welcome to geology B. Thank you. 18 00:01:06,800 --> 00:01:09,600 Happy to be here. I just gave a very high-level 19 00:01:09,600 --> 00:01:11,700 summary of what's inside of subducting. 20 00:01:11,700 --> 00:01:15,900 Oceanic slab before we talk about water in the slab. 21 00:01:16,100 --> 00:01:19,300 Can you flesh out the basic structure and composition of 22 00:01:19,300 --> 00:01:21,900 subducting oceanic slabs a bit more for us? 23 00:01:22,200 --> 00:01:26,300 Sure, almost everything that's adducts, as Oceanic lithosphere 24 00:01:26,400 --> 00:01:30,400 formed at a mid-ocean ridge, so that's where the story starts is 25 00:01:30,400 --> 00:01:32,800 two plates. Pull apart mantle Rises up. 26 00:01:33,100 --> 00:01:36,700 Rises above the melting temperature and some of it 27 00:01:36,700 --> 00:01:42,300 melts, those melts are basaltic in composition and they form the 28 00:01:42,300 --> 00:01:44,800 oceanic crust. It's about five six, seven 29 00:01:44,800 --> 00:01:48,900 kilometers, thick the top of those basalts reps on the 30 00:01:48,900 --> 00:01:51,600 seafloor makes these pillow basalts. 31 00:01:51,600 --> 00:01:54,400 Maybe you've seen movies of these things bubbling around 32 00:01:54,600 --> 00:01:58,100 their full of cracks and pores and they interact at very high 33 00:01:58,100 --> 00:02:01,000 temperatures with seawater. Let's see what it gets in these 34 00:02:01,000 --> 00:02:04,500 cracks and pores and react. To form alteration minerals 35 00:02:04,500 --> 00:02:07,700 minerals that have a lot of water in their structure deeper 36 00:02:07,700 --> 00:02:11,200 in the oceanic crust, the same basaltic material cools, much 37 00:02:11,200 --> 00:02:14,000 more slowly without a lot of water present as equality, 38 00:02:14,000 --> 00:02:17,200 gabbro and below. That is the mantle. 39 00:02:17,200 --> 00:02:18,700 What's left over from the melting? 40 00:02:18,800 --> 00:02:22,100 It's a peridotite rock, that's dominantly Olivine. 41 00:02:22,700 --> 00:02:26,300 So, that's the package that forms at the mid-ocean ridge as 42 00:02:26,300 --> 00:02:29,200 it moves away from the ridge, it cools and the sediments get 43 00:02:29,200 --> 00:02:31,400 deposited. They have a lot of minerals. 44 00:02:31,400 --> 00:02:34,600 I have a lot of water in there. Mineral structure things like 45 00:02:34,600 --> 00:02:37,400 Clays and so forth. How do we know what the 46 00:02:37,400 --> 00:02:39,400 structure of that oceanic plate is? 47 00:02:40,100 --> 00:02:43,600 Well, as a few different Clues, we go anywhere in the world in 48 00:02:43,600 --> 00:02:47,500 the, any ocean Basin, the depth to the seafloor and the heat 49 00:02:47,500 --> 00:02:49,000 flow, the right. The heat comes out like it's 50 00:02:49,008 --> 00:02:51,700 about the same everywhere and that's telling us that this 51 00:02:51,700 --> 00:02:54,400 basic process is very similar globally. 52 00:02:54,700 --> 00:02:56,800 The same package is getting delivered. 53 00:02:56,900 --> 00:03:00,300 The same five, six, seven kilometers of crust, the shallow 54 00:03:00,300 --> 00:03:02,300 Parts. We've drilled lots of places. 55 00:03:02,500 --> 00:03:05,800 The When drilling program samples, the sediments and the 56 00:03:05,800 --> 00:03:09,900 tops of the basalts one or two places we've drilled down to the 57 00:03:09,900 --> 00:03:12,800 top of the gabbro layers. We have not drilled through the 58 00:03:12,800 --> 00:03:15,500 Whole Crust anywhere. So mostly we know about the 59 00:03:15,500 --> 00:03:18,800 deeper parts of the section comes from exposed rocks. 60 00:03:19,000 --> 00:03:23,000 We call ophea lights, these are slivers of oceanic crust that's 61 00:03:23,000 --> 00:03:24,900 thrust on. Two continents through various 62 00:03:24,900 --> 00:03:26,200 geologic. Accidents. 63 00:03:26,900 --> 00:03:30,200 Among all of this is tied together by a tremendous amount 64 00:03:30,200 --> 00:03:32,700 of work done over the last several decades measuring the 65 00:03:32,900 --> 00:03:36,700 Dave speeds of seismic waves usually through some controlled 66 00:03:36,700 --> 00:03:40,200 Source, back in the 50s and 60s people throw, Dynamite off of 67 00:03:40,200 --> 00:03:44,700 boats, and recorded on hydrophones or seismometers at 68 00:03:44,700 --> 00:03:48,200 various distances more recently, a more controlled ways of 69 00:03:48,200 --> 00:03:51,800 creating the Sound Source. But those signals propagate for 70 00:03:51,800 --> 00:03:55,900 tens or hundreds of kilometers show, very distinct changes in 71 00:03:55,900 --> 00:03:59,200 wave speed as you go down through the sediments and the 72 00:03:59,200 --> 00:04:02,400 top of the basaltic layer in through the gabbros and then a 73 00:04:02,400 --> 00:04:06,100 big step in velocities to the faster mantle rocks. 74 00:04:06,700 --> 00:04:10,200 We can also tie these ways speeds, very carefully to 75 00:04:10,200 --> 00:04:14,100 experiments and labs done on minerals and on Rock samples to 76 00:04:14,100 --> 00:04:18,000 see what kinds of compositions correspond to what kind of way 77 00:04:18,000 --> 00:04:20,300 is speeds. And that gives us also some of 78 00:04:20,308 --> 00:04:24,700 the first hints of the effects of water water, when it reacts 79 00:04:24,700 --> 00:04:27,700 with most rocks will form other minerals. 80 00:04:27,700 --> 00:04:30,300 That have sort of the same Bowl composition, plus some water 81 00:04:30,300 --> 00:04:32,700 added, those minerals typically of slower. 82 00:04:32,900 --> 00:04:35,400 Is speeds. This gives us another tool for 83 00:04:35,400 --> 00:04:39,000 calibrating the extent of say, the alteration in the basalt. 84 00:04:39,000 --> 00:04:42,000 Or if any water gets down deeper, we can fingerprint that 85 00:04:42,000 --> 00:04:45,600 as well. Okay, so that's how we can tell 86 00:04:45,600 --> 00:04:49,500 something about what's in this lab as it forms. 87 00:04:49,500 --> 00:04:52,500 The ocean floor and moved away from the mid-ocean ridge towards 88 00:04:52,500 --> 00:04:54,400 the subduction zone. But after it, enters a 89 00:04:54,407 --> 00:04:57,900 subduction zone, and it starts to go deeper. 90 00:04:58,300 --> 00:05:01,600 Can we still get any picture of what the structure is? 91 00:05:01,600 --> 00:05:03,500 How do we look at it? Once it, It actually starts to 92 00:05:03,500 --> 00:05:06,500 dive down to any depth. Well a lot of the same tools 93 00:05:06,500 --> 00:05:10,400 exist for seeing deeper and again some of the main tools 94 00:05:10,400 --> 00:05:13,300 come from looking at seismic wave propagation. 95 00:05:13,600 --> 00:05:18,500 The speeds that waves travel at Oceana crustal section and see 96 00:05:18,700 --> 00:05:23,600 if they change say as the play heats up and minerals would hold 97 00:05:23,600 --> 00:05:25,200 water in their structure break down. 98 00:05:25,200 --> 00:05:28,400 Those are usually unstable at high temperatures and as you get 99 00:05:28,400 --> 00:05:31,600 deeper, people, sample water coming out of seeps in the 100 00:05:31,600 --> 00:05:33,100 ground, or old. Ali. 101 00:05:33,100 --> 00:05:36,500 Get to the volcanic Arc, the law visitor coming out of that to 102 00:05:36,500 --> 00:05:37,900 provide hints of what's gone down. 103 00:05:37,900 --> 00:05:40,400 Let's come back up again and this is very clear at 104 00:05:40,400 --> 00:05:43,300 Fingerprints of subduction zones of sediment. 105 00:05:43,300 --> 00:05:48,200 For example, let's talk about our theme today which is the 106 00:05:48,200 --> 00:05:52,400 geological water cycle. What do you mean when you talk 107 00:05:52,400 --> 00:05:56,200 about the geological water cycle, as distinct from the 108 00:05:56,200 --> 00:05:59,700 water cycle of the atmosphere, and oceans that most people are 109 00:05:59,700 --> 00:06:02,500 much more familiar with? Yeah, that's a great question. 110 00:06:02,800 --> 00:06:04,700 What's the water that we know about sits at the earth's 111 00:06:04,700 --> 00:06:09,100 surface in the oceans and rivers in the atmosphere in ice. 112 00:06:09,500 --> 00:06:11,800 And it moves around, you know, fairly quickly between these 113 00:06:11,800 --> 00:06:14,400 different reservoirs. But there's this other cycle 114 00:06:14,600 --> 00:06:19,100 where water is exchanged between the Surface Reservoir and the 115 00:06:19,100 --> 00:06:23,500 mantle, the Deep interior water comes out in tiny tiny 116 00:06:23,500 --> 00:06:26,300 quantities, in measured in your tens, hundreds of parts per 117 00:06:26,300 --> 00:06:30,400 million through mid ocean, ridges and hotspots, and other 118 00:06:30,900 --> 00:06:34,000 sources of volcanoes. That tap The Earth and then 119 00:06:34,000 --> 00:06:38,900 returns to the mantle primarily in subduction zones and so is 120 00:06:38,900 --> 00:06:41,700 deep water cycle and it's regulated at plate tectonics 121 00:06:41,700 --> 00:06:43,800 speed. So you know, CM tens of 122 00:06:43,800 --> 00:06:47,700 centimeters a year is a cycle between the surface water that 123 00:06:47,700 --> 00:06:50,400 we see and the water that's in the mantle. 124 00:06:50,900 --> 00:06:53,700 Okay? So can you step through what 125 00:06:53,700 --> 00:06:57,500 happens to the water in the subducting oceanic slab after it 126 00:06:57,500 --> 00:07:00,100 enters the trench? Some of the water comes out 127 00:07:00,100 --> 00:07:02,700 pretty quickly. What it's sitting in crack. 128 00:07:02,800 --> 00:07:04,600 Sand, pours. In the shallow part of the 129 00:07:04,600 --> 00:07:08,000 crust, gets squeezed in the upper few kilometers as it's 130 00:07:08,000 --> 00:07:11,000 going down and you can see that coming out and seeps and 131 00:07:11,000 --> 00:07:14,900 trenches. But then a lot of the water is 132 00:07:14,900 --> 00:07:18,500 bound in minerals that are attached to the downgoing plate 133 00:07:18,600 --> 00:07:21,800 and stays in them until these minerals reach conditions where 134 00:07:21,800 --> 00:07:25,000 they're unstable and this is true of nearly all hydrous, 135 00:07:25,000 --> 00:07:28,000 minerals, beat them up enough and you squeeze them enough 136 00:07:28,200 --> 00:07:31,500 water is released the big Flex of that water coming out that we 137 00:07:31,500 --> 00:07:35,200 know about is To the mantle sitting underneath volcanic 138 00:07:35,200 --> 00:07:37,300 arcs. And so that a lot of them will 139 00:07:37,300 --> 00:07:40,600 come out in volcanoes Arc, volcanoes have a few weight 140 00:07:40,600 --> 00:07:44,400 percent water in the Magma's that are coming out of the 141 00:07:44,400 --> 00:07:47,900 mantle that feed them. Those two things would get 142 00:07:47,900 --> 00:07:50,300 squeezed out of the trench comes on, the are probably cancer 143 00:07:50,300 --> 00:07:52,700 about two-thirds of the water coming into the trench. 144 00:07:52,900 --> 00:07:56,700 The rest of it seems to be able to stay in the downgoing slab to 145 00:07:56,700 --> 00:08:00,000 go into the deeper parts of the mantle where it can feed much 146 00:08:00,000 --> 00:08:02,700 longer scale, geologic, processes long term. 147 00:08:02,800 --> 00:08:05,000 Water cycle. Can you connect the dots a bit 148 00:08:05,000 --> 00:08:08,900 between the release of water from this downgoing? 149 00:08:08,900 --> 00:08:12,700 Subducting slab and the water that spewing out into the 150 00:08:12,700 --> 00:08:15,100 atmosphere from a volcanic eruption? 151 00:08:15,600 --> 00:08:17,200 Yeah. So plate. 152 00:08:17,200 --> 00:08:20,100 That's the sending the top of it is the stuff that has the 153 00:08:20,100 --> 00:08:24,200 sediments and that Oceanic basalts that have a lot of water 154 00:08:24,200 --> 00:08:27,000 down in it. That top surface at some point 155 00:08:27,000 --> 00:08:30,700 is going to encounter the hot flowing part of the mantle, he 156 00:08:30,700 --> 00:08:35,299 called the mantle wedge where Hot material is coming in and 157 00:08:35,299 --> 00:08:39,200 getting dragged down by the plate, as it starts to heat up 158 00:08:39,500 --> 00:08:41,900 the minerals that hold the water in. 159 00:08:41,900 --> 00:08:45,200 It will start to break down. That water is released that 160 00:08:45,200 --> 00:08:48,400 water then because it's much more buoyant Rises. 161 00:08:48,400 --> 00:08:52,900 And we call flexes the mantle up above at adding, a little bit of 162 00:08:52,900 --> 00:08:56,400 water, to a hot, rock will lower its melting temperature. 163 00:08:56,600 --> 00:08:59,400 And so the idea is that if it's hot enough, you get enough water 164 00:08:59,400 --> 00:09:02,700 and they'll actually trigger melting in the mantle above the 165 00:09:02,800 --> 00:09:06,700 Uh down going plate, then those melts then rise more or less 166 00:09:06,700 --> 00:09:09,500 vertically to form the volcanic Arc so we see all around the 167 00:09:09,508 --> 00:09:14,000 world and this is a key piece that ties subduction of oceanic 168 00:09:14,000 --> 00:09:17,000 plates going into the trench with this parallel chain of 169 00:09:17,000 --> 00:09:19,900 volcanoes that we see everywhere around the world in my 170 00:09:19,900 --> 00:09:21,900 introduction. I said that you couple your 171 00:09:21,900 --> 00:09:25,100 seismic observations with computer-based models of the 172 00:09:25,100 --> 00:09:27,400 physics and chemistry of subducting plates. 173 00:09:28,100 --> 00:09:31,900 What exactly do the models? Try to replicate my mean, 174 00:09:31,900 --> 00:09:34,200 colleague in this. Peter Van Kirk, and the Carnegie 175 00:09:34,200 --> 00:09:36,800 Institution is doing all the computer modeling. 176 00:09:37,200 --> 00:09:40,800 What we've been working on for the last 10 or 20 years has been 177 00:09:41,300 --> 00:09:45,800 doing largely two dimensional models of the thermal structure 178 00:09:45,800 --> 00:09:51,200 and flow fields of different sections of subduction zones 179 00:09:51,200 --> 00:09:55,100 around the world where we try to pattern inputs and geometries as 180 00:09:55,100 --> 00:09:57,800 closely as we can to real places. 181 00:09:58,300 --> 00:10:01,700 And then the few different things we can do one is is to 182 00:10:01,700 --> 00:10:04,800 make them predictions of the thermal structure of the Arctic 183 00:10:04,800 --> 00:10:08,800 temperatures. But then we put in a model of 184 00:10:08,800 --> 00:10:12,100 the rock composition of the mineralogy and petrology. 185 00:10:12,400 --> 00:10:16,500 Look at what minerals are stable, where given those 186 00:10:16,500 --> 00:10:19,300 temperatures and pressures. How much water they hold? 187 00:10:19,400 --> 00:10:23,000 And then make predictions from that of where we would expect 188 00:10:23,000 --> 00:10:25,600 water to be released at different depths both. 189 00:10:25,900 --> 00:10:27,700 Going down the surface of the slab. 190 00:10:27,900 --> 00:10:31,300 And then within the plate. So the model is then give us 191 00:10:31,500 --> 00:10:36,000 predictions of water, storage and water releases well and 192 00:10:36,100 --> 00:10:39,500 different subduction zones. I'm what do the results predict. 193 00:10:39,600 --> 00:10:42,600 There's a few different things at shallow depths, because 194 00:10:42,600 --> 00:10:46,100 you're being very cold, Oceanic lithosphere fairly quickly 195 00:10:46,100 --> 00:10:49,300 underneath some overriding upper plate, you're effectively 196 00:10:49,300 --> 00:10:52,200 cooling, it sort of refrigerating upper plate, and 197 00:10:52,200 --> 00:10:56,000 so temperatures, inside the down, going play, can stay 198 00:10:56,000 --> 00:10:59,600 pretty cold for several. Tens of kilometers depth at 199 00:10:59,600 --> 00:11:04,600 least probably around 70 or 80 kilometers this plate, starts to 200 00:11:04,600 --> 00:11:08,600 encounter the hot flowing part of the wedge as it goes down. 201 00:11:08,700 --> 00:11:12,100 The deeper Parts, drag down through this kissflow. 202 00:11:12,100 --> 00:11:15,800 The mantle and then hot mantle has to come in to replace it. 203 00:11:16,200 --> 00:11:20,000 So the first thing that we see is that at this depth of around 204 00:11:20,000 --> 00:11:25,300 70, 80 km, the top surface of the plate, undergoes, a fairly 205 00:11:25,300 --> 00:11:30,400 rapid heating pulse that Eating pulses, big enough that we think 206 00:11:30,400 --> 00:11:35,500 in most cases the water that's Bound in minerals in subducting 207 00:11:35,500 --> 00:11:39,400 sediment and probably the top of the oceanic crust is released 208 00:11:39,400 --> 00:11:40,700 there. And that's really the water 209 00:11:40,700 --> 00:11:43,800 that's available to feed. The oceanic volcanic Arc, the 210 00:11:43,808 --> 00:11:46,500 volcanic Arc is just you know the slabs a little bit deeper, 211 00:11:46,500 --> 00:11:50,300 maybe 100 110 120 kilometers deep when the volcanoes appear. 212 00:11:50,600 --> 00:11:53,300 So we think there's is there tight connection between this 213 00:11:53,300 --> 00:11:57,200 thermal structure and where the volcanic Arc is the other thing 214 00:11:57,200 --> 00:12:01,500 that we see Is that if water were to get deeper into the 215 00:12:01,500 --> 00:12:04,300 downgoing plate, and there is some hints to seaward of 216 00:12:04,300 --> 00:12:07,800 trenches at this happens. Water gets through the oceanic 217 00:12:07,800 --> 00:12:12,000 crust into the oceanic mantle that water can stay Bound in 218 00:12:12,000 --> 00:12:15,800 minerals like serpentines for quite a ways deeper than the 219 00:12:15,800 --> 00:12:17,300 plate. At those depths doesn't 220 00:12:17,300 --> 00:12:20,400 necessarily heat up to the point where serpentines breakdown 221 00:12:20,400 --> 00:12:23,800 that's temperatures like 600 and 650 degrees C. 222 00:12:24,300 --> 00:12:28,800 And so that's a pathway for water to bypass the volcanic Arc 223 00:12:28,900 --> 00:12:32,500 and go into the deeper part of the Earth where it can be stored 224 00:12:32,500 --> 00:12:37,000 for much longer, okay? So the models help you predict 225 00:12:37,100 --> 00:12:40,600 how much of it gets recycled out of fatty shot of depth and then 226 00:12:40,600 --> 00:12:44,400 goes to feed the volcanoes and spews out in the atmosphere 227 00:12:44,400 --> 00:12:46,700 again. And how much of it might 228 00:12:46,700 --> 00:12:49,100 actually just stick around in the slab for long enough in the 229 00:12:49,100 --> 00:12:52,200 form of minerals like 17 and then actually wind up going into 230 00:12:52,200 --> 00:12:55,200 the mantle. Yeah, one thing that we see 231 00:12:55,200 --> 00:12:58,900 though is that given the range of Ages Incoming plate which 232 00:12:58,900 --> 00:13:01,900 have to do with sort of how thick the cold part is and how 233 00:13:01,900 --> 00:13:04,300 long it takes to heat up how fast they're descending. 234 00:13:04,300 --> 00:13:08,200 The geometries is actually an incredible variability. 235 00:13:08,600 --> 00:13:13,100 In some subduction zones, the water boils off very quickly is 236 00:13:13,100 --> 00:13:16,600 really not much room for water to stay in the subducting plate. 237 00:13:16,800 --> 00:13:20,100 Past the volcanic Arc. This would include places like 238 00:13:20,100 --> 00:13:23,000 say where the Juan de Fuca plate off of Oregon. 239 00:13:23,000 --> 00:13:25,200 Washington. British, Columbia's subducting 240 00:13:25,400 --> 00:13:29,300 underneath the Cascades there. The Late is maybe only 8 million 241 00:13:29,300 --> 00:13:32,600 years old when it hits the trench and it's descending, very 242 00:13:32,600 --> 00:13:34,700 slowly. So we think they're a water goes 243 00:13:34,700 --> 00:13:37,600 off very quickly. Other places like Tonga, like 244 00:13:37,600 --> 00:13:39,600 Northern Japan, like the Marianas. 245 00:13:40,000 --> 00:13:42,700 He's very old plates subducting very quickly. 246 00:13:43,100 --> 00:13:45,900 The Interiors of those places to can stay cold, for hundreds of 247 00:13:45,900 --> 00:13:50,000 kilometers and so those becomes a highway is for water to go to 248 00:13:50,000 --> 00:13:53,100 the deep Earth. When we add this up globally, 249 00:13:53,100 --> 00:13:56,500 right now, at present rates, probably about a third of the 250 00:13:56,500 --> 00:13:59,800 water that's hitting trenches, is making it past the volcanic 251 00:13:59,800 --> 00:14:02,800 Arc into the deeper through these kinds of processes. 252 00:14:03,200 --> 00:14:06,300 And to get that average, two thuds boils off. 253 00:14:06,300 --> 00:14:10,000 If you like button goes back up and one-third goes deeply, you 254 00:14:10,000 --> 00:14:13,200 actually put real-world parameters for the temperatures 255 00:14:13,200 --> 00:14:16,200 and the thicknesses. How many subduction zones did 256 00:14:16,200 --> 00:14:20,500 you put in parameters for all? We've gone around the world to 257 00:14:20,500 --> 00:14:24,700 serve 55. 60 segments of subduction zones trying to look 258 00:14:24,700 --> 00:14:28,100 at every 500 ish kilometers of Arc. 259 00:14:28,100 --> 00:14:31,500 Segment around the world where we think we know the geometry 260 00:14:31,500 --> 00:14:34,500 and the subduction rate and something about how much 261 00:14:34,500 --> 00:14:38,400 sediments actually going in. I should say, 1/3 2/3 number. 262 00:14:38,600 --> 00:14:40,900 There's a lot of uncertainty were assuming a certain amount 263 00:14:40,900 --> 00:14:44,500 of water is coming in. The mantle of the incoming 264 00:14:44,500 --> 00:14:47,000 plate. We've seen in a few places. 265 00:14:47,100 --> 00:14:50,600 First place was off of Nicaragua and we've seen since that awful 266 00:14:50,600 --> 00:14:53,600 Parts of Alaska De and some other subduction zones where it 267 00:14:53,600 --> 00:14:58,000 looks like as the plate starts to bend for it. 268 00:14:58,000 --> 00:15:02,200 Reaches the trench sort of 50, 100 km seaward, of the trench 269 00:15:02,200 --> 00:15:05,800 that bending makes faults, that seem to act as conduits for 270 00:15:05,800 --> 00:15:10,000 water to get into the mantle. And so we see seismic waves 271 00:15:10,000 --> 00:15:12,900 speeds of the very top of the mantle, I should start to slow 272 00:15:12,900 --> 00:15:15,700 down before the plate. Hits the trench. 273 00:15:15,700 --> 00:15:19,500 We think that's a fingerprint of water, getting into the mantle 274 00:15:19,700 --> 00:15:22,800 reacting to form serpentines. Probably but we don't really 275 00:15:22,800 --> 00:15:25,900 know how deep that goes. How many places that goes is 276 00:15:25,900 --> 00:15:29,400 clearly places that does not happen and because the mantle 277 00:15:29,400 --> 00:15:32,600 part of the system is very big also because those minerals hold 278 00:15:32,600 --> 00:15:36,400 a lot of water, a serpentine old something like fourteen, weight 279 00:15:36,400 --> 00:15:40,600 percent water by mass, not really knowing how much water is 280 00:15:40,600 --> 00:15:42,800 in that. Subducting mantle gives a really 281 00:15:42,800 --> 00:15:47,000 big uncertainty into the system. Now that you've got this model, 282 00:15:47,000 --> 00:15:49,100 how can you? So cool ground truth. 283 00:15:49,100 --> 00:15:51,300 It two main ways we've been doing. 284 00:15:51,400 --> 00:15:55,600 This one observation that we see is a layer that looks like 285 00:15:55,600 --> 00:15:58,500 oceanic crust that has waves speeds. 286 00:15:58,800 --> 00:16:01,800 They look similar to those that we see on the seafloor. 287 00:16:01,800 --> 00:16:05,200 But underneath our 600 kilometers down. 288 00:16:05,500 --> 00:16:08,000 And then this goes away. When we think we're is going 289 00:16:08,000 --> 00:16:13,600 away is where these dehydration reactions occur, where the crust 290 00:16:13,600 --> 00:16:16,600 reacts to form a much denser higher speed, minerals, and we 291 00:16:16,600 --> 00:16:19,100 made these observations using a few different kinds of seismic 292 00:16:19,100 --> 00:16:23,000 waves in thin layers. So we I think we can see the 293 00:16:23,000 --> 00:16:25,600 depth distribution of water release, for instance, in 294 00:16:25,600 --> 00:16:27,600 Cascadia, which is a very hot place. 295 00:16:27,600 --> 00:16:30,800 We think we see this happening. Much shallower depths than we do 296 00:16:30,800 --> 00:16:34,800 say under Alaska, where the incoming plate is colder, 297 00:16:34,800 --> 00:16:38,500 subducting faster, and able to keep the water in the plate Down 298 00:16:38,500 --> 00:16:40,500 Deeper. The second test. 299 00:16:40,500 --> 00:16:44,000 We've been able to do is looking not at the speed of the ways but 300 00:16:44,000 --> 00:16:48,000 sort of the amplitudes of the waves are more precisely how the 301 00:16:48,000 --> 00:16:51,000 energy in the waves being absorbed as it travels through 302 00:16:51,000 --> 00:16:57,000 the Rock the absorption because seismic attenuation is fairly 303 00:16:57,000 --> 00:17:01,200 good function of temperature. As the temperature is especially 304 00:17:01,200 --> 00:17:04,900 approached the melting temperature of the rocks and 305 00:17:04,900 --> 00:17:08,500 what we observed is waves, propagate very efficiently, very 306 00:17:08,500 --> 00:17:11,900 little absorption in the downgoing plate and in the upper 307 00:17:11,900 --> 00:17:16,000 plate, when the diamond plate is less than about 70 or 80 km deep 308 00:17:16,200 --> 00:17:20,400 and very quickly, underneath the volcanic Arc centers is very 309 00:17:20,400 --> 00:17:23,099 high attenuation. Zone where we treat that too 310 00:17:23,099 --> 00:17:26,000 much higher temperatures. So this gives us a little bit 311 00:17:26,000 --> 00:17:29,400 more of a direct handle of testing, the thermal models 312 00:17:29,400 --> 00:17:32,900 themselves that are underlying the mineralogy, then the water 313 00:17:32,900 --> 00:17:35,400 release. We also, look at the depth of 314 00:17:35,400 --> 00:17:38,700 earthquakes, turns out a lot of earthquakes inside these down 315 00:17:38,700 --> 00:17:40,400 going plates. In fact, this is how they were 316 00:17:40,400 --> 00:17:43,100 first recognized. There's a lot of arguments to 317 00:17:43,100 --> 00:17:45,900 suggest the earthquakes are probably in some way due to the 318 00:17:45,908 --> 00:17:48,400 water. Release, either directly or 319 00:17:48,400 --> 00:17:50,900 maybe indirectly through increase in Pore pressure 320 00:17:50,900 --> 00:17:53,200 because the Water is suddenly released in the system. 321 00:17:53,500 --> 00:17:55,900 We've also seen good correlations between the depths 322 00:17:56,200 --> 00:17:59,500 and the geometry of where earthquakes are and where the 323 00:17:59,500 --> 00:18:01,800 thermal models predict the water really should happen. 324 00:18:02,500 --> 00:18:05,800 So when you make this comparison, then for your 325 00:18:05,800 --> 00:18:08,800 population of 50, odd, subduction zones. 326 00:18:09,000 --> 00:18:13,100 Generally speaking, you're able to reproduce the seismic wave 327 00:18:13,100 --> 00:18:15,900 speeds and the attenuation, that's observed by using 328 00:18:15,900 --> 00:18:18,000 reasonable parameters. The reason I'm pushing on this 329 00:18:18,000 --> 00:18:20,500 is there are those people who very skeptical of models that 330 00:18:20,500 --> 00:18:22,900 you have enough. So whatever you see, you can put 331 00:18:22,900 --> 00:18:24,900 together parameter set that will predict that. 332 00:18:24,900 --> 00:18:26,900 And then the question is, have you learned anything? 333 00:18:27,300 --> 00:18:31,100 Yes, as a great Point, our philosophy is been to try to 334 00:18:31,100 --> 00:18:35,000 keep the modeling. No more complicated than it. 335 00:18:35,000 --> 00:18:37,700 Absolutely has to be to explain these observations. 336 00:18:37,700 --> 00:18:41,200 So keeping this in two Dimensions were tying, the 337 00:18:41,200 --> 00:18:44,700 geometries things we know about, we can see from the seismicity 338 00:18:45,000 --> 00:18:46,900 where the plates go. We know how fast the plates go. 339 00:18:46,900 --> 00:18:48,800 We know a lot about the upper plates. 340 00:18:49,100 --> 00:18:51,200 There are these complicated transitions. 341 00:18:51,400 --> 00:18:55,800 The shallow part of the system is essentially a fault with one 342 00:18:55,900 --> 00:18:59,100 rigid plate, going underneath, another one, as you get deeper 343 00:18:59,100 --> 00:19:02,500 as things, warm up that fall transitions into probably a 344 00:19:02,508 --> 00:19:06,100 ductile Shear Zone. We see these things Exposed on 345 00:19:06,100 --> 00:19:10,100 Earth everywhere. We see deep rocks exposed and 346 00:19:10,100 --> 00:19:14,600 then the transitions to this hot flowing viscous mantle wedge. 347 00:19:15,300 --> 00:19:19,900 These kinds of transitions are very hard to replicate without 348 00:19:19,900 --> 00:19:23,700 at least a little bit. Of numerical equation solving. 349 00:19:24,200 --> 00:19:27,300 You could make different decisions but I don't think you 350 00:19:27,300 --> 00:19:30,300 could replicate this whole Suite of observations were talking 351 00:19:30,300 --> 00:19:33,000 about. But purely, simple analytical 352 00:19:33,000 --> 00:19:35,600 models to get there. You can certainly make model 353 00:19:35,600 --> 00:19:38,200 more complicated and there are people who do their people that 354 00:19:38,700 --> 00:19:41,300 make up all sorts of other processes of blobs that fly 355 00:19:41,300 --> 00:19:45,000 around and those, I suppose, help you imagine what might be 356 00:19:45,000 --> 00:19:48,000 down there. But I feel like we are at a 357 00:19:48,000 --> 00:19:50,900 point where these models are just complicated enough, can 358 00:19:51,000 --> 00:19:53,000 capture Sure. The essence of the first order 359 00:19:53,000 --> 00:19:55,300 things that are happening here. Let's come back to the 360 00:19:55,300 --> 00:19:58,700 geological water cycle. What are we talking about? 361 00:19:58,700 --> 00:20:02,100 In terms of quantities of water that we can subtract over 362 00:20:02,100 --> 00:20:04,400 geological time? Yeah, well of course this is a 363 00:20:04,400 --> 00:20:06,200 slow process. This isn't something that 364 00:20:06,200 --> 00:20:09,200 happens overnight. So our numbers suggest that at 365 00:20:09,200 --> 00:20:13,600 the trench we're seeing about the Notions worth of water 366 00:20:13,600 --> 00:20:19,400 subducting in 1 billion, maybe 1.3 billion years assuming 367 00:20:19,400 --> 00:20:23,500 everything stays steady. With today's rates, which is a 368 00:20:23,500 --> 00:20:26,400 big. If if two-thirds of this is 369 00:20:26,400 --> 00:20:30,700 coming out in arcs and for arcs, that means over the age of the 370 00:20:30,700 --> 00:20:32,700 Earth. We've probably selected one 371 00:20:32,700 --> 00:20:35,200 oceans worth of water into the mantle. 372 00:20:35,500 --> 00:20:37,200 Wow. So is that more water in the 373 00:20:37,200 --> 00:20:41,500 mantle normal water? In the oceans, is probably more 374 00:20:41,500 --> 00:20:45,400 water in the mantle than in the oceans right now. 375 00:20:45,800 --> 00:20:49,300 There are things that we know from the very small amounts of 376 00:20:49,300 --> 00:20:51,200 water that do come out at mid-ocean ridge. 377 00:20:51,300 --> 00:20:56,000 Judges tell us about the water and the uppermost mantle is more 378 00:20:56,000 --> 00:20:58,800 water that comes out and hot spots like Hawaii. 379 00:20:59,300 --> 00:21:02,700 These give us some Clues and there's a bunch of other 380 00:21:02,700 --> 00:21:06,700 geochemical arguments but numbers that Jake has to look at 381 00:21:06,700 --> 00:21:10,300 these director, indirect proxies for melting are at somewhere 382 00:21:10,300 --> 00:21:15,300 between one and five or ten. Oceans worth of water are 383 00:21:15,300 --> 00:21:18,000 sitting somewhere in the mantle, where exactly. 384 00:21:18,000 --> 00:21:21,200 That's it seems good question. A lot of water. 385 00:21:21,400 --> 00:21:24,900 Probably sits not in these hydrous minerals things like 386 00:21:24,900 --> 00:21:27,000 Mike has and Clay's and serpentines. 387 00:21:27,600 --> 00:21:31,500 But as hydrogen as defects and nominally anhydrous minerals, 388 00:21:31,800 --> 00:21:34,100 you can hold it. High pressure, is a fair bit of 389 00:21:34,400 --> 00:21:37,600 hydrogen, say in and Olivine, crystalline structure, the 390 00:21:37,600 --> 00:21:42,800 transition zone between about 410 and 670 km depth. 391 00:21:43,100 --> 00:21:45,800 As minerals, that seem to be very good at holding a lot of 392 00:21:45,800 --> 00:21:48,600 water in their structure. So a lot of people think that's 393 00:21:48,800 --> 00:21:52,600 probably the biggest sponge and Mantle to hold a lot of this 394 00:21:52,600 --> 00:21:56,400 water just to clarify what you said earlier about observational 395 00:21:56,400 --> 00:21:58,600 sampling of the matter, you said, at mid-ocean ridges. 396 00:21:58,600 --> 00:22:01,200 We see the water that might be coming from relatively shallow 397 00:22:01,200 --> 00:22:04,000 in the mantle, because that's where we think the material 398 00:22:04,000 --> 00:22:07,600 comes from. But it hot spots, we're sampling 399 00:22:07,600 --> 00:22:10,400 much deeper mantle potentially According to some people all the 400 00:22:10,400 --> 00:22:11,900 way down to the core-mantle boundary. 401 00:22:11,900 --> 00:22:14,500 So, it was that the point, you are making that we can actually 402 00:22:14,500 --> 00:22:18,100 look at these two different geological structures and we can 403 00:22:18,100 --> 00:22:21,200 therefore get a handle on. What water might be in there. 404 00:22:21,400 --> 00:22:24,900 Hello, and the Deep mantle, that's right in mid-ocean ridge, 405 00:22:24,900 --> 00:22:30,000 basalts around the global Ridge system, somewhere between 50 and 406 00:22:30,000 --> 00:22:34,600 200 parts per million by weight of those rocks, are probably 407 00:22:34,600 --> 00:22:36,600 water in the law, visit come out. 408 00:22:36,700 --> 00:22:39,000 And that tells us about the water in the source region. 409 00:22:39,300 --> 00:22:44,300 There seems to be more in hot spots and this is one of the 410 00:22:44,300 --> 00:22:47,900 lines of evidence that people have used to suggest that the 411 00:22:47,900 --> 00:22:51,000 hotspots source region may be over geologic time. 412 00:22:51,300 --> 00:22:54,100 Fed by subduction because we're bringing all this water down and 413 00:22:54,100 --> 00:22:57,300 subduction zones seems to be one of the main conveyors is, of 414 00:22:57,300 --> 00:22:59,700 course debate about this. Because these are very deep 415 00:22:59,700 --> 00:23:03,600 processes that are very hard to fingerprint, but that's at least 416 00:23:03,600 --> 00:23:08,200 a self-consistent model for this very long-term geological water 417 00:23:08,200 --> 00:23:12,000 cycle and the return flow. I'm intrigued by the fact that I 418 00:23:12,008 --> 00:23:15,200 would you logical time, you're talking about subducting and 419 00:23:15,200 --> 00:23:19,000 essentially locking up in the mantle of reservoir of the order 420 00:23:19,000 --> 00:23:21,100 of the same amount of water that we have in the oceans today. 421 00:23:21,300 --> 00:23:25,000 Would it be the case that if plate tectonics was just a bit 422 00:23:25,000 --> 00:23:27,300 more? Vigorous we might have subducted 423 00:23:27,300 --> 00:23:29,500 away the whole ocean. We might have a totally dry 424 00:23:29,500 --> 00:23:31,500 Planet. That's an interesting question. 425 00:23:31,500 --> 00:23:36,400 What happens as you go back in geologic, time or even slightly 426 00:23:36,400 --> 00:23:40,700 different scenarios in terms of what plates are subducting. 427 00:23:40,700 --> 00:23:44,900 When right now, we're in a setting where plates are on 428 00:23:44,900 --> 00:23:49,000 average older than they were 100 million years ago when they were 429 00:23:49,008 --> 00:23:51,200 subjecting, just because of how countenance of moved around, 430 00:23:51,300 --> 00:23:53,200 Round and how ocean basins have formed. 431 00:23:53,600 --> 00:23:56,600 So, even over a couple hundred million years, there may be some 432 00:23:56,600 --> 00:23:59,600 pretty big variations in. How much water is getting into 433 00:23:59,600 --> 00:24:02,000 the mantle. There are a bunch of competing 434 00:24:02,000 --> 00:24:05,600 effects. The so if you subtract things 435 00:24:05,600 --> 00:24:09,300 faster, your of course, bringing more water into the system, but 436 00:24:09,300 --> 00:24:14,000 you're also then spreading at ridges faster and the plates 437 00:24:14,000 --> 00:24:16,400 that hit, the trenches are younger. 438 00:24:16,800 --> 00:24:19,500 So they're going to be hotter and if they're hotter, there's 439 00:24:19,500 --> 00:24:21,800 less room in them to hold the Water. 440 00:24:22,200 --> 00:24:25,000 So they may start with less water going in, so it's not 441 00:24:25,000 --> 00:24:27,300 really clear. If just speeding up plate, 442 00:24:27,300 --> 00:24:31,100 tectonics is going to increase or decrease this. 443 00:24:31,100 --> 00:24:34,400 Net water flux secular cooling of the earth. 444 00:24:34,400 --> 00:24:36,700 Over geologic. Time is also another important 445 00:24:36,700 --> 00:24:39,900 factor, that's coupled to how fast plate tectonics Works. 446 00:24:40,400 --> 00:24:43,000 Some people have done this modelling suggests that the 447 00:24:43,008 --> 00:24:46,500 ability of the subducting mantle lithosphere to hold water and 448 00:24:46,500 --> 00:24:50,800 serpentines, again, has to be colder than about 600 degrees C. 449 00:24:51,200 --> 00:24:54,600 Before it's abducts, this is a relatively recent phenomenon, 450 00:24:54,600 --> 00:24:58,000 over the four and a half billion years of Earth history and so 451 00:24:58,000 --> 00:24:59,400 wasn't really, until that started. 452 00:24:59,400 --> 00:25:01,300 You start to drag down a lot of water. 453 00:25:02,000 --> 00:25:06,600 The key observation here though, is from what we can tell sea. 454 00:25:06,600 --> 00:25:12,100 Level hasn't really vary a lot, maybe tens, maybe a couple 455 00:25:12,100 --> 00:25:15,300 hundred meters over as much of geologic time, as we can tell 456 00:25:15,300 --> 00:25:18,400 anything ocean basins are about four kilometers deep. 457 00:25:18,400 --> 00:25:23,200 So, on average, this Hasn't radically changed. 458 00:25:23,600 --> 00:25:26,400 Interesting question is whether we actually have a stable 459 00:25:26,400 --> 00:25:30,300 equilibrium that? Yeah, I mean, it's hard to think 460 00:25:30,300 --> 00:25:33,000 of what a proper feedback mechanism would be. 461 00:25:33,300 --> 00:25:36,300 There's a lot of feedback mechanisms in the surface water 462 00:25:36,300 --> 00:25:40,000 cycle, that help regulate various parts of it. 463 00:25:40,600 --> 00:25:44,500 The very slow rates of things going into the mantle and what 464 00:25:44,500 --> 00:25:47,100 happens at subduction zones versus what's happening at the 465 00:25:47,100 --> 00:25:52,500 outgassing at ridges and plumes. It's Not so obvious. 466 00:25:52,500 --> 00:25:54,600 Why? Or how these things could talk 467 00:25:54,600 --> 00:25:57,400 to each other? And there could well, be an 468 00:25:57,400 --> 00:25:59,600 imbalance over time. That is changing. 469 00:25:59,600 --> 00:26:02,800 We just don't know this. So I think I'd be a little bit 470 00:26:02,800 --> 00:26:06,300 cautious thinking, there's actually a feedback mechanism in 471 00:26:06,300 --> 00:26:09,100 place, although maybe it's there and we just haven't figured out 472 00:26:09,200 --> 00:26:10,900 how that could work. What are you working on? 473 00:26:10,900 --> 00:26:13,900 At the moment lately, I've been very interested in taking the 474 00:26:13,900 --> 00:26:16,300 same kinds of approaches. We've been using to look at this 475 00:26:16,300 --> 00:26:19,500 sort of larger scale water cycle to look more closely at the 476 00:26:19,500 --> 00:26:24,900 volcanoes themselves. And thinking about, can we see 477 00:26:24,900 --> 00:26:29,600 better the Magma's where they actually form in the mantle that 478 00:26:29,600 --> 00:26:32,200 flux through the crust, to create the volcanic? 479 00:26:32,200 --> 00:26:34,200 Eruptions does this process happen? 480 00:26:34,200 --> 00:26:38,000 Steadily or episodically are their observations that you can 481 00:26:38,000 --> 00:26:41,600 make in the deeper parts of the crust or of the mantle that more 482 00:26:41,600 --> 00:26:44,300 directly tied to eruptive Cycles in volcanoes. 483 00:26:44,700 --> 00:26:48,700 We use these kinds of methods and tools deeper seismic Imaging 484 00:26:48,700 --> 00:26:52,300 to better predict volcano Behavior but Could be extremely 485 00:26:52,300 --> 00:26:55,600 useful if it gets as far as even Hazard prediction. 486 00:26:55,900 --> 00:26:57,800 Yeah, that's a lot of the motivation. 487 00:26:57,800 --> 00:27:00,700 As we understanding how these bigger systems work, then can we 488 00:27:00,700 --> 00:27:05,200 use that understanding and some more useful ways to worry about 489 00:27:05,200 --> 00:27:08,300 volcanic hazards? Understand also better things 490 00:27:08,300 --> 00:27:12,600 like or affirmation other, aspects of things that volcanoes 491 00:27:12,600 --> 00:27:16,000 do that are maybe a little bit more directly applicable to 492 00:27:16,100 --> 00:27:18,700 human existence. Jeff Abus. 493 00:27:18,700 --> 00:27:20,300 Thank you very much. Thank you. 494 00:27:20,300 --> 00:27:33,300 It's been a wonderful chatting. For more about geology b, as 495 00:27:33,300 --> 00:27:36,300 well as pictures and illustrations, that support this 496 00:27:36,300 --> 00:27:39,400 podcast, go to geology B.com.