WEBVTT Kind: captions Language: en 00:00:00.840 --> 00:00:04.470 Good morning, everybody. I think we’ll get going here. 00:00:04.470 --> 00:00:05.970 Couple of announcements. 00:00:05.970 --> 00:00:10.490 This Wednesday at our normal time, the Earthquake Science Center 00:00:10.490 --> 00:00:14.610 will host Jay Patton from Humboldt State University. 00:00:14.610 --> 00:00:19.270 He’ll present a talk titled Sedimentary and Geophysical 00:00:19.270 --> 00:00:24.279 Investigations Offshore Sumatra – Riddling the Seafloor for Evidence 00:00:24.279 --> 00:00:29.140 of Earthquakes and Submarine Landslides. So please join us for that. 00:00:29.140 --> 00:00:34.280 Today, for this special joint seminar, we are joined by Meredith Townsend. 00:00:34.280 --> 00:00:38.100 Meredith is a Ph.D. candidate at Stanford University 00:00:38.100 --> 00:00:41.829 in the geological and environmental sciences department. 00:00:41.829 --> 00:00:45.790 She did her undergraduate work at Washington and Lee University, 00:00:45.790 --> 00:00:47.989 also in geology. 00:00:47.989 --> 00:00:52.079 And I recently learned, while she was at Stanford, 00:00:52.079 --> 00:00:55.680 she completed something called a Ph.D. minor in the department 00:00:55.680 --> 00:00:58.690 of feminist, gender, and sexuality studies. 00:00:58.690 --> 00:01:01.000 So that’s pretty cool. Pretty incredible. 00:01:01.000 --> 00:01:04.670 Today she will share some of her graduate work 00:01:04.670 --> 00:01:07.539 on fracturing around magmatic dikes. 00:01:07.539 --> 00:01:10.799 And I’ll now turn things over to Meredith. 00:01:17.540 --> 00:01:19.620 - So thank you all for being here today. 00:01:19.630 --> 00:01:25.670 It’s a real treat for me to get to present my dissertation work here. 00:01:25.670 --> 00:01:31.270 So I just want to give a big thanks to Ole and Brooks for arranging my visit. 00:01:31.270 --> 00:01:36.069 The title of my talk is Fracturing Around Magmatic Dikes 00:01:36.069 --> 00:01:40.549 as a Precursor to the Development of Volcanic Plugs. 00:01:40.549 --> 00:01:44.219 But as a quick summary, this work is really about trying to 00:01:44.219 --> 00:01:49.880 understand deformation around volcanic conduits using a combination 00:01:49.880 --> 00:01:54.590 of field observations and structural geology along with 00:01:54.590 --> 00:02:00.369 models for the relevant thermo-poroelastic processes. 00:02:00.369 --> 00:02:06.120 So the first half of the talk will be mostly about the field observations. 00:02:06.120 --> 00:02:09.100 And then in the second half, I’ll get into some of the modeling 00:02:09.100 --> 00:02:11.501 I’ve been working on. 00:02:14.500 --> 00:02:17.939 So I actually want to start with acknowledgments. 00:02:17.939 --> 00:02:20.030 First and foremost, I want to acknowledge 00:02:20.030 --> 00:02:22.740 my adviser at Stanford, Dave Pollard. 00:02:22.740 --> 00:02:24.819 And I also want to acknowledge my co-authors 00:02:24.819 --> 00:02:29.120 on the first part of this project, Kendra Johnson, who is a Ph.D. student 00:02:29.120 --> 00:02:32.459 at Colorado School of Mines, and also Cansu Culha, 00:02:32.459 --> 00:02:36.310 who is a graduate student at Stanford. 00:02:36.310 --> 00:02:39.719 I also want to give a shout-out to Josie Nevitt, 00:02:39.719 --> 00:02:42.829 who has helped me tremendously in the field. 00:02:42.829 --> 00:02:47.629 And I also want to thank these folks for constructive feedback 00:02:47.629 --> 00:02:51.320 on the project throughout my time at Stanford. 00:02:51.320 --> 00:02:54.540 I also have to thank CoreLabs for performing the 00:02:54.540 --> 00:02:57.840 permeability measurements that I’ll be presenting. 00:02:57.840 --> 00:03:01.620 And I also have to acknowledge John Shaw, who let me work 00:03:01.629 --> 00:03:07.189 in his lab last fall and use some of the lab equipment at Harvard. 00:03:07.189 --> 00:03:11.780 Also, lately I’ve been receiving lots of online technical support 00:03:11.780 --> 00:03:16.010 for Hydrotherm, and so I just want to thank Steve Ingebritsen, 00:03:16.010 --> 00:03:18.640 Tom Driesner, and Paul Hsieh 00:03:18.640 --> 00:03:22.439 for communicating with me about those things. 00:03:22.439 --> 00:03:26.459 My work is funded through the Department of Geological Sciences 00:03:26.459 --> 00:03:31.059 at Stanford, the Rock Fracture Project, the DARE Fellowship, 00:03:31.060 --> 00:03:33.960 the McGee Grant, and the MacDiarmid Grant. 00:03:33.960 --> 00:03:36.680 So with that, let’s get started. 00:03:38.409 --> 00:03:44.620 So this work is couched in the broader context of the evolution of 00:03:44.620 --> 00:03:51.139 basaltic eruptions, which have perhaps been best documented in Hawaii. 00:03:51.139 --> 00:03:59.579 And eruptions tend to initiate as nearly planar features called fissures. 00:03:59.579 --> 00:04:06.090 And most of these fissure eruptions will freeze up within a few hours, 00:04:06.090 --> 00:04:10.829 but some of these eruptions will localize into a discrete conduit 00:04:10.829 --> 00:04:13.099 through which the eruption is sustained. 00:04:13.099 --> 00:04:18.109 So this is an example of one such sequence of events 00:04:18.109 --> 00:04:21.079 at Kilauea Iki in 1959. 00:04:21.079 --> 00:04:25.470 The fissure eruption initiated and grew to a length of about a kilometer, 00:04:25.470 --> 00:04:29.370 spewing magma about 30 meters into the air at an eruption rate 00:04:29.370 --> 00:04:33.039 of 100 cubic meters per second. 00:04:33.039 --> 00:04:38.419 And a few hours later, the eruption had ceased along most of the fissure except 00:04:38.419 --> 00:04:45.370 for one location back here through – and at this location, the eruption persisted. 00:04:45.370 --> 00:04:48.860 And four days later, shown on the right, 00:04:48.860 --> 00:04:54.250 the conduit had substantially widened and rounded out. 00:04:54.250 --> 00:04:57.289 And at this point, the eruption rate had more than tripled, 00:04:57.289 --> 00:05:02.360 and the fountaining height reached 350 meters. 00:05:02.360 --> 00:05:08.020 So the question I seek to answer, at least in part, is what are the 00:05:08.020 --> 00:05:11.069 physical mechanisms that lead to the growth of 00:05:11.069 --> 00:05:16.960 larger volcanic conduits from dikes and fissures? 00:05:16.960 --> 00:05:23.110 And two of the main mechanisms that have been proposed for this 00:05:23.110 --> 00:05:29.250 transition include thermal erosion – that is, melting of the host rock – 00:05:29.250 --> 00:05:36.810 and mechanical erosion, particularly via water – magma water interaction. 00:05:36.810 --> 00:05:42.090 And the work on thermal erosion has primarily been theoretical. 00:05:42.090 --> 00:05:48.979 That is, no geologic study has yet presented evidence that host rock 00:05:48.979 --> 00:05:54.379 melting plays a significant role in this conduit geometry evolution. 00:05:54.379 --> 00:06:01.250 However, there are advection-diffusion models for heat flow through fissures, 00:06:01.250 --> 00:06:08.300 and they generally show that, for giant dikes, such as the ones 00:06:08.300 --> 00:06:12.690 that feed flood basalts that can be hundreds of meters wide 00:06:12.690 --> 00:06:16.889 and thousands of kilometers long, there likely is enough heat flow 00:06:16.889 --> 00:06:21.409 to cause some significant melting of the host rock. 00:06:21.409 --> 00:06:26.750 However, for fissures of the sizes observed in places like Hawaii, 00:06:26.750 --> 00:06:30.050 which tend to be only, you know, 1 or 2 meters wide, 00:06:30.050 --> 00:06:32.879 under typical flow conditions, there doesn’t seem to be enough 00:06:32.879 --> 00:06:38.069 heat flow to cause significant melting of the host rock. 00:06:38.069 --> 00:06:44.840 And so the other mechanism that’s been suggested is magma-water interaction 00:06:44.840 --> 00:06:49.940 leading to brecciation of the host rock and erosion of the conduit. 00:06:49.940 --> 00:06:55.360 And this mechanism has been treated with less theoretical rigor but is 00:06:55.360 --> 00:06:59.580 perhaps better supported by the geologic evidence. 00:06:59.580 --> 00:07:05.500 For example, breccias are commonly associated with volcanic conduits. 00:07:05.500 --> 00:07:09.250 And so I got interested in honing in on this question. 00:07:09.250 --> 00:07:13.280 What is the role of magma-groundwater interaction 00:07:13.280 --> 00:07:17.860 in altering the geometry of dikes and fissures? 00:07:17.860 --> 00:07:23.280 And I sought to tie together the geology with robust models 00:07:23.280 --> 00:07:25.500 for host rock deformation. 00:07:27.000 --> 00:07:30.880 And it turns out there are some other good reasons 00:07:30.889 --> 00:07:35.729 to study magma-water interaction. We know it affects eruption dynamics. 00:07:35.729 --> 00:07:42.189 When water can directly invade the conduits, we can get explosive phases. 00:07:42.189 --> 00:07:45.330 And as I’ve already mentioned, and then as would be the focus 00:07:45.330 --> 00:07:53.699 of this talk, magma-water interaction can affect host rock deformation. 00:07:53.699 --> 00:07:57.949 And hydrothermal processes are also important at mid-ocean ridges 00:07:57.949 --> 00:08:03.159 where convection cells are set up and relate to the heat budget 00:08:03.159 --> 00:08:06.860 at this very important part of the crust. 00:08:06.860 --> 00:08:10.719 And magma-water interaction is also important for understanding 00:08:10.719 --> 00:08:15.669 contact metamorphism. And of course, hydrothermal processes 00:08:15.669 --> 00:08:20.889 are important for understanding ore deposits and geothermal energy. 00:08:23.259 --> 00:08:27.020 So if you really want to learn about deformation around dikes, 00:08:27.020 --> 00:08:32.409 one of the best things you can do is go visit an ancient volcano 00:08:32.409 --> 00:08:36.830 where enough erosion has taken place to expose the plumbing system. 00:08:36.830 --> 00:08:40.840 And so I visited the Navajo Volcanic Field, 00:08:40.840 --> 00:08:45.590 which is located in the central Colorado Plateau. 00:08:45.590 --> 00:08:55.250 And the most prominent feature in this area is this plug called Ship Rock, 00:08:55.250 --> 00:09:02.130 which is about 500 meters high and surrounded by dikes and smaller plugs. 00:09:02.130 --> 00:09:05.350 And just to give a little more geologic background, 00:09:05.350 --> 00:09:09.320 the Navajo Volcanic Field is 30 million years old, 00:09:09.320 --> 00:09:13.440 and it’s composed of a type of magmatic material 00:09:13.440 --> 00:09:18.290 that’s ultrapotassic, and it’s called a lamprophyre. 00:09:18.290 --> 00:09:25.570 And this lamprophyric magma intruded through flat-lying sediments 00:09:25.570 --> 00:09:31.210 of the San Juan Basin that were deposited during the Cretaceous. 00:09:31.210 --> 00:09:36.060 And since the time of eruption, erosion has cut down and 00:09:36.060 --> 00:09:39.160 exposed the plumbing system at different levels. 00:09:39.960 --> 00:09:45.620 And at Ship Rock, it’s estimated that about a kilometer of erosion 00:09:45.620 --> 00:09:49.680 has taken place since the time of eruption. 00:09:49.680 --> 00:09:54.700 And the current land surface around the dikes here is part of the 00:09:54.700 --> 00:09:58.430 Mancos Shale formation, which despite its name, 00:09:58.430 --> 00:10:02.550 is actually composed of layers with different grain sizes. 00:10:02.550 --> 00:10:07.290 And here it’s actually more like a siltstone. 00:10:07.290 --> 00:10:11.660 The dikes are about a few kilometers long. 00:10:11.660 --> 00:10:16.560 They dip vertically. They’re a meter or two wide. 00:10:16.560 --> 00:10:20.570 And they’re typically broken into echelon segments. 00:10:20.570 --> 00:10:27.900 And so the direction of propagation inferred here is vertical at this level. 00:10:27.900 --> 00:10:32.350 And the northeastern dike, which I’ve pointed out here, has been the focus 00:10:32.350 --> 00:10:39.090 of previous studies and is also going to be the focus of my work. 00:10:39.090 --> 00:10:44.840 And so the analogous question at these ancient volcanoes is, 00:10:44.840 --> 00:10:48.640 what are the mechanisms for the growth of these larger 00:10:48.640 --> 00:10:53.420 volcanic plugs like Ship Rock from these dikes? 00:10:58.220 --> 00:11:04.100 In the 1980s, Paul Delaney and Dave Pollard used field data from the 00:11:04.100 --> 00:11:07.970 northeastern dike to try to learn about the mechanics of dike propagation. 00:11:07.970 --> 00:11:12.850 And specifically they were interested in whether propagation is controlled 00:11:12.850 --> 00:11:17.880 by the local structure or whether propagation occurs 00:11:17.880 --> 00:11:21.700 through fractures formed at the dike tips. 00:11:21.700 --> 00:11:28.260 And they measured the fracture patterns around the dike and around the region. 00:11:28.260 --> 00:11:34.450 And they found that, at Ship Rock, the orientation of regional fractures 00:11:34.450 --> 00:11:39.060 is not consistent with the orientation of the dikes. 00:11:39.060 --> 00:11:43.580 However, what they did find is that locally around the dikes, 00:11:43.580 --> 00:11:50.480 within about 15 meters, there was a set of dike-parallel joints in the 00:11:50.480 --> 00:11:56.210 host rock that appeared to be related to the propagation of the dikes. 00:11:56.210 --> 00:12:02.320 And so they sort of concluded that the dikes in this area are propagating 00:12:02.320 --> 00:12:07.610 through their own fractures, and they used this analogy 00:12:07.610 --> 00:12:13.230 of ground cracks that tend to form ahead of propagating fissures. 00:12:17.150 --> 00:12:23.060 So when I visited Ship Rock, I saw the dike-parallel fractures 00:12:23.060 --> 00:12:27.820 that were discussed in that paper, which I’ve here outlined in blue. 00:12:29.440 --> 00:12:33.100 But I also saw another set of joints in the host rock 00:12:33.110 --> 00:12:36.580 that Dave and Paul had apparently missed. 00:12:36.580 --> 00:12:39.670 And much like the parallel fractures, 00:12:39.670 --> 00:12:44.160 these joints are localized within about a meter of the dikes. 00:12:44.160 --> 00:12:47.430 They’re not part of a regional set. 00:12:47.430 --> 00:12:50.760 And they dip vertically, and they’re consistently oriented 00:12:50.760 --> 00:12:54.800 perpendicular to the dike contacts. 00:12:54.800 --> 00:12:59.450 So I got interested in what these perpendicular fractures are, 00:12:59.450 --> 00:13:04.700 since they hadn’t been documented before and since they seemed 00:13:04.700 --> 00:13:10.120 at odds with what I knew about stress around dikes. 00:13:10.120 --> 00:13:15.110 So in an effort to be a good geologist, I carefully mapped out the 00:13:15.110 --> 00:13:20.300 fracture patterns using a technique called structure from motion. 00:13:20.300 --> 00:13:25.320 And this is where my colleague Kendra Johnson was a really big help. 00:13:25.320 --> 00:13:28.530 This is the focus of her Ph.D. thesis. 00:13:28.530 --> 00:13:32.990 So she came out with me, and what we did was we filled 00:13:32.990 --> 00:13:37.580 this balloon with helium gas and attached a camera to the bottom of 00:13:37.580 --> 00:13:43.230 the balloon and set the camera to take pictures at a 5-second interval. 00:13:43.230 --> 00:13:47.320 And then the balloon was tethered so that we could kind of control it, 00:13:47.320 --> 00:13:48.970 like flying a kite. 00:13:48.970 --> 00:13:53.620 And we walked up and down one of the dike segments along 00:13:53.620 --> 00:13:59.740 the northeastern dike while the camera took overlapping photographs. 00:13:59.740 --> 00:14:05.170 And we walked up and down once, and it took about 30 minutes. 00:14:05.170 --> 00:14:09.300 And then the overlapping photographs were uploaded into Agisoft Photoscan 00:14:09.300 --> 00:14:15.690 and automatically orthorectified with the help of a few ground control points. 00:14:15.690 --> 00:14:23.200 And the result was a high-resolution composite photograph of the dike, 00:14:23.200 --> 00:14:27.400 which here is this dark material that’s curving around like so. 00:14:27.400 --> 00:14:32.370 And to give you a sense for the resolution that we were able to achieve, 00:14:32.370 --> 00:14:38.980 this dike is about 150 meters long, but now we’re going to zoom in 00:14:38.980 --> 00:14:45.250 on the red box, and we can start to see a little more detail in the host rock, 00:14:45.250 --> 00:14:51.120 which is this buff-colored rock here that you can see has been sort of baked 00:14:51.120 --> 00:14:57.970 up against the dike and is therefore more resistant to erosion, so it stands out. 00:14:57.970 --> 00:15:01.370 But we’re going to zoom in again. 00:15:01.370 --> 00:15:06.330 And now we can actually start to pick out details of fractures in the host rock. 00:15:06.330 --> 00:15:09.950 So in the end, we achieved a resolution of about 3-1/2 millimeters 00:15:09.950 --> 00:15:14.880 per pixel for the cost of $200, which was the helium tank. 00:15:14.880 --> 00:15:18.970 And it took about 30 minutes to gather the data and a few hours 00:15:18.970 --> 00:15:23.280 to orthorectify and make the composite photograph. 00:15:23.280 --> 00:15:27.100 So this was a very helpful technique. 00:15:27.100 --> 00:15:31.450 And then, using the photograph, we mapped the dike contact, 00:15:31.450 --> 00:15:37.110 which is in white, and we mapped the dike-parallel fractures in blue 00:15:37.110 --> 00:15:40.920 and the dike-perpendicular fractures in red. 00:15:40.920 --> 00:15:46.260 And this kind of mapping was useful because it allowed us to gather, 00:15:46.260 --> 00:15:50.110 quite rapidly, statistics on the joint sets. 00:15:50.110 --> 00:15:56.740 For example, the joints’ lengths and geometry, spacing, 00:15:56.740 --> 00:15:59.780 and relationship to other fractures. 00:15:59.780 --> 00:16:05.980 And so, from this mapping, we concluded that the dike-perpendicular 00:16:05.980 --> 00:16:10.860 joints are definitely associated with the emplacement of the magma. 00:16:10.860 --> 00:16:14.910 Because wherever the contact of the dike changes orientation, 00:16:14.910 --> 00:16:20.980 these joints in red also change orientation to remain perpendicular. 00:16:20.980 --> 00:16:25.180 And we also found that the dike-perpendicular joints 00:16:25.190 --> 00:16:28.950 tended to terminate at the dike-parallel fractures. 00:16:28.950 --> 00:16:33.970 And so we inferred that they’re younger than the parallel set. 00:16:36.400 --> 00:16:40.310 This is a regular photograph looking up at a dike. 00:16:40.310 --> 00:16:45.410 And we can see the host rock baked on and preserving the fractures. 00:16:45.410 --> 00:16:47.040 And so these are the dike-parallel joints in red, 00:16:47.040 --> 00:16:49.040 and we see they’re vertically dipping. 00:16:49.040 --> 00:16:54.630 And they cut through many of the beds at once, which are highlighted in green. 00:16:54.630 --> 00:17:00.250 And there are also plumose structures on the joint faces that indicate that 00:17:00.250 --> 00:17:07.159 the joints initiated at the dike contact and then propagated up and away. 00:17:09.580 --> 00:17:14.860 The next key observation is that there are xenoliths of the host rock 00:17:14.860 --> 00:17:22.480 that appear to have this rectangular prismatic structure that appears to be 00:17:22.480 --> 00:17:28.350 formed by the combination of the two orthogonal joint sets and the bedding. 00:17:28.350 --> 00:17:32.879 And sometimes you can see in places where the rectangular blocks of 00:17:32.879 --> 00:17:38.819 host rock have just been rotated out of place before the magma froze. 00:17:41.080 --> 00:17:44.940 And in some places, if enough host rock has been removed, 00:17:44.940 --> 00:17:51.230 the dike has been widened into a structure that we call a bud. 00:17:51.230 --> 00:17:56.360 And the host rock around these buds tends to be highly brecciated. 00:17:56.360 --> 00:18:03.629 However, you can hopefully pick out that the clasts in this breccia 00:18:03.629 --> 00:18:09.139 preserve that rectangular prismatic shape, which suggests that the 00:18:09.139 --> 00:18:12.620 breccia is really just an area where the dike-parallel and 00:18:12.620 --> 00:18:16.460 dike-perpendicular fracture density is very high. 00:18:19.400 --> 00:18:24.720 So all together, the field observations suggest that the two orthogonal 00:18:24.720 --> 00:18:29.570 joint sets, along with the bedding planes, carve the host rock into isolated blocks 00:18:29.570 --> 00:18:35.090 that can be entrained by the flow of magma to widen the dike and form buds, 00:18:35.090 --> 00:18:40.559 which represent the insipient stages of volcanic plug formation. 00:18:40.559 --> 00:18:44.220 So since these dike-perpendicular joints seem to be such a crucial step 00:18:44.220 --> 00:18:47.710 in this process, and since they hadn’t been documented before, 00:18:47.710 --> 00:18:53.750 I was interested in trying to figure out why they’re there. [chuckles] 00:18:56.560 --> 00:19:02.560 So now we’re going to turn our attention to some fun problems 00:19:02.570 --> 00:19:06.399 related to deformation around dikes in the shallow crust. 00:19:07.700 --> 00:19:11.140 And we’re going to try to see if we can explain the formation 00:19:11.149 --> 00:19:13.779 of these dike-perpendicular joints. 00:19:13.779 --> 00:19:19.080 And I just first want to reinforce the notion that the joints provide some pretty 00:19:19.080 --> 00:19:23.740 hard constraints on the state of stress around the dike during emplacement. 00:19:23.740 --> 00:19:27.470 And in particular, the joint orientations 00:19:27.470 --> 00:19:32.460 constrain the orientations of the principal stress trajectories. 00:19:32.460 --> 00:19:36.570 And the fact that these joints are tensile features suggests 00:19:36.570 --> 00:19:41.120 that the effective stress was in the tensile regime. 00:19:41.120 --> 00:19:48.350 So we’re going to begin by simplifying the problem to 2D 00:19:48.350 --> 00:19:51.200 such that the plane we’re working in represents 00:19:51.200 --> 00:19:54.929 the horizontal slice through one of these dikes. 00:19:54.929 --> 00:19:59.139 And before the dike is in place, there is some initial state of stress 00:19:59.139 --> 00:20:05.999 characterized by two remote principal stresses, denoted sigma-1r and sigma-2r. 00:20:05.999 --> 00:20:12.549 And we’ll take a tension-positive convention so that sigma-1 is 00:20:12.549 --> 00:20:19.909 the maximum principal stress, or the least compressive principal stress. 00:20:19.909 --> 00:20:25.499 Since the emplacement depth at Ship Rock is estimated to be 1 kilometer, 00:20:25.499 --> 00:20:32.220 the lithostatic overburden contributes to about negative 25 MPa. 00:20:32.220 --> 00:20:38.120 And in hydrostatic conditions, the fluid pressure would be about 10 MPa. 00:20:38.120 --> 00:20:41.019 And so the effective stress at this depth was 00:20:41.019 --> 00:20:45.079 about 15 MPa of compression. 00:20:47.620 --> 00:20:50.820 When the dike is emplaced, there is a stress perturbation 00:20:50.830 --> 00:20:56.440 associated with the elastic distortion of the host rock. 00:20:56.440 --> 00:21:02.409 And there’s also heat transfer to the host rock, which causes thermal expansion of 00:21:02.409 --> 00:21:09.320 the grains and the pore fluids, which we’re going to be assuming to be water. 00:21:09.320 --> 00:21:14.409 And as a side note, the thermal expansion of water is about 00:21:14.409 --> 00:21:19.420 two orders of magnitude greater than the thermal expansion of quartz. 00:21:19.420 --> 00:21:22.120 So we’re really going to be focusing primarily on the effects 00:21:22.129 --> 00:21:25.709 of heat transfer to the fluids. 00:21:27.230 --> 00:21:31.710 So our failure criteria is that all of these stresses have to 00:21:31.710 --> 00:21:38.259 combine such that the effective stress reaches the tensile strength, 00:21:38.259 --> 00:21:43.299 which for most rocks is only a few MPa, or effectively zero. 00:21:43.299 --> 00:21:51.820 So we need about an additional 15 MPa of fluid pressure 00:21:51.820 --> 00:21:54.899 to overcome the lithostatic stress. 00:21:56.680 --> 00:22:01.700 So we’re going to start by looking at the stress perturbation 00:22:01.700 --> 00:22:05.820 due to the elastic displacements around a dike. 00:22:05.820 --> 00:22:10.980 And we’re here just using the solution from Pollard and Segall. 00:22:10.980 --> 00:22:17.090 And what I’ve plotted here, the blue tick marks are the 00:22:17.090 --> 00:22:24.059 orientations of the most compressive principal stress trajectory, which in 2D 00:22:24.059 --> 00:22:31.870 represents, more or less, the orientation that hypothetical joints would form. 00:22:31.870 --> 00:22:37.019 And the difference between the left and the right plots is this ratio 00:22:37.019 --> 00:22:40.809 of the difference in the two remote stresses 00:22:40.809 --> 00:22:44.870 relative to the driving pressure for dike opening. 00:22:44.870 --> 00:22:50.830 And so this left plot essentially represents an isotropic state of stress. 00:22:50.830 --> 00:22:56.870 And we see that joints that might form at the dike contact 00:22:56.870 --> 00:23:00.779 are perpendicular to the dike. 00:23:01.740 --> 00:23:05.440 However, when this ratio increases to a critical value, 00:23:05.440 --> 00:23:11.980 which happens to be about 0.6, the stress trajectories swap 00:23:11.980 --> 00:23:16.679 and become parallel with the dike contact. 00:23:16.679 --> 00:23:22.490 And so from this simple analysis, we can conclude that most likely 00:23:22.490 --> 00:23:26.460 the remote stress at the time of dike emplacement was isotropic, 00:23:26.460 --> 00:23:32.840 or nearly isotropic. However, the elastic stresses are 00:23:32.840 --> 00:23:38.789 compressive, so we still don’t have a mechanism to induce tension in the rock. 00:23:38.789 --> 00:23:43.450 So now we’re going to focus on the thermal expansion of pore fluids 00:23:43.450 --> 00:23:46.559 and see how that might elevate the pore pressure 00:23:46.559 --> 00:23:51.179 and perhaps bring the host rock into tension. 00:23:51.179 --> 00:23:55.470 So Paul Delaney was also interested in thermal pore fluid pressurization. 00:23:56.580 --> 00:24:01.159 And he envisioned a model for an instantaneously emplaced dike 00:24:01.159 --> 00:24:05.220 within a fluid-saturated host rock where the temperature change 00:24:05.220 --> 00:24:08.539 at the contact was about 500 C. 00:24:08.539 --> 00:24:14.889 And in his model, he assumes no fluid exchange across the contact. 00:24:14.889 --> 00:24:18.429 And for now, we’re also going to adopt that assumption since 00:24:18.429 --> 00:24:23.490 there’s not any obvious evidence that fluids were exchanged. 00:24:23.490 --> 00:24:28.679 So at the margins of the dikes at Ship Rock, there tends to be 00:24:28.679 --> 00:24:33.339 a glassy rind in most places that makes the contact pretty sharp. 00:24:33.340 --> 00:24:39.440 So we’re going to stick with this no-flow boundary condition for now. 00:24:39.450 --> 00:24:46.009 And so in this paper, Delaney presented an analytical solution 00:24:46.009 --> 00:24:50.460 to the coupled heat and fluid flow equations in porous media. 00:24:50.460 --> 00:24:55.700 And at short times, heat flow is dominated by conduction. 00:24:55.700 --> 00:25:04.070 And thermal expansion can increase the pressure and set up a Darcy-type flow. 00:25:04.070 --> 00:25:12.320 And the estimate for the increase in pore fluid pressure is this number, 00:25:12.320 --> 00:25:16.830 where the first term is essentially a driving term that describes 00:25:16.830 --> 00:25:20.919 the competition between thermal expansion of the fluids 00:25:20.919 --> 00:25:25.289 and their ability to compress within the pore space. 00:25:25.289 --> 00:25:31.409 And then that driving term is sort of regulated by this ratio 00:25:31.409 --> 00:25:35.519 of the thermal-to-hydraulic diffusivity. 00:25:35.519 --> 00:25:40.629 So for very permeable rocks with high hydraulic diffusivity, 00:25:40.629 --> 00:25:46.869 pressure can diffuse faster than it builds. So there’s not a huge pressure build-up. 00:25:46.869 --> 00:25:50.279 But for rocks with a low enough permeability, 00:25:50.279 --> 00:25:53.759 the pressure builds because it can’t diffuse quickly. 00:25:53.759 --> 00:26:01.440 So using this solution, we find that at Ship Rock, the pressure change 00:26:01.440 --> 00:26:04.220 due to thermal expansion of the fluids is anywhere 00:26:04.220 --> 00:26:07.990 from hundredths of MPa to hundreds of MPa. 00:26:07.990 --> 00:26:11.700 And as a side note, obviously the pressure would not actually 00:26:11.700 --> 00:26:15.139 increase to 500 MPa. Fracturing would occur first. 00:26:15.139 --> 00:26:21.269 But this is just sort of a hypothetical range of pressures. 00:26:21.269 --> 00:26:26.320 And so this wide range is arising because the permeability of rocks 00:26:26.320 --> 00:26:30.740 can span 10 orders of magnitude, even within the same lithology. 00:26:30.740 --> 00:26:35.110 So this just highlights the need to constrain the permeability 00:26:35.110 --> 00:26:42.269 if you want to evaluate the effects of thermal pressurization. 00:26:42.269 --> 00:26:47.549 So I collected samples of the fresh, unaltered Mancos shale 00:26:47.549 --> 00:26:52.070 from an outcrop a few tens of meters away from the dike. 00:26:52.070 --> 00:26:54.649 And I had core flooding measurements performed 00:26:54.649 --> 00:27:00.200 that showed that the permeability of the unaltered host rock is 00:27:00.200 --> 00:27:03.259 on the order of 10 to the minus 13th meters squared, 00:27:03.259 --> 00:27:06.549 which corresponds to hydraulic diffusivity of about 10. 00:27:06.549 --> 00:27:10.869 And when we input this into that analytical equation, 00:27:10.869 --> 00:27:15.549 we find that the pressure increase is only a few tenths of MPa. 00:27:15.549 --> 00:27:18.889 And if you recall, we need something more like 15 00:27:18.889 --> 00:27:24.129 to induce failure, so this is not enough. 00:27:24.129 --> 00:27:28.100 However, if you recall, the host rock is baked up 00:27:28.100 --> 00:27:32.580 against the dike and therefore is less permeable. 00:27:32.580 --> 00:27:38.029 And so I was curious if there was any way that this thermal alteration process 00:27:38.029 --> 00:27:45.899 was contributing to changing the pressurization and flow around the dikes. 00:27:45.899 --> 00:27:51.169 And so, in the SEM, we can get a sense for 00:27:51.169 --> 00:27:57.580 what the effects are of alteration of the Mancos shale. 00:27:57.580 --> 00:28:02.909 So compared to the unaltered Mancos shale on the left, the altered rock 00:28:02.909 --> 00:28:09.090 on the right has clearly experienced some quartz recrystallization. 00:28:09.090 --> 00:28:15.450 And furthermore, the space between the quartz is filled with this 00:28:15.450 --> 00:28:21.989 dark amorphous cement, which is rich in aluminum, magnesium, and iron, 00:28:21.989 --> 00:28:27.450 but which doesn’t take on any consistent mineral formula. 00:28:27.450 --> 00:28:33.710 And then, within this cement, there are tiny microclasts 00:28:33.710 --> 00:28:37.629 of quartz floating around. 00:28:37.629 --> 00:28:43.659 And the effect of this alteration on the hydraulic properties 00:28:43.659 --> 00:28:47.220 was to reduce the porosity from something that was 00:28:47.220 --> 00:28:51.369 initially about 22% to something like 8%. 00:28:52.900 --> 00:28:56.720 And there was a corresponding reduction in permeability. 00:28:56.730 --> 00:29:00.720 So we used a combination of pulse decay and core flooding 00:29:00.720 --> 00:29:08.080 experiments to find the permeability of the host rock 00:29:08.080 --> 00:29:11.739 as a function of distance from the contact. 00:29:11.739 --> 00:29:15.379 And what we found is that, right at the contact, 00:29:15.379 --> 00:29:18.159 the permeability of the host rock is on the order of 00:29:18.159 --> 00:29:20.309 10 to the minus 18th meters squared, 00:29:20.309 --> 00:29:27.139 which is a reduction of five orders of magnitude from the fresh host rock. 00:29:27.139 --> 00:29:31.249 And the permeability consistently increases 00:29:31.249 --> 00:29:35.570 with distance away from the dike. 00:29:35.570 --> 00:29:40.669 And this relationship of permeability with distance 00:29:40.669 --> 00:29:45.330 suggests the possibility that permeability and porosity change 00:29:45.330 --> 00:29:49.539 as a function of the amount of heat experienced 00:29:49.539 --> 00:29:53.529 as a function of position near the dike. 00:29:53.529 --> 00:30:01.489 And so I decided to entertain that idea, and I began with looking at 00:30:01.489 --> 00:30:07.789 what the temperature profiles look like around a dike. 00:30:07.789 --> 00:30:14.110 And so these are just results for a simple heat conduction model 00:30:14.110 --> 00:30:21.059 for a meter-wide dike at 1,000 degrees in a host rock that’s initially zero. 00:30:21.059 --> 00:30:29.600 And the blue line is the temperature in time 00:30:29.600 --> 00:30:32.360 for a position basically right at the contact. 00:30:32.360 --> 00:30:35.200 And what we see is that the temperature initially spikes 00:30:35.200 --> 00:30:39.019 to about 500 and then decays back down. 00:30:39.019 --> 00:30:42.909 And then, a half a meter away from the dike, in red, 00:30:42.909 --> 00:30:46.700 the temperature spikes to about 350 C. 00:30:46.700 --> 00:30:49.679 And then, at a meter away, the maximum temperature reached 00:30:49.679 --> 00:30:57.999 is only about 275. And for reference, at these pressures, 00:30:57.999 --> 00:31:03.139 quartz recrystallization would start at around 300. 00:31:03.139 --> 00:31:10.119 So I don’t think it’s any coincidence that the width of the baked zone corresponds 00:31:10.129 --> 00:31:16.709 to the width of the host rock that achieved a temperature or 300 or greater. 00:31:18.480 --> 00:31:23.300 So fueled by this potential relationship between temperature 00:31:23.309 --> 00:31:30.739 and hydraulic properties, I plotted the porosity and permeability 00:31:30.739 --> 00:31:36.200 as a function of this maximum temperature reached in the host rock. 00:31:36.200 --> 00:31:40.690 And what I found was that the porosity and max temperature 00:31:40.690 --> 00:31:43.590 have a more or less linear relationship, 00:31:43.590 --> 00:31:49.389 and the permeability with temperature have a log linear relationship. 00:31:49.389 --> 00:31:53.210 And this checks out when we realize that porosity and permeability 00:31:53.210 --> 00:32:00.669 vary together exponentially in the form of a Kozeny-Carman relationship. 00:32:00.669 --> 00:32:07.730 So I used these empirical relationships to motivate a suite of models 00:32:07.730 --> 00:32:11.989 for thermal pore pressurization near a dike using 00:32:11.989 --> 00:32:15.520 temperature-dependent hydraulic properties. 00:32:15.520 --> 00:32:20.360 And I did this using Hydrotherm, which is a finite difference code 00:32:20.369 --> 00:32:25.840 written here at the USGS and simulates a multi-phase 00:32:25.840 --> 00:32:29.399 groundwater flow and heat transport. 00:32:29.399 --> 00:32:35.559 So I used this to model an instantaneously emplaced dike 00:32:35.559 --> 00:32:40.809 a meter wide, initially 1,000 degrees C within a fluid-saturated host rock 00:32:40.809 --> 00:32:51.259 at 25 C and 10 MPa of fluid pressure to represent the conditions at 1 kilometer. 00:32:51.259 --> 00:32:57.820 So for now, I’ve just been solving 1D heat and fluid flow. 00:32:57.820 --> 00:33:03.179 And I’m solving for the pore pressure change at the dike contact. 00:33:03.179 --> 00:33:06.570 And for each simulation, I assigned hydraulic properties 00:33:06.570 --> 00:33:09.999 to the host rock, and I was able to make them a function of temperature. 00:33:09.999 --> 00:33:15.320 And in Hydrotherm, how you do this is you assign a value for the 00:33:15.320 --> 00:33:20.109 permeability at one temperature and a value for another temperature. 00:33:20.109 --> 00:33:26.909 And the relationship is interpolated between those two values. 00:33:29.330 --> 00:33:35.389 So it was not obvious to me what the individual effects would be 00:33:35.389 --> 00:33:39.450 of changing permeability and changing porosity. 00:33:39.450 --> 00:33:45.659 So to be systematic, I first ran a suite of models where I fixed the porosity 00:33:45.659 --> 00:33:49.259 and made the permeability a function of temperature. 00:33:49.259 --> 00:33:51.509 And then I did a suite of models where I fixed the permeability 00:33:51.509 --> 00:33:54.399 and made the porosity a function of temperature. 00:33:54.399 --> 00:33:57.509 And then finally, I made both a function of temperature 00:33:57.509 --> 00:34:01.730 to see how they add or don’t add. 00:34:01.730 --> 00:34:08.460 So to start, here are the results for fixing porosity at 25% 00:34:08.460 --> 00:34:11.000 and then changing permeability. 00:34:11.000 --> 00:34:15.280 So the colors represent the change in pore fluid pressure, where the 00:34:15.280 --> 00:34:21.540 blue is only a few MPa, and the dark red is getting into hundreds of MPa. 00:34:21.540 --> 00:34:26.400 And the permeability at 25 C, or the initial permeability 00:34:26.410 --> 00:34:28.790 of the host rock, is on the X axis. 00:34:28.790 --> 00:34:34.070 And the permeability defined at 500 C is on the Y axis. 00:34:34.070 --> 00:34:39.120 And so this black line represents no change, or no temperature dependence. 00:34:39.120 --> 00:34:42.980 Below the line represents a decrease in permeability, 00:34:42.980 --> 00:34:47.080 you know, due to something like mineralization. 00:34:47.090 --> 00:34:49.320 And above the black line would represent permeability 00:34:49.320 --> 00:34:53.190 enhancement due to something like fracturing. 00:34:53.190 --> 00:34:59.420 And what we find from the trend of the contours is that the initial – 00:34:59.420 --> 00:35:05.440 the initial permeability is what really dominates the pore fluid pressure. 00:35:05.440 --> 00:35:10.090 And that the change in permeability essentially has no effect 00:35:10.090 --> 00:35:14.730 unless that change gets to be really extreme and ends up 00:35:14.730 --> 00:35:18.090 at something like 10 to the minus 21st. 00:35:19.380 --> 00:35:22.160 So then we fix the permeability and make porosity 00:35:22.160 --> 00:35:25.510 as a function of temperature. 00:35:25.510 --> 00:35:29.530 And what we find is that the trend of the contours are now 00:35:29.530 --> 00:35:32.800 a little different and increase to the bottom right, 00:35:32.800 --> 00:35:39.340 which means that the porosity change actually does affect the fluid pressure. 00:35:41.460 --> 00:35:48.980 And the significance of this porosity reduction depends on the permeability. 00:35:48.990 --> 00:35:56.250 So here on the left are values for pore pressure, changing porosity, 00:35:56.250 --> 00:36:01.200 but for a permeability of 10 to the minus 18th, which is fairly low. 00:36:01.200 --> 00:36:06.120 And on the right is for a permeability of 10 to the minus 13th. 00:36:06.120 --> 00:36:10.450 And what I want to point out to you is that 00:36:10.450 --> 00:36:13.590 the values of the pore pressure are obviously very different. 00:36:13.590 --> 00:36:22.340 But more so, for the low permeability, the maximum pore fluid pressure change 00:36:22.340 --> 00:36:28.050 is about 6 times what it would be if porosity had not changed. 00:36:28.050 --> 00:36:33.430 Whereas, in the high permeability case, the max pore fluid pressure change 00:36:33.430 --> 00:36:39.290 is only about twice what it would be without porosity reduction. 00:36:39.290 --> 00:36:46.300 So this was sort of an interesting result for me that inelastic porosity reduction 00:36:46.300 --> 00:36:51.120 can increase the fluid pressure by decreasing the pore volume. 00:36:51.120 --> 00:36:54.540 And that the significance of this process depends on 00:36:54.540 --> 00:37:00.540 both the rate of change of the porosity and also the permeability. 00:37:02.920 --> 00:37:07.990 So when both permeability and porosity are a function of temperature, we find 00:37:07.990 --> 00:37:15.730 that, once again, the initial permeability really dominates the effects. 00:37:15.730 --> 00:37:23.050 And however, the values of pore pressure change are higher 00:37:23.050 --> 00:37:30.530 when we have the porosity changing, which here was from 25% to 5%. 00:37:32.180 --> 00:37:38.060 And so I plotted the permeability and porosity data from Ship Rock onto this 00:37:38.060 --> 00:37:45.710 solution space and found that, at most, the pressure increase is only a few MPa. 00:37:45.710 --> 00:37:52.860 So that is still not enough to explain the presence of these joints. 00:37:52.860 --> 00:37:57.080 So in the end, I sort of concluded that, you know, 00:37:57.080 --> 00:38:02.460 although thermal pore pressurization is perhaps not really a major contribution 00:38:02.460 --> 00:38:06.520 to the stress around dikes at Ship Rock, because the initial permeability 00:38:06.520 --> 00:38:15.670 is just very high, many of these values for permeability and porosity 00:38:15.670 --> 00:38:20.060 are realistic values for other environments in the Earth. 00:38:20.060 --> 00:38:23.580 So perhaps these processes might be important in other 00:38:23.580 --> 00:38:26.920 hydrothermal environments. 00:38:29.600 --> 00:38:32.420 So conclusions from the work. 00:38:32.420 --> 00:38:36.890 We found that fracturing and entrainment of the host rock 00:38:36.890 --> 00:38:40.850 is a mechanism for changing conduit geometry. 00:38:40.850 --> 00:38:45.610 That dike-perpendicular joints at the dike contact are promoted by 00:38:45.610 --> 00:38:51.650 a small difference in the remote stresses relative to the driving pressure. 00:38:51.650 --> 00:38:56.180 At Ship Rock, fluid pressure rose at least about 15 MPa 00:38:56.180 --> 00:38:59.590 as evidenced by the dike-perpendicular joints. 00:38:59.590 --> 00:39:04.500 That thermal pressurization alone does not explain these joints. 00:39:04.500 --> 00:39:10.100 But as an aside, I found that inelastic changes in porosity – 00:39:10.110 --> 00:39:15.080 that is, inelastic porosity reduction can significantly increase 00:39:15.080 --> 00:39:17.860 fluid pressures if the permeability is low enough. 00:39:17.860 --> 00:39:25.000 And this is an independent effect independent of thermal pressurization. 00:39:26.490 --> 00:39:33.250 So [chuckles] when all of your models fail to explain the geologic phenomena, 00:39:33.250 --> 00:39:38.260 you have to revisit some of the initial model assumptions. 00:39:38.260 --> 00:39:41.880 And so for example, you know, we assumed emplacement depth 00:39:41.880 --> 00:39:44.840 of 1 kilometer and hydrostatic conditions. 00:39:44.840 --> 00:39:51.100 But maybe if the dike was actually emplaced at a shallower depth, 00:39:51.110 --> 00:39:55.330 or if the fluids were over-pressured, then the effective stress 00:39:55.330 --> 00:39:58.430 would already be closer to failure. 00:39:59.880 --> 00:40:04.540 This is pretty difficult to constrain in the field. 00:40:04.540 --> 00:40:09.280 But another assumption that we made that I think is worth questioning 00:40:09.280 --> 00:40:14.010 was that there’s no fluid exchange between the dike and the host rock. 00:40:14.880 --> 00:40:17.880 However, at these shallow depths, we know that water will 00:40:17.890 --> 00:40:21.400 start to exsolve out of the magma. 00:40:21.400 --> 00:40:25.260 And so – and actually, the magmatic material in the dikes 00:40:25.260 --> 00:40:29.910 at Ship Rock have about 15% porosity. 00:40:29.910 --> 00:40:35.800 So one direction for future research that I’m very interested in pursuing 00:40:35.800 --> 00:40:40.570 is understanding this process of vesiculation and trying to 00:40:40.570 --> 00:40:46.570 quantify the amount of outgassing to the host rock. 00:40:46.570 --> 00:40:50.750 So I’m also interested in learning about how these hydrothermal processes 00:40:50.750 --> 00:40:53.610 contribute to deformation in other environments, 00:40:53.610 --> 00:40:58.650 such as at mid-ocean ridges or in metamorphic aureoles. 00:40:59.860 --> 00:41:05.220 And I also want to take this time to put in a plug for another project that I’m 00:41:05.220 --> 00:41:11.820 working on with Dave Pollard related to the stability of magmatic dikes. 00:41:11.820 --> 00:41:16.780 So we’re using new analytical solutions for some elastic boundary 00:41:16.780 --> 00:41:23.810 value problems to analyze things like the effects of gravity and 00:41:23.810 --> 00:41:32.460 crustal density layering on the size and propagation of magmatic dikes. 00:41:32.460 --> 00:41:36.820 And personally, I’m also interested in extending some of those models 00:41:36.820 --> 00:41:41.870 to think about how vesiculation – vesiculation rates might affect 00:41:41.870 --> 00:41:46.580 vertical propagation or stability. 00:41:46.580 --> 00:41:51.140 So I’ll be around today, and if anyone wants to discuss 00:41:51.140 --> 00:41:54.590 any of these projects in addition to the work I presented, 00:41:54.590 --> 00:42:02.350 I would encourage you to catch me or email me at my Stanford address. 00:42:03.900 --> 00:42:06.220 So thank you all for listening. 00:42:06.220 --> 00:42:12.460 [ Applause ] 00:42:16.980 --> 00:42:18.140 - Check. 00:42:20.920 --> 00:42:23.520 - Meredith, that was a really nice talk and a – and a – 00:42:23.530 --> 00:42:26.130 you know, nice body of work. 00:42:26.130 --> 00:42:30.880 But I’m really a little skeptical about how representative the 00:42:30.880 --> 00:42:34.780 permeability values you’re getting from the near surface are. 00:42:34.780 --> 00:42:37.920 That permeability of 10 to the minus 13th meters squared is sort of 00:42:37.920 --> 00:42:42.250 like a clean sand permeability. - Mm-hmm. 00:42:42.250 --> 00:42:46.300 - I think a typical Mancos shale permeability at depth 00:42:46.300 --> 00:42:51.280 would probably be less than 10 to the minus 17th meters squared. 00:42:51.280 --> 00:42:54.720 I mean, if you look at Chris Neuzil’s review papers of the permeabilities of – 00:42:54.720 --> 00:42:58.880 on the permeabilities of shales, I think you’ll find 10 to the 00:42:58.880 --> 00:43:01.970 minus 13th just practically doesn’t exist. 00:43:01.970 --> 00:43:06.010 It’s somewhere off of the … - Yeah. 00:43:06.010 --> 00:43:09.080 - So that may be part of the issue, that you’re just – you’re assuming 00:43:09.080 --> 00:43:13.920 those modern surficial samples are representative of 1-kilometer depth. 00:43:13.920 --> 00:43:20.400 And … - Yeah. So I can respond. 00:43:20.400 --> 00:43:24.120 So – right, so we did collect surface samples. 00:43:24.680 --> 00:43:27.000 I’ll say a couple things. 00:43:27.010 --> 00:43:31.980 One is that we did test the – how the compressibility of the rock 00:43:31.980 --> 00:43:35.020 affects the permeability. So we simulated the permeability 00:43:35.020 --> 00:43:42.590 measurements at the confining stress of 25 MPa and found that this 00:43:42.590 --> 00:43:47.690 didn’t change the permeability by more than an order of magnitude. 00:43:47.690 --> 00:43:52.730 The second thing is that the Mancos – the Mancos Shale is the name 00:43:52.730 --> 00:43:56.250 of the formation, but it’s over a kilometer thick in this area 00:43:56.250 --> 00:43:59.450 and it quite heterogeneous going up section. 00:43:59.450 --> 00:44:03.080 And the section that’s exposed at Ship Rock 00:44:03.080 --> 00:44:10.020 where these joints are occurring is a coarser-grained section. 00:44:10.020 --> 00:44:14.510 And it’s pretty homogenous throughout the entire Ship Rock area. 00:44:14.510 --> 00:44:20.580 And that, you know, I am saying it’s homogenous, 00:44:20.580 --> 00:44:25.660 both because the permeability results were homogenous and also that 00:44:25.660 --> 00:44:31.430 the joints are pretty consistently spaced along the dikes. 00:44:31.430 --> 00:44:40.140 And so – yeah, so I guess, you know, the question here is, you know, how much – 00:44:40.140 --> 00:44:47.370 how different could the permeability have been 30 million years ago? 00:44:47.370 --> 00:44:53.650 And, you know, I don’t suspect the grain size really changed that much. 00:44:53.650 --> 00:44:58.510 And we have already tested for conditions and compaction. 00:44:58.510 --> 00:45:03.740 So unless there was some massive regional cement that could seal 00:45:03.740 --> 00:45:09.090 these things up, I’m not sure that I can really make this – 00:45:09.090 --> 00:45:12.100 you know, suggest that the permeability was different 00:45:12.100 --> 00:45:15.799 by four or five orders of magnitude at the time. 00:45:15.799 --> 00:45:19.070 - There’s been a lot of groundwater models that include 00:45:19.070 --> 00:45:23.230 the Mancos shale as a – as one of the units that’s simulated. 00:45:23.230 --> 00:45:25.800 What sort of permeabilities do they invoke? 00:45:25.800 --> 00:45:29.780 - Yeah. So there are a lot of models for the Mancos shale in the parts of 00:45:29.780 --> 00:45:33.780 the formation where – you know, that are gas-bearing shales. 00:45:33.780 --> 00:45:38.840 And those tighter shales are quite – have very low permeability, like, 00:45:38.840 --> 00:45:43.690 10 to the minus 18th and 19th. Yeah. 00:45:43.690 --> 00:45:46.370 - I would just suspect that those kind of values are more representative 00:45:46.370 --> 00:45:48.480 of kilometer depth. Although it might be a really 00:45:48.480 --> 00:45:51.600 unusual pocket of the Mancos that you’re – that you’re studying there. 00:45:51.600 --> 00:45:54.720 - Yeah. I think it is, actually. 00:45:59.430 --> 00:46:03.750 - Well, the most amazing thing to me about walking around 00:46:03.750 --> 00:46:10.440 Ship Rock is the fact that the alteration – there’s no alteration. 00:46:10.440 --> 00:46:14.690 You get right on up to the dike, and there’s just no alteration. 00:46:14.690 --> 00:46:20.890 So that I’m surprised the thermal models are even relevant. 00:46:20.890 --> 00:46:25.700 And by the way, there’s an extreme helium field just 00:46:25.700 --> 00:46:29.490 very close by, which – whether that’s relevant or not, 00:46:29.490 --> 00:46:36.310 I don’t know about, but heat or the fluid content … 00:46:37.660 --> 00:46:42.160 I know there’s a little – Gene Shoemaker drew a profile 00:46:42.160 --> 00:46:46.920 of Ship Rock for me, and it does show a little sag on the top. 00:46:46.920 --> 00:46:50.920 And I don’t know if he used the word “minette” or whatever it is to describe it. 00:46:50.930 --> 00:46:55.530 Well, that teeny little structure has any relevance. 00:46:55.530 --> 00:46:59.820 And I did map a gravity low that’s quite – 00:46:59.820 --> 00:47:03.810 rather symmetric with Ship Rock. 00:47:03.810 --> 00:47:08.280 You would think that would enter into interpretation, 00:47:08.280 --> 00:47:13.440 but I haven’t studied it myself since quite a while ago, so I don’t know 00:47:13.440 --> 00:47:22.940 if others have taken that into account since that – since that time. 00:47:24.120 --> 00:47:29.100 There are a lot of – of course, [inaudible], Agathla, and various 00:47:29.100 --> 00:47:33.630 other plugs, but Ship Rock is in this great isolated place 00:47:33.630 --> 00:47:37.790 that you can really try to pin it down. 00:47:37.790 --> 00:47:44.510 But I don’t see how thermal enters into the discussion. 00:47:44.510 --> 00:47:54.920 Because this – the source – the magma source down below is of consideration. 00:47:54.920 --> 00:47:58.460 And how this dike whipped its way through [chuckles] – 00:47:58.460 --> 00:48:03.560 dramatically through the Mancos is merely the top-most layer. 00:48:03.560 --> 00:48:09.210 And then all that layered to the tip of the Ship Rock, that’s not exposed anymore. 00:48:09.210 --> 00:48:11.360 It’s kind of interesting. 00:48:11.360 --> 00:48:15.400 I don’t know if I have a specific point that you’d like to address. 00:48:16.120 --> 00:48:21.260 - Yeah. Well, I’m fascinated by the detail in which you can 00:48:21.260 --> 00:48:24.450 describe these outcrops. That’s really awesome. 00:48:24.450 --> 00:48:29.310 They are wonderful outcrops for looking at these processes – 00:48:29.310 --> 00:48:35.680 that is, deformation around dikes and the growth of buds and plugs. 00:48:35.680 --> 00:48:41.390 So you’re right that there has not been regional thermal alteration 00:48:41.390 --> 00:48:47.100 the way that there is in, you know, larger plutonic environments. 00:48:47.100 --> 00:48:55.920 But I just – we’ll come back to this picture to show that the amount of 00:48:55.930 --> 00:49:02.390 alteration is definitely low, but it does happen, and it’s really localized, though. 00:49:02.390 --> 00:49:07.230 It’s localized to this zone, you know, less than a meter wide around the dike. 00:49:07.230 --> 00:49:08.740 - Oh, I see. - Yeah. 00:49:08.740 --> 00:49:10.910 So that’s the alteration that I’m referring to. 00:49:10.910 --> 00:49:15.500 It’s just that very local alteration around the dikes that causes the rock 00:49:15.500 --> 00:49:20.740 to be sort of baked up and – against the dike and preserved. 00:49:22.820 --> 00:49:31.880 [ Silence ] 00:49:33.040 --> 00:49:37.600 - Early in your talk, you alluded to an interpretation that these 00:49:37.610 --> 00:49:44.470 radial dikes are fed vertically rather than laterally from the Ship Rock center. 00:49:44.470 --> 00:49:48.680 What’s the evidence for that? - Yeah. So ... 00:49:48.680 --> 00:49:51.700 - And how might it bear in your interpretations? 00:49:51.700 --> 00:49:54.580 - Mm-hmm. Yeah. Good question. 00:49:54.580 --> 00:50:05.580 So the northeastern dike, at least, is broken up into about 35 echelon 00:50:05.580 --> 00:50:11.140 segments across this entire length, which is about 2 kilometers. 00:50:11.140 --> 00:50:16.020 So, you know, one constraint is just that the segmented nature 00:50:16.020 --> 00:50:20.560 of the dike would prevent significant lateral flow. 00:50:21.580 --> 00:50:30.700 And there are also cusps where the dike is sort of rotating where you can 00:50:30.710 --> 00:50:35.910 measure the dip of the cusp, which indicates the propagation direction. 00:50:35.910 --> 00:50:40.420 And that is vertical for the northeastern dike. 00:50:41.620 --> 00:50:45.840 However, if, for some reason, we were thinking about 00:50:45.850 --> 00:50:54.540 lateral propagation, then much of the same things would actually apply. 00:50:54.540 --> 00:51:01.290 So the parallel joints would still be parallel with the dike contacts. 00:51:01.290 --> 00:51:07.450 And, you know, I don’t know about the perpendicular joints, but … 00:51:07.450 --> 00:51:14.230 - Okay. Two questions and a comment. If the – if you had a third dimension 00:51:14.230 --> 00:51:17.390 of exposure in any length, might you find that some of the 00:51:17.390 --> 00:51:24.090 on-echelon segments really connect at depth and are fingering out 00:51:24.090 --> 00:51:27.290 vertically rather than … - Mm-hmm. 00:51:27.290 --> 00:51:31.520 - … but connected so that they could have flowed laterally? 00:51:31.520 --> 00:51:37.180 And I just wondered whether you had seen any fluting or grooving in the walls 00:51:37.180 --> 00:51:41.240 that would indicate vertical directions or whether anybody had ever tried 00:51:41.240 --> 00:51:44.980 magnetic and isotropy measurements. 00:51:44.980 --> 00:51:50.440 Because in other radial dike systems of different composition that have 00:51:50.440 --> 00:51:56.550 minerals like amphibole that are linear, it’s very common 00:51:56.550 --> 00:52:00.960 to see sub-horizontal directions. - Mm-hmm. 00:52:00.960 --> 00:52:07.700 Yeah. So there are not well-preserved grooves. 00:52:07.700 --> 00:52:13.640 And to my knowledge, no one has done magnetic anisotropy. 00:52:13.640 --> 00:52:17.110 In some fin sections that I looked at, I cut some fin sections in 00:52:17.110 --> 00:52:19.040 different orientations of the dike. 00:52:19.040 --> 00:52:25.840 And I noticed that some of the bubbles were sort of deformed vertically. 00:52:25.840 --> 00:52:30.600 So that’s sort of an indication also that, at least at the last moments, you know, 00:52:30.600 --> 00:52:34.650 things were moving either up or down. 00:52:34.650 --> 00:52:38.330 But yeah, the segments probably do connect at depth. 00:52:38.330 --> 00:52:40.800 - The bubbles would rise vertically regardless of 00:52:40.800 --> 00:52:44.580 [how the magma was in place]. - Sure. But there are phenocrysts 00:52:44.580 --> 00:52:52.980 of the phlogopite that are also oriented – sort of imbricated vertically. 00:52:55.340 --> 00:52:57.500 - I know this is kind of – kind of – certainly beyond the scope of 00:52:57.510 --> 00:53:01.120 what you’ve been doing, but would you like to speculate on what we might look 00:53:01.120 --> 00:53:06.260 for in real-time data to see if – see – maybe see this process going on? 00:53:06.260 --> 00:53:10.740 - Yeah. [laughs] That’s a great question. 00:53:12.960 --> 00:53:18.200 Of course, active volcanoes are, you know, really difficult to get close to. 00:53:18.200 --> 00:53:21.690 And, you know, any deformation that’s this localized 00:53:21.690 --> 00:53:25.030 would get covered up by eruptive products. 00:53:25.030 --> 00:53:29.290 I suppose one thing that could be – you know, I don’t know if this 00:53:29.290 --> 00:53:32.010 would be fruitful or not, but I know that sometimes 00:53:32.010 --> 00:53:37.470 a lot of fissure eruptions will sort of drain back into the conduit. 00:53:37.470 --> 00:53:42.920 And if there were somehow, I guess, a way to get into the conduit and, like, 00:53:42.920 --> 00:53:46.340 look along the walls for any evidence of, like, 00:53:46.340 --> 00:53:50.930 deformation and – I don’t know, maybe something like that. 00:53:50.930 --> 00:53:54.670 - [inaudible] [laughter] 00:53:54.670 --> 00:53:59.620 - Maybe a few years after everything has cooled off. [laughs] 00:53:59.620 --> 00:54:02.740 Send a drone down there. 00:54:02.740 --> 00:54:05.540 - I had one final question, Meredith. 00:54:05.540 --> 00:54:10.760 I was curious about the timing of the dike-parallel joints 00:54:10.760 --> 00:54:12.800 and how that fits in with your model. 00:54:12.800 --> 00:54:17.000 It just seems like you need those to be later than the perpendicular joints 00:54:17.000 --> 00:54:19.970 because it might affect the permeability? - Ah. 00:54:19.970 --> 00:54:21.980 - Next to the dike? - Yeah. 00:54:21.980 --> 00:54:28.890 So the dike-parallel joints, we pretty much know are a result 00:54:28.890 --> 00:54:32.730 of the concentration at the tip of the dike as it was propagating 00:54:32.730 --> 00:54:38.060 and that they occurred prior to the dike-perpendicular joints. 00:54:38.070 --> 00:54:42.080 So the timing on that is pretty well-constrained. 00:54:42.080 --> 00:54:46.880 The parallel joints would affect the permeability. 00:54:46.880 --> 00:54:51.010 But not in every direction. 00:54:51.010 --> 00:54:57.610 And the direction that I’m interested in is perpendicular to the dike contact. 00:54:57.610 --> 00:55:03.840 So if these joints are parallel with the contact, then, you know, 00:55:03.840 --> 00:55:09.500 assuming there’s enough compression on those parallel joints due to the 00:55:09.500 --> 00:55:13.940 magma pressure, then I don’t – I suspect that the permeability 00:55:13.940 --> 00:55:19.019 wouldn’t be terribly altered in the direction perpendicular. 00:55:19.019 --> 00:55:24.840 - Maybe there’s mineralization in those joints as well 00:55:24.840 --> 00:55:32.360 that might inhibit permeability? - Yeah. I don’t see any, but I – 00:55:32.360 --> 00:55:35.240 you know, I don’t know what it looked like at the time. 00:55:35.240 --> 00:55:39.580 I don’t know how fast mineralization takes place. 00:55:39.580 --> 00:55:42.140 I suspect these – you know, these joints formed, you know, 00:55:42.140 --> 00:55:44.940 one after the other, like, pretty quick. 00:55:44.940 --> 00:55:47.060 - Okay. Thank you very much. 00:55:47.080 --> 00:55:51.040 And if you’d like to join us for lunch, I think we’re going to meet out front 00:55:51.040 --> 00:55:54.300 by the flagpole area in five or 10 minutes and walk over to the patio. 00:55:54.300 --> 00:55:56.640 Let’s thank Meredith one more time. - Thanks. 00:55:56.640 --> 00:56:00.340 [ Applause ] 00:56:00.340 --> 00:56:32.100 [ Silence ]