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Yellowstone Lake Geology Talk Transcript

Speaker: Lisa Morgan, USGS, February 10, 2004
Title: The floor of Yellowstone Lake is anything but quiet: Volcanic and hydrothermal processes in a large lake above a magma chamber

Hank Heasler (Yellowstone National Park Geologist): This isn’t the title that I distributed to all of you. That’s because Yellowstone’s geology is so dynamic that we’ve already had a change in the title.

Audience laughs.

Hank Heasler: Lisa will be covering the last 8 or 9 years of her work on Yellowstone Lake and we will leave time at the end for any questions you might have about the pending eruption of the Yellowstone Volcano.

Audience laughs.

Lisa Morgan:

I’m going to start this morning by just showing a diagram of mapping that had been done in Yellowstone Lake just so people can see the difference in the level of resolution of the kind of data that we’ve been able to collect. This is the previous map, that was available for Yellowstone Lake, that was done by Matt Kaplinski in 1991. Resolution on this data set is somewhere between 10 and 50 meters. In 2000 we mapped West Thumb Basin. You can see that the resolution here, at 1 meter, is much greater than we could see on the previous map. It’s almost like having the cataracts taken off of your eyes and you can see much greater details in terms of the features that are on the floor of Yellowstone Lake.

The title of my talk today is “The floor of Yellowstone Lake is anything but quiet: Volcanic and hydrothermal processes in a large lake above a magma chamber.” This is a joint venture between the US Geological Survey and the National Park Service. The floor of Yellowstone Lake is anything but quiet; when you think of most lake environments, they are catchment basins and they are very quiet. Yellowstone Lake is not like that. It is also a catchment basin, but it is also quite active.

Okay, this work was done with a true team effort. It involved many scientists including myself, Pat Shanks, Sam Johnson, Ken Pierce, a lot of people from the US Geological Survey, individuals from Eastern Oceanics and L-3 Communications, Elac Nautik.

We had many funding sources. Just the contract costs for this work came to about $600,000. We required a lot of partnership here. So we had multiple sources from within the US Geological Survey in the form of programs, and divisions, and different science centers, as well as, the National Park Service and also from the private sector through the Yellowstone Park Foundation and the Yellowstone Association Institute.

I don’t think I need to tell you exactly where Yellowstone is. But the main point of this particular slide is to locate for you, Yellowstone Lake in context with the 640,000 year old Yellowstone Caldera. Yellowstone Lake straddles the southeast margin of the Yellowstone Caldera.

We were definitely not the first people to map Yellowstone Lake. In fact, when the Hayden Survey came in 1871, they completed a map, that looks like this, of Yellowstone Lake. Henry Elliot was the Chief Mapper in the mapping of Yellowstone Lake. This was a … well it’s not exactly how we see Yellowstone Lake today. This was an incredible effort that took about 24 days, about twenty-some thousand dollars, and these guys collected over three hundred soundings during their time. They made a significant and monumental effort to do this. Everybody has seen Annie, and that’s the boat they collected their data in; pretty phenomenal, a blanket and a box.

In 1876, Hague, of the US Geological Survey, published a report, which it had a new configuration of Yellowstone Lake. During the Hague Survey, they did not collect any more soundings from Yellowstone Lake, but an individual named Chase re-triangulated the boundaries of Yellowstone Lake, making it appear much more like we see it today.

The next major effort that produced a bathymetric map was by Matt Kaplinski. This was a monumental effort. It was a very impressive map for the time. He was using state-of-the-art technology for the late 1980s. He used a single-channel echo-sounder and a mini-ranger, which allowed triangulation for locating his points, his location, and produced a map which is what we see over here.

Yellowstone Lake is the centerpiece of the Yellowstone Geoecosystem. At 341 square kilometers, Yellowstone Lake is the largest high-altitude lake in North America. High altitude being defined as any elevation greater than 7,000 feet. There are over 144 tributaries that flow into Yellowstone Lake. The major tributary is the Yellowstone River, which enters Yellowstone Lake at Southeast Arm. Only one tributary flows out of Yellowstone Lake, which is the Yellowstone River.

One of the reasons that the National Park Service was interested in our doing a higher resolution map of the lake floor was the issue of the cutthroat trout and them being consumed aggressively by the illegally introduced lake trout. It is estimated by fisheries biologists in Yellowstone that approximately 59 to 60 cutthroat trout are consumed by one lake trout on an annual basis. So it is basically decimating the cutthroat trout population. The lake trout live, all year, within Yellowstone Lake whereas the cutthroat trout go and spawn in all these different tributaries and are primary food sources for larger fauna, such as grizzly bear, osprey, bald eagles, and otters. It is also the last place to have any geologic map of the park. So it was like a big blue hole in the middle of an area where …. The park has been pretty extensively mapped.

What we are looking at here is an oblique view, looking in from the southeast, of the entire park. We have shaded topographic relief and on top of that we’ve draped a colored geologic map. The colors represent different geologic units and I am just going to focus on a couple of those.

The topographic margin of the Yellowstone Caldera comes in through Flat Mountain Arm, cuts across the south part of Frank Island, comes up across the Central Basin, and then comes out here and then comes on up and comes around and then comes on down like this.

The yellow units represent the youngest units on this map. They are primarily just sediments. The formation of the Yellowstone Caldera started with the eruption of the Lava Creek Tuff which is this lighter brown unit which is distributed in a radial pattern around the caldera. That occurred at around 640,000 years ago. It is estimated that a minimum volume of 1,000 cubic kilometers of ash, of pyroclastic flows, came out of that eruption. Just for comparison, Mt. St. Helens erupted about 1 cubic kilometer in 1980 and everyone remembers the havoc that Mt. St. Helens brought in 1980.

The next major geologic event within the Yellowstone Caldera was the implacement of these two domes, Mallard Dome and Sour Creek Dome, which occurred, somewhere on the order of, 50,000 to 100,000 years after the cataclysmic caldera-forming eruption. Later, we had emplacement of these pink units, which we refer to as post-caldera rhyolitic lava flows. They are the land forms that you see so prevalent in the Central Plateau and the Madison Plateau, that have these very steep sides and hummocky tops. They also control, within the caldera, the hydrology of the system.

Part of this post-caldera rhyolitic activity … these lavas came out between 150,000 to 70,000 years ago. Part of that activity included the eruption of the West Thumb Caldera, which is a smaller caldera, approximately the size of Crater Lake, within a larger caldera, the Yellowstone Caldera.

Later hydrothermal activity produced these smaller hydrothermal explosion craters, that you see around the lake and those are all post-glacial. The glacial ice receded from the Central Basin about 16,000 years ago. These large hydrothermal explosion craters have formed over the last 14,000 years. The youngest one is at Indian Pond at 3,000 years ago.

These blue, or purple, units over on the east side of the park are the Absaroka volcanic group. They are a different type of volcanic series. They are a different composition and they erupted somewhere between 47 and 54 million years ago.

We worked using the National Park Service’s RV, Cutthroat, and it is about a 26 foot long aluminum boat. We used multiple techniques to map the floor of the lake. We used a differential GPS, which gives our surface accuracy of less than one meter. We used a hull-mounted sonar swath mapping system. Basically this is a cartoon that shows the array of the multi-beams. It has a sonification angle of about 150 degrees allowing a swath width approximately 8 times the water depth. So for an area that is 10 meters deep, we are getting a swath width of about 80 meters. This allowed us to get continuous and overlapping data. This is the first time anyone has been able to collect this type of data in Yellowstone Lake. When Matt Kaplinski did his mapping, he collected a whole bunch of individual single points and then interpolated those points. So there was a lot of interpretation involved in there. We complimented out bathymetric survey with a sub-bottom seismic reflection survey as well. It is about a 2 to 15 KHz frequency allowing a maximum penetration of the lake floor between 20 and 30 meters. Previous people, Otis and Smith, had done some seismic work here in the 1980s but their seismic frequency allowed much deeper penetration and not as high a resolution in the upper 20 to 30 meters. Our two surveys, with Otis and Smith, and this recent survey, are very complimentary. The Otis and Smith lets us see deeper. The new survey lets us see, in very good detail the very upper 20 to 30 meters. All of these techniques, that I’ve just described, are imaging techniques. I am a true believer in ground-truthing and it was very important that we ground-truthed some of our findings in the lake. So we deployed a submersible, remotely operated, vehicle to collect solid samples, vent fluids, temperatures, institute chemistry, and photographically document what we were seeing on the floor of the lake.

This is the completed bathymetric map of Yellowstone Lake. Where the Hayden Survey collected over 300 data points for Yellowstone Lake our survey collected over 240 million data points. So it is a much higher resolution image. This was a major effort. It took us over 64 days over the course of 4 years.

We initiated our mapping of the floor of Yellowstone Lake in the Northern Basin in 1999. In 2000 we moved to West Thumb Basin. In 2001 we moved into the Central Basin. And we completed the survey in 2002 with Flat Mountain, South, and Southeast Arms.

This is one version of a bathymetric map. The colors depicted here; red represents shallow areas, dark blues or purples represent deeper areas. The range in depths in Yellowstone Lake range from less than 1 meter to about 131 meters. The deepest part of Yellowstone Lake is a 200 meter wide crater off of Stevenson Island at 131 meters. Clearly there are deep areas in the Central Basin as well as in the West Thumb Basin.

The resolution of our surveys allowed us to identify features that really had not been previously seen in the lake. We were able, for the first time, to accurately identify where the topographic margin of the Yellowstone Caldera is. This correlates very well with our high-resolution aeromagnetic data that the US Geological Survey collected in 1996. We were also able to identify, for the first time, the fact that rhyolitic lava flows are on the floor of Yellowstone Lake on the northern side of the caldera margin and that those lava flows control the distribution of hydrothermal features in the lake.

Previous to our work, workers had identified the Mary Bay explosion crater as a large hydrothermal explosion crater. Our work was able to identify several more hydrothermal explosion craters. Here is one that we refer to as Elliot’s Crater. Here is a second one that we refer to as Evil Twin. Here is another one that we don’t have a name for and these may be some other hydrothermal explosion craters.

With my colleague, Ken Pierce, we’ve been able to date some of these hydrothermal explosion events. The oldest hydrothermal explosion events started with the Mary Bay explosion at 13.8 thousand years ago. The next major event that we have dated radiometrically is Turbid Lake. Turbid Lake erupted about 8.3 thousand years ago. And then Indian Pond, which is right over here, is 3 thousand years old.

Storm Point, which is not an explosion crater, but definitely a hydrothermal feature, is 4 to 6 thousand years old. We estimate that the age of Duck Lake is somewhere between 6 to 8 thousand years, as is Evil Twin. We estimate that Elliot is about 6 to 8 thousand and that this older crater may be on the order of 10 to 11 thousand years old.

We were also able to identify a major hydrothermal field in Yellowstone Lake that had not been documented before. With my colleague Pat Shanks, he has been able to estimate that 10 percent of the total hydrothermal flux in Yellowstone National Park comes from the floor of Yellowstone Lake. He has estimated that this is slightly larger than the flux that comes out of Norris Geyser Basin. So they are comparable sized fields.

Dave Lovalvo identified spires back in 1997 using his submersible in Bridge Bay. We were also able to extend the young Eagle Bay Mountain Fault, which had been mapped by Ken Pierce, Phil Locke, and Grant Meyer, down here. But we were able to extend it up through the lake, up west of Stevenson Island and then coming out and connecting it to another fault system that we refer to as the Outlet Graben and this is a very young structure.

Our surveys further identified extensive landslides on the eastern – you can see one sticking out there, you can see them over here, and on the western side of Yellowstone Lake as well as large detachment blocks. This particular structure, we think, is a large piece of Absaroka volcanic block that slid down from this steep west side of the Absaroka Mountains which are right off of the map on the east side of the lake.

In the southern part of the lake, we were able to identify a series of glacial melt water features very similar to kettle terrains that we see in Jackson Hole. We were able to identify a whole series of submerged lake shorelines which record the heavy breathing of the inflation and the deflation of the Yellowstone Caldera, still an active structure. We were able to identify a new series of features that we are referring to as the Weasel Creek Storm Point Vent System.

This is the first geologic map of Yellowstone Lake. Here we have the caldera margin, the topographic margin of the Yellowstone Caldera, coming through the lake. On the north side of the lake you can see the geologic processes that were dominant were primarily volcanic and hydrothermal in nature. These blue and purplish units are all different rhyolitic lava flows that we’ve been able to correlate with rhyolitic lava flows, these units in pink, on land. You can see these kind of tan features are hydrothermal explosion deposits that are on the floor of the lake. And these tiny little red dots represent hydrothermal vents and small craters.

On the south side of the caldera margin we see a landscape that’s been dominated by glacial and river types of processes. At about 24 to 16 thousand years ago we had the development of a significant glacial ice cap over the central basin. The thickness of the ice there was estimated by Ken Pierce to be on the order of a thousand kilometers thick. This is the axis of maximum ice thickness at 11,000 feet. The lake surface is 7,733 feet right now and Ken’s estimated that this ice cap had about 3 thousand more feet or one kilometers of elevation there. So it was a significant ice feature in the lake. The last ice receded from the central lake basin around 16,000 years ago.

I need to update this particular map but one of the things that we were able to do with the seismic data and then combining the seismic data with the bathymetric data was to identify the distribution of these hydrothermal vents. We noted that there is a very close spatial relationship between the bathymetry, or areas of high relief, and the distribution of the hydrothermal vents. Except for within the large hydrothermal explosion craters, most of the hydrothermal vents are confined to the edges of the rhyolitic lava flows.

This is a bathymetric image of West Thumb Basin. The next slide I am going to show you is a seismic reflection profile going from north to south through this part of the basin. This is the seismic reflection profile. Here is north and here is south. What we are seeing here … We are able to penetrate into the lake sub-bottom and we are able to see, what we refer to as a high amplitude reflector, which we have interpreted as the top of a rhyolitic lava flow. We have a whole series of glacial, and later lacustrine sediments, or lake sediments, that were deposited on top of that rhyolitic lava flow and later hydrothermal activity has come in, created little gas pockets, that you can see as these dark areas, within the lake sediments. We think that these gasses are either steam, or CO2 or a combination of the two or it may be a gas charged fluid. You can see smaller domes. Here’s a gas bubble that is basically doming up some of these sediments. These lake sediments are generally pretty laminar or horizontal and the gas pockets have gently pushed up those lake sediments. We’ve also been able, with our seismic data, to identify all kinds of different faults.

Hank had said that I had started my lake work back in ’95 and while I hadn’t gotten into the lake, I started doing stratagraphic studies of the deposits on the shores of the lake. I had started this work with Ken Pierce. We were looking at the large hydrothermal explosion breccia deposits that are exposed on the shores of Yellowstone Lake. The top photograph shows the bottom of the Mary Bay explosion breccia deposit in contact with a pretty unusual sand.

Once you start mapping this, you see a very close spatial relationship between this sand and the base of the Mary Bay explosion crater. The Mary Bay explosion crater is the largest hydrothermal explosion crater yet documented in the world. It represents a very extreme event and so it’s a very rare occurrence. So you don’t get these kinds of things every day. When you go at different exposures and look at the base of the Mary Bay explosion breccia with the sand, these sands are quite variable in nature. You can map these sands as far north as 6 kilometers, into the southern Pelican Valley. We also have evidence that there were multiple events in the development of the Mary Bay explosion crater. It probably started out with a major explosion and then there were smaller explosions over a very short period of time, probably less than 100 years.

We can see at least one other sand layer within the Mary Bay explosion breccia. We know from the lake sediments that are within this breccia deposit that there was a least 10 to maybe 20 years of quiescence between two of these events.

In looking at what triggered the Mary Bay explosion crater, one possible scenario is that there may have been seismic activity along the Outlet Graben or the Lake Hotel Fault Zone that may be associated with the Mary Bay event. The Outlet Graben is right here, about 8 kilometers from Mary Bay. This work down here was worked up by one of my USGS colleagues, Sam Johnson. This is a seismic projection profile through the Outlet Graben. What you see here is the development of the Outlet Graben. The first major event along the Outlet Graben is estimated to have occurred about 12 to 15 thousand years ago with a total displacement of about one meter. A second major event occurred approximately 7.5 to 12.5 thousand years ago and total displacement of 15 centimeters. Then we estimate that there was another significant event somewhere between 100 and 2,100 years ago that resulted in a total displacement of about 3 meters.

One of the scenarios is that you had seismicity along this fault starting about 12 to 15 thousand years ago, 13.8 thousand fits in very well, and that that seismicity may have generated a large tsunami, or large wave deposit, that resulted in significant displacement of water and lowering of the hydrostatic head over what at that point and time was a sealed hydrothermal system in Mary Bay. And that that triggered the hydrothermal explosion event that we know now developed Mary Bay.

Another feature that our mapping has found is this structure that we call the Weasel Creek Linear Trend which now is on the Weasel Creek-Storm Point Vent System. What we see with Weasel Creek is that Weasel Creek is an unusual stream valley in that it’s very straight. It is very similar to the valleys you see up in Elephant Back if you’ve ever kind of gone off trail and gone down where these red lines are, that are mapped as faults, but I really think they are more like fissures. When the caldera has inflated in the past, it has created cracks, and then it comes down, and then it inflates again. These cracks, or fissures, act as conduits for hydrothermal fluids to come up and either alter the rock, help contribute to hydrothemal systems, and maybe even that system could develop into a structural dome like what we see at Storm Point or a hydrothermal explosion crater like we see at Indian Pond.

This Weasel Creek Linear Trend can be in the aeromagnetic data. This is a map of the high resolution aeromagnetic data and without going into to much detail here, suffice it to say that areas in blue represent low values of magnetic intensity, meaning that the rock is not magnetized anymore. These areas in red represent higher values of magnetic intensity. When you get these rocks within the caldera, all of them are high silica rhyolites which basically translates to a silica content of about 74 percent. These rhyolites have different crystals in them. The ones that carry the magnetic phases, or the magnetism, are magnetite and that is a magnetic mineral. When that is altered it alters the hematite, which really has no magnetization, or elmanite, which is very weakly magnetic. You can see where the hydrothermal activity has really focussed.

This is a map that I took out of a paper by another colleague at the US Geological Survey, Dan Dzurisin. You can see Yellowstone Lake, there is the outline of Yellowstone Park. Here you have the Yellowstone Caldera and these lines here represent areas of inflation of the caldera from 1923 to 1977. The maximum amount of inflation is represented by this dashed line filled in with orange that is about 700 millimeters of uplift in that 50-year period of time. If you look where this Weasel Creek Linear Trend is, it’s somewhere between the 400 and 500 millimeters up uplift in that period of time. We suggest that the Weasel Creek Linear Trend is really just another fracture system in the Elephant Back lava flow and part of the inflation and deflation of the Yellowstone Caldera.

This is the Weasel Creek-Storm Point Vent System. We discovered it in 1999. This is an oblique view. It was produced by another colleague at L-3 Communications. The vertical exaggeration is approximately 2 ½ times. Again, this is a bathymetric map. The areas in blue represent low areas. Areas in red represent shallower areas. This structure is about twenty meters below the lake surface.

We see in the seismic reflection data a whole series of domal structures. You can see here multiple, and what turn out to be, very vigorous hydrothermal vents. The dimensions of this structure are approximately 700 meters in diameter and a total positive relief of about 30 meters. We do not think that this total 30 meters of relief can be attributed only to hydrothermal activity. We believe that a significant portion of that is associated with the pre-existing topographic relief of a rhyolitic lava flow. Then you had the glacial sediments and lake sediments deposited on top of the lava flow. The hydrothermal system has been in the interface of the rhyolitic lava flow and that lake sediment.

We’ve been able to deploy the submersible, remotely operated vehicle to measure temperatures. Here I have it in excess of 85 degrees. This summer we were measuring temperatures near the boiling point. In 2002 we were out in the lake in late September and we noted the following observations. We were on top of this and we were ready to deploy the ROV. We smelled a very strong scent of hydrogen sulfide. We could see in the shallow sub-surface of the lake, a plume of finely disseminated sediments having a diameter of maybe 50 to 70 meters in diameter. The surface of the lake looked like a lot of little bubbles, kind of just coming to the surface. That was quite interesting and so we went back in August of 2003 and we really did not observe the same features. We saw some bubbling, but not as much as we had observed in the previous year in late September. Yet later in 2003, late in October, the entrained sediments came back, the strong smell of H2S came back and the bubbling on the surface of the lake had increased somewhat.

This is a map view, instead of an oblique view, of a gray-shaded relief of the Weasel Creek-Storm Point Vent System. Here you can see all of these active different of vents that are on this structure. This is what we refer to as a back-scatter diagram. And we collect that with our swath sonar bathymetry. An easy way to look at it is, how hard is the surface and how able is the sound to penetrate into the surface. Where you see the white areas, these are areas that we are interpreting as silicified, or part of a hydrothermal cap. These darker areas represent active hydrothermal vents and the soil, the lake floor, is not hard there.

We’ve been doing differential bathymetric analyses comparing our bathymetric data between 1999 and 2002. In order to do that, we first had to estimate what the lake level variation was from 1999 to 2002. The resolution of that data is going to come out at about 60 centimeters. Our analyses thus far indicate that there has been no differential movement on this structure, whatsoever.

We deployed in 2003, in working with the University of Minnesota, real-time in situ chemical sensors, which were able to measure CO2, H2S, hydrogen, and pH and also measure temperature in real-time. We believe that phenomenon observed in 2002 and 2003, may be associated with an annual disturbance, similar to what we see in Norris Geyser Basin. It has been documented ever since the first surveys came here in late summer/fall when the lake levels drop lower and lower the hydrostatic head over these vent systems.

This is a diagram that has been prepared by my colleague, Pat Shanks. On one axis you have sulfate over chloride versus pH. The main thing I want to note here is that the sulfate is derived from the hydrogen sulfide during oxidation, during mixing with the oxygenated lake water. This dilutes the chloride with the N member fluids, those being the H2S and lake water. The sulfide oxidation produces hydrogen which lowers the pH. The result from these analyses in ‘99, ‘02, and ’03 is that there is no apparent change in the vent chemistry from vents on the Weasel Creek-Storm Point Vent System.

So what are the possible scenarios for the Weasel Creek-Storm Point Vent System? One very, I think, remote possibility is that maybe it could develop into a hydrothermal explosion crater. But those are pretty rare events compared to these following events. One could be that the Weasel Creek-Storm Point Vent System is similar to Storm Point where you have a structural dome in the true sense of the word. You have beach sediments that are dipping to the east on the east side of the structure, to the south on the south side of the structure, to the west on the west side of the structure, that it’s been somewhat inflated by hydrothermal activity. You’ve developed a sealed cap and then over time that those sealed caps have collapsed in on themselves.

This is Collapse Crater over at West Thumb. When you go up and you look at the crater field that exists on Storm Point there is very little, if any, evidence for any kind of explosive activity. So up here on Storm Point what we have is a series of collapsed craters. This is the east side of Storm Point where you can see structures kind of emerging … This was done in late October of this year. I took this picture. Here you see these kind of domed up sediments that have been uplifted and then their axis has been breached allowing the vent fluids to escape. You see structures like this very common on the lake floor. There are dozens of these kinds of structures and today if you went over to Sedge Bay you would see one of these structures emergent right now.

Other findings in Yellowstone Lake included siliceous spires in Bridge Bay. We believe from their morphology and chemistry and their association with vents, that these are clearly hydrothermal in origin. This is a sample of a baby spire, half of which is here in the park and will eventually go on display. This was collected back in 2000 by National Park Service rangers here. What you see is the external coat of manganese and iron oxide. It is just a very thin coat and the interior is a pretty white. It looks almost like Styrofoam, very fine-grained but kind of porous. This is a scanning electron microscope image of a sample from the spire. One of the big surprises we found was that, instead of just pure amorphous silica, forming the spire, that the spire was formed by filamentous bacteria that was later silicified and diatoms. And there is amorphous silica holding that together.

This is PPMs versus various elements that we know are predominant in hydrothermal systems. There, basically, is not a whole lot of difference between the oxidized exterior and the interior.

So what are the implications for resource management for the national park? Our detailed map provides information regarding the geologic features on the lake floor. We have been able to identify the role of geologic processes on the biodiversity in the lake and the food chain. When you go to a lot of these hydrothermal vents, one of the things you observe are all of these sulfur-eating bacterial mats that are all around these vents. These sulfur-eating bacteria are prime food sources for amphipods and other similar type organisms in the lake. These are food sources for the cutthroat trout. The cutthroat are then eaten by the lake trout. If the cutthroat get out of the lake and go into the tributaries to feed, they have significant impact on the bear, otters, osprey, and bald eagles.

There has been work done by Pat Shanks, Bob Riot, the USGS, as well as, I think, Bob Schwartz up at MSU and USGS on chemistry of bear hair. In preliminary studies they have been able to identify elevated levels of mercury in the bear hair from two bears that live close to the lake as opposed to two other bears that do not live close to the lake. You can see a progression from the hydrothermal vents all the way up into the grizzly bears.

There are significant issues of preservation and management of the hydrothermal features. I know that when people and I have given this talk before, they said “Oh, well where can we get the coordinates to dive on Bridge Bay spires?” I think that is something that the National Park Service really has to address. Right now, there is a ‘No Boat Zone’ in the Bridge Bay area. I think that needs to continue to be enforced.

There are also the issues of monitoring different hydrothermal features and lake levels that I think the park would be interested in. Our detailed maps will inform Yellowstone on the spawning areas of the exotic and aggressive lake trout. Right now, we are working with individuals in fisheries on seismic reflection data in trying to characterize that seismic reflection data and looking at where they know are established spawning areas of lake trout and then using that seismic data to go to similar areas and see if lake trout are spawning there. Finally, but not least, we’ve identified potentially hazardous thermal and seismic features as well as landslide deposits, collapsed features, and large tsunami lake wave deposits.

Okay, so what we are going to do, none of you wore your swimming suits, but we are going for a swim. We are going to go into the Southeast Arm. This is a 3D animated view. We are going to fly through the lake. What we are doing, is looking into Southeast Arm. This is an oblique bathymetric image that was prepared by Boris Schultz at L-3 Communications and here we go.

(Screen goes black.) Oh, No. Well, this is a small enough group that we could look at this (laptop computer). I don’t know why it’s not working. This is the first… If people want to look at this, if you … We’re going to get to the end here. Sorry. We can go do that one more time because it’s really fun. We’re coming into West Thumb and there you see the edge of the rhyolitic lava flow. There are a bunch of hydrothermal vents at the edge of the rhyolitic lava flow.

If we can do this… We’ll make it go one more time. So, here we go. Here’s the glacial terrain of Southeast Arm. We’re coming down the Southeast Arm. There’s the caldera boundary, right there, those little vents. There’s Frank Island, we’re going to turn west here. We’re going to come on the north side of Frank Island. These are all rhyolitic lava flows that are draped with later glacial and lake sediments. We’re coming down through breeze bay and then into the West Thumb caldera. Our caldera inside of a caldera.

The USGS just acquired this program, which is a pretty expensive program. One of the things I would like to work on with the park is to create fly-throughs for research purposes as well as visitor enjoyment purposes. You can pre-program this to take it where you want people to go, or you can take it where you want to go. That’s something that we can talk about. It’s kind of fun.

So, that’s the end of my talk. I don’t know if there are any questions but I’d be happy to answer any questions.

Audience Member: Those lava flows that are under that lake at Storm Point and West Thumb. Are those sub-areal flows that had been covered by the lake or were they sub-patheous flows?

Lisa Morgan: I think that they were sub-areal flows when they were emplaced. Now the one in Storm Point and in Mary Bay, that has never been mapped. We think that that I either a shallow intrusive body, or an earlier post-caldera rhyolitic lava flow. Bill MacIntosh at the New Mexico Bureau of Mines has done an Argon 40-39 date on a couple of samples from that and we have dates of about 140 to 144 thousand years for that particular unit. It is the dominent lithic in the hydrothermal explosion breccia of Mary Bay. So we think in the case of the hydrothermal explosion of Mary Bay, that it was a pretty deep explosion and that it penetrated into that cap rock of the rhyolite. I should say, we have no evidence in Yellowstone National Park for a hydrothermal event system ever triggering a volcanic event. So we don’t have any evidence for that and I don’t think it exists right now.

Audience Member: The explosions, like Turbid Lake and Mary Bay, they’re not at the edge of rhyolitic lava flows? They’re actually underground explosions?

Lisa Morgan: Turbid Lake has what we are now referring to as the Pelican Creek lava, which is this unmapped … I brought Bob Christiansen up and showed him this and he had never seen it before. I just think, it’s not exposed. It’s there but it’s just not exposed and I think the only evidence we have is in the clast. We think that that flow probably goes to Turbid and is at the edge of Turbid. We also know though that at Turbid, we have a lot of Lava Creek tuff in there. I’ve been doing some really intensive studies of the different lithic clasts in the hydrothermal explosion breccias. There seems to be, at least for Turbid Lake, Mary Bay, and Indian Pond, that there seems to be this following rule of thumb. The wider the crater, the deeper it goes and the more complex is the lithic clasts composition in that deposit. For example Mary Bay, which is 2 ½ to 3 kilometers in diameter, has everything from this unmapped rhyolite, to solicified lake sediments, to beach sediments, to fractured breccias. We see evidence for multiple brecciation within one breccia piece. With respect to Turbid Lake, the stratigraphy there is just Lava Creek in the clast and the beach sediments and some lake sediments. For Indian Pond, all you get in there are primarily beach sediments and maybe a little bit of lake sediments. It wasn’t a very deep structure. I don’t know if that is going to hold for the rest of our studies. Mary Bay also coincides, and I forgot to say that earlier, with probably one of the highest areas of heat flow in the park. I think it wasn’t just being associated with the rhyolitic lava flow in the case of Mary Bay, but you had this very intense area of high-heat flow in that northern part of the lake which really helped to amplify that.

Audience Member: Did you put a date on the West Thumb explosion?

Lisa Morgan: I think it was about 140 thousand years. It is part of that whole post-caldera rhyolitic lava flow system. One of the units I didn’t show but that was shown in my map is the tuff of Bluff Point which is pyroclastic flow deposit and that is, I think, one of the few pyroclastic flow deposits that is associated with the post-caldera rhyolitic volcanism. Most of that are these big, viscous lava flows.

Audience Member: In terms of what you are talking about with the Weasel Creek-Storm Point System and of course with the news talking about the inflated plain, I assume these are the same. When interpreting to the general public or to classroom groups, things like that, how would you interpret that for the general public?

Lisa Morgan: It’s an active hydrothermal system. It has multiple, vigorous vents on it. These vents are very close to boiling temperatures. That they have high H2S. You know, it’s not an unusual vent system for Yellowstone. I mean, where are we? We’re on top of one of the world’s largest calderas and it is still an active caldera.

Audience Member: I think that’s the big point right there. That it is not unusual for Yellowstone. Comments that I’ve heard from people coming in is like “is it going to explode?” or whatever else. It’s in the media, it’s in the news.

Lisa Morgan: This whole thing that’s been in the news with respect, “Oh, there’s this big bulge or bubble on the bottom of Yellowstone Lake and that that’s going to trigger a major, caldera-forming eruption.” That is so untrue. One, as I said previously, we have no evidence of any interaction of water and magma in the park. Two, the USGS with University of Utah and the National Park Service has a pretty intensive seismic network system and monitoring system in place. If we saw anything that we saw as unusual, we would all know about that. We see no evidence of really unusual events, and I would say that the events that we have seen in the last year are pretty much normal for Yellowstone. The fact is, Yellowstone is one of the most dynamic places on earth and I think we have to understand that it is not the middle of Kansas.

(There was an audience question that was not recorded because video tapes were being changed in the camera. The following is a portion of the reply.)

Lisa Morgan: Along the Weasel Creek-Storm Point Vent System we only see this trough and this linear trend. I think if you went up to Elephant Back, while those features are mapped as faults, you are pretty hard-pressed to see any kind of displacement. Even if you talked with Bob Christiansen he would say those are probably more like fractures and not faults.

Audience Member: In that linear bulge that you were showing trending sort of northeast, would that have intersected the river at LeHardy Rapids. (Other visitors join in the description as Lisa tries to understand the question.)

Lisa Morgan: Oh, the inflation? That wasn’t a bulge! (Everyone laughs.) Those are isopachs or lines of equal amounts of inflation that Dan Dzurisin had mapped associated with his differential GPS surveys that he’s been doing, I think since, I don’t know, 1980 or maybe he was back here in the 70s. He’s been doing those for a long time. He’s been able to pretty much track the inflation and deflation.

Audience Member: And would that meet the river at LeHardy Rapids?

Lisa Morgan: Oh yeah. That was LeHardy Rapids and that’s basically what’s controlling the lake level.

I just want to say also, that we have different papers that are available and I’m hoping to have for display and distribution this summer different maps of Yellowstone Lake and if people are interested, I think Hank knows my email and my information.

Hank Heasler: On the Yellowstone Volcano Observatory Website we have links to many of your publications. Lisa, thank you.

Lisa Morgan: Thank you.

(Audience applause.)

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Last Updated:Tuesday, 30-Mar-04 14:29:29