David Rodgers, Professor of Geology, Idaho State University
John: Please give us a basic overview of what is happening in the Bechler area of Yellowstone.
David: The Bechler area is to the southwest of the main Yellowstone Park and Yellowstone and that area is a series of high volcanic plateaus made of really thick, viscous magma. When that magma froze it made these plateaus which were initially probably pretty smooth in texture but then a lot of snow and melt water and rain has carved those plateaus down and created lots of steep, narrow canyons and as those come down the plateau they sort of stair-step down into the meadow itself where some sediment has accumulated. It’s a little flatter in places and makes the beautiful meadow area.
J: What about all that water rushing through there creating the waterfalls and lakes? Where does that come from?
D: The water itself comes from several different sources. It comes from the snow melt because there is tons of snow that falls there every year. Then there is a lot of spring water that is really nothing more than the snow melt that initially sinks into the ground and stays there for years or decades or perhaps centuries and then finally comes back to the surface and creates the pools and so forth. And of course there is the direct rain that falls there a fair amount during the summer and spring.
J: What elevation changes are we looking at from the caldera area down to the Bechler area?
D: It’s about a thousand feet, maybe fifteen hundred feet all told but there are a lot of places that are higher and get up to total elevations of nine thousand, maybe ten thousand feet. I’m not quite sure. And so total topographic relief through there is on the order of three thousand, maybe four thousand feet.
J: How is the Bechler area different geologically from the main part of the park?
D: The perimeter of Yellowstone, especially on the west and the south is all these rhyolite plateaus of which Bechler is I think one of the prettiest. But if you get into the interior of the park you lose some of those young rhyolite plateaus, rhyolite volcanic plateaus and you get more into part of the actual caldera where the most recent large eruption was created. And then if you go elsewhere in the park you’ve got some areas that are pretty highly faulted by north trending faults and so that creates the local mountains like the Madison Range and so forth and then if you get all the way over to the north and east part of the park you’ve got some ancient volcanic, the Absoroka volcanic that create some very high mountains as you head off in that direction.
J: Is the Teton Range all part of an uplift that happened that then just falls off toward Idaho?
D: Yeah. The Tetons are an actively uplifting mountain range of course with the east side or the Jackson Hole side down and the Tetons up. It’s not a straight up and down motion. It’s actually sort of like a hinged motion so the east side of the Tetons is rising up and the backside or the Idaho side of the Tetons is just sort of sloping back into Idaho. And as you come northward the offset or displacement on that fault begins to diminish and so the Tetons aren’t as high as we head into Yellowstone Park and so the fault is active now. The fault will slip on a frequency of perhaps every few thousand years and as we go northward into Yellowstone some of those rhyolites are offset a little bit and the faults up there will slip perhaps more infrequently and so overall, the rhyolites are just offset less until finally you can’t tell that they are offset at all.
J: What about the geo-thermal activity? Why is there so much in the main part of the park and then it diminishes as you radiate out?
D: That is probably a reflection of the sources of heat. Of course for geo-thermal activity you need maybe three elements: you need a source of water, a continuous source of water. You need some sort of fracture system in the rocks so the water can move up and down quickly before it cools off and then you need a heat source down at the bottom and in the areas where you have the geyser fields and so forth you have all three elements all together. Maybe what is special about those areas is the fracture system. There is enough sort of north-south trending fractures over top of the deep line heat source and then the water can move up and down and create the geysers and the hot springs. As we come down into Bechler there are still some hot springs but not as many as in the main part of the park and that is probably for two reasons. We’re getting away from the deep line heat source which is only under the caldera portion and then there aren’t quite as many north-south fractures as there are elsewhere.
J: Can you talk about the hot spot and how that is moved and how that plays its role?
D: Yellowstone today is what it is because there is an unusually large source of heat at depth. That source of heat is so great that it has melted much of the rock from about 5 kilometers downward and it’s a pencil shaped I guess source of heat and magma so if you look down on it from on top it would be as though you are looking at the eraser of the pencil. And then as you go down into the earth it stays hot over top of that pencil profile. The ultimate source of the heat is probably down at five or six hundred kilometers depth and because of the way the earth is configured in terms of plate tectonics the upper hundred kilometers or so of the earth surface is a separate tectonic plate that moves – in the case of north America – at around three or four centimeters per year in a southwest direction.
So as the tectonic plate moves southwest over the source of heat that is much deeper it creates a track or a path as the heat burns its way up through the tectonic plate. An analogy we often use is taking a candle and passing a piece of paper of the top of it and as you pass that piece of paper over top you are going to burn a track through the paper. In our case that track is the Snake River plain and the Owyhee plateau.
J: How long is that hot spot going to be stable in the Yellowstone area?
D: As far as we know it is still moving at an average rate of around three centimeters per year. You can do the math and you can see that it is going to head on out to the northeast corner of Yellowstone in a matter of a million years and keep on cutting northward into Montana over the next many millions of years.
J: So it’s moving but in our small scale it looks more stable.
D: That’s right.
J: Talk about the caldera and how that’s the center of everything.
D: So as this heat source rises up from great depths it maintains a pretty narrow focus and I don’t think all the rocks at say five, ten and fifteen kilometers depth are molten but there are pockets of magma that accumulate and it’s a dynamic system. There is a lot of heat moving around, some rocks freeze a bit, some rocks get back up to their melting point and so from again about five kilometers depth downward, beneath what we call the Caldera is a location where most of the rocks are in a partially molten state and so that is the place where the magma is most dynamic and as it continues to melt there is going to be some volume expansion, there is going to be some gasses created and over time the pressures that cause any volcanic chamber to erupt are going to build and build and that will ultimately cause some sort of eruption in Yellowstone in the caldera area whether it’s another huge one or perhaps a smaller volume eruption, we really can’t predict but it is certainly going to be located pretty much in the same Caldera area where we was the action the last time. Things happen.
J: Talk about that larger eruption that formed the caldera.
D: Around 630 thousand years ago or so there was an eruption, a quite large eruption, enough that we now classify that as a super volcano and it started by probably some gasses and some magma venting out of a few cracks and then it started blasting out in several different places around the perimeter of the caldera and there was, in a geological term, an instantaneous de-gassing and eruption of all that magma up into the air blasting a huge cloud of rock fragments, of magma that froze as it went up into the air turning into glass and of course all the gasses that built up and so perched over Yellowstone that day and week and who knows, maybe even month that this eruption happened was this voluminous cloud of material and the winds would have distributed that downwind probably to the east and maybe to the south and ultimately that cloud which is perched up in the air gets cold and it gets really dense because of that and it collapses and so it’s not hot enough to stay up in the air and so it collapses and it catastrophically comes down, hits the earth’s surface and it spreads out laterally in one or more directions shooting across the landscape at very high speeds and finally dropping all these rock fragments and glass fragments and so forth on the ground and that is what we see today as the volcanic rock that formed at that time.
J: How unique are the Yellowstone features?
D: The process I just described where the cloud of gas and ash and rock particles is perched in the air and then comes back down is a pretty standard process that we’ve seen evidence of in the geologic record around the world. What is really special about Yellowstone is the size of all this. The magnitude or the volume of materials shot into the air is nearly unprecedented in earth history – not just North America but anything that is preserved on the earth surface. We don’t see very many of examples of this except along the Yellowstone hot spot track.
J: What about the geysers and hot springs? How unique are those features?
D: We have a few places on earth today where there are geysers and hot springs and so forth. One place is in the north island of New Zealand which actually has several geologic features that are similar to what we see in Yellowstone. Iceland is actually a very well developed geo-thermal area. In all these cases we have a heat source from underground that is rising up close to the surface, we have fracture patterns, we have a combination of volcanism and a lot of rainfall, all those things, again, combined to create the geyser system.
J: But in North America?
D: Pretty special. Down in Nevada there are geo-thermal fields that have been developed where there are isolated examples of major hot springs or even a few geysers. I know geo-thermal energy is becoming a popular energy source to exploit and we’re looking for more of these geysers – I think we know where every geyser is at this point – but in terms of hot springs there still may be a few out there waiting to be developed.
J: Talk about the process of a hot spring like what you see at Mammoth.
D: In once sense it’s a plumbing system like in your house or something like that where you’ve just got water circulating. The water comes into the ground from rain or snow and drips on down through the fractures in the rock until it gets to a point where the rock itself is hot, maybe even hot to boiling and while that is not going to hurt a rock too much it will change the water from a liquid to a vapor potentially but in any event the water heats up and it begins to then ascend or rise through the fractures in the rock because of its buoyancy and all that sort of stuff. So you get a circulation system. Just like water boiling in a pot on your stove you get a circulation system where the cold water goes down and the hot water goes up. So that’s the plumbing system and then hot water rises and potentially bubbles out to the ground.
What also happens in addition to that are chemical changes because hot and cold water are capable of dissolving or even precipitating different elements and minerals depending on what the chemical conditions are. And so in a place like Mammoth you have a system where the water goes down into the ground and as it heats up it is more and more capable of dissolving the actual rock that it flows through. And then as it rises upward and cools off it begins to precipitate some of the same minerals near the surface.
J: What kind of rock and geology are we seeing specifically in that area compared to other areas we’ve talked about?
D: In Bechler the dominant rock type is a slightly different kind of volcanic rock than I talked about previously. Sometimes when there is not very much intermix gasses in the underground magma, instead of blasting out into the air it flows in a more effusive manner or just more slowly and this particular kind of lava rock or volcanic rock is really think and gooey. Incredibly thick and gooey and so even though it’s temperature is quite hot – 800 degrees centigrade or something – it just oozes and spreads in a slow fashion across the surface of the earth and that’s mostly what you have in the Bechler meadows area, is this rhyolite volcanic rock which spread very slowly across the earth’s surface. So that is in contrast to some of the what we call pyroclastic deposits elsewhere in the park which were created as the cloud up in the sky collapsed and dumped the material.
J: So it is all volcanic rock?
D: That’s right and other than the volcanic rocks the only thing that is there is a thin layer of loosely consolidated gravel and sand and so forth. But if you look at the components of the gravel and the sand it’s all pieces of the volcanic rock which is the bedrock there.
J: How does it take on the lush appearance?
D: I think a couple of issues. Any where you can get a good soil built up you have the capability of supporting that kind of vegetation and so it’s been several hundred thousand years at Bechler since the rhyolite volcanic rock was created and in that time we’ve had the ability to create soils on top of that and that soil isn’t really thick but it’s there and the other thing is that it rains and snows a heck of a lot. In contrast if you go to Craters, some of those basaltic lava flows are only two thousand years old as opposed to several hundred thousand years. That doesn’t give it enough time really to develop a soil cover. And the second thing is that Craters gets about 8 inches of precipitation a year compared to, I don’t know, it must be 50 or 60 at Bechler Meadows.
J: Rainfall and precipitation really creates what you see in Bechler.
D: Yeah. Time and rainfall more than anything.
J: Do you personally spend any time up there?
D: Yeah, I’ve been to Bechler Meadows hiking with friends two or three times. One of the best hikes I ever had was with young kids going up there. Three families and young kids and hiking through the meadows and the wildlife that we saw over those two or three days were spectacular.
J: With your background what is your appreciation for the Yellowstone area?
D: I think twenty, thirty years of training in geology has taught me the element of time. A geologist can look at a landscape and can realize that the processes that create that landscape may be spread out in time over millions of years but it doesn’t make it any less exciting. And so what it takes to create a Yellowstone type landscape is just a phenomenal series of processes that if we could see them over the course of human life time would be shocking. And I guess a geologist is trained to compress geologic time and think about it almost in a human timeframe and that is something that is really special.