Scott Hughes – Professor of Geology and Chair of Dept. of Geosciences, Idaho State University
John: Give us a brief overview of the Yellowstone volcano caldera area.
Scott: The Yellowstone, and I like the way you say caldera area because it’s not one caldera. It’s a series of calderas and three main eruptions produced these calderas. These are what are called super eruptions and one occurred about 2.1 million years ago. Another one occurred about 1.3 million years ago and then the last one about 600,000 years ago.
Now these super eruptions are the big ones but in between there have been a lot of smaller eruptions, lava flows and domes and so forth that are active in Yellowstone today. And so what happens is that during a caldera eruption a very large magma chamber beneath the crust slowly reaches a level and an accumulation of volatiles meaning the gasses and driving forces, the pressure necessary to squirt out on fractures in sort of a ring-like pattern and then once the top part of this magma chamber is evacuated it is evacuated by eruptions of ash and pumice and the hot stuff coming from a super volcano. There is nothing to support the roof and so the roof over the chamber sort of sags. It collapses down and it collapses down into a depression and you are left with an escarpment all the way around this depression and that is what the caldera really is and this depression can be ten or twenty miles in diameter.
J: And that’s what you have in the main part of Yellowstone Park?
S: Right. And it is a series of nested calderas. The first one that occurred over 2 million years ago produced what was called the Henry’s Fork Caldera which is up in Island Park and then the second caldera eruption produced what was called the Mesa Falls eruption, the Mesa Falls Tuff and then the final one was the Lava Creek Tuff from which is the caldera we see in the Yellowstone itself which encompasses much of the park, the north side around Mammoth. The scarf is visible up there; it’s partially visible around the west entrance and so forth.
J: So this greater Yellowstone area is actually a lot bigger than the park.
S: The greater Yellowstone area – it’s a terrain that is made up of a lot of older rocks, meaning older than two or three million years – and a lot of folded sedimentary rocks and older volcanic rocks too that have been part of the broader processes in the region for the last several millions of years and what has happened here is that our tectonic plate, the North American plate is overriding a hot spot and a hot spot is purported to be a mantle plume. It’s a region of higher heat flow, a mass of hotter material that is rising from great depths in the earth’s mantle, through the earth’s mantle and up into the earth’s lithosphere. And so as this thing impinges on the lithosphere it bulges up and so what we have at Yellowstone, it’s now beneath Yellowstone and so the whole bulge is right there may be hundreds of miles across, this bulge, and so that is why we have the continental divide going right through Yellowstone.
J: How big an area are we talking about?
S: Oh, the effect of that bulge? Oh, seven, eight hundred miles across.
J: So it covers a lot of Idaho.
S: A lot of Idaho, Wyoming, Montana and it used to be here just outside of Pocatello 10 million years ago we had the same sort of system out here on what is now the Snake River Plain and 15 million years ago it was down over in southwest Idaho and southeast Oregon.
J: What causes some of the geothermal activity? The features that you see in the park?
S: The geysers in the park are related to circulating meteoric water. Now meteoric water means anything that falls out of the sky and is in circulation. So that would be snow, rain and everything else, water that is flowing in streams. But what happens is that because we have this magma chamber that is only a couple of miles below the surface of the earth just sitting there stewing, heating up all the rocks above it and also going into what is called a resurgent phase, meaning that parts of this magma chamber are bulging up in local areas what you have is hot rocks coming in contact with ground water. And any of this water is circulating, it goes down into these cracks and it gets down deeper and starts to come in contact with hot rocks and then so it migrates back up.
And so as it is migrating back up in a heated state what will happen is it will build up the right kind of pressure conditions for a geyser to go off. Many of the geysers are just hot springs that once they have evacuated a chamber let’s say, a chamber of hot water then through an eruption of a geyser – it takes a little time for it to recharge again and for that hot water to replenish itself and then to go for the next cycle. And of course you can’t think about Yellowstone without thinking of Old Faithful. That’s how Old Faithful works. Every so many minutes plus or minus a few minutes this cycle starts all over again.
J: How is a place like Mammoth Hot Springs different?
S: The interesting thing about Yellowstone, about hot springs is there are three different types of thermal features or at least the hot water features.
One is what is called a PH neutral geyser or hot pool or hot spring and that is what you see at Morning Glory Pool and Old Faithful and these are the clear pools that have a lot of dissolved silica Si02 and so around those kinds of pools you’ll see sinter – that white deposit that is plastered on the rocks and you can see some of it on the trees because the trees as they die they are still capable of drawing up some of that water and the water evaporates leaving this sinter, this white coating. And so that is one kind of a pool.
Another kind of a pool would be called an acidic, mud pot type thing and that is related to the water, the water table is a little bit lower and so you have a lot of sulfur dioxide gasses that are emitting and permeating the soil and what happens is that it reacts with the rocks and soil changing a lot of those feldspar minerals into clay and it changes into clay, mixes with water and creates mud. So that is a mud pot.
The third type is what we see at Mammoth. And the third type is really a carbonate system and so the water is going, actually not going through volcanic rocks per se but actually going through some older sedimentary rocks in this case limestone. Limestone is made up calcium carbonate and so instead of getting an acidic system we get a basic system with a lot of calcium carbonate dissolved in the water and that calcium carbonate starts to precipitate out as the water evaporates. So this water comes up to the surface and it’s flowing out over the surface and you get these travertine pools and the travertine pools are formed - they are ridges, little dams if you will that are formed by evaporation at the margin that deposits calcium carbonate in the form of travertine. It’s the same sort of process that you get with stalagmites and stalactites in caves.
J: How is the main Yellowstone area different than some of the peripheral areas? What’s the contrast?
S: What is going on beneath in Yellowstone proper at the two main geyser basins is what is called the resurgence of the caldera and this is what happens between the catastrophic eruptions that caused the caldera itself. Subsequent to one of those eruptions you have a series of lava flows. As a matter of fact the lava flows in Yellowstone are represented in those big, gray cliffs that you see around Madison Junction. Those are big rhyolite lava flows. They are very thick, hundreds of feet thick. There have been thirty or forty – I don’t remember the exact number – that have occurred in the caldera itself and they sort of fill up the caldera. But what has happened here is that as the magma chamber starts to evolve you start to get places where it is bulging up and more of this accumulation of gasses and volatiles and of course whatever is driving this buoyancy in two locations now in Yellowstone, the two geyser basins and as these two doming effects are pushing up in those two areas they are bringing hot rock, magma in contact with shallower and shallower regions and so that is why we have a high concentration of thermal features there.
Surrounding the caldera, outside the caldera there’s not much activity at all in terms of volcanic. That doesn’t mean you can’t get a volcano out there it just means it’s not concentrated out there as much.
J: When you are coming off the high plateaus you are basically getting a lot of snow melt, rain melt coming off the caldera area and working its way down toward Idaho.
S: Yeah. As you work down towards Idaho you are coming off that plateau and you are going outside the caldera proper but you might be inside one of those older calderas. Those older calderas like the one that formed 2 million years ago, that produced what is called the Huckleberry Ridge Tuff. It’s a huge outflow sheet of what we call ash flow tuff or ignium bright and a lot of that material is exposed or present around the periphery of Yellowstone proper along with the Mesa Falls Tuff which was produced 1.3 million years ago and in those regions – the region you are talking about – there is a lot of what is called volcanic breccia because during these eruptions whatever was being erupted it sort of comes out in a semi-solid state and if you can imagine something that is not quite liquid but not quite solid. It’s sort of very viscous and very sluggish but if you blow it apart it makes chunks. And so we get these deposits of chunks mixed in with ash and the ash is the really very, very tiny chunks. Ash is really magma that has been blown apart and blown into smithereens into very small particles, smaller than you can see with the naked eye. And so what you are seeing around those areas are deposits that are either just outside the caldera or just inside the caldera representing this very broken up – we say breccia, brecciated zone because it is made up of class chunks that have been broken up, mixed in together with other constituents. A little bit of obsidian in there as well.
J: So the whole eastern Idaho and the border area, that is all volcanic material underlying it but for the layman they would look at that and see how treed and lush it is. They may not understand the volcanic background.
S: It is interesting that humans are only around for a few decades and the volcanic processes are going on for millions of years and geology is a field where we study events that happen over long periods of time but what is happening are the processes that are causing the eruptions and so forth to be working and working and working but then culminating in a catastrophic event.
So volcanic eruption is catastrophic, earthquake is catastrophic. In between those catastrophic events the earth has time to readjust with weather, the normal processes of streams, transport, weathering, erosion and so forth and the sole developments will occur and so even though this is an active volcanic system there is plenty of time between eruptions to have trees and meadows and wildlife to come back. So what we are seeing is an active system but it is right now dormant. It could blow any minute of course and just like you see around Mount St. Helens for example or any other volcanic system, it looks devastated but that is only during the active phases.
J: In that corner of Idaho you also have the convergence of the Teton Range with the Yellowstone system.
S: The Tetons, now a structural geologist might be able to elaborate a little bit more on the Tetons – and here’s a misnomer – a lot of people say the Grand Tetons. There is no such thing as the Grand Tetons. It is the Tetons and there is one Grand. I made that mistake on a field trip years ago and a scientist friend of mine from the U.S. Geological Survey corrected me.
So any way, the point here is that the Tetons are being uplifted as masses of older rock that are being pushed up by tectonic forces. Now the tectonic forces are not necessarily related to the hot spot although there is going to be some peripheral activity. What we have here at Yellowstone is a convergence of three major terrains.
We have the Wyoming thrust belt which is how all the Rockies in that area got folded and pushed up and then we have the Basin and Range where everything off to the west is sort of relaxing and getting chopped up as the crust is extending and them we have the hot spot which is producing Yellowstone. So it’s a fascinating area just because of that. So the interaction of the Tetons with Yellowstone having them right there together is an indication that there is a lot of dynamic crustal motion going on. Some up, some down and some sideways.
J: Working in Pocatello what is it like having such a dynamic system so close?
S: Could it get any better than this? We think of a lot of good places to go and I’m a volcanologist and so I like Hawaii and I like other places where there are active volcanoes. I love the Snake River Plain because it’s got volcanic systems. They are not active at the moment but it’s an active system and it could erupt any day now and it would be nice to see that but I also like other planets. I like Mars, I like the moon. We don’t get to go there – not everybody any way- and it’s just so fascinating to be in a place where we can study these things even though we’re not able to get in all the places we’d like to see we at least have a place that it is representative.
And quite frankly, we live in a special part of the world. Not too many places in the whole solar system do you have a large system like Yellowstone representing a hot spot beneath continental crust. And it’s just a fascinating system because it is so unique on the planet as well as in the solar system.
J: Anything else unique about volcanology or Yellowstone we haven’t touched on that people don’t appreciate or understand?
S: That’s a very good question. I think everybody – and I mean everybody should know a little bit about how the earth works. We’re supposed to know how the seasons work and we’re supposed to know what an element is and we’re supposed to know all these things. And it doesn’t matter if you are a scientist or not. What really matters is that you have a curiosity about how things work and what are the processes that are involved. One of the things Yellowstone brings home to us – and this whole eastern Idaho area – is that we have a combination of internal processes and external processes. The internal processes are driving it. It’s that engine that drives tectonic forces and magmatism and earthquakes and all that. It’s the external forces, whether climate, solar driven processes that create snow and ice and all the forces that change the surface of the earth and so it’s the interaction of internal versus external forces that I think is the take-home message and everybody should be sort of curious. And if you are curious about something then you start to learn about it.
S: I just want to say one more thing. Idaho is a great place because we have the outdoor classroom. All these things that we can study, they are out there to look at and we can use that as our lecture hall for example.