See Also: BBC; Science & Nature
..Narration: Seventy four thousand years ago in Indonesia a gigantic volcanic eruption shook the earth. It blasted a cloud of volcanic ash out to a distance of three thousand kilometres and triggered a global volcanic winter.
Narration: The crater left behind by this eruption was larger than the City of London.
This was no ordinary eruption.
It was a super eruption.
At the time the human race was in it’s infancy with just a few thousand people occupying the Planet.
But what if the next super eruption occurred in the heart of a country that is
home to three hundred million people?
Bill McGuire: If we see one of these huge cataclysmic events today, particularly in an industrialised country, for example another super eruption at Yellowstone, then the effects would be devastating. Not only for that country but for the whole Planet.
Steve Sparks: If a super eruption happened tomorrow the consequences could be catastrophic. An area the size of a Continent would be completely devastated and there would be global effects for years afterwards.
Narration: By consulting with leading Geologists and Volcanologists from
Research Institutes all around the world this programme asks the question “How likely is it that the Yellowstone Super Volcano will erupt in our lifetime?
And what would the warning signs be?”
When most people think about volcanoes they think about ones like Mount St. Helens.
In 1980 it re-awakened after lying dormant for over one hundred and twenty years.
With the explosive power of one Hiroshima bomb every second, it blasted a huge cloud of volcanic debris twenty kilometres high into the atmosphere.
And obliterated the surrounding landscape.
It was the most violent eruption in modern American history.
Nearly twenty five years later the area still has not recovered. But this is nothing compared with the devastation that could be unleashed by a super eruption.
A super eruption is the world’s biggest bang. It is a volcanic explosion big enough to dwarf all others and with a reach great enough to effect everyone on the Planet.
Narration: Apart from scale another characteristic that sets super volcanoes apart is their invisibility.
Bill Mcguire: Normal volcanoes have a classic cone shape and
Mount St. Helens is an excellent example of that. Super volcanoes on the other hand are very flat lying structures. They are very difficult to detect, and we may not even know they are there.
Narration: Super volcanoes are best detected after they have erupted. When the underground chamber of molten rock collapses leaving behind a crater in the ground called a “caldera”.
So vast are these calderas that they can often only be identified from space.
Super volcanoes have been found in Indonesia where the world’s most recent super eruption took place.
In New Zealand. And in South America. There are even very ancient calderas in Britain.
But like most super volcanoes they are long extinct.
One of the exceptions however is the Yellowstone Super Volcano.
Yellowstone is America’s most famous and popular National Park. Over three
million visitors are drawn here every year to admire the biggest collection of hydrothermal features on the Planet.
What few people realise however is that this spectacular display is powered by a potentially deadly force that lies just a few kilometres beneath their feet.
Jake Lownstern: There is such a tremendous collection because this is after all sitting on top of a large reservoir of magma….
…It heats the rocks. It heats the waters and it fuels the system.
Steve Sparks: I should think people going to Yellowstone on holiday or vacation would be really surprised to know that they were walking in a volcano which was close to a hundred kilometres across.
Narration: Today the Yellowstone Super Volcano even in it’s dormant state is an extremely hazardous and volatile environment.
The huge reservoir of magma beneath the ground heats the overlying water giving rise to the boiling hot springs, geysers and mud pots that injure and kill unsuspecting tourists every year.
Then of course there is the ever present threat of a future volcanic eruption. An eruption that could be anything from a small flow of lava to the biggest and most explosive eruption of them all.
Bill Mcguire: If we look at the recent eruptive history of Yellowstone there were small eruptions maybe every twenty to thirty thousand years. Now we have not seen anything for seventy thousand and that might tell you something was overdue. It is also certain that there is going to be another super eruption somewhere on the Planet. Whether there will be another one at Yellowstone is still questionable. There is a reasonable chance there will be. They will in fact have occurred throughout earth history. They are not going to stop occurring just because we have appeared on the scene.
Narration: Because the world’s last super eruption occurred over seventy thousand years ago it is difficult to know what to expect from the next one.
Intriguingly it was Archaeologists working in the Town of Orchard in Nebraska who first discovered evidence of just how devastating super eruptions can be.
They unearthed the skeletons of hundreds of pre-historic animals who lay buried in a thick blanket of volcanic ash.
When Scientists eventually traced the eruption that had killed the animals, they had a shock.
The volcano responsible was at a place called Bruno Jarbridge in Idaho an astonishing one thousand six hundred kilometres away from the fossil bed in Nebraska.
The disturbing thing is the power that fuelled the Bruno Jarbridge eruption is now fuelling the volcanic system under Yellowstone National Park.
Bill McGuire: Many of the world’s volcanoes are located around the margins of the tectonic plates which make up the rigid outer carapace of the Planet. But Yellowstone is a little bit different. Yellowstone is sitting above what is called “a hot spot” and here you have hot material from the earth’s mantle coming upwards, hitting the base of the earth’s crust and melting it.
Narration: The hot spot is a fixed point deep within the earth’s interior over which the North American Plate slowly moves. It melts the overlying crust to create a huge chamber of molten rock or magma, which eventually accumulates to produce a super eruption
Over millions of years these super eruptions have left a trial of calderas on the earth’s crust. Bruno Jarbridge is one of them.
Over two million years ago the hot spot arrived beneath Yellowstone and a new cycle of super eruptions began.
As the Scientists in charge of the Yellowstone Volcano Observatory, Jake Lowenstern is an expert in the Park’s volcanic potential.
Jake Lowenstern: Across the Valley we see this snow clad range, it is the Gallatin Range, and these are rocks that are far older than the Yellowstone volcano itself. When you get to the south end of the Range that is where the caldera begins for the two point one million year old eruption.
Narration: There is an enormous gap in the Mountain Range where it crosses the caldera formed by the first super eruption. For years Scientists could not work out why.
Jake Lowenstern: It appears that what happened is that the Range did exist during that time and that it was essentially swallowed up during formation of the caldera two point one million years ago. So the mountain has essentially disappeared.
Narration: An eruption capable of swallowing an eighty kilometre stretch of mountains is beyond anything we have ever witnessed before.
Since then there have been two more super eruptions at Yellowstone. One point three million years ago.
And six hundred and forty thousand years ago.
Disturbingly these eruptions appear to be on a cycle of between six hundred to seven hundred thousand years.
And the last one was six hundred and forty thousand years ago.
Could another be imminent?
Steve Sparks: It is actually very difficult to say for sure what is happening underneath a volcano like Yellowstone. It could be that the next super eruption is another hundred thousand years from now or it could be tomorrow.
Narration: If an eruption were brewing at Yellowstone, what would the warning signs be?
Norris Geyser Basin is one of Yellowstone’s most popular tourist attractions.
In the summer of 2003, strange things started to happen here.
Several geysers started to erupt more frequently and a new series of vents opened up, spewing out scorching volcanic gases.
Hank Heasler: A closure was put into place immediately over the area that had the new thermal feature that was spewing boiling temperature acidic mud on to the trail. So we viewed that it was unsafe for visitors to be walking through the area. Trees were starting to die.
The trail was looking very, very different because steam was actually condensing on the surface of the trail.
Narration: These strange events also appeared to have an effect on the local wildlife.
Hank Heasler: And just about a little bit more than a half mile or kilometre behind me, five Bison, all within about forty five metre radius of each other, that were just falling over dead right where they stood.
Hank: And so it turned into a bit of a mystery.
Narration: Scientists descended onto the area to try to work out what was going on.
Hank Heasler: We went there and started looking at the geology and measured very, very high levels of hydrogen sulphide. And then when Jake was out here earlier this summer he measured some extremely high concentration levels of carbon dioxide.
Narration: Their test results pointed to the Bison dying of gas poisoning and hydrogen sulphide and carbon dioxide are both gases found in magma.
Could magma have escaped from the chamber and be making it’s way to the surface?
There is another more obvious sign that magma is rising.
Jake Lowenstern: Prior to eruptions we are looking for ground deformation. Movement of the ground upwards. It indicates that magma is rising towards the surface and deforming the rock as it goes.
Narration: To keep track of ground movement at Yellowstone Scientists use GPS instruments, and a new highly advanced technique called “InSar”.
Jake Lowenstern: InSar is a satellite based technology. You are looking at the distance between the satellite and the plate on the ground. You take one image and then you can compare it to another image taken at a different time. And by comparing those two images you can see how much plate in the ground has moved with respect to the satellite.
Narration: When Scientists analysed the InSar data for the Norris Basin they were surprised.
Jake Lowenstern: One of the Scientists here looked at the data and compared it with an earlier year, 1996, and saw that there had been some uplift in the northern part of the Yellowstone caldera centred around here, not far from the Norris Geyser Basin. And this was unusual because it was a different area than we had ever witnessed up-lifting before.
Narration: Their data showed that over the last seven years the ground at Norris had been steadily up-lifting.
Should they be worried?
Jake Lowenstern: There are at least two possibilities for why we might get up-lift at Norris. And one possibility is that somewhere deep in the system, ten to fifteen kilometres, there is a small injection of magma that is pushing rock out of the way and causing some up-lift.
Another possibility is that gas build up is causing some of the up-lift and that gas could either be coming from the magma or from the hydrothermal systems.
Steve Sparks: If you take the phenomena going on at Norris it could be that magma is moving but it will never reach the surface so this activity will die down.
That is a very common situation. It could be that magma will reach the surface at some stage and we will get a small eruption. Or of course it is at least possible that if magma reached the surface we could go into a much bigger eruption.
Narration: Norris is not the only area of the Park that has been causing concern.
In 1999 a team of Scientists from the US Geological Survey began a detailed study of the floor of Yellowstone Lake.
One striking new feature in particular caught their attention.
It was a seven hundred metre long bulge which rose thirty metres above the Lake floor. They called it “the inflated plane”.
To get a better look at this new feature, they used a remotely operated camera.
Their images revealed that the sediments resting on the top of the bulge showed signs of disturbance.
And indication that it could be swelling.
Bill Mcguire: What we are seeing at the moment is this so-called inflated plane which is a sort of up-domed area beneath Yellowstone Lake. Now that may at some point in the future herald a hydrothermal explosion.
Which is a steam drive blast. It will only effect the immediate area. Or it may subside or it may just stay as it is. It is really impossible to say what is going to happen.
Narration: Hydrothermal explosions can indicate that a volcano is stirring. However they have occurred throughout Yellowstone’s history without triggering eruptions.
Bill McGuire: The thing about Yellowstone is it is a restless caldera. And that means that it is always doing something. So you clearly have magma down there, it is heating the ground water. That is giving you the geysers, the hot mud pools. It is also inflating the ground surface which is going up and down over periods of decades or centuries. But this activity is not unusual for Yellowstone.
Narration: The difficulty for Scientists is that many of the signs they rely on to predict eruptions at other volcanoes occur all the time at Yellowstone.
There is another warning sign however the Scientists cannot ignore.
Bill McGuire: Magma is actually formed tens of kilometres beneath the surface, if not below that. To get from there to the surface it has to make a path for itself and it does that by breaking rock. And that breaking the rock generates earthquakes. So we can detect those. So we know the magma’s on it’s way up.
Jake Lowenstern: There are many earthquakes at Yellowstone. It is a very seismically active region. There are anywhere between about one thousand and three thousand earthquakes here in any given year.
Narration: Most of the earthquakes are so small that they can only be detected by seismometers.
But there can be exceptions.
Broadcaster: One of America’s worst series of earthquakes in this century felt throughout the north west. In Madison River Canyon fifty million tons of earth and rock, the top of an eight thousand foot fell and thundered across the forest camp ground in one of the biggest landslides. Helicopters rescued three hundred campers who were trapped between the titanic slide at Hebden Dam. At least sixty are known injured, mostly vacationers, with at least a dozen fatalities.
The grim search for victims or survivors continues throughout the area as evacuation of those who did escape proceeds. The earth trembles and one of American’s favourite vacation spots comes devastation and terror.
Narration: Although the Hebden Lake earthquake was devastating, it could have been so much worse.
Because earthquakes of magnitude seven and higher have the power to trigger eruptions.
Jake Lowenstern: At other volcanoes around the world people have found a correlation between large earthquakes and volcanic eruptions.
That may have happened in the past at Yellowstone in the distance past. And it potentially could happen in the future.
Narration: Large earthquakes pose a risk because they can fracture the heart of the volcano, the magma chamber.
And allow its contents to escape.
Understanding what is going on inside the magma chamber is the only reliable way to predict the behaviour of the volcano.
Steve Sparks: The magma chamber is really the fundamental cause. It is at the heart of the process of leading up to an eruption.
And that is why Scientists really need to understand what is going on at depth below the volcano.
Narration: To build up an image of the magma chamber Professor Bob Smith is harnessing the many earthquakes that take place at Yellowstone.
Using a network of seismometers positioned all around the Park.
Bob Smith: The seismographs provide the data of waves passing through the earth. And Seismologists use that data as a window. That is we are looking down into the earth to get a three dimensional image.
Narration: Earthquakes generate sound waves which travel slower through the hot material inside the magma chamber than they do through the cold surrounding rock.
By measuring the different times taken for waves to reach the seismographs Bob has revealed the size of the magma chamber.
Bob Smith: The volume of Yellowstone’s magmatic system is about twenty to twenty five thousand cubic kilometres.
Narration: It is about eighty kilometres long, forty kilometres wide and eight kilometres deep.
In other words, the magma chamber is over three times the size of New York City.
But size alone does not dictate the possibility of a super eruption.
Steve Sparks: Many people might think that Yellowstone would be under-laying by a huge volume of molten rock. That is not really quite the case, because seismic studies underneath Yellowstone show that in actual fact the liquid rock is distributed between solid rock and there may be no more than thirty percent of liquid rock. What you need for a super volcanic eruption is for that liquid rock to separate and form a large layer of almost pure liquid right at the top of the chamber. And that is when you can get super conditions for a super volcanic eruption.
Narration: To qualify as a super eruption one thousand cubic kilometres of magma needs to be erupted.
Yellowstone’s magma chamber contains around five times that amount.
So, is the molten rock at the top of the chamber?
Frustratingly this is one question Scientists cannot currently answer.
Steve Sparks: When they are trying to find out exactly where the molten rock is underground, the images we get of the magma chamber are very inexact.
So we do not really know in detail of how this molten rock is distributed and we cannot exclude of course that at the moment there are not some pockets of purely molten rock which could erupt.
Narration: Although there is more than enough molten rock to fuel another super eruption, statistically it is much more likely that the next eruption will be a small, or moderate, one.
Steve Sparks: I do not think that the signs that the Scientists are monitoring now are any evidence that a super eruption is imminent. Far more likely that if there is an eruption it will be on a small scale, perhaps comparable to Mount St. Helens.
Narration: The Yellowstone Scientists do not believe that even a small eruption is brewing.
Jake Lowenstern: The kinds of things that we see have been going on ever since people have been coming to the Park and geologic evidence seems to indicate that they have been going on for at lest the last ten thousand years. So at this point in time we do not really see indications from ground information, from earthquakes, or from gas, that there is any sort of really unusual activity going on at Yellowstone that would be precursor to a volcanic eruption.
Narration: So, is there a chance that an eruption could happen in our lifetime?
Bob Smith: I could say “No”. But I will not! Because this is a volcanic system.
Hank Heasler: I do not know. But I am very interested in finding out!
Bill McGuire: We could have an eruption next year, or we could have to wait many thousands of years to come. Now I would not be surprised if there was another eruption in my lifetime but I also would not be surprised if there was not.