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NARRATOR: Yellowstone was on a 600,000 year cycle and the last eruption was just 600,000 years ago. Yet there was no evidence of volcanic activity now. The volcano seemed extinct. That reassuring thought was about to change. The vulcanologist realised only one thing could make the Earth heave in this way: a vast living magma chamber. The Yellowstone supervolcano was alive and if the calculations of the cycle were correct, the next eruption was already overdue.
MICHAEL RAMPINO: If Yellowstone goes off again, and it will, it'll be disastrous for the United States and eventually for the whole world.
BILL McGUIRE: This would generate sheets of magma which will be probably rising up to 30, 40, 50 kilometres sending gigantic amounts of debris into the atmosphere.
Supervolcanoes
BBC2 9:30pm Thursday 3rd February 2000
http://www.bbc.co.uk/science/horizon/supervolcanoes_script.shtml
NARRATOR (SINÉAD CUSACK): Yellowstone is America's first and most famous National Park. Every year over 3 million tourists visit this stunning wilderness, but beneath its hot springs and lush forests lies a monster of which the public is ignorant.
PROF. ROBERT CHRISTIANSEN (US Geological Survey): Millions of people come to Yellowstone every year to see the marvellous scenery and the wildlife and all and yet it's clear that, that very few of them really understand that they're here on a sleeping giant.
NARRATOR: If this giant were to stir, the United States would be devastated and the world would be plunged into a catastrophe which could push humanity to the brink of extinction.
PROF. ROBERT SMITH (University of Utah): It would be extremely devastating on a scale that we've probably never even thought about.
PROF. BILL McGUIRE (Benfield Greig Centre, UCL): It would mean absolute catastrophe for North America and the problem is we know so little about these phenomena.
NARRATOR: In 1971 heavy rain fell across much of east Nebraska. In the summer palaeontologist Mike Voorhies travelled to the farmland around the mid-west town of Orchard. What he was to discover exceeded his wildest dreams.
PROF. MIKE VOORHIES (University of Nebraska): Well I was walking up this gully looking for fossils, the way I'd walked up a thousand gullies before, keeping my eye on the ground looking for pieces of fossils that might have washed down in the rain the previous night and I scrambled up to the top and I saw something that completely astounded me, a sight that no palaeontologist has ever seen.
NARRATOR: It was a sight of sudden, prehistoric disaster. Voorhies's digging revealed the bones of 200 fossilised rhinos, together with the prehistoric skeletons of camels and lizards, horses and turtles. Dating showed they had all died abruptly 10 million years ago.
MIKE VOORHIES: It suddenly dawned on me that this was a scene of a mass catastrophe of a type that I'd never, never encountered before.
NARRATOR: The cause of death, however, remained a mystery. It was not from old age.
MIKE VOORHIES: I could tell by looking at the teeth that these animals had died in their prime. What was astounding was that here were young mothers and their, and their babies, big bull rhinos in the prime of life and here they were dead for no, no apparent reason.
NARRATOR: For the animals at Orchard death had come suddenly. There was another strange feature to the skeletons, an oddity which offered a crucial clue about the cause of the catastrophe.
MIKE VOORHIES: We saw that all of these skeletons were covered with very peculiar growth, soft material that I first thought was a mineral deposit. Then we noticed that it was cellular. It's biological in origin so there was something actually growing on those bones. I had no idea what that stuff was, never seen anything like it.
NARRATOR: A palaeo-pathologist, Karl Reinhard, was sent a sample of the bones.
PROF. KARL REINHARD (University of Nebraska): This specimen is typical of the rhino bones. You see this material, in this case it's a whitish material that's deposited on the surface of the original bone. This is peculiar to me, but as I thought back in my experience I realised that this was similar to something that turns up in the veterinary world, a disease called Marie's disease.
NARRATOR: Marie's is a symptom of deadly lung disease. Every animal at Orchard seemed to be infected.
KARL REINHARD: One of the clues was that all of the animals had it. Now that is a very important observation for all the diseases, all the animals to exhibit this disease there had to be some universal problem.
NARRATOR: Scientists discovered the universal problem was ash. 10 million years ago ash had choked them to death.
KARL REINHARD: It may have been a bit like pneumonia with the lungs filling with fluid, except in this case the fluid would have been blood for the ash is very sharp. There'd be microscopic shards of ash lacerating the lung tissue and, and causing the bleeding. I would imagine these animals as stumbling around the thick ash, spitting up blood through their mouths and gradually dying in a most miserable way.
NARRATOR: Only a volcano could have produced so much ash, yet the wide flat plains of Nebraska have no volcanoes.
MIKE VOORHIES: I remember some of my students and I sitting around after a day's digging and just speculating where did this stuff come from? There, there are no volcanoes in Nebraska now. As far as we know there never have been. We, we obviously had to have volcano somewhere that, that produced enough ash to completely drown the landscape here, but where that was really was anybody's guess.
NARRATOR: One geologist in Idaho realised there had been a volcanic eruption which coincided with the disaster at Orchard 10 million years ago, but the site was halfway across North America.
PROF. BILL BONNICHSEN (Idaho Geological Survey): It seemed like a really fascinating story which made me think, because I had been working on volcanic rocks in south-western Idaho that potentially could make lots of ash and, and there was some age dates on that that were around 10 million years and I began to wonder wow, could this situation in Nebraska have really been caused by some of these large eruptions that evidently had happened in south-western Idaho.
NARRATOR: The extinct volcanic area, Bruneau Jarbridge, was 1600 kilometres away, a vast distance. How could this eruption have blasted so much ash so far? Bonnichsen was sceptical.
BILL BONNICHSEN: Volcanoes will spew ash for a few tens or maybe a few hundreds of miles. This ash, and it's like two metres thick, in Nebraska is 1600 kilometres or more away from its potential source, so that's an amazing thing. There really had been no previous documentation, to my knowledge, of phenomenon like that.
NARRATOR: Despite his doubts Bonnichsen decided to compare the chemical content of ash from the two sites. He analysed samples from both Bruneau Jarbridge and Orchard and plotted their mineral composition on a graph looking for similarities.
BILL BONNICHSEN: if you have a group of rocks that are very similar to one another they should be a closely spaced cluster of pods. We had these analyses come out from the Orchard site and I thought I'd try the clock again and see how close they were to one another. By golly, they fall right in the same little trend as the Bruneau Jarbridge samples.
NARRATOR: Bonnichsen's hunch had proved correct. Bruneau Jarbridge was responsible for the catastrophe at Orchard. An eruption covering half of North America with two metres of ash was hundreds of times more powerful than any normal volcano. It seemed almost unbelievable, but then Bruneau Jarbridge was that rarest of phenomena which scientists barely understand and the public knows nothing about: a supervolcano.
ROBERT SMITH: Supervolcanoes are eruptions and explosions of catastrophic proportions.
BILL McGUIRE: When you actually sit down and think about these things they are absolutely apocalyptic in scale.
PROF. MICHAEL RAMPINO (New York University): It's difficult to conceive of a, of an eruption this big.
NARRATOR: Scientists have never witnessed a supervolcanic eruption, but they can calculate how vast they are.
BILL McGUIRE: Super eruptions are often called VEI8 and this means that they sit at point 8 on what's known as a volcano explosivity index. Now this runs from zero up to 8. It's actually a measure of the violence of a volcanic eruption and each point on it represents an eruption 10 times more powerful than the previous one, so if we take Mount St. Helens, for example, which is a VEI5, we can represent that eruption by a cube of this sort of size, this represents here the amount of material ejected during that eruption. If you go up step higher and look at a VI6, something of the Santorini size for example, then we can represent the amount of material ejected in Santorini by a cube of this sort of size, but if we go up to VEI8 eruptions then we're dealing with something on an altogether different scale, a colossal eruption and you can represent a VI8, some of the biggest VI8 eruptions by a cube of this, this sort of size. It's absolutely enormous.
NARRATOR: Normal volcanoes are formed by a column of magma, molten rock, rising from deep within the Earth, erupting on the surface and hardening in layers down the sides. This forms the familiar dome or cone-shaped mountains.
BILL McGUIRE: Most people's idea of a volcano is a lovely symmetrical cone and this involves magma coming up, reaching the surface, being extruded either as lava or as explosive eruptions as, as ash and these layers of ash and lava gradually accumulate until you're left with a, a classic cone shape.
NARRATOR: Vulcanologists know this smooth flowing magma contains huge quantities of volcanic gases, like carbon dioxide and sulphur dioxide. Because this magma is so liquid these gases bubble to the surface, easily escaping. There are thousands of these normal volcanoes throughout the world. Around 50 erupt every year, but supervolcanoes are very different in almost every way.
First, they look different. Rather than being volcanic mountains, supervolcanoes form depressions in the ground. Despite never having seen a supervolcano erupt, by studying the surrounding rock scientists have pieced together how supervolcanoes are formed. Like normal volcanoes they begin when a column of magma rises from deep within the Earth. Under certain conditions, rather than breaking through the surface, the magma pools and melts the Earth's crust turning the rock itself into more thick magma.
Scientists don't know why, but in the case of supervolcanoes a vast reservoir of molten rock eventually forms. The magma here is so thick and viscous that it traps the volcanic gases building up colossal pressures over thousands of years. When the magma chamber eventually does erupt its blast is hundreds of times more powerful than normal draining the underground reservoir. This causes the roof of this chamber to collapse forming an enormous crater. All supervolcano eruptions form these subsided craters. They are called calderas.
BILL McGUIRE: The main factor governing the size of eruptions is really the amount of available magma. If you've accumulated an enormous volume of magma in the crust then you have at least a potential for a very, very large eruption.
NARRATOR: The exact geological conditions needed to create a vast magma chamber exist in very few places, so there are only a handful of supervolcanoes in the world. The last one to erupt was Toba 74,000 years ago. No modern human has ever witnessed an eruption. We're not even sure where all the supervolcanoes are. Yellowstone National Park, North America. Ever since people began to explore Yellowstone the area was known to be hydrothermal. It was assumed these hot springs and geysers were perfectly harmless, but all that was to change.
ROBERT CHRISTIANSEN: I first came to Yellowstone in the mid-1960s to be a part of a major restudy of the geology of Yellowstone National Park, but at that point I had no idea of what we were to find.
NARRATOR: When geologist Bob Christiansen first began examining Yellowstone rocks he noticed many were made of compacted ash. But he could see no extinct volcano or caldera crater, there was no give-away depression.
ROBERT CHRISTIANSEN: We realised that Yellowstone had been an ancient volcanic system. We suspected that it had been a caldera volcano, but we didn't know where the caldera was or specifically how large it was.
NARRATOR: As he searched throughout the Park looking for the volcanic caldera Christiansen began to wonder if he was mistaken. Then he had a stroke of luck. NASA decided to survey Yellowstone from the air. The Space Agency had designed infrared photography equipment for the moon shot and wanted to test it over the Earth. NASA's test flight took the most revealing photographs of Yellowstone ever seen.
ROBERT CHRISTIANSEN: What was so exciting about looking at the remote sensing imagery was the sense that showed it in one, one sweeping view of what this truly was.
NARRATOR: Christiansen hadn't been able to see the ancient caldera from the ground because it was so huge. It encompassed almost the entire Park.
ROBERT CHRISTIANSEN: An enormous feature. 70 kilometres across, 30 kilometres wide. This had been a colossal supervolcano. Certainly one of the largest known anywhere on earth.
NARRATOR: Bob Christiansen was determined to find out when Yellowstone had last erupted. He began examining the sheets of hardened ash, dozens of metres thick blasted from the ground during the eruption. What he found was 3 separate layers. This meant there had been 3 different eruptions. When Christiansen and his team dated the Yellowstone ash he found something unexpected. The oldest caldera was formed by a vast eruption 2 million years ago. The second eruption was 1.2 million years old and when he dated the third and most recent eruption he found it occurred just 600,000 years ago. The eruptions were regularly spaced.
ROBERT CHRISTIANSEN: Quite amazingly we realised that there was a cycle of caldera-forming eruptions, these huge volcanic eruptions about every 600,000 years.
NARRATOR: Yellowstone was on a 600,000 year cycle and the last eruption was just 600,000 years ago. Yet there was no evidence of volcanic activity now. The volcano seemed extinct. That reassuring thought was about to change. There was another geologist who was fascinated by Yellowstone's volcanic history. Like Bob Christiansen, Professor Bob Smith has been studying the Park for much of his career. In 1973 he was doing field work, camping at one end of Yellowstone Lake.
ROBERT SMITH: I was working at the south end of this lake at a place called Peal Island. I was standing on the island one day and I noticed a couple of unusual things. The, the boat dock that we normally would use at this place seemed to be underwater. That evening as I was looking over the expanse of the south end of the lake I could see trees that were being inundated by water. I took a look at these trees and they were be, being inundated with water a few inches, maybe a foot deep and it was very unusual for me to see that because nowhere else in the lake would the lake level have really changed. What did it mean? We did not know.
NARRATOR: Smith commissioned a survey to try to find out what was happening at Yellowstone. The Park had last been surveyed in the 1920s when the elevation, the height above sea-level, was measured at various points across Yellowstone. 50 years later, Smith surveyed the same points.
ROBERT SMITH: The idea was to survey their elevations and to compare the elevations in the mid-70s to what they were in 1923 and the type of thing that we did is to make recordings at a precision level of, of a few millimetres.
NARRATOR: The two sets of figures should have been similar, but as the survey team moved across the Park, they noticed something unexpected: the ground seemed to be heaving upwards.
ROBERT SMITH: The surveyor said to me there's something wrong and he said it's not me, it's got to be something else, so we went through all the measurements again trying to be very careful and the conclusion kind of hit me in the face and said this caldera has uplifted at that time 740 millimetres in the middle of the caldera.
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