NARRATOR: As the measuring continued, an explanation for the submerged trees began to emerge. The ground beneath the north of Yellowstone was bulging up, tilting the rest of the Park downwards. This was tipping out the sound end of the lake inundating the shoreside trees with water. 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.
ROBERT CHRISTIANSEN: Well this gave us a real shiver of nervousness if you will about the fact that we have been through this 600,000 year cycle and that the last eruption was about 600,000 years ago.
ROBERT SMITH: I felt like telling people, that is we basically have on our hands a giant.
NARRATOR: The scientists had found the largest single active volcanic system yet discovered. There were many things they needed to find out. How big was the magma chamber deep underground, how widespread would the effects of an eruption be and crucially, when would it happen? To answer any of these questions vulcanologists knew they first had to understand Yellowstone's mysterious magma chamber.
ROBERT SMITH: It's incredibly important to understand what's happening inside of the magma chamber because that pressure and that heat, the fluid is what's triggering the final eruption. It's like understanding the primer in a bullet.
NARRATOR: Understanding the magma chamber would be very difficult. Smith and his team needed to discover the size of something 8 kilometres below the ground. They began harnessing information from an ingenious source: earthquakes.
ROBERT SMITH: Well, what we have here is a seismometer. This is the working end of a seismograph, the device that's used to record earthquakes. It is able to pick up the smallest of earthquakes in, in Yellowstone plus it picks up moderate to large earthquakes around the world, it is so sensitive. This forms one of a network of 22 seismograph stations in Yellowstone that is used for monitoring and all the data are transmitted to a central recording facility at the University of Utah.
NARRATOR: Like many thermal areas, Yellowstone has hundreds of tiny earth tremors each year. They are harmless, but in his seismographic lab Smith has been using them to trace the size of the magma chamber.
ROBERT SMITH: Earthquakes are essentially telling you the pulse. They tell you the real time pulse of how the caldera is deforming, of how faults are fracturing.
NARRATOR: Bob Smith's 22 permanent seismographs are spread across the Park. They detect the sound-waves which come from earthquakes deep underground. These waves travel at different speeds depending on the texture of what they pass through. Soundwaves passing through solid rock go faster than those travelling through molten rock or magma. By measuring the time they take to reach the seismographs Smith can tell what they've passed through. Eventually this builds up a picture of what lies beneath the Park.
ROBERT SMITH: The magma chamber we found extends basically beneath the entire caldera. It's maybe 40-50 kilometres long, maybe 20 kilometres wide and it has a thickness of about 10 kilometres. So it's a giant in volume and essentially encompasses a half or a third of the area beneath Yellowstone National Park. NARRATOR: The magma chamber was enormous. If it erupted it would be devastating. To discover the extent of the devastation scientists had to understand the force of the eruption. The clues to this could be found in a much smaller volcano halfway across the world: the Greek island of Santorini. The eruption here 3,500 years ago, although not VEI8 in scale, did have a small magma chamber. Professor Steve Sparks has spent much of his career studying Santorini.
PROF. STEVE SPARKS (University of Bristol): When I first came to Santorini and started to look at the pumice deposits from these caldera forming eruptions I found evidence of a dramatic change in the power and violence of the eruption.
NARRATION: By examining the layers of Santorini pumice Sparks discovered magma chambers could erupt with almost unimaginable force and spread their devastation widely.
STEVE SPARKS: There's dramatic evidence of a sudden increase in the power. Huge blocks about 2 metres in diameter were hurled out of the volcano reaching 7 kilometres and smashing into the ground and to do that the blocks must have been thrown from the volcano at hundreds of metres per second, about the speed of Concorde and you can imagine this enormous red rock crashing in and breaking up on impact.
NARRATOR: To understand why caldera volcanoes could erupt with such power Sparks replicated their violence at one trillionth of the scale.
STEVE SPARKS: OK, so we need this…
NARRATOR: In the lab he modelled a reaction which occurs in the magma chamber of an erupting caldera.
STEVE SPARKS: The problem is we can't go into a magma chamber so the next best thing to do is to go to the laboratory and try and simulate what happens in the magma chamber and in the pathway to the surface.
NARRATOR: Sparks believed escaping volcanic gas trapped in the magma might be responsible for the violence of the eruptions. Into a glass flask - the magma chamber - he poured a mixture of pine resin and acetone. the pine resin mimicked the magma, the acetone modelled trapped volcanic gases like carbon dioxide and sulphur dioxide.
STEVE SPARKS: Pine resin is a very sticky, stiff material so it has some properties which are rather like magma and we thought that if we could get a, a gas which dissolved in pine resin, like acetone, then we could get a, a laboratory system which would represent the, the natural case.
NARRATOR: Sparks then created a vacuum above the flask to mimic the depressurisation that occurs in the magma chamber when a supervolcano begins its eruption and the dissolved volcanic gas can expand. When the vacuum reached the liquid it caused a dramatic change. The dissolved acetone suddenly became a gas. This made the resin expand causing violent frothing and blasting the contents out of the chamber.
STEVE SPARKS: These experiments give us tremendous insight into the tremendous power of gases coming out of solution and enabled to drive these very dramatic explosive flows.
NARRATOR: Unlike supervolcanoes, normal volcanoes don't have this vast reservoir of magma and trapped volcanic gases and don't have the potential for such powerful eruptions. But experiments in the laboratory cannot answer the biggest question of all surrounding Yellowstone: when will it next erupt? Scientists face a problem. They have never seen a supervolcano erupt. Until a VEI8 eruption is observed and analysed no-one knows what the telltale precursors would be to a Yellowstone eruption.
BILL McGUIRE: We can actually model volcanoes and their activity. We can do it in the laboratory on computer, but we need observational data in order to make those models realistic.
ROBERT SMITH: What the precursors might be for a giant volcanic eruptions they've never been observed scientifically and they've never been documented, so we don't know what to look for.
ROBERT CHRISTIANSEN: Nobody wants to see a global disaster of course and yet we'll never really fully understand the processes involved in these supervolcanic eruptions until one of them happens.
NARRATOR: A terrible truth underlies all mankind's efforts to understand the vast mechanisms which drive VEI8 eruptions. Ultimately trying to find out what makes supervolcanoes work may be pointless. Consider the last one. 74,000 years ago a supervolcano erupted here in Sumatra. It would have been the loudest noise ever heard by man. It would have blasted vast clouds of ash across the world.
The resultant caldera formed Lake Toba, 100 kilometres long, 60 kilometres wide. it was, in short, colossal. Scientists are only now beginning to understand the effects of so much ash on the planet's climate. This is the ocean core repository at Columbia University in America. It contains thousands of drill samples from seabeds round the world, a historical keyhole through which scientists, like Michael Rampino can view volcanic history.
MICHAEL RAMPINO: The size of the Toba eruption was enormous. We're talking about, about 3,000 cubic kilometres of material coming out of that volcano. That's about 10,000 times the size of the 1980 Mount St. Helens eruption which people think of as a large eruption, a truly super eruption.
This is an ocean drilling core from the central Indian Ocean. It's about 2,500 kilometres from the Toba volcano and here are 35 centimetres of ash deposited after the Toba eruption. It shows that this Toba eruption was a supervolcanic event, it was much, much bigger than any other volcanic eruption we see in the geological record. Chemical analysis of the ash tells us that this eruption was rich in sulphur, would have released a tremendous amount of sulphur dioxide and other gases into the stratosphere which would have turned into sulphuric acid aerosols and affected the climate of the Earth for years.
NARRATOR: For a long time scientists have known that volcanic ash can affect the global climate. The fine ash and sulphur dioxide blasted into the stratosphere reflects solar radiation back into space and stops sunlight reaching the planet. This has a cooling effect on the Earth. In the year following the 1991 eruption of Mount Pinatubo for instance the average global temperature fell by half a degree Celsius. By comparing the amount of ash ejected by past volcanoes with their effect on the Earth's temperature, Rampino has estimated the impact of the Toba eruption on the global climate 74,000 years ago.
MICHAEL RAMPINO: I'm plotting a simple graph where one side there's sulphur released in millions of tons by volcanic eruptions and on the other side there's a cooling in degree Celsius that we saw after these volcanic eruptions. I'm plotting as points the historical eruptions like Mount St. Helens, Krakatoa, Pinatubo, Tambora. There's a nice correlation between the sulphur released into the atmosphere and the cooling.
NARRATOR: Because of this relationship between the sulphur released by large volcanoes and global cooling, Rampino can calculate the drop in temperature caused by the Toba eruption.
MICHAEL RAMPINO: We can see this kind of plot predicts that the Toba eruption was so large that the temperature change after Toba in degrees Celsius would have been about a 5 degree global temperature drop, very significant, very severe global cooling. NARRATOR: Five degrees Celsius average drop in global temperature would have been devastating causing Europe's summers to freeze and triggering a volcanic winter.
MICHAEL RAMPINO: Five degrees globally would translate into 15 degrees or so of summer cooling in the temperate to high latitudes. The effects on agriculture, on the growth of plants, on life in the oceans would be catastrophic.
NARRATOR: This global catastrophe would have continued for years, dramatically affecting life on Earth, but what impact did it have on humans? The answer may be buried not inside the ancient rocks, but deep within us all. Lynn Jorde and Henry Harpending are scientists specialising in human genetics. Since the early 1990s they have been studying mitochondrial DNA using the information to investigate mankind's past. Most of our genetic information is stored in the nuclei of our cells, but a small, separate quantity exists in another component, the part which produces the cells' energy, the mitochondria.
PROF. LYNN JORDE (University of Utah): Mitochondria have their own genes. It's a small number of genes, a small amount of DNA, but it's distinct from the rest of the DNA in the cell and because of the way mitochondria are transmitted from one generation to the next, they're, they're inherited only from the mother so they give us a record of the maternal lineage of a population.
NARRATOR: Mitochondrial DNA is inherited only by the mother. All mutations are passed on from mother to child, generation after generation at a regular rate. Over time, the number of these mutations accumulate in a population.
LYNN JORDE: Every event that takes place in our past, every major event, a population increase, a population decrease, or the exchange of people from one population to another changes the composition of the mitochondrial DNA in that population, so what happens is that we have a record of our past written in our mitochondrial genes.
NARRATOR: By knowing the rate of mutation of mitochondrial DNA and by a complex analysis of the distribution of these mutations, the geneticists can estimate the size of populations in the past. Several years ago they began seeing a strange pattern in their results.
LYNN JORDE: We expected that we would see a pattern consistent with a relatively constant population size. Instead, we saw something that departed dramatically from that expectation. We saw a pattern much more consistent with a dramatic reduction in population size at some point in our past.
NARRATOR: This confirmed what other geneticists have noticed. Given the length of time humans have existed, there should be a wide range of genetic variation, yet DNA from people throughout the world is surprisingly similar. What could have caused this? The answer is a dramatic reduction of the population some time in the past: a bottleneck.
LYNN JORDE: We imagine the population diagrammed like this. In the distant past back here we have a large population, then a bottleneck looking like this and then a subsequent enlargement of population size again, so we would have families of people in the distant past with a significant amount of genetic diversity, but when the bottleneck occurs, when there's a reduction in population size perhaps only a few of those families would survive the bottleneck.
We have a dramatic reduction in genetic diversity during this time when the population is very small and then after the bottleneck the people who would we, who we would see today would be descendants only of those who survived, so they're going to be genetically much more similar to one another reducing the amount of genetic variation.
NARRATOR: Human DNA is so similar the scientists concluded the population reduction had been catastrophic. PROF. HENRY HARPENDING (University of Utah): It seemed so incredible, you know the idea that all of us, now there's 6 billion people on Earth, and what the data were telling us was that we, you know our species was reduced to, you know, a few thousand. Suddenly it hit us, we had something to say about human history.
LYNN JORDE: Our population may have been in such a precarious position that only a few thousand of us may have been alive on the whole face of the Earth at one point in time, that we almost went extinct, that some event was so catastrophic as to nearly cause our species to cease to exist completely.
NARRATOR: It is an astonishing revelation, but the key was to find out when and why it happened. Because mitochondrial DNA mutates at an average rate these scientists believe, controversially, that they can narrow down the date of the bottleneck.
LYNN JORDE: Mutations in the mitochondria take place with clocklike regularly, so the number of mutations give us a clock essentially that we can use to approximately date the major event. In the case of a population bottleneck we think that this would have occurred roughly 70-80,000 years ago, give or take some number of thousands of years. So then the real question is: what could have caused such a reduction, an extreme reduction, in the human population down to as few as 5 or 10,000 individuals?
NARRATOR: As for what caused this dramatic reduction in population the geneticists had no idea. Henry Harpending began touring universities to talk about the bottleneck. He was invited by anthropologist Stanley Ambrose to give a lecture to his students.
HENRY HARPENDING: Well Stanley is full of ideas, he's the kind of scientist that plucks things from all over and puts them together.
PROF. STANLEY AMBROSE (University of Illinois): I sat in on the lecture and he start4ed talking about this human population bottleneck and I thought what could have caused it and at that point I broke out into a sweat. I went up to Henry and said I've just read a paper, and it's on the top of my desk now, that may have an explanation for why this population bottleneck occurred.
HENRY HARPENDING: I didn't read it till a week later and when I read it you know it was like somebody kicking you in the face. There it was.
STANLEY AMBROSE: The paper was about the super eruption of a volcano called Toba in Sumatra.
NARRATOR: This team of scientists believe the bottleneck occurred between 70 and 80,000 years ago, although this date is hotly debated. Toba erupted in the middle of this period, 74,000 years ago. If there really is a connection this research has terrifying implications for a future Yellowstone eruption. It could well be of a similar size and ferocity to Toba. Like Toba, it would have a devastating impact, not just on the surrounding region, North America, but on the whole world.
MICHAEL RAMPINO: If Yellowstone goes off again, and it will, it'll be disastrous for the United States and eventually for the whole world.
NARRATOR: Vulcanologists believe it would all start with the magma chamber becoming unstable.
BILL McGUIRE: You'd start seeing bigger earthquakes, you may see parts of Yellowstone uplifting as magma intrudes and gets nearer and nearer the surface.
ROBERT SMITH: And maybe an earthquake sends a rupture through the brittle layer, you've broken the lid of the pressure cooker.
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.
ROBERT CHRISTIANSEN: Where we are right now would be gone. We would be instantly incinerated.
MICHAEL RAMPINO: Pyroclastic flows will cover that whole region, maybe kill tens of thousands of people in the surrounding area.
BILL McGUIRE: You're getting a, an eruption which we can barely imagine. We've never seen this sort of thing. You wouldn't be able to get within 1,000 kilometres of it when it was going like this.
ROBERT CHRISTIANSEN: The ash carried in the atmosphere and deposited over large areas of the United States, particularly over the great plains, would have devastating effects.
BILL McGUIRE: The area that would be affected is, is the bread basket of North America in effect and it produces an enormous amount of grain on a global scale really. That's, that's, that's the problem and you would see nothing. The harvest would vanish virtually overnight.
ROBERT CHRISTIANSEN: All basic economic activity would certainly be impacted by this and let alone changes in the climate that could possibly be induced.
MICHAEL RAMPINO: The climatic effects globally from that eruption will be produced by the plume of material that goes up into the atmosphere. That'll spread worldwide and will have a cooling effect that will probably knock out the growing season on a global basis. We can't really overstate the effect of these huge eruptions. Civilisation will start to creak at the seams in a sense.
ROBERT SMITH: The fact that we haven't seen one in historic time or documented means the human race really is not attuned to these things because they're such a rare event.
MICHAEL RAMPINO: It's really not a question of if it'll go off, it's a question of when because sooner or later one of these large super eruptions will happen.
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