Imagine the worst possible vision of hell. The vile stench of sulphurous gas pervading a world of darkness broken only by a dull red glow on a distant, invisible horizon. Heavy, grey ash pours from above like snow, clogging the eyes, nose, and ears as swiftly as you can remove it. Choking and retching you ram your fingers into your mouth to try and gouge out the ashy, gritty slime that forces its way in with every struggling breath, but to no avail. Suddenly a blinding flash reveals the nightmare landscape of Tolkien's Mordor-all familiar features blotted out and buried by ash accumulating at half a metre an hour. A titanic crash of thunder heralds the return of the darkness and the onset of a truly biblical deluge. Within seconds the ashy drifts are transformed into rushing torrents of mud that almost sweep your feet from under you. As the falling rain and ash combine, you are battered by pellets of mud that begin to weigh you down under a sticky, ever-thickening carapace of Vulcan's ordure. There is no sign that the Sun ever bathed the landscape in itswarming rays, but it is far from cold. In fact, your body is slowly roasting in the stifling heat of nature's own oven, your sweat sucking you dry as it drains from every pore to mix with the muddy rivulets covering every inch of your skin.
Some of the half a billion inhabitants of the danger zones around the world's 500 or so historically active volcanoes don't need to use their imagination. They have already experienced hell. Show the above description to survivors from the 1991 eruption of Pinatubo (Philippines) or the twin eruptions of Vulcan and Tavurvur at Rabaul (Papua New Guinea) three years later, and they will nod their heads and say 'I have been there!' However awful it might sound to those of us who live far from the slopes of an erupting volcano, there is nothing unusual about the above scene. But what if it was enacted 1,500 kilometres from the eruption? Then it really would be something very special, because it would mean that the Earth was being rent by one of nature's greatest killers — a volcanic super-eruption. These gigantic blasts dwarf even the greatest eruptions of recent times, and in comparison the cataclysmic detonation that blew Krakatoa (Indonesia) apart in 1883, killing around 36,000 of the inhabitants of Java and Sumatra, pales into insignificance. Even the titanic blast that tore the Greek island of Thera to pieces one and a half millennia before the birth of Christ (thereby engineering the demise of the Minoan civilization and launching the enduring legend of Atlantis) would be little more than a firecracker alongside such an Earth-shattering event.
Fortunately for us, super-eruptions are far from common, and it is estimated that throughout the last two million years of Earth history, there have been perhaps two such blasts every hundred millennia. While natural phenomena never stick rigorously to a timetable, it is nevertheless slightly disconcerting — given this frequency — that the last such
cataclysm occurred a good 73,500 years ago. The really scary thing, however, is that, unlike 'normal' volcanic blasts, there is no possibility of avoiding the devastating consequences of a volcanic super-eruption. Those of us tucked away in the most geologically friendly countries will still find our cosy world turned upside down by the next super-eruption, even if it occurs in a distant land on the other side of the planet. This is because of the severe impact it will have on the climate, the ash and gas ejected high into the atmosphere dramatically reducing the solar radiation reaching the surface and triggering a freezing volcanic winterworldwide.
Before examining the truly terrifying consequences of the next volcanic winter, let me take a more detailed look at the scale of volcanic super-eruptions, compared with the common-or-garden variety of volcanic blast. A number of scales have been devised in recent years to allow the sizes of volcanic events to be compared. One of the earliest and most commonly quoted is the Volcanic Explosivity Index or VEI devised by volcanologists Chris Newhall and Steve Self in 1982, primarily to allow estimation and comparison of the magnitudes and intensities of historical eruptions. Eruption magnitude refers to the mass of material erupted, while eruption intensity is a measure of the rate at which material is expelled. The index is logarithmic (like the better-known Richter Scale for earthquakes) which means that each point on the scale represents an eruption ten times larger than the one immediately below. Thus a VEI 5 is ten times larger than a 4, a VEI 6 a hundred times larger, and a VEI 7a thousand times larger. At the bottom of the index, the gentle effusions of lava that characterize most eruptions of Kilauea and Mauna Loa on Hawaii score a measly o, while mildly explosive eruptions that release sufficient ash to perhaps cover London or New York in a light dusting would register at 1 or 2. To a volcanologist, however, things don't really start to get exciting until higher values are reached. VEI 3 and 4 eruptions are described, respectively, as 'moderate' and 'large'. This translates into blasts big enough to cause local devastation, sending columns of ash up to 20 kilometres into the atmosphere and burying the surrounding landscape under piles of volcanic debris a metre or more deep. In 1994, the town of Rabaul in New Britain (Papua New Guinea) was destroyed by an eruption of this size, and a few years later — in 1997 — Plymouth, the capital of the Caribbean island of Montserrat, suffered the same fate. Eruptions that score a 5 on the scale, such as the much-televised 1980 blast of Mount St Helens (Washington State, USA) typically cause mayhem on a regional scale, while VEI 6 eruptions can be regionally devastating and the effects long-lasting. The 1991 Pinatubo eruption in the Philippines was probably the largest eruption of the twentieth century, ejecting sufficient ash and debris to bury central London to the depth of a kilometre and making hundreds of thousands homeless. For years afterwards, mudflows continued to pour down the flanks of the once-again dormant volcano, clogging rivers, burying farmland, and flooding towns and cities. For the last VEI 7 eruption we have to go back almost two centuries to 1815 — the year of the battle of Waterloo. As the armies of Wellington and Napoleon jostled for position across Europe, on the distant
Indonesian island of Sumbawa, the long-dormant volcano Tambora ripped itself apart in a gargantuan eruption that may have been the largest since the end of the Ice Age 10,000 years ago. Sir Stamford Raffles, the then British Lieutenant Governor of Java, reported a series of titanic detonations loud enough to be heard in Sumatra 1,600 kilometres away. When the eruption ended, after 34 days, it left 12,000 dead. In the ensuing months, however, a further 80,000 Indonesians succumbed to famine and disease as they struggled to find food and uncontaminated water across the ash-ravaged landscape.
Utterly devastating though the Tambora event no doubt was to the people of Indonesia, its direct effects were nonetheless confined to one part of South East Asia. Indirectly, however, much of the world was to suffer the consequences of this huge blast. Along with some 50 cubic kilometres of ash, the climactic explosions of the Tambora eruption also lofted around zoo million tonnes of sulphur-rich gases into the stratosphere, within which high-altitude winds swiftly spread them across the planet. The gases combined readily with water in the atmosphere to form 150 million tonnes of sulphuric acid aerosols—tiny particles of liquid that are very effective at blocking out solar radiation. Within months the northern hemisphere climate began to deteriorate and temperatures fell to such a degree that 1816 became known as the 'year without a summer.' Global temperatures are estimated to have fallen by around 0.7 degrees Celsius—perhaps a seventh of the drop required to plunge the planet into full ice age-causing summer frosts, snows, and torrential rains.
The miserable weather conditions may have set just the right mood for Mary Shelley's vivid imagination to spawn its most famous offspring, Frankenstein, while the spectacular ash and gas-laden sunsets are said to have inspired some of J. M. W. Turner's most brilliant works.
Certainly the weather conditions in Europe and North America during 1816 were awful, but could a volcanic eruption in a far-off part of the world really change the climate so much as to cause a breakdown in society and end the world as we know it? Evidence from the past suggests that there is no doubt that it can. Far back in the geological record — during the Ordovician period some 450 million years ago — an enormous volcanic explosion in what is now North America ejected sufficient ash and pyroclastic flows that, if it happened today, it would obliterate everything over an area of at least a million square kilometres. This is broadly the size of Egypt or four times the area of the UK. In addition the amount of gas and debris pumped into the atmosphere must have been phenomenal. A little nearer our time, just 2 million years ago, a mighty eruption at Yellowstone in Wyoming was violent enough to leave behind a gigantic crater (or caldera) up to 80 kilometres across, and pump out ash that fell across 16 states. Another huge eruption occurred at Yellowstone around 1.2 million years ago and yet another just years ago. If this last cataclysm occurred today it would leave the United States and its economy in tatters and the global climate in dire straits.
The eruption scoured the surrounding countryside with hurricane-force blasts of molten magma and incandescent gases —known as pyroclasticflows—with a volume sufficient to cover the entire USA to a depth of 8 centimetres. Ash fell as far afield as sites that are now occupied by the cities of El Paso (Texas) and Los Angeles (California), and Yellowstone ash from this eruption is even picked up in deep-sea geological cores from the Caribbean seabed. Although no eruptions have been recorded at Yellowstone for >70,000 years, the hot springs, spectacular geysers, and bubbling mud pools provide testimony that hot magma still resides not far beneath the surface. This is further supported by the numerous earthquakes that regularly shake the region and the periodic swelling and subsiding of the land surface. No one knows
when — or even if—Yellowstone will experience another devastating super—eruption. It is a little worrying, however, to note that these huge blasts seem to occur every 650,000 years or so. Perhaps then, we are due another any time now?
It would be easy to sit back and say — that's all very well, but these horrific events took place deep within the mists of time. Surely they can't happen today? Thinking along these lines would be a very big mistake. In 186 AD a massive eruption at New Zealand's Lake Taupo ejected pyroclastic flows that dev— astated a substantial portion of the North Island. 73,500 years ago—considerably older but still well within the time span of modern humanity—perhaps the greatest volcanic explosion ever tore a hole 100 kilometres across at Toba in northern Sumatra. This huge caldera, which is now lake filled, is very much a tourist attraction, but there is evidence of a much more sinister legacy. The eruption of Toba may have come within a hair's breadth of making the human race extinct. Estimates of the size of the blast vary, but there is no question that — along with the Yellowstone eruptions—Toba qualifies as a VEI 8 super—eruption. It was thought that the total amount of debris ejected during the eruption was of the order of 3,000 cubic kilometres, sufficient to cover virtually the whole of Indiawith a layer of ash one metre thick. Recent evidence from deep—sea geological cores suggests, however, that the eruption might have lasted longer than previously thought and ejected considerably more debris, perhaps up to cubic kilometres. Almost unbelievably, this would be enough to bury the entire United States to a depth of two— thirds of a metre.
Any of our ancestors living on Sumatra at the time would without question have been obliterated. For the human race as a whole to suffer the threat of extinction, though, the effects of the eruption would have to have been severe across the whole planet, and this they seem to have been. Along with the huge quantities of ash, the Toba blast may have poured out enough sulphur gases to create up to 5,000 million tonnes of sulphuric acid aerosols in the stratosphere. Thiswould have been sufficient to cut the amount of sunlight reaching the surface by go per cent, leading to global darkness and bitter cold. Temperatures in tropical regions may have rapidly fallen by up to 15 degrees Celsius, wiping out the sensitive tropical vegetation, while over the planet as a whole the temperature drop is likely to have been around 5 or 6 degrees Celsius, broadly the equivalent of plunging the planet into full ice age conditions within just a few months. Temperature records from Greenland ice cores suggest that the eruption was followed by at least six years of such volcanic winter conditions, which were in turn followed by a thousand-year cold 'snap'. Soon afterwards the planet entered the last Ice Age, and there is some speculation that in this respect, the cooling effect of the Toba eruption may have been the final straw, tipping an already cooling Earth from an interglacial into a glacial phase from which it only fully emerged around 10,000 years ago.
What then of our unfortunate ancestors: could this period of volcanic darkness and cold really have brought them to their knees? It certainly seems possible. Studies of human DNA contained in the sub-cellular structures known as mito-
chondria reveal that we are all much too similar—genetically speaking—to have evolved continuously and without impediment for hundreds of thousands of years. The only way to explain this extraordinary similarity is to invoke the occurrence of periodic population bottlenecks during which time the number of human beings was, for one reason or another, slashed and the gene pool dramatically reduced in size. At the end of the bottleneck, all individuals in the rap— idly expanding population carry the inherited characteristics of this limited gene pool, eventually across the entire planet. Mike Rampino, a geologist at New York University, and anthropologist Stanley Ambrose of the University of Illinois have proposed that the last human population bottleneck may have been a consequence of the Toba super-eruption. They argue that conditions after the Toba blast would have been comparable to the aftermath of an all-out nuclear war, although without the radiation. As the soot from burning cities and vegetation would result in a nuclear winter following atomic Armageddon, so the billions of tonnes of sulphuric acid in the stratosphere following Toba would mean per-
Sulphur aerosols (megatonnes)
34 Sunlight reduction due to Toba: worst-case estimates suggest that sulphuric acid aerosols from Toba may have cut out so much sunlight that the entire Earth was as dark as on a night of full moon
Sulphur aerosols (megatonnes)
34 Sunlight reduction due to Toba: worst-case estimates suggest that sulphuric acid aerosols from Toba may have cut out so much sunlight that the entire Earth was as dark as on a night of full moon petual darkness and cold for years. Photosynthesis would slow to almost nothing, destroying the food sources of both humans and the animals they fed upon. As the volcanic winter drew on, our ancestors simply starved to death leaving fewer and fewer of their number, perhaps in areas sheltered for geographical or climatological reasons from the worst of the catastrophe. It has been suggested that for 20 millennia or so there may have been only a few thousand individuals on the entire planet. This is just about as close to extinction as a species is likely to get and still bounce back, and—if true— must have placed our ancestors in as vulnerable a position as today's White Rhinos or Giant Pandas. Against all odds it seems that the dregs of our race managed to struggle through both the aftermath of Toba and the succeeding Ice Age, bringing our numbers up to the current 6 billion.
Could a future super-eruption wipe out the human race? It is highly unlikely that any eruption would be of sufficient size to completely obliterate today's teeming billions, but it is perfectly possible that our global technological society would not survive intact. Before the fall of the Berlin Wall, many national governments were quite prepared to plan for the terrible possibility of all-out nuclear war. With the threat now largely dissipated, however, there has been little enthusiasm for maintaining a civil defence plan to address the threat of a global geophysical catastrophe. In the absence of such forward thinking, the impact of a future super-eruption is likely to be appalling. With even developed countries such as the United States, the UK, Germany, and Australia having sufficient stores to feed their populations for a month or two at most, how would they cope with perhaps another six years without the possibility of replenishment? In the world's poorer countries, where famine and starvation are never far away, the situation would be magnified a thousand times, and death would come swiftly and terribly. From London to Lagos the law of the jungle would quite likely prevail as individuals and families fought for sustenance and survival. When the skies finally cleared and the Sun's initially feeble rays brought the first breath of warmth to the frozen Earth, maybe a quarter of the current population would have died through famine, disease, and civil strife.
Bearing in mind that over 70 millennia have elapsed since the Toba cataclysm it would be no surprise, statistically speaking, if another super-eruption struck within the next hundred years. But where? Restless calderas, which are constantly welling and shaking, are clear candidates, and both Yel-and Toba belong in this category. Large volumes of magma still reside beneath these sleeping giants that may well be released in future cataclysms. It is likely, however, that the warning signs of these giants' awakening-large earthquakes and severe welling of the surface—will continue for decades or even centuries before they finally let loose. As neither volcano is displaying such ominous behaviour at the moment we need not lose too much sleep over the imminence of a super-eruption at either Toba or Yellowstone. Only a tiny percentage of the Earth's 1,500 or so active volcanoes are currently, however, being monitored. Furthermore, the next super-eruption may blast itself to the surface at a point where no volcano currently exists. Perhaps even as
I write this some gigantic mass of magma that has been accumulating deep under the remote southern Andes may be priming itself to tear the crust apart — and our familiar world with it.
The super—eruptions I have talked about so far have all been cataclysmically explosive affairs. There is, however, another much less common species. One that — every few tens of millions of yearcerupts even greater volumes of magma, but with relatively little violence. Flood basalt erup— tions involve the effusion of gigantic volumes of low—viscosity lava that spread out over huge areas. These spectacular out— pourings have been identified all over the world, including India, southern Africa, the northwest United States, and northwest Scotland, but the greatest breached the surface nearly 250 million years ago in northern Siberia. Estimates vary, but it looks as if the lavas erupted by this unprecedented event covered over 25 million square kilometrecan area three times that of the United States.
Several similar outpourings have occurred throughout the Earth's long history and have been correlated with mass extinctions. Before the Siberian outburst, for example, the Earth of the Permian period teemed with life. During the succeeding Triassic period, however, when the great flows had cooled and solidified, fully 95 per cent of all species had vanished from the face of the planet. A similar mass extinc— tion 65 million years ago, at the end of the Cretaceous period, has been linked to the huge Deccan Trap flood basalt eruption in northwestern India. As I will address in the next chapter, however, there is incontrovertible evidence that the
Earth was struck at this time by a comet or asteroid, and many scientists now believe that this was the primary cause of the extinction of the dinosaurs and numerous other species at the end of the Cretaceous. Nevertheless, the Deccan lavas may also have had a role to play, pumping out gigantic quantities of carbon dioxide that may have led to severe greenhouse warming and the demise of organisms that were unable to adapt quickly enough. As our polluting society continues to do the same, perhaps we should take this as a salutary warning of what the future might hold for us, our world, and life upon it.
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