(8) Global Catastrophes

Tsunami - Earthquake Japan 2011

Pacific Tsunami Museum

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The Enemy within Super-Eruptions, Giant Tsunamis, and the Coming Great Quake

Hell on Earth

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 gritty 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 slime those 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 meter 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 its warming 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 halves 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 of 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 kilometers 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. The last such cataclysm shattered the crust at Taupo in New Zealand’s North Island, 26,500 years ago. This, however, does not mean we can sit back and relax for another 24 millennia or so.

Like buses, natural phenomena display scant regard for a timetable, so another super-eruption could be with us in 10 years – or even 100,000 years down the line. 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 cozy 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 winter worldwide.

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 7 a 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 0, 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 kilometers into the atmosphere and burying the surrounding landscape under piles of volcanic debris a meter 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 kilometer and making hundreds of thousands homeless. For years afterwards, ash-fed 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 jockeyed 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 kilometers 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 kilometers of ash, the climactic explosions of the Tambora eruption also lofted around 200 million tones 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 tones 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 to obliterate everything across an area of at least a million square kilometers. 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 kilometers 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 640,000 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 the hurricane-force blasts of molten magma and incandescent gases known as pyroclastic flows, whose gross volumes were sufficient – if spread across the nation – to cover the entire USA to a depth of 8 centimeters. 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 drill 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. The return periods between the three greatest Yellowstone blasts range from 660,000 to 800,000 years, so we could reasonably expect another sometime soon or have to wait well over a hundred millennia. It is also perfectly possible that Yellowstone may not ever produce another super-eruption, and that the giant volcanic system will gradually fade away into final extinction.

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 181 ad a massive eruption at New Zealand’s Lake Taupo ejected pyroclastic flows that devastated a substantial portion of the North Island, while the world’s last super-eruption did even worse damage to the island not much more than 24,000 years earlier. 74,000 years ago – considerably older but still well within the time span of modern humanity – perhaps the greatest volcanic explosion ever tore a hole 100 kilometers 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 on the order of 3,000 cubic kilometers, sufficient to cover virtually the whole of India with a layer of ash one meter 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 6,000 cubic kilometers. Almost unbelievably, this would be enough to bury the entire United States to a depth of two-thirds of a meter.

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 seems to have been the case. Along with the huge quantities of ash, the Toba blast may have poured out enough sulphur gases to create up to 5,000 million tones of sulphuric acid aerosols in the stratosphere. This would have been sufficient to cut the amount of sunlight reaching the surface by 90 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 mitochondria 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 rapidly 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 tones of sulphuric acid in the stratosphere following Toba would mean perpetual 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 6.5 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 civil defense plans 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.

It is extremely unlikely, but not impossible, that another super eruption might strike within the next hundred years. But where? Restless calderas, which are constantly swelling and shaking, are clear candidates, and both Yellowstone 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 swelling of the surface – will continue for decades or even centuries before they finally let loose. As neither volcano is displaying such ominous behavior 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 years – erupts even greater volumes of magma, but with relatively little violence. Flood basalt eruptions involve the effusion of gigantic volumes of low-viscosity lava that spread out over huge areas. These spectacular outpourings 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 kilometers – an 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 extinction 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. However, there is incontrovertible evidence that the Earth was struck at this time by a comet or asteroid, and  many scientists 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.

A watery grave

Although by no means the largest volcanic event of the twentieth century, the spectacular 1980 eruption of Mount St Helens, in Washington State (USA), was certainly the most filmed. Perhaps because it occurred in the world’s most media-attuned country, the explosions as the volcano blew itself apart were almost drowned out by the whir of cameras and the scribbling of journalists’ pencils. From a scientific point of view, however, the eruption was a watershed, because it drew attention to a style of eruption that had previously attracted little interest from volcanologists. Most eruptions involve the vertical ejection of volcanic debris from a central  vent, but the climactic eruption of Mount St Helens was quite different. Lava and debris from the previous eruption – all of 120 years earlier – had blocked the central conduit ensuring that the fresh magma rising into the volcano could not easily escape.

Instead it forced its way into the volcano’s north flank, causing it to swell like a giant carbuncle. By mid-May the carbuncle was 2 kilometers across and 100 meters high, and very unstable. Just after 8.30 in the morning on 18 May, a moderate earthquake beneath the volcano caused it to shrug off the bulge, which within seconds broke up and crashed down the flank of Mount St Helens as a gigantic landslide. With this huge weight removed from the underlying magma, the  gases contained therein decompressed explosively, blasting northwards with sufficient force to flatten fully grown fir trees up to 20 kilometers away and obliterating, in all, over 600 square kilometers of forest. The landslide material rapidly mixed with river and lake water forming raging mudflows that poured down the river valleys draining the volcano, while pyroclastic flows tore down the flanks and ash fell as far afield as Montana 1,000 kilometers away.

The Mount St Helens blast killed 57 people and was a disaster for the region, but its scientific importance lies squarely in its elucidation of the mechanism known as volcano lateral collapse. Most of us view volcanoes as static sentinels: bastions of strength and rigidity that are unmoving and unmovable. In fact, they are dynamic structures that are constantly shifting and changing. Far from being strong they are often rotten to the core; little more than unstable piles of ash and lava rubble looking for an excuse to fall apart. The numerous studies that followed the Mount St  Helens eruption revealed that collapse of the flanks and the formation of giant landslides is a normal part of the lifecycle of many volcanoes, and probably occurs somewhere on the planet around half a dozen times a century. Furthermore, they showed that the Mount St Helens landslide was tiny compared to the greatest known volcano collapses – with a volume of less than a cubic kilometer compared with over 1,000 cubic kilometers for the prodigious chunks of rock that had, in prehistoric times, sloughed off the Hawaiian Island volcanoes.

At this stage you might be asking yourself, so what? Surely a hunk of rock – however large – falling off a volcano can’t have a global impact – cans it? Well it probably can, provided that the collapse occurs into the ocean. In 1792, a relatively small landslide flowed down the side of Japan’s Unzen volcano and into the sea. The water displaced formed tsunamis tens of metres high that scoured the surrounding coastline, killing over 14,000 inhabitants in the small fishing villages that lined the shore. Just over a century later, in 1888, part of the Ritter Island volcano off the island of New Britain (Papua New Guinea) fell into the sea, generating tsunamis up to 15 meters high that crashed into settlements on neighboring coastlines taking over 3,000 lives. Clearly, the combination of a volcanic landslide and a large mass of water is a lethal one, but – you are no doubt thinking – how can it affect the vast majority of the Earth’s population who live far from an active volcano? The answer lies partly in the size of the largest collapses and partly in the scale of the tsunamis they generate.

Underwater images of the seabed surrounding the Hawaiian Islands show that they are surrounded by huge aprons of debris shed from their volcanoes over tens of millions of years. Within this great jumbled mass of volcanic cast-offs, nearly 70 individual giant landslides have been identified, some with volumes in excess of 1,000 cubic kilometers. The last massive collapse in the Hawaiian Islands occurred around 120,000 years ago from the flanks of the Mauna Loa volcano on the Big Island. Giant tsunamis resulting from the entry of this huge mass of rock into the Pacific Ocean surged 400 meters up the flanks of the neighboring Kohala volcano – higher than New York’s Empire State Building. Deposits of a similar age, which may be tsunami-related, have also been recognized 15 meters above sea level and 7,000 kilometers away on the southern coast of New South Wales in Australia. While the nature and provenance of both deposits is still debated, the scale of the waves generated appears to be realistic, with computer models developed to simulate the emplacement of giant volcanic landslides into the ocean coming up with similar sized tsunamis.

It seems, then, as if major collapses at ocean island volcanoes are perfectly capable of producing waves that are locally hundreds of meters high and remain tens of meters high even when they hit land half an ocean away. The next collapse in the Hawaiian Islands is likely, therefore, to generate a series of giant tsunamis that will devastate the entire Pacific Rim, including many of the world’s great cities in the United States, Canada, Japan, and China. In deep water, tsunamis travel with velocities comparable to a Jumbo Jet, so barely 12 hours will elapse before the towering wave’s crash with the force of countless atomic bombs onto the coastlines of North America and eastern Asia.

Nor is the problem confined solely to the Pacific. Scientific cruises around the Canary Islands, together with detailed geological surveys on land, have revealed a picture very similar to that painted for Hawaii. Huge masses of jumbled rock stretching for hundreds of kilometers across the seabed, and gigantic cliff-bounded collapse scars on land, testify to enormous prehistoric collapses from the islands of Tenerife and El Hierro. Of more immediate concern, it looks as if a new giant landslide has recently become activated on the westernmost Canary Island of La Palma, and is primed and ready to go. During the eruption before last, in 1949, much of the western flank of the island’s steep and rapidly growing volcano – the Cumbre Vieja – dropped 4 meters towards the North Atlantic and then stopped. Some UK and US scientists believe that this gigantic chunk of volcanic rock – with an estimated volume of a few hundred cubic kilometers, just about double the size of the UK’s Isle of Man – is now detached from the main body of the volcano and will eventually crash en masse into the sea. The problem at the moment is that we don’t have a clue when this will happen. It will probably be soon – geologically speaking – but whether it will be next year or in 10,000 years we simply don’t know. Measurements undertaken during the late 1990s using the satellite Global Positioning System  proved somewhat inconclusive but suggested that the landslide might still be creeping slowly seawards, perhaps at only a centimeter a year or even less. Even if this is the case, however, the rock mass is unlikely to complete its journey into the North Atlantic without the trigger of a new eruption.

What is certain is that at some point in the future the west flank of the Cumbre Vieja on La Palma will collapse, and the resulting tsunamis will ravage the entire Atlantic rim. Steven Ward of the University of California at Santa Cruz and Simon Day of University College London’s Benfield Hazard Research Centre created quite a stir in 2001 when they published a scientific paper that modeled the future collapse of the Cumbre Vieja and the passage of the resulting tsunamis across the Atlantic. Within two minutes of the landslide entering the sea, Ward and Day show that – for a worst case scenario involving the collapse of 500 cubic kilometers of rock – an initial dome of water an almost unbelievable 900 meters high will be generated, although its height will rapidly diminish.

Over the next 45 minutes a series of gigantic waves up to 100 meters high will pound the shores of the Canary Islands, obliterating the densely inhabited coastal strips, before crashing onto the African mainland. As the waves head further north they will start to break down, but Spain and the UK will still be battered by tsunamis up to 7 meters high. Meanwhile, to the west of La Palma, a great train of prodigious waves will streak towards the Americas. Barely six hours after the landslide, waves tens of meters high will inundate the north coast of Brazil, and a few hours later pour across the low-lying islands of the Caribbean and impact all down the east coast of the United States. Focusing effects in bays, estuaries, and harbors’ may increase wave heights to 50 meters or more as Boston, New York, Baltimore, Washington, and Miami bear the full brunt of Vulcan and Neptune’s combined assault. The destructive power of these skyscraper-high waves cannot be underestimated. Unlike the wind driven waves that crash every day onto beaches around the world, and which have wavelengths (wave crest to wave crest) of a few tens of meters, tsunamis have wavelengths that are typically hundreds of kilometers long. This means that once a tsunami hits the coast as a towering, solid wall of water, it just keeps coming – perhaps for ten or fifteen minutes or more – before taking the same length of time to withdraw. Under such a terrible onslaught all life and all but the most sturdily built structures are obliterated.

The lessons of the Indian Ocean tsunami have taught us that without considerable forward planning it is unlikely that the nine hours it will take for the wave’s advance guard to reach the North American coastline will be sufficient to facilitate effective, large-scale evacuation, and the death toll is certain to run into many millions. Furthermore, the impact on the US economy will be close to terminal, with the insurance industry wiped out at a stroke and global economic meltdown following swiftly on its heels. In this way, a relatively minor geophysical event at a remote Atlantic volcano will affect everyone on the planet. Like volcanic super eruptions, these giant tsunamis constitute perfectly normal, albeit infrequent, natural phenomena. At some point in the future one will certainly wreak havoc in the Atlantic or Pacific Basins, but when? The frequency of collapses on the Hawaiian volcanoes has variously been estimated to be between 25,000 and 100,000 years, but if giant landslides at all volcanic islands are considered, it may be that a major collapse event occurs every ten millennia or so. On a geological timescale this is very frequent indeed and should provide us with serious cause for concern. Even more worryingly, the rate of collapse may not be constant and the current episode of global warming engendered by human activities may in fact bring forward the timing of the next collapse. My own research team has linked increased incidences of past volcano collapse with periods of changing sea level, while others have suggested that a warmer and wetter climate might result in greater numbers of large volcanic landslides. Given that sea levels are forecast to continue to rise for the foreseeable future, while studies of past climate change show that a warmer planet results in heavier rainfall on many of the world’s largest volcanic island chains, perhaps we should all be thinking of moving inland and uphill, or at least of investing in a good-quality wet suit.

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