Us vs the volcano
In 1883 the island of Krakatoa in Indonesia blew apart, killing at least 40,000 people and throwing out 20 cubic kilometres of rock, ash and pumice, in an ear-splitting explosion that was heard 3,500 kilometres away in Perth, Australia, where it was mistaken for cannon-fire. Barograph readings showed that the pressure wave from the event circled the Earth seven times before fading out. The settlement of Merak on the north-western tip of Java was washed away in seconds by a wave towering almost 50 metres high, and ships as far away as South Africa were rocked as the residues of other tsunamis buffeted their hulls.
More violent still was the fate of the Aegean island of Santorini 3,600 years ago. A giant central lagoon is all that remains of the great volcano that erupted around 1620 BC, spewing vast clouds of dust and ash, and generating a huge tsunami that inundated the nearby island of Crete. This is the event, according to some historians, that triggered the demise of one of the world’s great early civilisations – the Minoans. The obliteration of Santorini, it’s also been theorised, may be the historical counterpart to the legend of Atlantis. At least 30 cubic kilometres of magma, rock bombs, dust and other debris poured out of Santorini, ranking the eruption among the top seven or eight biggest of the past 10,000 years, and making it perhaps the most influential in terms of its effects on the course of Western history.
It’s hard to imagine that there could be volcanic eruptions that dwarfed those of Krakatoa and Santorini. But there have been – and there will be again. One way volcanologists classify eruptions is in terms of the Volcanic Explosivity Index (VEI). Devised in 1982 by Chris Newhall of the US Geological Survey and Stephen Self of the University of Hawaii it gauges the relative violence of volcanic blow-outs. Factors that go into determining the VEI are the volume of material ejected, the height of the cloud of debris from the eruption and qualitative observations about the nature of the explosion. The VEI range goes from 0, for a non-explosive, gentle burbling or spraying of lava, to 8 in the case of the most prodigious, mega-cataclysmic supervolcanoes.
None of the world’s most famous eruptions has reached supervolcano status. Mount Etna, for example, on the east coast of Sicily, received a rating of 3 for its 2002-3 efforts, while the 2010 air traffic-disrupting effusions of Iceland’s Eyjafjallajökull (every newsreader’s nightmare) went one better with a 4. With an index of 5 comes the Mount St Helens eruption of 1980, followed in category 6 by Pinatubo (1991), Krakatoa (1883) and Novarupta (1912).
Santorini, for its circa 1620 BC detonation, earns an impressive VEI of 7. Matching this score is the little-known 186 AD eruption of Taupo on New Zealand’s North Island. Lake Taupo, the second largest freshwater lake in Oceania, is actually the submerged caldera of a colossal volcano that had its last major eruption more than 1,800 years ago. Only one explosion in recent history has made it to 7 on the Volcanic Explosivity Index. This is Mount Tambora, on the island of Sumbawa in Indonesia. Tambora began to rumble ominously in 1812, and reached a crescendo with a mind-numbing eruption in April 1815. Roughly 160 cubic kilometres of ejecta issued from Tambora’s 1815 outburst, making it the largest volcanic eruption in recorded history. The vast quantities of dust and ash entering the atmosphere lowered temperatures worldwide for months afterwards, and the following year, 1816, became known as the “year without a summer”.
“It’s hard to imagine that there could be volcanic eruptions which dwarfed those of Krakatoa and Santorini. But there were – and there will be again”
But what we know about Tambora and its effects was learned in retrospect. In fact, astonishingly, it took 160 years for the truth to come to light. Examining ash layers found in Greenland ice cores, scientists were amazed to discover the extent of the Tambora phenomenon – an eruption ten times more powerful than any other in the past two centuries, including the awesome Krakatoa.
Tambora, just east of the island paradises of Bali and Lombok, once rose to a height of over 4,000 metres. That was before its spectacular awakening after a long period of dormancy lasting five millennia. For three years, beginning in 1812, Tambora spewed steam and ash, and was rocked almost incessantly by earthquakes. Unbeknown to anyone at the time, seawater had found its way through cracks in the rock into the magma chamber deep under the volcano. Disaster was now inevitable. Gradually the pressure of superheated steam, in the heart of the mountain, built up and up until it could no longer be contained. On 10th April 1815, Tambora was blown apart in a series of devastating explosions. Volcanic ash and gas shot high into the stratosphere, while at ground level that deadliest of all volcanic phenomena, a pyroclastic flow – a fast-moving plume of superheated rock and ash – raced across the surrounding landscape at speeds that would have overtaken a Ferrari, incinerating and burying everything in its path. About 10,000 people died immediately. When the pyroclastic flow reached the ocean, it set off tsunamis which ravaged the populations on nearby islands. Ash from the eruption landed as far as 1,300 kilometres away. Where it fell thickest it killed crops and other plants, eventually leading to the deaths of another 80,000 to 90,000 people from famine.
The vast quantities of gas and aerosols injected into the atmosphere by Tambora blocked sunlight and caused temperatures to tumble around the world, most acutely over the northern hemisphere. Crop failures became common in Europe, with hunger and disease following in their wake.
One of the earliest modern works of horror and science-fiction came about indirectly because of the Tambora eruption. The poets Lord Byron and Percy Shelley were staying by Lake Geneva in 1816, and, as a result of the bad weather, spent much of the summer with their friends and family entertaining indoors. During one such gathering, Byron proposed that everyone present try to write a ghost story. Byron himself, inspired by the brooding skies of that gloomy season wrote the poem Darkness. Shelley’s wife, Mary, came up with an even more menacing creation – her novel Frankenstein, a landmark in gothic literature often credited as the first major sci-fi work.
When all hell breaks loose
The eruptions of Tambora, in 1815, and Taupo, New Zealand, in 186, were the two biggest of the past 10,000 years – roughly the period for which humans have been civilised. Both score 7 on the Volcanic Eruptivity Index. Both blasted out around 100 cubic kilometres of rock, dust and ash. But neither were supervolcanoes. That term, popularised by a BBC Horizon programme in 2000, is generally reserved for eruptions involving at least 1,000 cubic kilometres of ejecta – ten times the size of Tambora and Taupo, and similar in destructive capacity to a one-kilometre-wide asteroid barrelling into the Earth. Supervolcanoes have a VEI of 8, the maximum value recognised. According to the geological record, one of these monsters explodes on average every 100,000 years (making it ten times more likely than an asteroid impact capable of similar devastation). The last happened about 74,000 years ago on the island of Sumatra in western Indonesia. Known as the Toba super-eruption, its site is marked today by Lake Toba – at 100 kilometres long by 30 kilometres wide, the largest volcanic lake in the world.
Even by supervolcano standards Toba was impressive, unrivalled by any other eruption over the past 25 million years. The amount of molten rock and other stuff that poured out came to around 2,800 cubic kilometres – more than the double the volume of Mount Everest. Toba occurred at a time when Neanderthals and Paleaolithic humans coexisted in Europe and much of Asia, and its after-effects may have brought immense hardship to our ancestors, perhaps, according to some research, pushing them to the brink of extinction.
Immediately following the blast, global temperatures fell – by how much and for how long is a matter of debate. A key factor is the amount of sulphur dioxide and aerosols released by the eruption and what happened to them once they rose high into the atmosphere. Different theoretical models produce different results. What seems likely is that the Toba event put significant stresses on the humans who were around at the time.
“Roughly 160 cubic kilometres of ejecta issued from Tambora’s
1815 outburst, making it the largest volcanic eruption in recorded history”
It’s generally accepted that the Toba super-eruption caused a worldwide slump in average temperatures of between three and five degrees Celsius, and as much as 15 degrees at higher latitudes. The ash fallout was prodigious. A blanket of ash at least 15 centimetres deep covered all of South Asia, and in places the deposition was much greater – six metres (20 feet) at one site in India, and nine metres (30 feet) in parts of Malaysia. Flora and fauna in South-East Asia must have been devastated, and there may have been a planet-wide die-off.
Certainly, the Toba blow-out happened slap bang in the middle of a period, between 100,000 and 50,000 years ago, when the human population plummeted. This has led to the Toba catastrophe theory, according to which the effects of the eruption were so severe that the global population of Homo sapiens was slashed to 10,000 individuals or less. The catastrophe theory, although controversial, is backed up by genetic evidence, which suggests that all of us alive today, despite our apparent diversity, are descended from a very small group of people, perhaps as few as 1,000 breeding pairs who were alive about 70,000 years ago. The environmental pressures on this perilously tiny group of human survivors could only have been made worse as a result of the Toba super-eruption.
Somehow our species clawed its way back from its low-population crisis, and not only Homo sapiens, but also, for a while, the Neanderthals in Europe and, it seems, a small-brained relative of ours, Homo floresiensis (popularly known as Hobbits). The Toba disaster, the suggestion goes, would have forced a lifestyle change on humans at the time. The die-off of vegetation, and the cooling and drying of the climate resulting from the volcanic fallout, might have altered the migratory habits of our ancestors and compelled them to adopt new and ingenious methods to gain access to whatever scarce food sources were available. As separate little colonies developed independently, they would have sown the seeds for the differences between races seen today. Ultimately, our species may have benefited from the ordeal of Toba; it made us tougher, smarter, more reliant on our wits and our latent talents for communication and co-operation. The Neanderthals evidently didn’t fare so well in the final analysis, as they wound up extinct around 35,000 years later. But we came desperately close to annihilation in those troubled times.
The Yellowstone menace
There’s no doubt that the Earth will experience more super-eruptions. Some of the supervolcanoes that have erupted in the past and caused such global mayhem retain the capacity to do so again. Possible contenders for the next super-eruption include the Phlegrean Fields volcano west of Naples, Italy, and Lake Taupo in New Zealand, as well as locations in Indonesia, the Philippines, Central America, Japan and the Kamchatka peninsula in eastern Russia. But of all the candidates none has been so talked about, or instilled such popular fear, as one of the most visited, picturesque and unique areas of the planet, right in the heart of the United States: the Yellowstone caldera in the north-west corner of Wyoming.
The Yellowstone caldera, occupying about half of Yellowstone National Park, measures some 72 kilometres (45 miles) by 55 kilometres (34 miles) and is the site of numerous past eruptions, many of them in the range of ordinary volcanoes but a few in the supervolcano class. The most recent of these gargantuan outbursts, about 640,000 years ago, is thought to have been responsible for the demise of many of the larger mammals in North America at the time, including camels, rhinos and elephants, which choked on toxic gases or starved to death following the die-back of vegetation blanketed by the continent-wide ash cloud from the event. Other super-eruptions of the Yellowstone supervolcano happened 1.3 million and 2.1 million years ago. The average period between eruptions, geologists have determined, is about 600,000 years, which means we’re due for another one at any time.
This wouldn’t be good news for America. Blanketed in a layer of ash a metre thick (and a great deal more in areas close to the eruption), the country would be rendered virtually unfit for human habitation. Midwestern states, home to much of the nation’s food production and industry, would be hit especially hard. But the effects would be felt worldwide. As in the case of the Toba eruption, global cooling, the mass dying of plants and then the mass dying of animals and people would follow in the days, months and years after the cataclysm. In 2005, a working group of the Geological Society of London assessed the consequences of a contemporary super-eruption. “Pronounced deterioration of the global climate would be expected for a few years following the eruption,” it said. “Such events could result in the ruin of world agriculture, severe disruption of food supplies and mass starvation. The effects could be sufficiently severe to threaten the fabric of civilisation.”
“Beneath the picture-perfect scenery of Yellowstone is a giant magma chamber, which is slowly but surely filling with molten rock”
Not surprisingly, having seen dramatisations on TV of what a super-eruption could do, people get jittery when reports come through of fresh activity in the nation’s favourite national park. “Run for your lives… Yellowstone’s going to explode!” read a (slightly tongue-in-cheek) news headline from the Associated Press on 10th January 2009. Hundreds of minor earthquakes in the preceding weeks had set nerves jangling, reminding everyone that the photogenic landscape in this corner of Wyoming, with its geysers, mud-pots and hot springs, sits atop a supervolcano that will burst into life again, sooner or later.
Small earthquakes aren’t unusual in Yellowstone, and the signs of geothermal activity are everywhere to be seen, from the predictable appearances of the Old Faithful geyser to the constantly bubbling, burbling, sulphurous cauldrons of hot water and mud to be found all over the park. These crowd-pleasing features are just mild allusions to the colossal forces that are steadily building up below.
Between six and 16 kilometres beneath the picture-perfect scenery of Yellowstone is a giant magma chamber, which is slowly but surely filling with molten rock from the underlying mantle. It’s an estimated 50 kilometres long, 30 kilometres wide and ten kilometres deep, and is fed by a magma plume that rises at a 60-degree angle from at least 660 kilometres beneath Earth’s surface. The deepest part of the plume lies under the town of Wisdom, Montana, about 240 kilometres from Yellowstone National Park. Trapped gases are steadily increasing the pressure inside the magma, and although some of that pressure is gently relieved on a daily basis by the various geothermal features that attract visitors to the park, it isn’t enough. There may be minor eruptions first: no one can be sure that the next blow-out will be a monster one. But, at some point in the future, the pressure inside the subterranean chamber will reach a critical level, the overlying rock will be split apart, and the gas-laden magma will erupt explosively over a wide area at the surface. On that fateful day, more than 1,000 cubic kilometres of magma might burst into the light of day and bring hell to North America and beyond.
David Darling and Dirk Schulze-Makuch are the authors of ‘Megacatastrophes: Nine Strange Ways the World Could End’ published by Oneworld Publications.
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