by Alok Jha
And it would continue. In the following years, billions would die of starvation as crops failed and supplies of fresh water became contaminated. Modern life would stop. And despite the technological mastery humans have of the world, there would be nothing we could do to prevent it.
What is a supervolcano?
There are plenty of volcanoes on Earth capable of big, explosive eruptions and of shutting down entire countries for weeks. Supervolcanoes, though, are something very much out of the ordinary. They are so huge, so potentially devastating, that an eruption would have implications for global civilization, effects comparable to the aftermath of a 900 m (3,000 foot) wide asteroid hitting the Earth: fires, dust clouds, debris and tsunamis. Except that a supervolcano eruption is around 5–10 times more likely to occur within the next few thousand years than an asteroid impact.
Geologists estimate that every 100,000 years or so, there should be several eruptions large enough to cause global disaster. There have been none in the time that people have kept records—the giant eruptions at Tambora (1815), Krakatoa (1883) and Pinatubo (1991) all caused major local and climatic problems in the months and years afterward. But human civilization always remained intact afterward.
Supervolcanoes would be hundreds of times bigger than these eruptions, however, and their effects much worse. Entire continents would be covered in mud, ash and fire. For interminable years afterward, the Earth’s average temperature would drop as the fine particles ejected by the volcano traveled around the world and blocked out sunlight. Global agriculture would be devasted, food supplies disrupted and billions of people would die of starvation.
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Entire continents would be covered in mud, ash and fire ... and billions of people would die of starvation.
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At its simplest, a volcano is an opening in the Earth’s crust that allows magma, ash and hot gas from the planet’s interior to escape. They are normally found at the boundaries between tectonic plates, and erupt when the pressure underneath the cap is too high to be contained. In scientific definition, a supervolcano is something that ejects more than a trillion tons of material when it erupts. The last known example, the Toba eruption in Sumatra, happened around 74,000 years ago according to geological records, and everything we know about it and other supervolcanoes has been inferred by scientists from the historical effects of such eruptions on the Earth. Toba was the largest eruption in the past 2 million years, releasing around 2,500 Km3 (600 cubic miles) of material, around twice the volume of Mount Everest. A layer of ash 15 cm thick settled on India and southern China—just 1 cm is enough to ruin crops.
A volcano is the surface result when molten rock and magma rise from the core and penetrate the Earth’s crust. The pressure of the material under the crust causes the crust to buckle and, subsequently, release the molten rock and ash high into the air.
Toba ejected over 300 times more volcanic ash than even the biggest eruption of modern times, Tambora in Indonesia in 1815. Following the latter eruption was the “year without summer” in which Lord Byron wrote his poem “Darkness” and Mary Shelley composed Frankenstein. Temperatures in the northern hemisphere were depressed for around two years.
Back when Toba erupted, there was nothing in the way of civilization or infrastructure to be damaged, but still the event brought the fledgling human race to the edge of survival. As global temperatures dropped by up to 10°C in some places, the resulting ecological devastation left only a few thousand individuals alive at a time when Homo sapiens was first leaving Africa.
And there have been bigger than Toba: the biggest event of all time ejected 9,000 Km3 (2,200 cubic miles) of rock and ash. This was the so-called Fish Canyon Tuff event in Colorado, United States, around 27 million years ago. Super-eruptions are so devastating that some geologists believe they might explain some of the Earth’s mass extinctions, such as the one that occurred 250 million years ago in the Permian, when more than 90 percent of the world’s marine species were wiped out after the eruption of the Siberian Traps.
What happens on the Earth? What happens in the sky?
As part of an exercise to raise awareness of the potential dangers of supervolcanoes, the Geological Society of London contemplated what might happen if there was an eruption in the (admittedly highly unlikely) location of just a mile from the Houses of Parliament.
“A super-eruption in Trafalgar Square, London, yielding 300 cubic kilometers of magma would produce enough volcanic deposits to bury all of Greater London to a depth of about 150 meters (nearly 500 feet) thick,” said the working group’s report. “A larger super-eruption (1,000 cubic kilometers) would bury the same area to a depth of 420 meters (almost 1,400 feet). These thicknesses do not include extensive ash-fall deposits, which could cover an area greater than all of Europe.”
The immediate vicinity of the eruption would be destroyed beyond repair, and life would be difficult there as the ash quickly washed into the supplies of fresh water. Mudflows would block rivers and lead to floods.
Beyond that, for tens of thousands of square miles, waves of devastation would arrive in the form of incandescent hurricanes of gas and rock, called pyroclastic flows. These can reach 1,000°C in temperature and travel at speeds approaching those of jet aircraft. “No living beings caught by a pyroclastic flow survive,” wrote the geologists.
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A super-eruption in Trafalgar Square, London, yielding 300 Km3 of magma would produce enough volcanic deposits to bury all of Greater London to a depth of about 150 meters.
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And that is not even the worst of it. “Globally, most repercussions will come from the effects of the volcanic ash and volcanic gases suddenly released into the atmosphere,” they continued. This cloud of aerosols would reach high into the atmosphere to block out sunlight and absorb water vapor.
“The gases [...] commonly include significant amounts of sulphur dioxide, carbon dioxide and chlorine,” said the report. “Dust and gases injected by an eruption into the stratosphere reflect solar radiation back to space or themselves absorb heat, cooling the lower atmosphere. This fact has led to the concept of ‘volcanic winter.’ Silicate dust (tiny ash particles) is thought to be less important, because its residence time in the stratosphere is quite short (only a few weeks to months at most). The main thing causing global cooling after a major eruption is sulphur dioxide gas, which reacts with water to form tiny droplets of sulphuric acid, which remain in the stratosphere for two or three years as an aerosol.”
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PYROCLASTIC FLOW
Temperature 1,000°C Speed 700 km/h
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A super-eruption would devastate human civilization, which depends on trade and food that moves around the world. Ash clouds in the atmosphere would prevent air travel indefinitely and hamper satellite communications.
The Earth’s climate would eventually return to normal once the aerosols had disappeared, but given the complexity of the climate, it is hard for scientists to predict how long this would take. Observations of relatively small historic eruptions, such as Krakatoa and Pinatubo, showed that the aerosol levels dropped after a couple of years. “In principle, putting twice as much aerosol in the stratosphere should double the predicted climatic effect. But climate systems are complex, with important feedback processes. Thus the consequences of very much larger injections cannot be forecast with much confidence, said the Geological Society of London.”
Recent analysis of ice cores shows that the Toba eruption’s aerosol fallout could have lasted up to six years, with global temperatures dropping by 3–5°C. It may not sound like much, but a 4°C cooling sustained over a long enough period can cause a new ice age. According to the Geological Society, however, “great caution is needed in attributing causes and effects in a system as complex as global climate, and more detailed modeling research is required. Initial computer climate-model runs by scientists at the UK Meteorological Office’s Hadley Centre for a Toba-sized er
uption suggest Northern Hemisphere temperature drops of 10°C. This would freeze and kill the equatorial rainforests.”
Where will it strike?
There are plenty of contenders for the next supervolcano. The Yellowstone volcano in Wyoming rumbles from time to time, the result of a plume of molten rock that starts deep within the Earth. The area has experienced huge eruptions in the past: 2.1 million years ago, it suffered a Toba-sized blast at the Huckleberry Ridge that created the Island Park caldera and covered most of the continental United States in ash; 1.3 million years ago, a smaller eruption formed in the Henry’s Fork caldera.
Other danger spots around the world include Lake Taupo in New Zealand and the Phlegrean Fields volcano west of Naples in Italy. There is also the more familiar region of volcanic activity around Indonesia and the Philippines, while geologists continue to keep an eye on Japan, Central American countries and the Kamchatka peninsula in eastern Russia.
Can we stop it?
In short, no. Neither can we cross our fingers and hope it goes away as an issue. Another supervolcano eruption is inevitable. “It is not a question of ‘if’—it is a question of ‘when,’” Bill McGuire, director of the Aon Benfield Hazard Research Centre at University College, London, told New Scientist magazine.
Problems such as global warming, impacts by asteroids and comets, rapid use of natural resources and nuclear waste disposal require world leaders and governments to address issues with very long-term consequences for the global community. Countries already have disaster plans in place in case of emergency, and they should put one in place for supervolcanoes too, concluded the working group for the Geological Society of London. What would happen, they asked, if several billion people needed to be evacuated from most of Asia, while at the same time, Europe and North America were threatened with several years of agricultural devastation?
“This is not fanciful, but the kind of acute problem and inevitable consequence of the next super-eruption,” says the Geological Society’s working group. “Sooner or later a super-eruption will happen on Earth and this is an issue that also demands serious attention. While it may in future be possible to deflect asteroids or somehow avoid their impact, even science fiction cannot produce a credible mechanism for averting a super-eruption. The point is worth repeating. No strategies can be envisaged for reducing the power of major volcanic eruptions.”
Oxygen Depletion
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The world’s plants and animals need oxygen to survive. If for some reason the amount of oxygen in the air or sea dropped, it is not hard to conclude that life would become difficult in its current form. Even if the level fell just a small amount, billions of individual living things would die straight away, and billions more that depended on them in the interconnected ecosystem would eventually die too.
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Oxygen is important for life, but don’t take it for granted that the Earth will keep a ready supply available for the living things that cover its surface. In the past, the levels of this crucial gas have fluctuated enough to cause major problems. There was a marked period of deficiency in oceanic oxygen during the Cretaceous period, for example, which was the heyday of the dinosaurs. That deficiency, probably the result of an increase in the activity of undersea volcanism, led to mass extinction of life. As scientists learn more about such phenomena and the geological events that led up to them, they see alarming parallels with the warming world of today. History could be readying to repeat itself.
What is an anoxic event?
Around 93 million years ago, the Earth was going through an unusual episode of volcanic activity. Huge piles of lava collected along the seabed, creating island chains such as the Caribbean. The planet was so hot that there were palm trees in Alaska and large reptiles wandered around northern Canada.
At the same time, the seas were extremely low on oxygen, probably due to the very same volcanic activity. Scientists believe that these huge lumps of lava caused ocean circulation to slow down, so that carbon and oxygen did not move around so quickly. On the sea floor, conditions became highly toxic to life, devoid of the oxygen required to keep things alive.
“Episodes of anoxia, known as oceanic anoxic events (OAEs), have occurred periodically during Earth’s history, but none was more severe than that which occurred 93 [million years] ago, during the Cretaceous period,” wrote Timothy J. Bralower of the department of geosciences at Pennsylvania State University, in a 2008 article for Nature. “This OAE caused the extinction of large clams known as inoceramids and tiny protists called foraminifera that lived on the sea floor. Profound changes in ocean circulation also led to the production and preservation of enormous quantities of marine organic material that was subsequently transformed into oil during its burial.”
He also speculated how the volcanic activity of the time might have caused this strange anoxic environment. “One possibility is that the volcanism seeded the upper ocean with metal micronutrients, increasing phytoplankton production, which in turn led to increased oxygen use during the decay of organic matter. Another, not mutually exclusive, possibility is that a consequence of the global warming stemming from volcanically produced CO2 was a more stratified ocean, in which oxygen delivery to deep waters became restricted,” wrote Bralower.
So, if heat and gases emanating from the Earth and the sea caused this and several other ancient mass extinctions, could the same killer greenhouse conditions develop once again? Peter D. Ward, a professor in the University of Washington’s biology department and an expert on ancient mass extinction events, thinks so.
He argues that during mass extinctions of the past, green and purple sulfur-loving bacteria colonized seas that had been depleted of oxygen but were rich in hydrogen sulfide. Chemical evidence found in bands of stratified rock that mark out mass extinction events in the geological record makes it clear that cataclysmic extinctions in the past—an asteroid hitting the Earth, say—were the exception rather than the rule. “In most cases, the earth itself appears to have become life’s worst enemy in a previously unimagined way,” wrote Ward in a 2006 article for Scientific American. “And current human activities may be putting the biosphere at risk once again.”
He continued: “Scientists have long known that oxygen levels were lower than today around periods of mass extinction, although the reason was never adequately identified. Large-scale volcanic activity, also associated with most of the mass extinctions, could have raised CO2 levels in the atmosphere, reducing oxygen and leading to intense global warming—long an alternative theory to the impacts; however, the changes wrought by volcanism could not necessarily explain the massive marine extinctions of the end Permian [period]. Nor could volcanoes account for plant deaths on land, because vegetation would thrive on increased CO2 and could probably survive the warming.”
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OCEANIC DEAD ZONES
405 worldwide
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But in the oceanic sediments from the latest part of the Permian and Triassic periods, scientists have found chemical clues to an ocean-wide bloom of the bacteria that consumed hydrogen sulfide (H2S). “Because these microbes can live only in an oxygen-free environment but need sunlight for their photosynthesis, their presence in strata representing shallow marine settings is itself a marker indicating that even the surface of the oceans at the end of the Permian was without oxygen but was enriched in H2S.”
Modern oceans contain oxygen in almost equal amounts from top to bottom, because the gas dissolves from the atmosphere at the surface and ocean circulations take it further down. In some places, such as the Black Sea, anoxic conditions exist below a certain level. These conditions are perfect for organisms that can live without oxygen and that pump out H2S, which also dissolves in seawater. This gas bubbles upward, eventually meeting the oxygen diffusing downward from the surface. The place where these two domains meet is called the chemocline, and here the green and purple sulfur-loving bacteria enjoy a supply of H2S from below and sunlight from a
bove.
“Yet calculations by geoscientists [...] have shown that if oxygen levels drop in the oceans, conditions begin to favor the deep-sea anaerobic bacteria, which proliferate and produce greater amounts of hydrogen sulfide,” wrote Ward. “In their models, if the deepwater H2S concentrations were to increase beyond a critical threshold during such an interval of oceanic anoxia, then the chemocline separating the H2S-rich deepwater from oxygenated surface water could have floated up to the top abruptly. The horrific result would be great bubbles of toxic H2S gas erupting into the atmosphere.”
Calculations show that enough H2S was produced by such ocean belches at the end of the Permian to create extinctions on both land and sea. Ward adds that further scientific models show that the H2S would also have attacked the ozone layer, which protects life from the Sun’s ultraviolet radiation. “Evidence that such a disruption of the ozone layer did happen at the end of the Permian exists in fossil spores from Greenland, which display deformities known to result from extended exposure to high UV levels. Today we can also see that underneath ‘holes’ in the ozone shield, especially in the Antarctic, the biomass of phytoplankton rapidly decreases. And if the base of the food chain is destroyed, it is not long until the organisms higher up are in desperate straits as well.”