Many large volcanoes on Earth are capable of explosive eruptions much bigger than any experienced by humanity over historic time. Such volcanoes are termed super-volcanoes and their colossal eruptions super-eruptions. Super-eruptions are different from other hazards such as earthquakes, tsunamis, storms or floods in that – like the impact of a large asteroid or comet – their environmental effects threaten global civilisation. Events at the smaller-scale end of the super-eruption size spectrum are quite common when compared with the frequency of other naturally occurring devastating phenomena such as asteroid impacts. The effects of a medium-scale supereruption would be similar to those predicted for the impact of an asteroid one kilometre across, but super-eruptions of this size are still five to ten times more likely to occur within the next few thousand years than an impact.

Several of the largest volcanic eruptions of the last few hundred years (Tambora, 1815; Krakatau, 1883; Pinatubo, 1991) have caused major climatic anomalies in the two to three years after the eruption by creating a cloud of sulphuric acid droplets in the upper atmosphere. These droplets absorb and reflect sunlight, and absorb heat from the Earth, warming the upper atmosphere and cooling the lower atmosphere. The global climate system is disturbed, resulting in pronounced, anomalous warming and cooling of different parts of the Earth at different times.

Super-eruptions, however, are hundreds of times larger than these recent events and their global effects are likely to be much more severe. An area the size of North America or Europe could be devastated, and pronounced deterioration of global climate would be expected for a few years following the eruption. 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.

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Simulation Results

For erupted volume, we chose a fixed value of 330 km3 dense-rock equivalent (DRE) of magma. The three major caldera-forming eruptions from Yellowstone that produced the Huckleberry Ridge Tuff, Mesa Falls Tuff, and Lava Creek Tuff expelled about 2450, 280, and 1000 km3 DRE of magma, respectively. But only a fraction of this volume rose in buoyant ash columns that could be carried by winds to form fall deposits.

Fig. Simulated tephra fall thickness resulting from a month-long Yellowstone eruption of 330 km3 using 2001 wind fields for (a) January, (b) April, (c) July, and (d) October. In (a), the bold red line delineates the extent of the Huckleberry Ridge Tuff Bed (HR); the brown line delineates the extent of Lava Creek B Tuff (LCB).

A general decrease in thickness with distance is apparent, although cities west of Yellowstone receive much less ash overall than cities farther east.

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For eruption duration, we explore values from days to a month, reflecting durations that have been inferred and observed for moderate to large eruptions.

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For the height of ash injection, several factors are considered. Plume height is known to correlate with eruption rate, suggesting that a high-flux Yellowstone eruption would produce a very high plume. But such correlations are based mainly on plumes that emanate from single, central vents, whereas a large, complex Yellowstone plume is more likely to rise from multiple vents, or as an elutriated ash cloud from pyroclastic flows. Based on these observations, most of our simulations use an umbrella cloud whose top is at 25 km.

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Immediate Ash Thickness Versus Long-Term Impact

North America's highest population density lies along its coastlines. Deposit thicknesses on the coasts from nearly all simulations is millimeters to a few centimeters. Thicknesses of this magnitude seem small but their effects are far from negligible. A few millimeters of ash can reduce traction on roads and runways, short out electrical transformers and cause respiratory problems. Ash fall thicknesses of centimeters throughout the American Midwest would disrupt livestock and crop production, especially during critical times in the growing season. Thick deposits could threaten building integrity and obstruct sewer and water lines. Electronic communications and air transportation would likely be shut down throughout North America. There would also be major climate effects. Emission of sulfur aerosols during the 1991 Pinatubo eruption produced global cooling by an average of 1° C for a few years, while the 50 km3 Tambora eruption of 1815 cooled the planet enough to produce the famed “year without a summer” in 1816, during which snow fell in June in eastern North America and crop failures led to the worst famine of the 19th century. Other indirect effects include wind reworking of tephra into migrating dunes that bury roads and structures; or increased sediment load to streams that exacerbates flooding and impedes river traffic.…

The Scale of the Deposits left by Super-Eruption

To envisage the scale of the deposits left by a super-eruption, we can consider this familiar (but unlikely) example. A super-eruption in Trafalgar Square, London, yielding 300 cubic kilometres of magma would produce enough volcanic (pyroclastic flow) deposits to bury all of Greater London to a depth of about 210 metres. A larger super-eruption (1000 cubic kilometres) would bury the same area to a depth of 700 metres. These thicknesses do not include extensive ash-fall deposits, which could cover an area greater than all of Europe.

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A Yellowstone–size super-eruption would produce enough volcanic (pyroclastic flow) deposits to bury all of Greater London beneath 1.7 kilometers of ash.

Comparisons with nuclear winter

Study of large volcanic eruptions’ climatic effects was boosted during the 1980s by the issue of the potential long-term environmental effects of thermonuclear war. From this research, the concept of “nuclear winter” – pronounced cooling in the few years following such a war – emerged. In a nuclear war, large amounts of dust and smoke would be injected into the atmosphere. Data from volcanic eruptions like Mount Pinatubo later provided opportunities to test “nuclear winter” models. It emerged that the main cause of disaster would be destruction of global agriculture and food supply. After a year of severely reduced food supply, there would be mass starvation. Because a nuclear winter might last two or three years, scientists concluded that this would threaten the continued existence of civilisation (and possibly even our species). Casualties from the immediate consequences of nuclear winter due to direct destruction and radioactive contamination would be few compared with those due to mass starvation. This apocalyptic depiction of the consequences of nuclear war by the scientific community had a profound influence in governmental efforts to reduce the world’s nuclear arsenals and the threat of conflict.

The possibility that super-eruptions might have the same effects as nuclear war, by causing severe volcanic winters, is one reason why our working group wishes to draw attention to this natural volcanic phenomenon.

Q: What kinds of hazards are associated with volcanic eruptions?

A: Debris flows, or lahars, are slurries of muddy debris and water caused by mixing of solid debris with water, melted snow, or ice. Lahars destroyed houses, bridges, and logging trucks during the May 1980 eruption of Mount St. Helens, and have inundated other valleys around Cascade volcanoes during prehistoric eruptions. Lahars at Nevado del Ruiz volcano, Colombia, in 1985, killed more than 23,000 people. At Mount Rainier, lahars have also been produced by major landslides that apparently were neither triggered nor accompanied by eruptive activity. Lahars can travel many tens of miles in a period of hours, destroying everything in their paths.

Tephra (ash and coarser debris) is composed of fragments of magma or rock blown apart by gas expansion. Tephra can cause roofs to collapse, endanger people with respiratory problems, and damage machinery. Tephra can clog machinery, severely damage aircraft, cause respiratory problems, and short out power lines up to hundreds of miles downwind of eruptions. Explosions may also throw large rocks up to a few miles. Falling blocks killed people at Galeras Volcano in Colombia in 1992, and at Mount Etna, Italy, in 1979.

Pyroclastic surges and flows are hot, turbulent clouds of tephra (known as surges), or dense, turbulent mixtures of tephra and gas (known as flows). Pyroclastic flows and surges can travel more than a hundred miles per hour and incinerate or crush most objects in their path. Though most extend only a few miles, a pyroclastic surge at Mount St. Helens in 1980 extended 18 miles (28 km) and killed 57 people. Pyroclastic surges at El Chichón volcano in Mexico in 1982 killed 2000 people, and pyroclastic flows at Mount Unzen, Japan, in June, 1991, killed 43 people. Speeding vehicles cannot outrun a pyroclastic flow or surge.

Lava flows erupted at explosive stratovolcanoes like those in the Pacific Northwest and Alaska are typically slow-moving, thick, viscous flows. Kilauea volcano on the Island of Hawaii has produced thin, fluid lava flows throughout its history, and almost continuously since 1983. Lava flows destroyed a visitor center at Kilauea in 1989 and overran the village of Kalapana on the volcano's southeast flank in 1991.

Q: What is the chance of another catastrophic volcanic eruption at Yellowstone?

A: Although it is possible, scientists are not convinced that there will ever be another catastrophic eruption at Yellowstone. Given Yellowstone's past history, the yearly probability of another caldera-forming eruption could be calculated as 1 in 730,000 or 0.00014%. However, this number is based simply on averaging the two intervals between the three major past eruptions at Yellowstone — this is hardly enough to make a critical judgment. This probability is roughly similar to that of a large (1 kilometer) asteroid hitting the Earth. Moreover, catastrophic geologic events are neither regular nor predictable.

(Reuters) - Yellowstone National Park, which sits atop one of the world's largest super-volcanoes, was struck on Sunday by a magnitude 4.8 earthquake, the biggest recorded there since February 1980, but no damage or injuries were immediately reported.

The tremor, a relatively light event by seismic standards, struck the northwest corner of the park and capped a flurry of smaller quakes at Yellowstone since Thursday, geologists at the University of Utah Seismograph Stations said in a statement.

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The ancient super-volcano, or caldera, that lies beneath the surface of the park was discovered by scientists in recent years to be 2.5 times larger than previously thought, measured at 30 miles wide, according to the park.

Sunday's quake occurred near the center of an area of ground uplift that geologists have been tracking for several months, University of Utah seismologists said. Elevated seismic activity was also found in the area during a previous period of uplift from 1996 to 2003.

The recent spike in earthquake activity at Yellowstone is linked to the uplift, which in turn is caused by the upward movement of molten rock beneath the Earth's crust, according to the U.S. Geological Survey.

Fortunately, there was no indication that the recent seismic activity signaled an impending eruption of the Yellowstone Caldera, scientists said.

Researchers with the observatory have said in the past that catastrophic eruptions by the super-volcano are unlikely for tens of thousands of years, though less extreme lava releases could occur within thousands of years...