Super-volcanoes and super-eruptions
Most volcanoes that produce super-eruptions are very long-lived (active over millions of years), produce very large explosive eruptions, and remain dormant for long periods (from thousands to hundreds of thousands of years) between major eruptions. In general, the longer the dormancy (“repose period”) the larger the eventual eruption.
Volcanologists use two measures of eruption size:
To give some comparison, Mount St. Helens in 1980 erupted less than one cubic kilometre of magma (See image at top of page. This is an example of an explosive eruptions producing vertical eruption columns, and rose to 29 kilometres. These columns can carry ash and gas into the upper atmosphere, allowing ash fall to be carried by wind over a wider area. Super-eruption ash columns would be higher).
Vesuvius (AD 79) erupted about five cubic kilometres, and Krakatoa* (1883) about 12 cubic kilometres. The biggest eruption of the past few hundred, perhaps one thousand, years, that of Tambora volcano (Indonesia, 1815) released about 30 cubic kilometres of magma.
- the magnitude of the eruption (the volume or mass of magma erupted)and
- the intensity (the rate of magma eruption). (Magma is the hot, molten, often gas-laden rock material stored under volcanoes.)
To give some comparison, Mount St. Helens in 1980 erupted less than one cubic kilometre of magma (See image at top of page. This is an example of an explosive eruptions producing vertical eruption columns, and rose to 29 kilometres. These columns can carry ash and gas into the upper atmosphere, allowing ash fall to be carried by wind over a wider area. Super-eruption ash columns would be higher).
Vesuvius (AD 79) erupted about five cubic kilometres, and Krakatoa* (1883) about 12 cubic kilometres. The biggest eruption of the past few hundred, perhaps one thousand, years, that of Tambora volcano (Indonesia, 1815) released about 30 cubic kilometres of magma.
*Editor's note:
In the literature on this subject readers may sometimes find Krakatoa referred to as "Krakatau" which is the correct Javanese spelling. "Krakatoa", the most familiar form to most English speaking readers, has been the accepted form in vernacular English since reports were first made by British newspapers in 1883, and is used throughout this Web site for that reason.
But even Tambora is small beer compared to a super-eruption,, which may involve the release of hundreds to a few thousand cubic kilometres of magma.
The biggest super-eruption recognised so far produced approximately 5000 cubic kilometres, creating the so-called “Fish Canyon Tuff event” in Colorado, USA. There is no strict definition of a super-volcano, because in reality there is a continuum of volcanoes and volcanic eruptions from the very small and weak to the very large and violent. However, the definition of a super-eruption, a term introduced to describe the Toba eruption, has value in that it encapsulates the fact that there have been explosive volcanic eruptions very much larger than eruptions that have so far affected humanity in the short course of recorded human history.
The super-eruption of Toba volcano, Sumatra, some 75,000 years ago ejected about 300 times more volcanic ash than the eruption of Tambora in Indonesia in 1815. Tambora’s eruption had significant impact on global climate, producing the “Year Without a Summer” (1816) when Lord Byron wrote his poem 'Darkness' and Mary Shelley wrote 'Frankenstein', and anomalously cool summers in the Northern Hemisphere for the following two years.
It is easy to imagine that an eruption of the scale of Toba would have devastating global effects. An layer of ash estimated at 15cm thick fell over the entire Indian sub-continent, with similar amounts over much of SE Asia. Most recently, the Toba ash has been found in the South China Sea, implying that several centimetres also covered southern China. Just one centimetre of ash is enough to devastate agricultural activity, at least in the growing season when it falls. An eruption of this size would have catastrophic consequences. Many millions of lives throughout most of Asia would be threatened if Toba erupted today. The UK might not receive any ash fall directly, but it would be affected by the effects on global economic and political stability, as well as by worldwide climate effects.
The biggest super-eruption recognised so far produced approximately 5000 cubic kilometres, creating the so-called “Fish Canyon Tuff event” in Colorado, USA. There is no strict definition of a super-volcano, because in reality there is a continuum of volcanoes and volcanic eruptions from the very small and weak to the very large and violent. However, the definition of a super-eruption, a term introduced to describe the Toba eruption, has value in that it encapsulates the fact that there have been explosive volcanic eruptions very much larger than eruptions that have so far affected humanity in the short course of recorded human history.
The super-eruption of Toba volcano, Sumatra, some 75,000 years ago ejected about 300 times more volcanic ash than the eruption of Tambora in Indonesia in 1815. Tambora’s eruption had significant impact on global climate, producing the “Year Without a Summer” (1816) when Lord Byron wrote his poem 'Darkness' and Mary Shelley wrote 'Frankenstein', and anomalously cool summers in the Northern Hemisphere for the following two years.
It is easy to imagine that an eruption of the scale of Toba would have devastating global effects. An layer of ash estimated at 15cm thick fell over the entire Indian sub-continent, with similar amounts over much of SE Asia. Most recently, the Toba ash has been found in the South China Sea, implying that several centimetres also covered southern China. Just one centimetre of ash is enough to devastate agricultural activity, at least in the growing season when it falls. An eruption of this size would have catastrophic consequences. Many millions of lives throughout most of Asia would be threatened if Toba erupted today. The UK might not receive any ash fall directly, but it would be affected by the effects on global economic and political stability, as well as by worldwide climate effects.
A J.M.W. Turner landscape (of the Chichester Canal) painted in 1818 shows skies of typical turbidity for the post-Tambora period, including, possibly, a secondary glow at sunset caused by stratospheric aerosols.For the purposes of this report, we consider that an eruption of over 100 cubic kilometres of magma (equivalent to about 250 cubic kilometres of volcanic ash) would produce effects that would have global consequences, and can be taken as representing the smallest scale end of the super-eruption spectrum. This is more than three times larger than the Tambora eruption, but, interestingly, we do not have a firm idea on how many such eruptions have taken place in the past. One probably has to go back at least 6000 years, and perhaps 10,000 years, before eruptions approaching this size range (which occurred in Japan, Oregon (USA), and Kamchatka (Russia)), are recognised. In the last 40,000 years, super-eruptions with volumes in the 300 cubic kilometre range occurred in Italy, New Zealand, and Japan.
Intensity (effectively, the “violence”) of an eruption is measured in cubic metres or mass of magma erupted per second. Vesuvius (AD 79) erupted a staggering one hundred thousand cubic metres of magma per second over a 24-hour period. Yet this pales into insignificance alongside super-eruptions, where intensities of tens to even a hundred million cubic metres per second have been deduced from geological evidence and models. The entire volume of magma erupted at the Soufrière Hills volcano, Montserrat, in five years (about one third of a cubic kilometre) can be discharged in a few minutes during a super-eruption.
To envisage the scale of the deposits left by a super-eruption, we can consider a familiar (but unlikely!) example.
A super-eruption in Trafalgar Square, London, yielding 300 cubic kilometres of magma would produce enough volcanic deposits to bury all of Greater London to a depth of about 150 metres (nearly 500 feet) thick. A larger super-eruption (1000 cubic kilometres) would bury the same area to a depth of 420 metres (almost 1400 feet). These thicknesses do not include extensive ash-fall deposits, which could cover an area greater than all of Europe.
However, the severity of environmental effects is not simply determined by the amount of erupted material. The mass of erupted gas, which is related to the mass of magma, is crucial. It is now known that the most important factor in determining the impact of eruptions on global climate is the amount of sulphur and halogen gases (chlorine and fluorine) erupted. Not all volcanoes erupt magmas with large amounts of sulphur or halogen gas. Therefore a critical scientific issue is the mass of these key gases released, which is unfortunately not yet well constrained.
A super-eruption in Trafalgar Square, London, yielding 300 cubic kilometres of magma would produce enough volcanic deposits to bury all of Greater London to a depth of about 150 metres (nearly 500 feet) thick. A larger super-eruption (1000 cubic kilometres) would bury the same area to a depth of 420 metres (almost 1400 feet). These thicknesses do not include extensive ash-fall deposits, which could cover an area greater than all of Europe.
However, the severity of environmental effects is not simply determined by the amount of erupted material. The mass of erupted gas, which is related to the mass of magma, is crucial. It is now known that the most important factor in determining the impact of eruptions on global climate is the amount of sulphur and halogen gases (chlorine and fluorine) erupted. Not all volcanoes erupt magmas with large amounts of sulphur or halogen gas. Therefore a critical scientific issue is the mass of these key gases released, which is unfortunately not yet well constrained.
An example of volcanic sulphuric acid aerosol droplets in the 0.5-1 micrometer size-range sampled on filters carried by high-flying aircraft after the El Chichón eruption of 1982. The filters on the left are from pre-eruption flights and those on the right are from post-eruption missions. (Figure adapted from Eos, Transactions of the American Geophysical Union).
Continue to next section, Frequency, location and types of super-eruptions; or look at Examples