Frequency, locations and sizes of super-eruptions

Two types of super-eruption occur on Earth: huge outpourings of lava and great explosive eruptions. Geological processes presently operating within the Earth are capable of producing only the explosive type; but the Laki eruption is considered to be a small-scale example of the type of activity that has formed huge lava flows in the geologic past.

Fortunately, there is an inverse relation between frequency and size of eruption; the bigger they come, the less often they go off. Eruptions obey an approximate law similar to that for earthquakes – of which you can also say the larger the magnitude, the less frequent. There are only one or two eruptions the size of Pinatubo (five cubic kilometres) or Krakatoa (12 cubic kilometres) every century. Three to five eruptions with a volume of a few tens of cubic kilometres, such as Tambora, occur every thousand years. The great Bronze age eruption of Santorini in Greece is another example in this size range.

The frequency of a volcanic eruption of any size falls by about a factor of 7 for every ten-fold increase in eruption size. For this reason, volcanologists describe the sizes of volcanic eruptions using a logarithmic scale (in just the same way that earthquakes are described by the logarithmic Richter scale of magnitude). The table above, indicating the range of super-eruption magnitudes above 100 cubic kilometres of magma, is based on a scale suggested by the Cambridge Volcanology Group, (after Mason, B.G. et al. Bulletin of Volcanology , in press, 2004). Note that the largest eruptions shown are very rare indeed.

Note: volumes of ash ejected during a volcanic eruption are typically two to three times greater than the equivalent volume of magma. This is a source of frequent confusion: for example, the Krakatoa eruption in 1883 produced a total of about 30 cubic kilometres of ash, or 12 cubic kilometres of magma.

By defining the low end of the super-eruption size spectrum as “those exceeding 100 cubic kilometres of magma erupted”, we bring attention to events that are common enough to have reasonable odds for recurring within a human lifetime, but rare enough not to have been witnessed by our civilization. It is expected that such an eruption would produce significant, widespread and devastating effects, affecting the whole globe if it came from a volcano located within a zone from 30o North to 30o South of the Equator – where many volcanoes lie.

We stress that the number of past eruptions exceeding this size is poorly known, for various reasons. Our current understanding is that average recurrence rates could well be as many as one such eruption every 3000 years. There is, therefore, a 1 in 10 to 1 in 6 chance that an eruption of this scale could occur during the 21st Century. An eruption in the range size range 200-300 cubic kilometres is estimated to occur with a frequency of one every 10,000 years, resulting in a 1.0 % chance of such an eruption occurring in this century. The largest eruptions, such as that which caused Toba caldera in Sumatra, Indonesia, are very rare indeed – perhaps once every 500,000 years.

Where are the source vents of super-eruptions?

The sites of super-eruptions are mostly found where the Earth’s tectonic plates collide or where hot material wells up from the deep Earth's interior below a continent. Yellowstone volcano in the USA, the Phlegrean Fields volcano West of Naples, Italy, and Lake Taupo in New Zealand are three examples of active volcanoes capable of producing super-eruptions. However, there are many other areas of the Earth where such volcanoes are found or suspected, including Indonesia, the Philippines, several Central American countries, the Andes, Japan, and the Kamchatka Peninsula in eastern Russia. A key point is that super-volcanoes are often located within or near continents. World population expansion means that these vents are often close to areas with dense populations.


Source volcanoes of known super-eruptions that have happened within the last 2 million years are shown as dots on the map above. Note the concentration around the Pacific tectonic plate margin – the so-called “Ring of Fire”. Squares mark previous super-eruption sites, including those of some of the great, but ancient, flood lava provinces on Earth.

As to non-explosive eruptions involving the outpouring of copious amounts of lava, the most likely place today where such an eruption is possible is Iceland; and so this kind of eruption is of particular interest to Europe. The last one occurred in 1783 when the Laki fissure disgorged about 15 cubic kilometres of lava over 6 months. Enormous quantities of sulphur dioxide were released and caused climatic anomalies in Europe and North America, including a dry acid fog over Europe, and severe crop failures. Many of the livestock in Iceland died from poisoning by halogen gases (chlorine and fluorine). As a consequence about one third of the Icelandic people died from famine. Related food-shortages in France may even have been one of the factors that eventually triggered the French Revolution. Eruptions of this scale seem to take place in Iceland on average every 1000 years. The next large lava eruption on Iceland will not be of super-eruption scale by the definition given here, but it could be environmentally and economically devastating for Europe.

Link to next section, Hazardous effects of super-eruptions