Volcanology: the mysteries of mega-eruptions

Mount Tambora on Sumbawa Island in Indonesia blew up 200 years ago this weekend. The world's biggest volcanic explosion since medieval times killed an estimated 70,000 people on Sumbawa and neighbouring islands - and caused deaths and suffering far further away over the following two years, as sulphurous gases and particles emitted by the eruption temporarily cooled the global climate.

In Europe and North America 1816 was known as the "year without a summer". Persistently wet and cold weather, with temperatures 2-4C below normal, led to crop failures and what some agricultural historians call the last widespread subsistence crisis in the western world. (The best known cultural by-product is Frankenstein, which Mary Shelley began writing while sheltering from the endless summer rain in a villa by Lake Geneva.)

Yet the Tambora catastrophe is surprisingly little known, according to volcanologists, and deserves greater study. The April issue of the journal Nature Geoscience publishes a series of papers by scientists who hope the bicentenary will raise awareness of the threat posed by super-eruptions.

"Given the widespread and devastating impacts of this eruption, there is a surprising paucity of volcanological studies on Tambora," write Stephen Self of the University of California, Berkeley, and Ralf Gertisser of Keele University in the UK. "Virtually no field research has been conducted there since the 1980s because Tambora is extremely remote and inaccessible."

There is also little information about other large eruptions over the past few centuries or indeed millennia. "An eruption of that size today would certainly have major effects on air traffic as well as atmospheric circulation around the globe, so we would like to know when the next big one is coming," says Self. "But we can't predict that if we don't know the size of past eruptions and when they took place.

"Even in a country with well-studied volcanoes, like Japan, at least 40 per cent of the big eruptions are missing from the record. And if you look back . . . to 3,000 or 4,000 years ago, the record gets worse and worse. We know there are big eruptions hiding from the record."

Judging by the size of the caldera (volcanic bowl) at the top of Mount Tambora today, the 1815 eruption ejected 30-50 cubic kilometres of magma. That is three times more than the better known 1883 explosion of Indonesia's Krakatoa (pictured below in 2007), which killed 34,000 people and caused spectacular sunsets globally.

By comparison, the largest late 20th-century eruption, Mount Pinatubo in the Philippines, ejected about 5 cu km of material in 1991, while Mount St Helens in the US managed just 1 cu km in 1980. Going further back, an eruption comparable in size to Tambora took place at Samalas on the nearby island of Lombok in 1257. (Southeast Asia is the global hotspot for volcanic catastrophes.) The explosion on Santorini in the Mediterranean, which devastated Minoan civilisation 3,600 years ago, was probably larger than Tambora.

Volcanologists are struggling to work out the location and frequency of other mega-eruptions thousands of years ago that would cause serious global disruption if they occurred today. Ones that eject large amounts of sulphates high into the atmosphere may have left datable traces in ancient ice cores in Antarctica or Greenland but this is not a reliable indicator. Geological processes erode and erase local evidence such as calderas, lava flows and ash deposits.

"The current global volcanic eruption record is incomplete and difficult to interpret," Self and Gertisser conclude. "It is high time for a systematic exploration of all the available eruption archives - ice cores, ocean sediments, remotely sensed caldera volumes and geochronological analysis of eruption deposits - so we have a better chance to understand potential future hazards."

Photographs: Katie Preece; Getty Images

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