They rank among the world's most powerful and uncontrollably destructive forces. When they erupt, supervolcanoes wreak colossal damage on a planetary scale and may temporarily reshape the global climate.
To make matters worse, they are notoriously tough to predict. But that may change following new research revealing clues as to what happens in the lead-up to a cataclysmic eruption.
After analysing the composition of rock crystals from the Greek island of Santorini, an international team reports that eruptions are preceded by a relatively rapid build-up of magma, the hot molten rock in the Earth's crust. The hope is that, in future, the build-up might be detected as much as decades in advance using state-of-the-art seismological equipment.
Lava spews from a volcano. Photo: Reuters
''We get better at eruption prediction as we get more detailed data from each eruption and as the sophistication of monitoring equipment improves,'' says Tom Hubble, an associate professor of engineering and environmental geology at Sydney University.
''The new findings improve our understanding of the timescale over which large eruptive events reach their culmination.'' This is particularly so in case of more destructive volcanoes such as Santorini, which erupted about 1630BC.
But volcanoes are complex natural structures, Professor Hubble warns. ''Each one is slightly different and they're likely to trick us with idiosyncrasies.''
Other earth scientists agree. ''The new research provides another tool to enable us to understand volcanic processes,'' says Melbourne University seismologist Gary Gibson. ''A range of indicators will be needed for successful forecasts or predictions. Short-term earthquake and volcano forecasts based on current activity are improving, and long-term forecasts give results better than random chance, but not much better.''
Eruptions: how they work
Volcanoes are the surface expression of magma ‘‘plumbing’’ systems that lie deep within the planet’s crust. Magma originating from Earth’s mantle gets stored in chambers several kilometres below the surface. Pressure builds as more magma enters the system, resulting in fracturing of the crust. Eventually the pressure gets too great and magma is erupted.
Vulcanologists, who study the vents or fissures in the crust where molten lava erupts onto the surface, use a particular volcano’s eruption history to get some idea of when it might blow again.
‘‘What makes a volcano potentially ‘super’ is that it releases an immense volume of magma from a reservoir that has been accumulating at quite shallow levels in the crust for centuries,’’ says Museum Victoria geologist Bill Birch. Eventually, he says, crystallisation of the magma builds sufficient pressure to force an eruption, with destructive global consequences.
Most of the world’s large volcanoes, occurring on plate boundaries, are fed by magmas from the Earth’s upper mantle and lower crust.
‘‘Generally these erupt more often and don’t accumulate a magma reservoir for as big and as long-lived as a ‘supervolcano’,’’ Dr Birch says. ‘‘They can still be violently explosive — Mount St Helens and Krakatoa, for example — but without catastrophic global effects.’’
Volcanoes, like earthquakes, are very hard to predict. ‘‘Non-scientific predictions are often made, and sometimes are correct in location but not occurrence in time or size,’’ says Mr Gibson. ‘‘Predictions that are correct are normally announced after the event has occurred.’’
The success or failure of forecasts can be measured, he explains, allowing forecasting models to be developed, improved — or abandoned. ‘‘It is possible to test predictions or forecasts using past events, but the true test is with future events,’’ Mr Gibson says.
The best way to predict the eruption of any volcano is through intensive monitoring of earthquake activity, changes in heat flow and distribution, the build-up of stresses in overlying rocks, groundwater temperatures and gas emissions, explains Dr Birch.
Although it is an inexact science, vulcanology may gain some predictive power as a result of the new research, reported in the British journal Nature.
‘‘It seems that the subterranean plumbing system opens up and allows a relatively rapid and efficient transfer of magma from the zone of production in the lower crust and upper mantle to the near-surface magma chambers beneath the volcano,’’ Professor Hubble explains.
‘‘The new work shows that this transfer of magma occurs on time frames measured in months, years and decades as opposed to the multi-centurial and multi-millennial scales envisioned by previous generations of geologists.’’
This means that continual and careful monitoring of ground surface shape and the background low-level seismic movements near dormant volcanoes might help pinpoint magma moving in the subsurface.
Large volcanic eruptions, he says, are usually preceded by an uplift of the ground surface as magma from chambers tens of kilometres down is transferred to higher chambers, two or three kilometres beneath the surface.
Volcanoes often signal their reawakening from a dormant state with steam blasts, and small eruptions of ash, which increase in intensity over months to years.
‘‘As the ground swells, the volcanic plumbing system usually unclogs as well,’’ Professor Hubble says. ‘‘Steam blasts open up the conduits linking the near-surface magma chamber to the volcano.’’
Rhythmic earthquake activity, known as harmonic tremor, often accompany such events. ‘‘These processes build up and die away and then build up again, several or even tens of times before an eruption takes place,’’ he says.
‘‘Making the final call to evacuate people living around a volcano is particularly difficult to get right, because if you evacuate the population and nothing happens or the activity fizzles out, then you lose people’s trust and they’ll ignore subsequent warnings or alerts. So public education about the complexity of the process and the difficulty of producing an accurate prediction is vital.’’
What’s in a name?
Vulcanologists and their igneous petrologist colleagues (who analyse the make-up of rocks such as granite or basalt that have crystallised from magma) study the formation of magma, its injection into the crust and its ejection and eruption as ash, spatter and lava flows during volcanic eruptions.
The term ‘‘supervolcano’’ was first used and popularised by a BBC documentary that examined the cataclysmic eruptions in Yellowstone Park in the north-west of the US.
The term has been used to identify volcanic caldera eruptions that yielded more than 1000 cubic kilometres of ash, ejected rock and lava, and is associated with the largest eruptive events that score an eight on the open-ended Volcanic Explosivity Index, Professor Hubble explains.
Mega-calderas are large depressions, developed above magma chambers within a few kilometres of the surface.
The depressions are usually occupied by lakes. In addition to Yellowstone Park, well-known mega-calderas include Lake Taupo in New Zealand, Lake Toba in Sumatra and Long Valley, east of San Francisco, in California.
‘‘Luckily for us, the mega-volcanoes erupt fairly infrequently,’’ Professor Hubble says. ‘‘But, when they do, it’s usually an enormous, extremely destructive event that often modifies the world’s climate for several years, decades and possibly centuries. Apocalypse is probably too gentle a word to describe the violence of one of these events.’’
Plate tectonics: a primer
Beneath the Earth’s relatively thin, rigid crust lies a mantle of solid crystalline rock. At a depth of more than 100 kilometres or so, the rock is so hot that it’s malleable and squishy and can flow like plasticine.
Over aeons, the covering crust has broken into massive brittle sections called plates. There are seven main ones and several smaller crustal plates, all moving and shoving against each other as they slide across the hot, soft mantle.
The plates are continuously colliding and being forced under one another, a process called subduction. Occasionally they fracture, causing earthquakes and volcanoes.
Volcanoes come in three varieties: active (likely to explode again), dormant (no activity for many years) and extinct (no sign of activity for ages). ‘‘We should probably should add a fourth category, ‘erupting’, to indicate dangerous and currently ongoing volcanism,’’ Professor Hubble says.
About 1900 or so of Earth’s volcanoes are active. So which are the ones to watch? Vulcanologists say there is evidence for magma accumulating beneath Yellowstone and Mount St Helens in the US, for example.
The last supervolcanic eruption occurred about 1200 years ago at Lake Taupo in New Zealand. Before that, Indonesia’s Lake Toba erupted 73,000 years ago.
Check out the latest Santorini research in Nature here.
Learn more about volcanoes at IMAX Melbourne Museum, which periodically screens Forces of Nature, as part of its education program. Details here.
A supervolcano to end all volcanoes might bring about the end of life, as we know it. Find out more about these scary scenarios in Bill McGuire’s book, A Guide to the End of the World (Oxford University Press).
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