Every so often the Earth chooses to remind us that we really aren't in control of this planet. The volcanic eruption in Iceland, which began on Wednesday, is just such a reminder. As ash spews out across northern Europe, grounding all flights across Scandinavia and the UK, we begin to realise how powerless we humans are.
But as volcanic eruptions go, the fireworks on Iceland are small fry. Scientists rank volcanoes according to how explosive they are, using the volcanic explosivity index (VEI), which goes from zero to eight.
Like the scale used to measure earthquake size, the VEI is logarithmic -- meaning that a volcano with a VEI of five is 10 times more powerful than one with a VEI of four.
As yet, scientists haven't managed to gather enough data to calculate the VEI of Eyjafjallajokull, but Thorvaldur Thordarson, an expert on Icelandic volcanism at the University of Edinburgh, estimates that this one is probably a two or three -- similar to the eruptions seen on Mount Etna on Sicily in 2002 and 2003, and the kind of eruption we expect to see somewhere on Earth at least once every year.
By contrast, the eruption of Mount St Helens, in the north-west of the US in May 1980, was a one-in-10-year event, with a VEI of around four.
Bigger still was the eruption of Tambora in 1815, on the island of Sumbawa, Indonesia, whose ash was responsible for some of the spectacular sunsets painted by JMW Turner. Rated as a seven on the VEI scale (a one-in-1,000-year event), it was the most deadly eruption in recorded history, killing over 70,000 people.
But as the Eyjafjallajokull event is showing, even baby eruptions can cause quite a nuisance. The last time Iceland experienced an eruption of this size was in 2004, when the Grimsvotn volcano blew. "On that occasion the ash cloud went to the north, but this time the jet stream has carried it south-east, towards the UK," Thordarson says.
If Eyjafjallajokull is anything like Grimsvotn, the eruption will peter out in a day or two, but there is a chance that things could go on for a lot longer. "We suspect it will end today or tomorrow, but it could last for weeks, months or even years," Thordarson says.
And even if the volcano goes quiet again soon, that doesn't mean she has finished yet. "The last time Eyjafjallajokull erupted [in 1823], it lasted for more than a year, so we could see more of the same disruption over the coming months," says Bill McGuire, director of the Aon Benfield UCL Hazard Research Centre at University College London.
For volcanologists, this most recent eruption is no surprise. "There has been lots of unrest under this particular volcano for the past 10 years, which picked up in intensity at the beginning of this year," says Thordarson.
To volcanologists, Iceland is heaven. There are 30 active volcanic systems on the island and very frequent firework displays.
However, Iceland has failed to live up to its reputation in recent decades. "The volcanoes have been very quiet over the last half of the 20th century," Thordarson says. But in the past 10 years, volcanologists have noticed increased rumblings from below, suggesting that Iceland might be entering a more active phase again and brewing some really big bangs.
If the volcanologists are right, we could be in for a bumpy ride. The last time Iceland had a colossal eruption was in 1783. Laki, a fissure close to the Grimsvotn volcano, burst open and threw up fountains of lava and clouds of ash for eight months, rating a six on the VEI.
The poisonous sulphur dioxide gas killed over half of Iceland's livestock, led to a famine that wiped out about a quarter of the country's population, and produced a noxious haze that caused the deaths of thousands in western Europe during 1783 and the winter of 1784.
F URTHER afield, the effects were also severe. "There is evidence that Laki may have caused the failure of the rice harvest in Japan that year, and weakened the African and Indian monsoon circulation," Thordarson says.
So how would we cope if Iceland produced another Laki tomorrow? "I think modern society is better equipped to deal with the health and environmental effects, but the economic consequences of halting air traffic for five months or so would be very severe," Thordarson says.
But when it comes to truly big threats, Iceland's volcanoes are mere distractions. Every 100,000 years or so a catastrophic eruption occurs, known as a "supervolcano". Over 1,000 cubic kilometres of material are blasted into the air and the ash and gas cloud sends Earth into a chill for years. The last time one erupted was 74,000 years ago, when Toba, on the Indonesian island of Sumatra, threw out nearly twice the volume of Mount Everest in magma.
Previous supervolcanic eruptions have been linked to mass extinction events, such as the Permian extinction 250m years ago -- which wiped out more than 90pc of marine species.
And unfortunately, there is no way of avoiding the next super-eruption. "It is not a question of if, it is a question of when," says McGuire.
Possible contenders for the next eruption include Yellowstone volcano in Wyoming, the Phlegrean fields volcano west of Naples, Italy, and Lake Taupo in New Zealand.
In 2005, a working group investigated the threat of a supervolcano and concluded that "an area the size of North America or Europe could be devastated, and pronounced deterioration of global climate would be expected. Such events could result in the ruin of world agriculture, and mass starvation. The effects could be sufficiently severe to threaten the fabric of civilisation."
This week's fireworks on Iceland are just sparklers compared to what is to come. (© Daily Telegraph, London)