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 Britain, 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. The measurement is based on how much material is thrown out of the volcano, how high the eruption goes and how long it lasts.

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 Eyjafjoll, 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 about four.

Meanwhile, Pinatubo’s boom in the Philippines in 1991 was a one-in-50- to 100-year spectacle, with a VEI of about five or six.

Bigger still was the eruption of Tambora in 1815, on the island of Sumbawa, Indonesia. Its ash was responsible for some of the spectacular sunsets painted by Turner.

Rated as a seven on the VEI scale (a one-in-1000-year event), it was the most deadly eruption in recorded history, killing more than 70,000 people.

But as the Eyjafjoll 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.

Having its ash carried into some of Europe’s busiest flight paths has made Eyjafjoll big news.

No aircraft can risk flying through the cloud – the chances of choking the engines and stalling the plane are high. And so we have to twiddle our thumbs until Eyjafjoll decides she has let off enough steam.

If Eyjafjoll 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 Eyjafjoll goes quiet again soon, that doesn’t mean she has finished yet.

“The last time Eyjafjoll 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.

Thordarson agrees, saying that we may well get a number of intermittent explosive events at Eyjafjoll over the next couple of years.

For vulcanologists, 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.

In fact, many vulcanologists are rubbing their hands with glee.

“It is a nice surprise for us, as this one hasn’t erupted for a while,” says Dougal Jerram, from the University of Durham.

To vulcanologists, Iceland is heaven.

There are 30 active volcanic systems on the island and very frequent firework displays.

Geologists believe the reason for Iceland’s explosive nature is that it sits over a “mantle plume” – a rising column of abnormally hot molten rock, originating at the edge of the earth’s core.

To make matters even more spectacular, this particular mantle plume has positioned itself under the mid-Atlantic ridge: the crack that runs down the middle of the Atlantic, where the ocean floor divides and continuously spews out fresh lava.

Despite its perfect position, 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, vulcanologists 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 vulcanologists 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.

The poisonous sulphur dioxide gas killed over half of Iceland’s livestock population and led to a famine that wiped out about a quarter of the country’s population.

Meanwhile, as the cloud blew south it wreaked havoc over Europe, too.

“This outpouring of sulphur dioxide during unusual weather conditions caused a thick haze to spread across western Europe, resulting in thousands of deaths throughout 1783 and the winter of 1784,” says Jerram.

The fog was so thick that boats across Europe were forced to stay in port.

Further 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.

On the explosivity index, Laki is judged to have been a six – the kind of volcano that occurs once every century, on average.

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.

Worse still would be a repeat of an eruption such as that of Oraefajokull, Iceland’s largest active volcano, which last erupted in 1362.

“This eruption was two or three orders of magnitude larger than Laki and the largest eruption in Europe in the past 2000 years,” says Thordarson.

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”.

More than 1000 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.

Toba was more than 5000 times as explosive as the eruption of Mount St Helens in 1980 and comes in as an eight on the VEI scale.

There is much debate about how devastating Toba was for humans, but without a doubt it will have had a very severe impact.

Some scientists argue that it may even have caused a bottleneck in human evolution.

Previous supervolcanic eruptions have been linked to mass extinction events, such as the Permian extinction 250 million years ago – which wiped out more than 90 per cent of marine species and was associated with an eruption at the Siberian Traps.

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.

Unlike conventional volcanoes, supervolcanoes are not always obvious from the surface, making it difficult for scientists to predict where the next one might be simmering.

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.

However, there are many other areas where a supervolcano could pop up, including Indonesia, the Philippines, several Central American countries, the Andes, Japan, the Kamchatka peninsula in eastern Russia, and even Europe (the area around Kos and Nisyros in the Aegean Sea might be a supervolcano).

In 2005, a working group (commissioned by the Geological Society of London) 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 for a few years following the eruption”.

“Such events could result in the ruin of world agriculture, severe disruption of food supplies, 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.

The Telegraph, London