Alastair Dalton: How forces combined to form a 'perfect storm' that occurs only once every few thousand years

ICE, wind direction and high pressure meant the eruption from an Icelandic volcano resulted in an unprecedented air travel shutdown across north western Europe.

Scottish scientists said the Eyjafjallajokull eruption was relatively minor, but a series of factors had produced a far-reaching impact.

Dr Adrian Finch, a reader in geology at St Andrews University, said: "Several things came together. The volcano is on a glacier, where hot lava hit ice, causing a lot of frothing and boiling which assisted material being carried into the atmosphere."

When the volcano began erupting on 20 March, it was on an ice-free flank of the mountain. Dr Finch added: "The wind direction also played a part, carrying the ash towards us rather than blowing it towards the Arctic and Greenland.

"In addition, high pressure over the UK means the ash has not been shaken about much, so it hasn't dissipated so quickly."

The Met Office said this also meant the ash cloud remained high in the atmosphere, with little possibility of dust reaching the ground in the UK.

It will not be visible from the ground and is not expected to affect the weather, apart from producing spectacular sunsets.

Dr Finch added that Iceland's typically runny magma, containing a high proportion of dissolved gases, along with the volcano's narrow fissure, or opening, also contributed to the amount of ash being hurled high into the atmosphere.

It has been sent up to ten miles into the sky, covering the 35,000ft (6.5 miles) cruising height of passenger aircraft.

Dr Finch said such events were regular but infrequent, occurring every few thousand years. Icelandic volcanic ash has covered Scotland in the past, with Eyjafjallajokull last erupting in 1823.

In 1783-84, an eruption at Laki spewed out 120 million tonnes of sulphur dioxide – three times as much as was produced by European industry in 2006 – which caused a thick haze that killed thousands of people across the continent.

In the more recent past, aircraft have been diverted away from volcanoes, such as during eruptions of Mount Etna in Sicily and Mount St Helens, in Washington state in the United States.