http://news.bbc.co.uk/2/hi/uk_news/8627253.stmpage last updated at 20:04 GMT, Saturday, 17 April 2010 21:04 UK
Q&A: Volcanic ash cloud
Aircraft across much of Europe have been grounded as a result of an ash cloud caused by an erupting volcano in Iceland.
What is the history of this volcano?
Dr Dave Rothery, a volcanologist from Open University, said records show the volcano last erupted between December 1821 and January 1823 however it is unknown how much of that 14-month period the volcano was producing an eruption column several kilometres in height.
The current eruption beneath the Eyjafjallajoekull glacier in south-western Iceland began on 20 March, where it forced about 500 people in the sparsely populated area from their homes.
Is the size of this eruption an unprecedented event in history?
No. It's relatively small, but has happened in a very busy airspace. Airspace over Alaska, Kamchatka in Russia and Indonesia are not frequently closed because of airborne ash.
Is there a risk of the eruption spreading to nearby volcanoes?
Activity may migrate east to Katla volcano, which lies below the adjacent ice cap however there is no sign of that at present.
We know the ash can clog jet engines, but is the ash dangerous to humans? Is there any risk of toxic gas from the eruption drifting into populated areas?
Dr Rothery said fine ash can exacerbate asthma or lung diseases. In the UK, ash falling from the cloud is not likely to be a problem while still airborne. If it settles to the ground, he said it should not be stirred up by scuffing it with feet as that would make it hazardous for breathing.
Once traces of ash have been rained on, the hazard is over.
Dr Rothery said the gas did not pose a danger this far down wind.
Why is so much ash being produced?
British glaciologist Dr Matthew Roberts, who is working at the Icelandic Met Office, said a great deal of ash is being produced because the eruption is taking place beneath the Eyjafjallajokull icecap.
He said: "It's the interaction of the molten rock, the magma, and the glacial ice which is causing the magma to cool very quickly and to be pulverised into tiny fragments of rock.
"And these updrafts of fine volcanic ash are being lifted into the sky by the enormous steam plumes that have been created by the vast quantities of ice that's been melted."
Why is the eruption so explosive?
Dr Rothery said two factors may have contributed to the explosive nature of the eruption.
The amount of gas - made up of water, vapour, carbon dioxide and sulphur dioxide - in the magma now arriving in the volcano may have become greater than it was in the original batch of magma when it first started erupting in March, and the ice/water from the glacier is being turned into steam when it meets the magma.
The expansion of gas, which is mainly water vapour if it is from magma/ice interaction, drives the ash skywards, and then convection takes over. The plume is warmer than the surrounding air, and so is less dense despite the ash it contains.
How high it rises depends on the initial impetus from the expanding gas plus its heat content. When it reaches neutral buoyancy, it ceases to rise and is then at the mercy of the wind.
Coarse ash (more than 1mm) falls out, but fine ash (less than 0.1mm) stays airborne for a long time.
Why is the ash plume still rising?
The ash has been moving at a height of about 30,000ft due to water going into the vent.
According to Dr Rothery, as Thursday's eruption progressed, the best hope was that the interaction between melt water and the continually arriving magma would lessen.
Ideally, solidified magma would isolate the vent from the ice and/or water, so that the eruption would become less explosive, and the plume height would decrease until it was no longer a threat.
However, currently the ash column is higher than it was on Friday. A possible explanation is that fractures caused by magma movement and small earthquakes keep allowing water into the vent, at a sufficient depth that steam expansion has no option but to drive the ash fragments skywards.
When will it stop?
Volcanologists have said it is impossible to predict when the eruptions might cease, pointing out that eruptions in Iceland could continue for months.
Prof Brian Golding, of the Met Office, said once all the water has evaporated, it should die down.
What should we look out for?
Dr Rothery said one thing to watch for on webcams was if the plume was grey or brown as then it has ash in it - a white plume is not a problem.
If it reaches above the top of the webcam field of view, then it is reaching high enough to pose a problem.
Is it possible to estimate how long the ash cloud will be in the air?
This depends on how long the eruption continues with its current explosive vigour. So long as an ash column rises nearly 10km high, it will be problematic. If it subsides to below 3km, and stays that low, then airspace can return to normal.
