Airports across much of Europe have faced major disruption because of ash from an erupting volcano in Iceland. Experts say it is unclear how long the problems to aviation will continue.
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 for how much of that 14-month period the volcano was producing an eruption column several kilometres in height.
The current eruption beneath the Eyjafjallajokull glacier in south-western Iceland began on
when it forced about 500 people in the sparsely populated area from their homes.
Is the size of this eruption unprecedented?
No. It's relatively small, but it has happened in a very busy airspace. Airspace over Alaska, Kamchatka in Russia and Indonesia is not infrequently closed because of airborne ash.
Is there a risk of the eruption spreading to nearby volcanoes?
Some experts say there is chance that Katla volcano, to the east of the Eyjafjallajokull glacier, may erupt. 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?
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 it should not be stirred up by scuffing it with feet as that would make breathing hazardous.
However the gas is unlikey to pose a danger this far downwind from the volcano.
Ash cloud footage and advice from Met Office Scientist Derrick Ryall
Why is so much ash being produced?
British glaciologist Dr Matthew Roberts, who is working at the Icelandic Met Office, said huge amounts aof ash are being produced because the eruption is taking place beneath the Eyjafjallajokull icecap.
He said the interaction of the molten rock, the magma, and the glacial ice caused the magma to cool very quickly, pulverising it into tiny fragments of rock.
"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," Dr Roberts said.
Why was the eruption so explosive?
Dr Rothery said various factors may have contributed to the explosive nature of the eruption.
The proportion of gas - made up of water vapour, carbon dioxide and sulphur dioxide - in the volcanic magma may have increased, compared to the original batch of magma when it first started erupting in March.
The mixture of ice and water from the glacier turns into steam when it meets the magma, and the ash is driven skywards.
Once airborne, convection takes over and the plume - which is warmer than the surrounding air - rises.
How high the plume rises depends on the initial impetus from the expanding gas, plus its heat content. Eventually the plume cools, reaches neutral buoyancy, and then ceases to rise. It is then at the mercy of the wind.
The magma from the more recent eruption has proved to be higher in silica than that from the earlier stages of the eruption, which means it is more liable to fracture, forming ash.
Coarse ash which is more than 1mm in diameter falls to earth. However fine ash - less than 0.1mm in diasmeter - stays airborne for a long time.
When will it stop?
Volcanologists say it is impossible to predict when the eruptions might cease, pointing out that eruptions in Iceland can continue for months
How can we tell if the ash cloud is potentially dangerous?
Dr Rothery said one sign was whether the plume was grey or brown as then it has ash in it. A white volcanic plume does not pose the same problems for aircraft.
Is it possible to estimate how long the ash cloud will be in the air?
This depends on how long the eruptions continue. So long as an ash column rises nearly 10km high, it can cause problems . If it subsides to below 3km, and stays that low, then airspace can return to normal.
Who makes the decision to close UK airspace when there is an ash cloud?
Met Office forecasters monitor volcanic eruptions as part of their role in the global network of nine Volcanic Ash Advisory Centres. It provides forecasts every six hours giving an 18-hour prediction of where the volcanic ash cloud is and details of the differing concentration levels.
The Civil Aviation Authority uses this information to advise the air industry of any resulting no-fly zones. It also advises on "red zones" - were ash is present but flights can continue with tougher safety rules.
Air traffic authority Nats then uses this information to brief airline operators and airports on the specific areas that will be affected.
What are the rules guiding when airspace is closed?
Before the initial grounding of UK flights in April, the rules were set by an international body called the International Civil Aviation Organisation. It had a limit of "no tolerance" for any concentration of volcanic ash.
But since then, the Civil Aviation Authority (CAA) has raised the threshold for the concentration of ash based on test flights and analysis of the cloud.
The CAA worked with scientists and engine and aircraft manufacturers to set a safe threshold for the concentration of ash of 0.004g per cubic metre of air.
The authority initially set "conservative" threshold of 0.002g per cubic metre, which incorporated a "buffer" to ensure that planes could fly safely.
New data, including reports from flights that have encountered ash, has now allowed manufacturers to confirm that the higher ash concentration will not damage aircraft or engines.
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