sumber : http://news.bbc.co.uk/2/hi/science/nature/8621992.stm
By Victoria Gill
Science reporter, BBC News
The volcanic ash cloud
The volcanic ash cloud reached about 55,000ft, Eurocontrol says
More than 1,000km from the event itself, Iceland's second volcanic eruption in the space of a month has caused flights in the UK to be grounded.
Scientists and aviation authorities are continuing to monitor a plume of volcanic ash that is moving southwards over the UK.
The entirety of UK airspace closed from noon on Thursday.
National Air Traffic Services said: "The current restrictions across UK controlled airspace due to the volcanic ash cloud will remain in place until at least 0100 BST on Tuesday 20 April."
The eruption ejected the plume, which is made up of fine rock particles, up to 11km into the atmosphere.
"This ash cloud is now drifting with the high altitude winds," said Dr David Rothery, a volcano researcher from the UK's Open University.
"The main mass is over Scandinavia, but it is also over the north of Great Britain and is likely to spread south over the whole island by the end of [Thursday]."
It developed into something more than we'd ever seen before... it was, yeah, a little bit frightening
Capt Eric Moody, who piloted a 747 through a volcanic dust cloud
Read more here
The plume is so high that it will neither be visible nor pose a threat to the health of humans on the ground, although Dr Rothery added that we may have a "spectacularly red sunset" on Thursday evening.
The major concern is that the ash could pose a very serious hazard to aircraft engines. The latest maps showing the spread of the volcanic ash cloud can be found here.
Dr Dougal Jerram, an earth scientist at the University of Durham, UK, explained: "Eruptions which are charged with gas start to froth and expand as they reach the surface.
"This results in explosive eruptions and this fine ash being sent up into the atmosphere.
"If it is ejected high enough, the ash can reach the high winds and be dispersed around the globe, for example, from Iceland to Europe. These high winds are exactly where the aeroplanes cruise."
Airports operator BAA confirmed that all flights at Heathrow, Stansted and Gatwick would be suspended from midday.
"Air traffic restrictions have very properly been applied," said Dr Rothery. "If volcanic ash particles are ingested into a jet engine, they accumulate and clog the engines with molten glass."
Graphic showing effects of volcanic ash on a jet engine
In 1982, British Airways and Singapore Airways jumbo jets lost all their engines when they flew into an ash cloud over Indonesia.
Reports said that the ash sandblasted the windscreen and clogged the engines, which only restarted when enough of the molten ash solidified and broke off.
A KLM flight had a similar experience in 1989 over Alaska.
Stewart John, a fellow of the Royal Academy of Engineering and former president of the Royal Aeronautical Society, explained that the ash can cause severe damage.
"This dust really is nasty stuff," he told BBC News. "It's extremely fine and if it gets into a jet engine, it blocks up all of the ventilation holes that bleed in cooling air.
"Jet engines operate at about 2,000C, and the metals can't take that. The engine will just shut down."
In the case of the 1982 British Airways flight, Mr John explained, when the plane emerged from the cloud, the pilot repeatedly tried and failed to restart the engines.
"They were going down and down, and had just about accepted that they would have to ditch.
"But, at the last minute, one engine started. By repeatedly turning the engine over and having a clean airflow going through, he managed to blow the ash out."
Dr Rothery explained that as a result of those incidents, emergency procedure manuals for pilots were changed.
"Previously, when engines began to fail the standard practice had been to increase power. This just makes the ash problem worse," he said.
"Nowadays, a pilot will throttle back and lose height so as to drop below the ash cloud as soon as possible. The inrush of cold, clean air is usually enough to shatter the glass and unclog the engines.
"Even so, the forward windows may have become so badly abraded by ash that they are useless, and the plane has to land on instruments."
Mr John concluded: "We do not know how long this will last.
"It's like a typhoon - because you can't fly through it, you can't directly monitor it, so we have rely on satellite images and to err on the side of extreme caution."
Height comparison graphic