|The plume of ash has grounded aircraft and closed airports across Europe [AFP]
The eruption of Iceland's Eyjafjallajokull volcano has brought travel chaos to Europe and around the world, grounding thousands of flights and stranding millions of travellers. Here we look at some of the key questions in the crisis:
What danger does volcanic ash pose to aircraft?
The fine particles of rock and glass ejected high into the atmosphere by a volcano have the potential to cause serious and possibly catastrophic damage to jet aircraft in flight.
Particles sucked into aircraft engines and rapidly heated can quickly clog vital intake valves with molten glass, causing the engine to overheat and fail.
|The clouds of ash thrown out by the volcano contain highly abrasive particles [AFP]
In addition, the sandblasting effect created by the highly abrasive particles can badly scratch the aircraft's windows, reducing pilot visibility to near zero.
Little was known about the ash danger until the early 1980s when two aircraft - 747s, one operated by British Airways and another by Singapore Airlines - suffered complete engine failure after flying through an ash cloud above an Indonesian volcano.
Both aircraft eventually recovered and carried out safe emergency landings even though, in the case of the British Airways aircraft at least, the cockpit window was so badly scratched that the pilots were unable to see through it.
In 1991 ash clouds generated by the eruption of Mount Pinatubo in the Philippines caused damage to more than 20 aircraft, most of which were flying 1,000km or more from the volcano.
Volcanoes erupt fairly often in other parts of the world, so why has this one caused so much chaos?
Not all volcanoes behave in the same way, and in the case of the Eyjafjallajokull volcano a combination of circumstances made it particularly disruptive.
In essence it comes down to four factors: geography, geology, glaciology and meteorology.
Iceland itself owes its physical existence to volcanoes and straddles the highly volatile mid-Atlantic fault created where the European and North American plates are pulling apart from each other.
In addition, the Eyjafjallajokull volcano itself is buried beneath a massive glacier, the meltwater from which is fuelling the power of the eruption.
Because of the heat generated by the volcano, the melting ice is quickly turned to steam which then makes the eruption even more explosive, flinging fine particles of debris up to 11km into the atmosphere.
In turn these particles – virtually invisible to the naked eye – are being picked up by easterly winds, spreading the plume high and wide across the European continent.
What are the health dangers?
The people most at risk are farming communities living in the vicinity of the volcano itself and the thick clouds of ash spewing from it.
|Inhaling ash can damage health, particularly among the most vulnerable [AFP]
Although called volcanic ash, the particles ejected from the volcano are not soft like wood ash, but jagged and highly abrasive and can cause health problems, especially for children and the elderly.
Many people have already been evacuated from the danger area.
For most people elsewhere in Europe, the health risks are thought to be minimal.
However, medical officials have said people with pre-existing coronary or breathing conditions could be at risk of complications from inhaling the fine particles.
Those people have been advised to stay indoors if they notice any worsening of their condition and seek medical advice, and to consider wearing a mask when outside.
Why can't aircraft just fly around or below the ash plume?
The main reason is the vast size of the ash plume, which has grown dramatically since Wednesday's eruption and is continuing to spread.
|Aircraft jet engines can be shut down by injesting volcanic ash [AFP]
Flying at a lower altitude below the ash plume presents several problems.
It requires much more intensive management from air traffic control, because aircraft will be closer together, and far fewer aircraft would be able to use the lower flight paths.
In addition modern jet engines are designed to perform at their optimum at high altitudes.
Flying lower uses far more fuel, raising the cost significantly which would be uneconomical in anything beyond a very short-term solution.
On Sunday, aircraft from Dutch airline KLM and Germany's Lufthansa were reported to have conducted test flights to assess the possibility of resuming a limited service.
How long will this last?
The simple answer is that no one knows.
Volcanoes have been studied for centuries and today scientists are equipped with advanced technology to monitor even the slightest changes in activity.
But despite years of experience, predicting when a volcano will erupt - or in this case when it might stop - remains an inexact science.
The last major eruption of the Eyjafjallajokull volcano, some two centuries ago, lasted for more than two years.
I'm stuck abroad and can't get home. What should I do?
|Residents living close to the volcano have been evacuated by Iceland's civil defence [AFP]
If your flight has been cancelled, or if your departure airport falls within the affected area, the advice from most officials is not to travel to the airport before first checking with your airline or the airport concerned.
If you do, you should be prepared to be disappointed.
Most airlines are posting regular information updates on their websites and are also asking passengers to register their contact details so that they can be kept up to date via SMS, phone or email.
Policies on changing or refunding tickets vary by airline, and you should also check the wording of your travel insurance policy to see what cover you may be entitled to.
Some airlines are offering refunds for cancelled flights and/or are waiving fees for date changes or rerouting flights.
Information will usually be found on the relevant airlines' websites.