What is volcanic ash?
Volcanic ash consists of plumes of dust spewed out by volcanoes. These plumes usually contain tiny particles of glass, pulverised rock and silicates. The result is a cloud of material resembling sandpaper.
What happens when volcanic ash is detected?
A 120 nautical mile danger zone is declared around the original plume and flights are banned in contaminated areas as winds move the ash across flight paths.
How long will this problem last?
The answer depends on the weather. Prevailing winds have so far spread the cloud eastwards from Iceland towards Irish, British and Scandinavian airspace.
Irish airspace is expected to be closed until at least 6am tomorrow.
On its website, Met Éireann says: ‘Our weather models are indicating that it is going to remain dry over the next couple of days so we do not expect any significant surface deposition of ash in the short term. Ash and dust normally reach the surface through being washed out of the atmosphere by rain’.
'Next week the airflow will be more directly from Iceland to Ireland and the weather forecasts indicate that there will be precipitation so, if the eruptions continue, it is possible that there will be deposition at that stage.'
Why is it a hazard to aviation?
The real problem is not visibility but the abrasive effect of the ash, which can strip vital surfaces and clog up an engine.
Pulverised rock colliding at high speed with a speeding jetliner can blast away surfaces inside the engine.
Due to intense heat in the motor, the particles can fuse together when they penetrate the engine and stop it from working.
Aircraft avionics and electronics can also be damaged. Aviation authorities say clouds of ash are often accompanied by clouds of gas such as sulphuric acid and hydrochloric acid.
How do controllers plan for events such as this?
The International Civil Aviation Organisation, a UN body, maintains detailed contingency plans that were activated today.
This incident came just six weeks after European authorities carried out the first of two 2010 exercises for such an event, aimed at preventing a catastrophe due to volcanic dust.