Volcanic ash can cause aircraft engine failure
Volcanic ash, which consists of the pulverised rock and glass created by the eruptions, can jam aircraft machinery if a plane flies through the plume, shutting down the engines.
Ash can also be can be sucked into the cabin itself, contaminating the passengers' environment as well as damaging the plane's electronic systems.
There have been a number of incidents where aircraft have reported damage from ash, including one involving a British Airways Boeing 747 in June 1982.
The aircraft ran into difficulties after the eruption of a volcano at Galunggung, Indonesia. Ash jammed all four engines briefly, and the aircraft plummeted 24,000 feet before they could be restarted.
Because of the threat to aviation, a global early warning system, known as the International Airways Volcano Watch, has been established. Iceland is considered as particularly vulnerable to volcanic disruption.
Authorities there yesterday evacuated 800 residents from around the Eyjafjallajokull glacier as water gushed down the mountainside and rivers rose by up to 10 feet (3 meters).
The Eyjafjallajökull volcano erupted for the first time in 200 years on March 20, in a dramatic display that sent fountains of lava spewing into the air.
The first eruption did not trigger any major flooding, as was initially feared, because the active vents were in a mainly ice-free part of the volcano.
But Tuesday's eruption came from a different vent beneath a 650-ft (200m) thick block of ice, unleashing a torrent of glacial meltwater.