Disruption of air traffic because of the spread of volcanic ash from Iceland continues to ground aircraft and travellers for a fourth day.  Eurocontrol officials told a news conference between 12,000 to 13,000 flights were likely to operate in European airspace on Friday, compared with about 29,500 normally. Experts are predicting the ash cloud to spread further south and east.

"What we are seeing in Iceland is that as the magma particles get towards the surface, they interact with the very cold water and they chill to form glassy fragments and these glassy fragments are small, they have sharp edges and when those get up in the air that is what is causing the risk to aviation," says Colin Macpherson, a professor of Earth sciences at Britain's Durham University.

Sharp volcanic particles can damage a jet engine or cause it to shutdown. Nearly twenty years ago a Boeing 747 encountered volcanic ash and had all four engines stop.  Fortunately, the crew was able to start several engines and fly to a nearby airport just barely avoiding a disaster.