Volcanic Ash and Airplanes

All airline services have been cancelled over the most of the European airspace today due to an eruption of a volcano under the Eyjafjallajökull Glacier in Iceland.

As the molten lave goes through the ice of the glacier, it quickly cools down and forms small glass shards. These shards practically sandblast any aircraft through them and can cause anything from engine failures to windscreens becoming opaque.

A Wikipedia article regarding the British Airways Flight 9 on June 24th 1982 illustrates the dangers very clearly. Quoting the captain of that flight, as a perfect example of british understatement some 30000ft from the ground:

Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them under control. I trust you are not in too much distress.