Interesting report on an analysis of the volcanic ash emitted by Eyjafjallajokull which closed European airspace. Work undertaken by an Icelandic-Danish team.
BBC News - Volcanic ash air shutdown the 'right' decision
Looks scientific enough to warrant acceptance. The link to the PNAS paper is of particular interest.
Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment
On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days.