NASA's Terra and Aqua spacecraft, both in "ball of yarn" polar orbits, carry a detector called MODIS. On April 15, the Terra spacecraft used its onboard MODIS instrument to capture the following image of a volcanic plume from the Eyjafjallajökull volcano in southern Iceland.
In the photo, the Shetland Islands are at the photo's lower right corner.
In southern Iceland, the glacier over top of the volcano caused the upwelling lava to cool very quickly, forming a silica glass which fragmented into fine particles, and which in turn were carried into the atmosphere by the eruption plume.
The height of the plume occupies the normal range of cruise altitudes for commercial airliners. The presence of the the fine, sharp, jagged, abrasive silica particles in the volcanic ash cloud would cause engine failure if the material were sucked into an intake turbine. For this reason, air travel over most of northern Europe was cancelled for the day, and likely over the coming weekend.
Not since the World Trade Center attacks on September 11, 2001 has there been such a large disruption in air travel.
UPDATE - The Astronomy Picture of the Day website has a great picture of the volcano for their April 19 entry.