John Timmer in Ars Technica:
The initial images of the Eyjafjallajökull eruption showed the sort of dramatic spires of molten rock that we associate with Hawaiian volcanoes. The next time it made the news, it was because air travel throughout Northern Europe had been shut down as a huge cloud of ash spread slowly across the UK and Scandinavia—very un-Hawaiian. To get a better sense of why this Icelandic volcano was showing such a split personality, we got in touch with the American Geophysical Union, which handed us on to Dr. Jeff Karson, who's chair of the Earth Sciences department at Syracuse University. Dr. Karson patiently explained what makes volcanism in Iceland distinct.
If you're like me, and know just enough Geology to be dangerous, you'd probably divide volcanoes into two categories: hotspots like Hawaii, where molten rock pours out as gently as anything that's 1,200°C possibly can, and volcanism associated with subduction zones, which tends to produce massive, explosive eruptions, such as the ones at St. Helens and Pinatubo. There is, of course, a third kind, the eruptions associated with the spreading of mid-ocean ridges. But these generally take place deep underwater, and are rarely captured on film.
Iceland is distinct because it's the product of a huge hot spot located directly under the Mid-Atlantic ridge. It's not unique in that regard; Karson said that the Azores and Galapagos Islands are the product of similar situations. But Iceland is much hotter and more active than the others.
If it's not on a subduction zone, however, you might not expect it to produce the sorts of explosive eruptions that send clouds of ash across large areas of the North Atlantic. Karson said there are two factors that can make Icelandic volcanoes pack a punch. The first is that, in addition to the basaltic magma associated with mid-ocean ridges, Iceland's volcanoes produce significant amounts of rhyolite, which is silica-rich and, more significantly, contains a lot more volatile substances. As the rhyolitic magma reaches the surface and pressure is released, the result can be an explosive eruption. As Karson put it, the cause is different from the explosive volcanic eruptions that occur at subduction zones, but the result can look very similar.
Individual magma chambers beneath Iceland can have varying mixes of basaltic and rhyolitic materials, which means that individual volcanoes on the island may have a complicated eruption history.
The other factor that adds to the explosiveness of Iceland's volcanoes is the ice itself.