There is always a mollusc involved it seems. The July 13 issue of Science reports on major research titled "Magmatic Gas Composition Reveals the Source Depth of Slug-Driven Strombolian Explosive Activity"
Clearly, there is no reason to actually read this paper, it is clear from the title the slugs are driving highly explosive volcanic processes from deep with the Earth! Quite amazing really. From This Week in Science,
"The explosions are thought to be caused by gas slugs that rise faster than the surrounding magma and generate seismic activity near the surface of the volcano.[...]. they show that gas slugs form 3 kilometers beneath the summit craters, at the base of the volcanic pile and away from seismological processes at the surface."