GEOPHYSICS Violent volcanic eruptions are thought to be triggered, in part, by the rapid expansion of gases (and gas bubbles) in a magma chamber or conduit, but the causes of this process and what leads to a violent eruption versus a less hazardous outpouring of bubble-rich lava are subjects of debate. Because of the high pressures and temperatures at which real magmas exist, most laboratory simulations (and many science fair projects) have used analog systems that foam at conditions close to ambient, and thus there has been concern that the important dynamics of eruptions have not been simulated accurately. Martel et al. have performed a systematic study of which factors contribute to fragmentation of a bubble-rich magma. In their experiments, they hydrated a sample of rhyolite (a Si-rich volcanic rock), raised it to high pressure and temperature where foaming occurred, then rapidly lowered the pressure by puncturing a diaphragm. The degree of decompression was more important in fragmenting the magma than the quantity of bubbles or their shape. These results are consistent with explosive dome-building eruptions, seen for example in Soufriere Hills, Montserrat.— BH Earth Planet. Sci. Lett. 178 , 47 (2000).