Some eruptions take place on the surface of white-dwarf stars.
Its volcanoes do not have the explosive oomph of some eruptions in the "ring of fire" around the Pacific, such as those of Tambora and Krakatoa in the 19th century, let alone the prehistoric eruptions of Taupo, in New Zealand, and Toba, in Indonesia, which were large enough to have severe, if short-lived, effects on the global climate (see chart).
Although volcanologists now agree with that general picture, it leaves open the crucial question of why some eruptions are explosive and others -- often in the same volcano, as with Mount Mazama -- merely effusive, spitting up magma like overheated oatmeal on a stovetop.
In some eruptions, foaming magma wells to the surface as an emulsion of hot gases and incandescent particles; the shredded pumaceous material spreads swiftly, even over gentle gradients, as a glowing avalanche (nuée ardente) that may move many kilometres at speeds greater than 160 km (100 miles) per hour.
Some eruptions have already occurred, among Sumatra's 36 active volcanoes, which adjoin the responsible fault.
But Katla has produced 208 ash layers in the past 8400 years, some eruptions rating a four or even a five.
Some eruptions cease after this stage, leaving broad maars or explosion craters typically 1-2 kinometres in diameter.
Now researchers have a new idea how this happens, and the findings may shed light on why some eruptions are much more violent than others.
The results help explain why geophysicists haven't found simmering magma chambers under volcanoes like Yellowstone, and why some eruptions are more violent than others.
