VOLCANOES have more in common with monster thunderstorms than we thought. Their plumes swirl like cyclones, which could explain why they have been seen spawning tornadoes and sheet lightning.
It is conventionally thought that a volcanic plume rises straight up and spreads out in a rough circle. But when Pinaki Chakraborty at the University of Illinois at Urbana-Champaign and colleagues studied hourly satellite photos of the 1991 eruption of mount Pinatubo in the Philippines, they noticed that the margins gradually separated into five lobes. They saw similar shapes in photos of five other volcanoes.
The team's calculations show that this is caused by anticlockwise rotation in the plume, which creates a centrifugal force that throws the lobes outwards. The rotation occurs due to interactions between the updraught of the plume and horizontal wind patterns (Nature, DOI: 10.1038/nature07866).
Such rotation may be the driving force behind small tornadoes and waterspouts seen near plumes, which would form in the same way as dust devils around a thunderstorm. It may also explain the "sheaths" of lightning spotted in the 2008 eruption of mount Chaitén in Chile, because the cyclonic motion throws charged particles to the plume margins.
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