By analysing satellite images of volcanic eruptions, researchers found the ash swirls like a tornado.
SYDNEY: Ash ejected from major volcanic eruptions doesn't just billow into the air but forms tornado-like funnels sheathed in lightning, which cause the ash to reach further into the atmosphere.
The study, published in Nature today, combines evidence from a 198-year-old description from a ship's captain, who witnessed a volcanic eruption from the sea in the Azores archipelago in the Atlantic Ocean, and satellite imagery of the 1991 eruption of Mt Pinatubo in the Philippines.
Geologists have long studied how volcanoes erupt by looking at the way deposits of ash and rock form. It is only recently that satellite imagery has revealed exactly how ash and smoke is ejected into the air as a volcano erupts.
Now researchers from the University of Illinois at Urbana-Champaign, Illinois, led by geologist Pinaki Chakraborty, for the first time present evidence that the plume of ash and smoke forms into mesocyclones - rotating updraughts that also form within supercells, the birthplace of severe thunderstorms.
"What happens in tornadic thunderstorms is analogous to what happens in strong volcanic plumes," said Chakraborty.
Predicting effects
Knowing how plumes rotate can help scientists predict how far ash and smoke is ejected into the atmosphere and how long it will stay there. When Mt Pinatubo erupted, it injected tonnes of sulphur dioxide into the stratosphere, about 40 km kilometres above the Earth, increasing the size the ozone hole and causing a drop in temperature of 0.5°C for two years after the event.
"One of the effects of a volcanic mesocyclone is on the height of a volcanic plume, which is an important parameter for forecasting the effects of a volcanic eruption," Chakraborty said. "A volcanic mesocyclone may result in an increase in the height of a volcanic plume."
Any change in the height of a plume affects where the ash from the volcano will end up, which affects the planning of air-traffic routes, for example, he added.
Finding a storm
The researchers became suspicious mesocyclones could form in volcanic plumes after spotting a spectacular image of the May 2008 eruption of Chaitén Volcano in the southern Andes sheathed in lightning. This lightning sheath also forms at the edges of a mesocyclone within a thunderstorm: a vigorous updraught rises in the presence of horizontal, spinning tubes of air, and entrains them into the beginnings of a vertical vortex of air. The Earth rotation's ultimately favours those funnels that spin in the same direction as cyclones in the southern Hemisphere and hurricanes in the northern Hemisphere (a phenomena caused by the Coriolis effect).
The researchers' suspicion was confirmed when they found a witness' description of a volcanic eruption in 1811 that spun ''like an horizontal wheel'', as well as in analysis of satellite images taken at hourly intervals of the Pinatubo eruption. These showed the umbrella-like top of the ash column forming becoming lumpy or lobate as the plume spun.
Vulcanologist Scott Bryan from the University of Queensland said the rotation is an "interesting phenomenon" but "not surprising".
"We see the Coriolis effects in water and ocean currents, so it's not too surprising to see [that] in a volcanic plume that you get this rotation occurring and forming dust devils," Bryan said.
"What will be interesting is to see what effect latitude will have. Pinatubo is a very low-latitude volcano, near the equator. It may be the plume rotation is stronger at higher latitudes," he said.
