First author

The hot gases and dust that erupt from a volcano form a plume with a vertical column topped by a horizontal cloud, or umbrella. The umbrella is known to change shape over time from a circle to a wavy, lobular form, but no one knew why. Postdoc Pinaki Chakraborty of the University of Illinois at Urbana-Champaign and his colleagues set out to learn what caused the change. Using satellite images of recent eruptions and an obscure 1811 paper that describes a volcanic plume erupting from the sea, the team concludes that the volcanic plume actually spins about its axis, a phenomenon they have dubbed a volcanic mesocyclone. This rotation model explains the umbrella's changed shape and unravels other mysteries, including why lightning and waterspouts or dust devils usually accompany volcanic eruptions (see page 497). Chakraborty tells Nature more.

What is a volcanic mesocyclone?

It's a vortex of air, hot gases and dust that rises and rotates, just like in a violent thunderstorm that spawns a tornado. Everyone had assumed that a volcanic plume doesn't spin. But we discovered that as the hot gases and dust erupt, they engulf atmospheric winds and create this narrowing tube of wind, gases and dust that rotates the column and umbrella.

Why was the 1811 paper significant?

It tied everything together. A sea captain sailing in 1811 in the Azores, west of Portugal, says that the rising plume was rotating — the first and only direct mention of rotation I have ever seen in the literature. He also reports seeing lightning and waterspouts along with the rotation. This was a crucial piece of the mystery for us, and helped us draw an analogy with tornadic thunderstorms.

What did you learn from images of the 2008 eruption of Chile's Mount Chaiten?

They seem to show the whole plume covered in lightning. We found that charged particles in the plume are spat out of the centre of rotation and line up around the edge of the mesocyclone, creating a lightning sheath. Also, the rising plume forms an updraught that begins to spin and may create tornadoes — seen as waterspouts at sea or dust devils on land.

How did you confirm your ideas on rotation?

Using satellite records of the June 1991 Mount Pinatubo eruption in the Philippines, we analysed the umbrella's edge at different times to define its expansion, its rotation and its movement as one element. The umbrella's shape changes because the centrifugal force of its rotation makes the umbrella's edge unstable, and the circle breaks down into a lobular shape.