Two mechanisms for generating rotation in a volcanic plume are shown here. In one, as the plume shoots up at 200 to 600 meters a second, winds surrounding the volcano form a horizontal vortex tube that is tilted and stretched as it travels up - this is similar to what is seen in thunderstorms. In the second example, eddies and vortices from the volcanic environment itself can form creating a horizontal vortex ring. Click on image for full size (120 Kb) Image Courtesy of the Zina Deretsky/NSF, after Chakraborty et al., Volcanic mesocyclones, Nature, 3/26/09

A 200-year-old report by a sea captain and photographs of the 2008 eruption of Mount Chaiten are helping scientists better understand strong volcanic plumes.

In a paper published this week in the journal Nature, the scientists show that the spontaneous formation of a "volcanic mesocyclone"--a rotating, column-shaped vortex--causes the volcanic plume to rotate on its axis.

The rotation, in turn, triggers a sheath of lightning and creates waterspouts or dust devils. The origins of these volcanic phenomena were previously unexplained.

"These results solve a long-standing mystery about the relationship between volcanic plumes and associated tornadoes, waterspouts and lightning, showing for the first time that rotation of a volcanic plume may be the primary cause of these effects," said Sonia Esperanca, program director in the National Science Foundation's (NSF) geosciences directorate.

The research is supported by NSF, via three directorates: geosciences; mathematical and physical sciences; and engineering.

"Rotation is an essential element of a strong volcanic plume," said Pinaki Chakraborty, a computational scientist and engineer at the University of Illinois and the paper's lead author. "By taking into account the rotation, we can better predict the effects of volcanic eruptions."

In 2008, a photograph of the Mount Chaiten eruption in southern Chile showed what appeared to be a volcanic plume wrapped in a sheath of lightning.

A search for references to other occurrences of lightning sheaths led Chakraborty, mechanical engineer Gustavo Gioia and geologist Susan Kieffer to an obscure paper by a sea captain, published in 1811.

In that paper, the sea captain reported his observations of a volcanic vent that emerged from the sea in the Azores archipelago and formed a large volcanic plume.

According to the captain, the plume rotated on the water "like an (sic) horizontal wheel" and was accompanied by continuous "flashes of lightning" and a "quantity of waterspouts."

This conjunction of rotation, lightning and waterspouts (or dust devils on land) is characteristic of a familiar meteorological phenomenon seemingly unrelated to volcanic plumes: a tornadic thunderstorm.

The same process that creates a mesocyclone in a tornadic thunderstorm also creates a volcanic mesocyclone in a strong volcanic plume, Chakraborty said. "What happens in tornadic thunderstorms is analogous to what happens in strong volcanic plumes."

A strong volcanic plume consists of a vertical column of hot gases and dust topped with a horizontal "umbrella." A volcanic mesocyclone sets the entire plume rotating on its axis.

The mesocyclone spawns waterspouts or dust devils, and groups the electric charges in the plume to form a sheath of lightning, as was so prominently displayed in the eruption of Mount Chaiten.

On June 15, 1991, the eruption of Mount Pinatubo in the Philippines was recorded by a satellite snapping hourly images. The images show that the edge of Pinatubo's umbrella was rotating about its center, consistent with the presence of a volcanic mesocyclone, say the scientists.

Satellite images of future volcanic plumes taken at intervals of a few minutes would make it possible to trace the evolution of umbrellas in detail, Gioia said.

In addition, some of the tools commonly used in the study of thunderstorms could be deployed for the study of volcanic eruptions.

"The structure and dynamics of volcanic mesocyclones, as well as the presence of lightning sheaths, might be verified with Doppler radar and lightning mapping arrays, two technologies that have been scarcely used in volcanology," Gioia said.

Text above is courtesy of the National Science Foundation

This image illustrates two different ways that rotation can happen in a volcanic plume. Click on image for full size (120 Kb) Image Courtesy of the Zina Deretsky/NSF, after Chakraborty et al., Volcanic mesocyclones, Nature, 3/26/09

Scientists are using information they have from past volcanic eruptions to better understand strong volcanic plumes. They have observed that in some volcanic eruptions, there is a spontaneous formation of a "volcanic mesocyclone," which is a rotating, column-shaped vortex that causes the volcanic plume to rotate on its axis. This rotation triggers the formation of lightning and creates waterspouts or dust devils. Waterspouts are like dust devils on land, but they contain water when they are over the ocean.

This phenomena was photographed during the 2008 eruption of Mount Chaiten in southern Chile. In addition, scientists have found a paper written by a sea captain in 1811. In the paper, the captain reported that he saw a volcanic vent coming out of the sea in the Azores archipelago that contained a large volcanic plume. According to the captain, the plume rotated on the water like a wheel and there were continuous flashes of lightning and a lot of waterspouts.

The scientists that worked on this project think that the rotation of these volcanic plumes is the primary cause of tornadoes, waterspouts, and lightning that have been seen in the plumes. They also think this process is similar to how a strong thunderstorm creates a tornado. The plume contains a column of hot gases and dust topped with an "umbrella."

In the future, scientists plan to look at satellite images of volcanic plumes taken every few minutes so they can study the formation of these umbrellas, with the rotation and lightning that go along with them. They also plan to study the volcanic plumes using Doppler radar, which is something meteorologists use to study thunderstorms but hasn't been used to study volcanoes before.

This image illustrates two different ways that rotation can happen in a volcanic plume. Click on image for full size (120 Kb) Image Courtesy of the Zina Deretsky/NSF, after Chakraborty et al., Volcanic mesocyclones, Nature, 3/26/09

Scientists have observed that in some volcanic eruptions, a rotating, column of dust and gas forms and causes the volcanic plume to rotate on its axis. This rotation causes the formation of lightning and creates waterspouts or dust devils.

This phenomena has not been seen very much. It was photographed during the 2008 eruption of Mount Chaiten in southern Chile. In addition, scientists have found a paper written by a sea captain in 1811. In the paper, the captain reported that he saw a volcanic vent coming out of the sea that had a plume coming out of it. The plume rotated on the water like a wheel and there were continuous flashes of lightning and a lot of waterspouts.

The scientists that worked on this project think that the rotation of these volcanic plumes is the primary cause of tornadoes, waterspouts, and lightning that have been seen in the plumes. They also think this process is similar to how a strong thunderstorm creates a tornado. The plume contains a column of hot gases and dust topped with an "umbrella."