Rare Volcanic Plumes Create Uncommonly Dangerous Ash Flows

WASHINGTON - Three unique photographs of a recent volcanic eruption in a remote part of Ecuador show a plume unlike any previously documented and hint at a newly recognized hazard, say scientists at the University of Illinois at Urbana-Champaign.

"The usual volcanic plume consists of a stalk capped with an umbrella and resembles the mushroom of an atom bomb blast," says geology professor Susan Kieffer, "but the umbrella on this plume was wavy, like the shell of a scallop." Kieffer noted that "we had never seen a scalloped umbrella before. Unusual conditions must have existed in the volcanic plume that formed this umbrella."

In a paper scheduled to appear 15 March in the journal Geophysical Research Letters, Kieffer, mechanics professor Gustavo Gioia, and graduate student Pinaki Chakraborty explain what might have caused the umbrella to scallop, a task made more difficult by the scarcity of information.

Located about 100 kilometers [60 miles] from Quito, Ecuador, Volcan Reventador ("one that explodes") lived up to its name on the morning of 3 November 2002. Following seven hours of seismic activity and billowing steam, the summit cone exploded and sent a stream of ash, called a pyroclastic flow, several kilometers [miles] down nearby valleys. While traveling close to the ground, the ash heated the surrounding air, which became buoyant as in a hot-air balloon. The air rose in a volcanic plume, carrying the ash with it.

"A volcanic plume rises until the atmosphere becomes so thin that the mixture of air and ash loses buoyancy and starts to spread laterally, forming an umbrella," Gioia said. "The umbrella spreads and cools for a long time before the ash begins to fall gradually."

But instead of the usual hot ash, the Reventador eruption appears to have been laden with steam and fairly cool ash from the destruction of the summit cone, the researchers say. The unusually cool umbrella could not spread for a long time. It rapidly became a heavy mixture of air, steam, and ash hovering precariously over the lighter air below.

"When a heavier fluid is placed on top of a lighter one, you might say that the fluids want to be reversed," said Chakraborty, the paper's lead author. "The ensuing tug-of-war between gravity and the viscosity of the fluids results in a wavy instability that pulls the heavier fluid on a fast sinking course."

In laboratory experiments, the fluids are initially at rest, and the wavelength of the instability is a fraction of an inch [a few millimeters]. But the mixture of air, steam, and ash in the Reventador umbrella was turbulent, with many fast, locally swirling motions. "Turbulence magnifies the wavelength," Chakraborty said. "It gave the Reventador umbrella its distinctive scallops, which were hundreds of meters [yards] in wavelength."

While most umbrellas produce gradual ash falls, scalloped umbrellas behave differently and might represent a previously unrecognized hazard. "Our analysis suggests that the Reventador umbrella collapsed rapidly, forming new and especially dangerous ash flows," says Kieffer.

Originating far from the summit cone, these new flows must have helped spread the damage caused by the eruption, the researchers concluded. They must have been uncommonly energetic, they say, because the ash fell from the umbrella, which was 10 kilometers [six miles] high.

"For all we know, these flows were responsible for broken petroleum pipelines," Chakraborty said. The principal highway to Ecuador's oil fields, as well as two crude oil pipelines and one refined products pipeline, lie at the eastern foot of the Reventador caldera; this infrastructure was severely damaged by the 2002 eruption. According to an official report, "The longest pyroclastic flow...crossed the principal oil and gas pipelines, destroying the new, empty oil pipeline and carrying away small bridges on the main gravel highway leading to the oilfields."

The flows "might also have contributed to the early phases of a shutdown of Quito airport that lasted more than a week," Chakraborty said.

Notes for Journalists

Journalists (only) may obtain a pdf copy of this paper upon request to Jonathan Lifland: jlifland@agu.org. Please provide your name, name of publication, phone, and email address. The paper and this press release are not under embargo.

Title: "Volcan Reventador's unusual umbrella"

Authors:
Pinaki Chakraborty, Department of Theoretical and Applied Mechanics, University of Illinois, Urbana, Illinois, USA;
Gustavo Gioia, Professor, Department of Theoretical and Applied Mechanics, University of Illinois, Urbana, Illinois, USA;
Susan Kieffer, Professor, Department of Geology, and Professor, Center for Advanced Study, University of Illinois, Urbana, Illinois, USA.

Citation:
Chakraborty, P., G. Gioia, and S. Kieffer (2006), Volcan Reventador's unusual umbrella, Geophys. Res. Lett., 33, L05313, doi:10.1029/2005GL024915.

Contact information for authors:
Susan Kieffer, +1 217-244-6206;
Gustavo Gioia, +1 217-333-3173;
Pinaki Chakraborty, +1 217-333-3197;

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