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"The usual volcanic plume consists of
a stalk capped with an umbrella and resembles the mushroom of an
atom bomb blast," said geology professor Susan Kieffer, "but the
umbrella on this plume was wavy, like the shell of a scallop." In
a paper for the March 15 journal Geophysical Research Letters,
Kieffer, theoretical and applied 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.
"We had never seen a scalloped umbrella before," said Kieffer,
who holds a Charles R. Walgreen Jr. Chair at Illinois. "Unusual
conditions must have existed in the volcanic plume that formed this
umbrella."
Located about 100 kilometers from Quito, Ecuador, Volcan
Reventador -- Spanish for "one that explodes" -- lived up to its
name on the morning of Nov. 3, 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
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 a fairly cool ash from the
destruction of the summit cone. 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.
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 "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. 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 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," said
Kieffer, who is also a professor in the university's
Center for
Advanced Study, one of the highest forms of campus recognition.
Originating far from the summit cone, these new ash flows must
have helped spread the damage caused by the eruption. They must have
been uncommonly energetic, because the ash fell from the umbrella,
which was 10 kilometers high.
"For all we know, these flows were responsible for broken
petroleum pipelines," Chakraborty said. "The flows might also have
contributed to the early phases of a shutdown of Quito airport that
lasted more than a week."
[James E. Kloeppel, physical sciences editor,
University of Illinois at Urbana-Champaign news bureau] |
