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Settling-driven gravitational instabilities associated with volcanic clouds: new insights from experimental investigations
Language
English
Obiettivo Specifico
5V. Dinamica dei processi eruttivi e post-eruttivi
Status
Published
JCR Journal
JCR Journal
Title of the book
Issue/vol(year)
/79 (2017)
Pages (printed)
39
Issued date
April 22, 2017
Abstract
Downward propagating instabilities are often observed
at the bottom of volcanic plumes and clouds. These
instabilities generate fingers that enhance the sedimentation of
fine ash. Despite their potential influence on tephra dispersal
and deposition, their dynamics is not entirely understood,
undermining the accuracy of volcanic ash transport and dispersal
models. Here, we present new laboratory experiments
that investigate the effects of particle size, composition and
concentration on finger generation and dynamics. The experimental
set-up consists of a Plexiglas tank equipped with a
removable plastic sheet that separates two different layers.
The lower layer is a solution of water and sugar, initially
denser than the upper layer, which consists of water and particles.
Particles in the experiments include glass beads as well
as andesitic, rhyolitic and basaltic volcanic ash. During the
experiments, we removed the horizontal plastic sheet separating
the two fluids. Particles were illuminated with a laser and
filmed with a HD camera; particle image velocimetry (PIV) is
used to analyse finger dynamics. Results show that both the
number and the downward advance speed of fingers increase
with particle concentration in the upper layer, while finger
speed increases with particle size but is independent of particle
composition. An increase in particle concentration and turbulence
is estimated to take place inside the fingers, which could
promote aggregation in subaerial fallout events. Finally, finger
number, finger speed and particle concentration were observed
to decrease with time after the formation of fingers.
A similar pattern could occur in volcanic clouds when the
mass supply from the eruptive vent is reduced. Observed evolution
of the experiments through time also indicates that there
must be a threshold of fine ash concentration and mass eruption
rate below which fingers do not form; this is also confirmed
by field observations.
at the bottom of volcanic plumes and clouds. These
instabilities generate fingers that enhance the sedimentation of
fine ash. Despite their potential influence on tephra dispersal
and deposition, their dynamics is not entirely understood,
undermining the accuracy of volcanic ash transport and dispersal
models. Here, we present new laboratory experiments
that investigate the effects of particle size, composition and
concentration on finger generation and dynamics. The experimental
set-up consists of a Plexiglas tank equipped with a
removable plastic sheet that separates two different layers.
The lower layer is a solution of water and sugar, initially
denser than the upper layer, which consists of water and particles.
Particles in the experiments include glass beads as well
as andesitic, rhyolitic and basaltic volcanic ash. During the
experiments, we removed the horizontal plastic sheet separating
the two fluids. Particles were illuminated with a laser and
filmed with a HD camera; particle image velocimetry (PIV) is
used to analyse finger dynamics. Results show that both the
number and the downward advance speed of fingers increase
with particle concentration in the upper layer, while finger
speed increases with particle size but is independent of particle
composition. An increase in particle concentration and turbulence
is estimated to take place inside the fingers, which could
promote aggregation in subaerial fallout events. Finally, finger
number, finger speed and particle concentration were observed
to decrease with time after the formation of fingers.
A similar pattern could occur in volcanic clouds when the
mass supply from the eruptive vent is reduced. Observed evolution
of the experiments through time also indicates that there
must be a threshold of fine ash concentration and mass eruption
rate below which fingers do not form; this is also confirmed
by field observations.
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article
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