68 research outputs found
Controls on andesitic glaciovolcanism at ice-capped volcanoes from field and experimental studies
Glaciovolcanic deposits at Tongariro and Ruapehu volcanoes, New Zealand, represent diverse styles of interaction between wet-based glaciers and andesitic lava. There are ice-confined lavas, and also hydroclastic breccia and subaqueous pyroclastic deposits that formed during effusive and explosive eruptions into meltwater beneath the glacier; they are rare among globally reported products of andesitic glaciovolcanism. The apparent lack of hydrovolcanically fragmented andesite at ice-capped volcanoes has been attributed to a lack of meltwater at the interaction sites because either the thermal characteristics of andesite limit meltwater production or meltwater drains out through leaky glaciers and down steep volcano slopes. We used published field evidence and novel, dynamic andesite-ice experiments to show that, in some cases, meltwater accumulates under glaciers on andesitic volcanoes and that meltwater production rates increase as andesite pushes against an ice wall. We concur with models for eruptions beneath ice sheets showing that the glacial conditions and pre-eruption edifice morphology are more important controls on the style of glaciovolcanism and its products than magma composition and the thermal properties of magmas. Glaciovolcanic products can be useful proxies for paleoenvironment, and the range of andesitic products and the hydrological environments in which andesite erupts are greater than hitherto appreciated
Conduit flow experiments help constraining the regime of explosive eruptions
It is currently impractical to measure what happens in a volcano during an explosive
eruption, and up to now much of our knowledge depends on theoretical models. Here we
show, by means of largeâscale experiments, that the regime of explosive events can be
constrained on the basis of the characteristics of magma at the point of fragmentation
and conduit geometry. Our model, whose results are consistent with the literature, is a
simple tool for defining the conditions at conduit exit that control the most hazardous
volcanic regimes. Besides the wellâknown convective plume regime, which generates
pyroclastic fallout, and the vertically collapsing column regime, which leads to pyroclastic
flows, we introduce an additional regime of radially expanding columns, which form when
the eruptive gasâparticle mixture exits from the vent at overpressure with respect to
atmosphere. As a consequence of the radial expansion, a dilute collapse occurs, which
favors the formation of density currents resembling natural base surges. We conclude that
a quantitative knowledge of magma fragmentation, i.e., particle size, fragmentation
energy, and fragmentation speed, is critical for determining the eruption regime.Research was partially funded by DPC-INGV
agreement 07â09 and MUR PRIN 06.PublishedB042043.6. Fisica del vulcanismoJCR Journalrestricte
Electrostatic field variations related to the big Sumatra earthquake
Electrical effects in correlation with earthquakes have been reported by many authors
and different theories are discussed about the origin of these seismo-electrical effects.
The actually most popular models consider piezoelectric effects, electro-kinetic effects,
surface charge on crack wall, and rock/magma fragmentation as probable mechanism
for the generation of electromagnetic emissions. Recently also laboratory experiments
have been performed to study the mechanisms of rock fracturing, frictional
sliding, and stick-slip phenomena. In this context our group has developed a method
for monitoring of instable mountain flanks, which is presently tested at several sites.
Here we report on extraordinary electrical signals, recorded by a station in Italy, that
clearly corresponds to the Mw=9.3 earthquake of December 26, 2004, which occurred
at 00:58:50.7 (UTC) âoff the west coast of northern Sumatra, Indonesiaâ at 3.50 N,
95.72 E. Electrical monitoring with this method can be an additional tool for the
global detection of very strong earthquakes. As this signals travel at the speed of light,
the alert window will be significantly increased
Conduit flow experiments help constraining the regime of explosive eruptions
It is currently impractical to measure what happens in a volcano during an explosive
eruption, and up to now much of our knowledge depends on theoretical models. Here we
show, by means of largeâscale experiments, that the regime of explosive events can be
constrained on the basis of the characteristics of magma at the point of fragmentation
and conduit geometry. Our model, whose results are consistent with the literature, is a
simple tool for defining the conditions at conduit exit that control the most hazardous
volcanic regimes. Besides the wellâknown convective plume regime, which generates
pyroclastic fallout, and the vertically collapsing column regime, which leads to pyroclastic
flows, we introduce an additional regime of radially expanding columns, which form when
the eruptive gasâparticle mixture exits from the vent at overpressure with respect to
atmosphere. As a consequence of the radial expansion, a dilute collapse occurs, which
favors the formation of density currents resembling natural base surges. We conclude that
a quantitative knowledge of magma fragmentation, i.e., particle size, fragmentation
energy, and fragmentation speed, is critical for determining the eruption regime
Conduit flow experiments help constraining the regime of explosive eruptions
It is currently impractical to measure what happens in a volcano during an explosive eruption,
and up to now much of our knowledge depends on theoretical models. Here we show, by means of
large-scale experiments, that the regime of explosive events can be constrained based on the
characteristics of magma at the point of fragmentation and conduit geometry. Our model, whose
results are consistent with the literature, is a simple tool for defining the conditions at conduit exit
that control the most hazardous volcanic regimes. Besides the well-known convective plume
regime, which generates pyroclastic fallout, and the vertically collapsing column regime, which
leads to pyroclastic flows, we introduce an additional regime of radially expanding columns, which
form when the eruptive gas-particle mixture exits from the vent at overpressure with respect to
atmosphere. As a consequence of the radial expansion, a dilute collapse occurs, which favours the
formation of density currents resembling natural base surges. We conclude that a quantitative
knowledge of magma fragmentation, i.e. particle size, fragmentation energy and fragmentation
speed, is critical for determining the eruption regime
Ash from the Eyjafjallajokull eruption (Iceland): Fragmentation processes and aerodynamic behavior
The fragmentation process and aerodynamic behavior of ash from the Eyjafjallajökull
eruption of 2010 are investigated by combining grain-size, Scanning Electron Microscopy (SEM), and quantitative particle morphology. Ash samples were collected on land in
Iceland at 3â55 km distance from the volcanic vent, and represent various phases of the pulsating eruption. The grain size is fine even for deposits close to the vent, suggesting that the parent particle population at fragmentation consisted of a substantial amount of fine
ash. SEM investigation reveals that ash produced during the first phase of the eruption consists of juvenile glass particles showing key features of magma-water interaction,
suggesting that phreatomagmatism played a major role in the fragmentation of a
vesicle-poor magma. In the last phase of the eruption, fragmentation was purely magmatic and resulted from stress-induced reaction of a microvesicular, fragile melt. The shape of ash, as determined by quantitative morphology analysis, is highly irregular, rendering
the settling velocity quite low. This makes transportation by wind much easier than for other more regularly shaped particles of sedimentary origin. We conclude that the
combination of magmaâs fine brittle fragmentation and irregular particle shape was the main factor in the extensive atmospheric circulation of ash from the mildly energetic
Eyjafjallajökull eruption
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