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Volcano interactions
The way volcanoes affect and are influenced by their surroundings has remained
a hot topic in volcanology research. The Pollena eruption of Somma-Vesuvius
(Campania, Italy) in A.D. 472 was only an intermediate-scale volcanic event,
but its impact on environmental and human history was greatly amplified by secondary
catastrophic sedimentary processes. Volcanologists and archaeologists working
with Guiseppe Mastrolorenzo of the Vesuvian Volcano Observatory argue that the
hydromagmatic character of the eruption created extensive debris flows that
spread across the surrounding alluvial plains. Although the fall of the Roman
Empire in A.D. 476 was caused by considerable population decline during the
barbarian incursions, the destructive nature of the eruption most likely discouraged
significant resettlement of the territory for centuries (Journal of Volcanology
and Geothermal Research, v. 113, p. 19-36).
Large-scale Plinian eruptions affect Earth's atmosphere by releasing tremendous
amounts of ash and gas. Volcanic-ash clouds such as those released by Pinatubo
in 1991 are also a hazard for aircraft. New grain-size analysis of Pinatubo
ash reveals that this ash cloud deposited predominantly homogeneous fine material
in the micrometer-to-millimeter range, regardless of the distance from the source.
Sébastian Dartville, McGill University, and co-workers argue that such
a fine ash cloud is expected to entrain large quantities of water as it moves
through the troposphere. The water vapor will condense and freeze onto the fine
ash, masking its infrared spectral signature - cause for concern for aircraft
that rely on automated infrared methods to detect ash in the atmosphere (Geology,
v. 30, p. 663-666).
Comparatively gentle, effusive outpourings of basalt can also have a severe
impact, particularly on the scale of the Siberian Flood Basalts that occurred
250 million years ago, coincident with the largest known mass extinction event,
the Permo-Triassic (P/T) crisis. This eruption certainly released large quantities
of gases; it has been argued that an event such as P/T could trigger climatic
disruption and the destabilization of ecosystems, leading to documented mass
extinction. This case is strengthened by new argon-40/argon-39 dates and geochemical
data from basalts of the West Siberian Basin. Marc Reichow, University of Leicester,
and colleagues report that these basalts were erupted synchronous with the Siberian
Flood Basalts and, hence, double the confirmed area of the volcanic province
(Science, v. 296, p. 1846-1849).
Determination of gas emissions from volcanoes relies heavily on ground-based,
remote-sensing measurements of sulfur dioxide in volcanic plumes. Such measurements
are now easier because of new technological advances in ultraviolet spectroscopy.
A miniaturized spectrometer, weighing less than a pound and powered by a laptop,
has been tested successfully at active volcanoes for the first time. Bo Galle,
University of Cambridge, and colleagues show that this instrument performs as
well as the much larger instrument used for the past 30 years and has immense
potential for geochemical surveillance of volcanoes and estimates of global
volcanic gas emissions (Journal of Volcanology and Geothermal Research,
v. 119, p. 241-254).
Volcano triggers
Volcanic activity not only changes the environment but may be triggered by environmental
changes. On the rift zones through Iceland, for example, a maximum in volcanic
production coincided with rapid crustal rebound during and after glacier melting
at the Pleistocene/Holocene boundary. Gudmundur Sigvaldson, University of Iceland,
documents the high volcanic production with new tephrachronological dates and
volume estimates. He argues that a Plinian eruption that occurred 10,000 years
ago and is possibly related to the formation of the Askja Caldera was caused
by pressure release associated with rapid glacier melting. Over-pressure decrease
resulted in volatile supersaturation in the rhyolitic magma, leading to explosive
eruption (Bulletin of Volcanology, v. 64, p. 192-205).
In the tropics, where 45 percent of the world's 1,500 active or potentially
active volcanoes are located, intense rainfall onto unstable lava domes can
trigger their collapse, resulting in pyroclastic flows. Adrian Matthews, University
of East Anglia, and colleagues observed that dome collapse on Montserrat in
July 2001 followed unusually heavy rains that probably percolated into the dome
rock, vaporizing into high-pressure steam and destabilizing the dome. Recent
rainfall records and current weather forecasts could be combined with other
monitoring techniques to produce early warning signals for hazardous eruptions
of tropical volcanoes (Geophysical Research Letters, v. 29, p. 22-1 -
22-4).
Arenal in Costa Rica is one arc volcano that has seen continuous multi-decade
activity, producing monotonous basaltic andesite magma since 1968. Detailed
analyses of minerals, however, reveal complex chemical zoning patterns that
suggest multiple magma replenishment events of an evolving magma chamber. Martin
Streck, Portland State University, and co-workers show that while basalt injection
events do not trigger explosive eruptions, they are an essential component of
the mass and energy balance of this very active volcanic system (Bulletin
of Volcanology, v. 64, p. 316-327).
Pressurization of a lava dome as a result of magmatic gas accumulation is another
recognized eruption trigger. At Merapi Volcano in Indonesia, new analyses of
very long-period seismic signals are interpreted to directly result from gas
accumulation in the conduits below the lava dome. Dannie Hidayat, Pennsylvania
State University, and his research group suggest that these seismic signals
provide a quantitative method to evaluate gas pressurization of lava domes that
can eventually trigger explosive events (Geophysical Research Letters,
v. 29, p. 33-1 - 33-4).
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