Geotimes
Highlights
Volcanoes
Tobias Fischer


Volcanic activity in 2002 began relatively slowly but built up to several major eruptions toward the end of the year. In February, new lava flows poured out of Colima Volcano, Mexico; a lava dome in the crater generated several pyroclastic flows and led to the preemptive evacuation of towns nearby. Airline pilots flying over Papua New Guinea in August reported an ash plume rising rapidly from Pago, eventually reaching a height of nine kilometers and forcing the evacuation of 8,000 people. The eruption continued through January 2003, with dacitic lava flows issuing from a chain of craters. On Nov. 11, Java's Papandaya Volcano experienced a dramatic phreatic eruption that produced a landslide and lahars one hour later. The eruption progressed into a magmatic phase, culminating in 17 explosive events on Nov. 14 and 15.

Probably the most spectacular eruption of 2002 occurred in October on a flank of Mount Etna in Italy, with lava flows two kilometers long and explosive events producing ash plumes rising five kilometers. Subplinian explosions abruptly changed to the typical strombolian style on Nov. 12.

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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Fischer teaches volcanology in the Department of Earth and Planetary Sciences at the University of New Mexico. E-mail: fischer@unm.edu.

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