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Volcanoes
Michael Ramsey

Last year was a time of moderate volcanic activity punctuated by several larger eruptions, including first time events at volcanoes with no previously recorded historical eruptions. Toward the end of 2002, focus was on the eruptions of Mount Etna in Italy, Pago Volcano in Papua New Guinea and Reventador Volcano in Ecuador. Activity at Reventador was the first in 26 years and produced an eruption, which was possibly the largest ever seen in Ecuador. More recently active volcanoes, such as Kilauea in Hawaii, Soufrière Hills on Montserrat, Stromboli in Italy and Piton de la Fournaise in the Indian Ocean all continued their moderate levels of eruptive activity throughout most of 2003.

Two larger eruptions captured scientific attention by the middle of 2003. On April 18, Chikurachi Volcano on Paramushir Island (part of the Kurile volcanic chain, Russia) erupted. The activity sent large ash plumes into the western Pacific air routes and continued until July. Three weeks later on May 10, the first historical eruption of Anatahan Volcano in the Mariana Islands occurred. Scientists deploying seismographs for the Subduction Factory Imaging Project of the Margins Program were in the region and observed the eruption. Satellite images over the next weeks revealed plumes rising as high as 13 kilometers, resulting in closure the Guam International Airport for two days later in May.

Eruptions at both of these remote volcanoes were imaged by numerous satellite sensors, which revealed the thermal output of their craters, the extent and drift direction of their ash clouds, and the flux of sulfur dioxide emanating from their plumes. Such measurements commonly provide the first confirmation of a remote eruption and are critical tools for the world's Volcanic Ash Advisory Centers (VAAC), which are charged with issuing warnings to aircraft.

Global observations of volcanic unrest and eruption hazards are also key programs for many of the satellite instruments orbiting Earth. These observations vary depending on the wavelength region observed and the availability and timing of the data received. One such program is the MODIS Thermal Alert website (http://modis.higp.hawaii.edu/). Described by Robert Wright and colleagues at the University of Hawaii in a recent Remote Sensing of the Environment (v. 82, p. 135- 155), it is the first truly global high-temperature thermal monitoring system for volcanic activity.

Commonly, these satellite observations are the only means to positively confirm an eruption has occurred and to monitor the extent of the subsequent hazards. Two noteworthy examples of such observations were reported in 2003.

Data collected by the MODIS thermal alert program has been archived since early 2000. Dave Rothery and Diego Coppola recently reexamined the data for remote volcanoes in Melanasia and the Indian Ocean (Bulletin of the Global Volcanism Network, v. 28, n. 1, p. 2-16). The program recorded thermal alerts at Big Ben Volcano on Heard Island in the southern Indian Ocean between May 13, 2000 and January 30, 2003. Although several sub-glacial eruptions were reported for this volcano in historical time, observations are rare. Based on the MODIS data, it appears likely that a lava flow was emplaced at Big Ben.

Starting in late 2001, a series of thermal alerts also occurred at another remote southern hemisphere volcano and continued through early 2003. Mount Belinda on Montagu Island in the South Sandwich Islands had no record of Holocene or historical eruptive activity until the detections by MODIS. These hot pixels were confirmed to be of volcanic origins by spectacular images acquired of Montagu with the higher spatial resolution ASTER instrument. Reported by John Smellie, Matthew Patrick and colleagues at the fall 2003 meeting of American Geophysical Union (AGU), the ASTER image shows a series of lava flows and ash falls on the glacier-covered Mount Belinda (Eos Transactions, v. 84).

Geophysical observation of volcanic activity

Observing volcanic activity from hundreds of kilometers above the surface of the Earth by way of remote sensing is one form of the broader field of remote geophysical observations. Any parameter of volcanic activity captured by a noncontact instrument falls into this field of study.

Bo Galle and coworkers unveiled a new instrument which will likely change the way sulfur dioxide flux is measured at degassing volcanoes. For three decades, the correlation spectrometer (COSPEC) has been the principal tool for remote monitoring of these fluxes. Now a new instrument has been tested at Masaya Volcano, Nicaragua, and Soufrière Hills Volcano, Montserrat. The low cost ultraviolet differential optical absorption spectrometer (mini-DOAS) is small, operated from a laptop computer, and is a potential replacement for the cumbersome COSPEC (Journal of Volcanology and Geothermal Research, v. 119, p. 241-254).

Another remote geophysical monitoring tool that is relatively new to volcanology is measurement of infrasound. Capturing the very low frequency acoustic emissions of volcanic activity enables seismicity associated with gas release to be understood and separated from that of subsurface seismicity. Jeffrey Johnson and colleagues report on the utility of infrasonic measurements at active volcanoes in Antarctica, Russia, and Ecuador in order to assess the relative explosion sizes and other activity (Journal of Volcanology and Geothermal Research, v. 121, p. 15-63).

Interferometric synthetic aperture radar (InSAR) measurements from space promises the potential of being one of the most exciting remote observational tools available to volcanologists. The principle of measuring local to regional scale ground deformation by way of multiple radar images of a volcano has been well documented for the past decade.

In 2003, several investigations used InSAR data to examine or reexamine volcanic activity. Rosana Romero and coworkers focused on Lanzarote in the Canary Islands using InSAR over an eight-year period and found little deformation to have occurred (Bulletin of Volcanology, DOI: 10.1007/s00445-002-0232-3, p. 1-7). Whereas, Zhong Lu and others have interpreted the magma supply dynamics at Westdahl Volcano, Alaska, from the 1991-1992 eruption onward. They report the volcano has been steadily reinflating following the eruptions of the early 1990s (Journal of Geophysical Research, DOI: 110.1029/2002JB002311). And Scott Rowland and coworkers have combined InSAR with multispectral satellite data to infer the volumetric characteristics of lava flows from the 1995 and 1998 eruptions of Fernandina and Cerro Azul Volcanoes in the western Galápagos Islands (Bulletin of Volcanology, DOI: 10.1007/s00445-002-0262, p. 311-330).

Remote geophysical observations continue to be a focus in 2004, with the publication of a special issue of the Journal of Volcanology and Geothermal Research and a special session at the spring AGU meeting in Montreal, Canada, both of which highlight new techniques and volcanic observations.

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Ramsey is an assistant professor in the Department of Geology and Planetary Science at the University of Pittsburgh. E-mail: ramsey@ivis.eps.pitt.edu.

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