Lascar, the most active volcano in northern Chile, erupted on 4 May 2005. Although the eruption was substantial, thus far there is an absence of reports from anyone who saw the eruption at close range. Preliminary assessments came mainly from satellite sensors and distant affects witnessed in Argentina. This report is based on one sent to us by Chilean Observatorio Volcanológico de los Andes del Sur (OVDAS) scientists José Antonio Naranjo and Hugo Moreno, discussing events around 4 May, with brief comments on some of Lascar's behavior in the past several years, and suggestions for future monitoring.

Lascar sits ~ 70 km SW of the intersection between Chile, Argentina, and Bolivia, ~ 300 km inland from the Chilean port city of Antofagasta. This part of the coast lies along the Atacama desert, and on flat terrain tens of kilometers W of Lascar resides a large salt pan, the Salar de Atacama (about 50 x 150 km). The settlement of Toconao is ~ 33 km NW of Lascar. Previous reports discussed field observations during 13 October 2002 to 15 January 2003, and fine ash discharged from fumaroles on 9 December 2003 (BGVN 28:03 and 29:01).

Naranjo and Moreno concluded that at roughly 0400 on 4 May an explosive eruption ejected an ash cloud to a tentative altitude on the order of 10 km that dispersed to the SE. About 2 hours later the cloud began dropping ash on Salta, Argentina. Satellite images portrayed the ash cloud's dispersal. An aviation 'red alert' was issued by the Buenos Aires Volcanic Ash Center; they saw the plume over Argentina at altitudes of 3-5 km.

Shortly after atmospheric impacts of the 4 May eruption became apparent, the Buenos Aires VAAC notified OVDAS that NW Argentine cities had reported falling ash. These cities, all SE of Lascar, included Jujuy, Salta, Santiago del Estero, and Santa Fe—locations with respective approximate distances from Lascar of 260, 275, 580, and 1,130 km. The Argentine province of Chaco, along the country's NE margin, was also noted as receiving ash. Buenos Aires (~ 1,530 km SE of Lascar) remained ~ 400 km beyond the point of the farthest detected ashfall.

Patricia Lobera, a professor in Talabre, Argentina, 17 km E of Lascar, said that eruption noises were not heard there on the morning of 4 May. When observers saw the plume from Talabre that morning they reportedly thought the plume looked similar to those on previous days.

Remotely sensed hot spots were detected on a GOES satellite image for 0339 (0639 UTC) on 4 May, showing the first evidence of an eruption. In a later image, at 0409, the thermal anomaly had increased, and the image suggested a growing, ash-bearing cloud then trending ~ 23 km to the SE. The thermal anomaly diminished in intensity by 0439, remaining diminished thereafter, but by that time the plume's leading margin extended over ~ 100 km SE and its tail had detached from the volcano. At 0509 the plume reached 170 km SE. According to a press report, at around 0600 ash fell in Salta (~ 275 km SE of Lascar).

Rosa Marquilla, a geologist at the University of Salta, reported that residents there noticed a mist attributed to the eruption, which hung over the city until at least to 1600, after which, the sky gradually cleared. Preliminary description of the petrography of the ash that fell in Salta came from Ricardo Pereyra (University of Salta) who saw crystal fragments (pyroxenes, feldspars, and magnetite) and fragments of volcanic glass containing plagioclase mircrolites. Lithic fragments were not observed.

The OVDAS authors concluded that, apparently since the year 2000, Lascar underwent constant degassing from an open vent within the ~ 780-m-diameter active central crater. Sporadic explosions as in July 2000 and October 2002, and in this case, 4 May 2005, could be due to diverse causes. For example, there may have been temporarily obstructed conduits at depth, local collapses blocking the vent at the crater floor, or fresh magma injection contacting groundwater. Extrusion of a viscous dome lava also might explain the sudden explosions. That circumstance would presumably lead to visibly increased fumarolic output.

Naranjo and Moreno had several suggestions for ongoing monitoring. First, they suggested developing closer long-term contacts, including people able to visually monitor the volcano directly, as well as continued systematic contact with the Buenos Aires VAAC and their satellite analysts. They recommended ongoing relations with the University of Hawaii (MODVOLC) program to remotely sense hot-spots. They went on to suggest a campaign of stereo aerial photography to detect changes in the active crater. They advocated notifying local inhabitants of the possibility of ash falls before another explosive episode. They pointed out that mountaineers should be made aware of elevated risks within 8 km of the active crater.

References. Gardeweg, M., 1989, Informe preliminar sobre la evolución de la erupción del volcán Láscar (II Región): noviembre 1989: Servicio Nacional de Geología y Minería, Informe Inédito (unpublished report), 27 p.

Gardeweg, M., and Lindsay, J., 2004, Lascar Volcano, La Pacana Caldera, and El Tatio Geothermal Field: IAVCEI General Assembly Pucón 2004, Field Trip Guide-A2, 32 p.

Gardeweg, M., Medina, E., Murillo, M., and Espinoza, A., 1993, La erupción del 19-20 de abril de 1993: VI informe sobre el comportamiento del volcán Láscar (II Región): Servicio Nacional de Geología y Minería, Informe Inédito (unpublished report), 20 p.

Geological Summary. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: José Antonio Naranjo and Hugo Moreno, Programa Riesgo Volcanico, Servicio Nacional de Geologia y Mineria, Avda. Santa Maria 0104, Casilla 1347, Santiago, Chile; Gustavo Alberto Flowers, Buenos Aires Volcanic Ash Advisory Center (Buenos Aires VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php).