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Report on Sierra Negra (Ecuador) — 19 October-25 October 2005

Sierra Negra

Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
19 October-25 October 2005
Managing Editor: Gari Mayberry

Please cite this report as:

Global Volcanism Program, 2005. Report on Sierra Negra (Ecuador). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 19 October-25 October 2005. Smithsonian Institution and US Geological Survey.

Weekly Report (19 October-25 October 2005)

Sierra Negra


0.83°S, 91.17°W; summit elev. 1124 m

All times are local (unless otherwise noted)

A large eruption began at Sierra Negra on 22 October. According to the Washington VAAC, the eruption began around 1645, sending an ash cloud to ~12.8 km (42,000 ft) a.s.l. that drifted SW. A large hotspot was visible on satellite imagery. On 23 October around 0215 the cloud appeared to be at ~6.1 km (20,000 ft) a.s.l. on satellite imagery. Another eruption occurred on 23 October around 0700, and it's cloud also rose to ~12.8 km (42,000 ft) a.s.l. according to a pilot's report. By 1215, the cloud height was estimated to be at ~7.6 km (25,000 ft) a.s.l. based on satellite imagery. Although ash was no longer visible on satellite imagery on 24 October at 1215, a hotspot was seen. 25 October satellite imagery through 1315 showed the ash plume extending 220 km SW of the summit. A news article reported that three lava flows were emitted during the eruption.

Geological Summary. The broad shield volcano of Sierra Negra at the southern end of Isabela Island contains a shallow 7 x 10.5 km caldera that is the largest in the Galápagos Islands. Flank vents abound, including cinder cones and spatter cones concentrated along an ENE-trending rift system and tuff cones along the coast and forming offshore islands. The 1124-m-high volcano is elongated in a NE direction. Although it is the largest of the five major Isabela volcanoes, it has the flattest slopes, averaging less than 5 degrees and diminishing to 2 degrees near the coast. A sinuous 14-km-long, N-S-trending ridge occupies the west part of the caldera floor, which lies only about 100 m below its rim. Volcán de Azufre, the largest fumarolic area in the Galápagos Islands, lies within a graben between this ridge and the west caldera wall. Lava flows from a major eruption in 1979 extend all the way to the north coast from circumferential fissure vents on the upper northern flank. Sierra Negra, along with Cerro Azul and Volcán Wolf, is one of the most active of Isabela Island volcanoes.

Sources: Washington Volcanic Ash Advisory Center (VAAC), Associated Press