Report on Sierra Negra (Ecuador) — 26 October-1 November 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 26 October-1 November 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, 26 October-1 November 2005. Smithsonian Institution and US Geological Survey.
0.83°S, 91.17°W; summit elev. 1124 m
All times are local (unless otherwise noted)
According to IG, an eruption began at Sierra Negra on the S end of Isabela Island on 22 October at 1730 when an explosion was heard by many people in the town of Villamil, 20 km SE of the volcano. The eruption was preceded by a seismic event on 22 October at 1438, and by earthquakes on 19 October and 2 weeks earlier. The Washington VAAC recorded an ash cloud on satellite imagery at 1745 at a height of ~15.2 km (50,000 ft) a.s.l. moving SW, and a very large hotspot. The ash cloud may have reached a height of 20 km (65,600 ft) a.s.l. Extensive lava fountains were seen rising to heights of 200-300 m along a segment of the Sierra Negra rim. Incandescent lava flowed several kilometers down the outer NW flank of the volcano's edifice and tourists reported seeing two lava flows descending the N flank. The exact orientation of flows on the volcano's flanks was not clear from early reports. Scientists did not see active lava flows in this area or evidence of flows entering the sea during an overflight on 23 October.
During a visit to Sierra Negra on 23 October, scientists saw that the eruption originated from four adjacent craters aligned along a 500-m-long fracture at the base of the inner wall of the volcano's caldera in the NE sector. Lava traveled from four principal vents southwards with exceptional force, volume, and speed downslope in several main channels. Based on observations, the main lava river traveled nearly 20 m/sec as it left its source vents. Two vents mainly supplied lava to the many lava rivers flowing southward over the northern caldera bench and then down onto the caldera floor. The feeding fracture apparently extended westward along the inner wall, but then climbed up onto the caldera rim itself where its trace was not obvious. However, small vents with fountaining and incandescent lava were observed on the rim along this general fracture system, implying that the active fracture extended for about 2 km W of the main vents.
By 23 October around 1530, the lava formed one large flow that was 1-1.5 km wide and had progressed ~7 km southeastward along the base of the eastern interior wall of the caldera, then westward along the southern wall reaching a point almost halfway across the caldera. The volume of lava ejected at this time was estimated at 25 million cubic meters. On 26 October, there were reports that lava was no longer emitted from one of the four principal vents. No populated areas on the island were threatened by the eruption.
Geologic Background. 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.