Report on Sierra Negra (Ecuador) — 25 July-31 July 2018
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 25 July-31 July 2018
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2018. Report on Sierra Negra (Ecuador). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 25 July-31 July 2018. Smithsonian Institution and US Geological Survey.
0.83°S, 91.17°W; summit elev. 1124 m
All times are local (unless otherwise noted)
IG reported that the eruption at Sierra Negra continued during 25-31 July. Sulfur dioxide flux was as high as about 1,400 tons per day (on 28 July), and daily counts of volcano-tectonic and long-period events were 24-65 and 3-32, respectively. Nightly incandescence from advancing lava flows was visible. Gas plumes rose more than 1.8 km above the vents and drifted N, NW, and W.
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.