Based on satellite data and wind model data, the Washington VAAC reported that on 11 July gas-and-ash plumes from Sierra Negra rose 3.7 km (12,000 ft) a.s.l. and drifted almost 85 km W. Gas plumes on 13 July drifted N, NW, and SW. On 16 July gas plumes with minor amounts of ash rose 2.4 km (8,000 ft) a.s.l. and drifted SW. Incandescence was visible in webcam images.

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.

Source: Washington Volcanic Ash Advisory Center (VAAC)