IG reported that the eruption at Sierra Negra continued through 23 July, and continued to vary in intensity. Four fissures had initially opened on 26 June, at the start of the eruption. Fissure 1 is 4 km long and located at the edge of the caldera in the area of Volcán Chico. Lava flows from this fissure traveled 7 km down the flanks, and 1.7 km within the interior of the caldera, covering an area of 16.1 square kilometers. Fissure 2 is about 250 m long, located NW of the caldera, and produced lava flows that traveled 3 km and covered an area of 2.3 square kilometers. Fissure 3, located W of the caldera, is 250 m long and produced lava flows that descended the flanks 2 km, covering an area of 0.3 square kilometers. Fissures 1-3 were active until 27 June. Fissure 4, 250 m long and located on the NW flank, continued to be active. Lava from this fissure reached the ocean between 9 and 10 July, and by 16 July had covered an area of 11.6 square kilometers (30.4 square kilometers was covered by lava from all four fissures).

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. Although it is the largest of the five major Isabela volcanoes, it has the flattest slopes, averaging less than 5° and diminishing to 2° 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. The Volcán de Azufre fumarolic area 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.

Source: Instituto Geofísico-Escuela Politécnica Nacional (IG-EPN)