Volcano Type: Shield volcano
Volcano Status: Historical
Last Known Eruption: 2005
Summit Elevation: 1124 m 3,688 feet
Latitude: 0.83°S 0°50’0″S
Longitude: 91.17°W 91°10’0″W
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 Sierra Negra 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.
Sierra Negra
Sierra Negra’s caldera is somewhat unusual as well. It is elliptical, with its long axis aligned northeast southwest, whereas calderas on the other western volcanos are nearly circular. At 7 by 10 km, it has easily the largest caldera in the Galapagos. At is 100 and 140 m deep, is also the shallowest caldera. The caldera is structurally complex and has clearly collapsed in several stages. Much of the caldera floor is covered by quite young lava flows.
Sierra Negra Eruption
The southern flank of Sierra Negra is the windward side and often bathed in clouds and mist. As a result, it is heavily vegetated. This area is the site of two of the oldest settlements in the Galapagos, the coastal village of Villamil and the town of Santo Tomas, located 20 km inland high on the flank of the volcano. Santo Tomas was orginally established to mine sulfur from the fumeroles in the area. Some cattle and goat ranching occurs on the southern slopes, and there are many feral cattle and goats as well.
Originally posted 2010-08-09 20:22:39.
