Sierra Negra (Galápagos)

Sierra Negra
Eruption of Sierra Negra, 2005-10-25
Highest point
Elevation	1,124 m (3,688 ft)
Coordinates	0°50′S 91°10′W / 0.83°S 91.17°W / -0.83; -91.17
Geography
Location	Isabela Island, Galápagos Islands, Ecuador
Geology
Mountain type	Shield volcano
Last eruption	October 2005

Sierra Negra (Spanish: Black Mountain) is a large shield volcano at the South eastern end of Isabela Island in the Galapagos that rises to an altitude of 1124m. It coalesces with the volcanoes Cerro Azul to the west and Alcedo to the north. It is one of the most active of the Galapagos volcanoes with the most recent historic eruption in October 2005.

Guided tours of the volcano typically start at Puerto Villamil and traverse the rim of the caldera along its East side before heading into the fresh lava fields north east of the main crater.

The Sierra Negra like the other volcanoes on Isabela is believed to have been created from a mantle plume which has created the hotspot. The age of Sierra Negra and the other volcanoes on Isabela is hard to determine as they are in a North-South line to the east of the hotspot, which is believed to be under Fernandina volcano, and the Nazca plate is moving east. This puts the volcanoes perpendicular to the hotspot. The surface of Sierra Negra and its neighbouring volcanoes are also covered by young lavas, adding to the difficulty of aging them. An estimate based on volume (588 km³) and eruption rates suggest that Sierra Negra is approximately 535,000 years old.

The morphology of Sierra Negra is the upturned soup bowl shape of the other Isabela volcanoes, however it does not have the steep sloping sides that are on others. Instead the slope goes from approximately 2 degrees at its base and although increasing averages only 5 degrees. The volcano has the largest caldera of all of the Galapagos volcanoes, with dimensions of 7.2 x 9.3 km, with the long axis being south west to north east. The caldera is also the shallowest of the Isabela volcanoes at only 100m. The caldera is structurally complex with a 14 km long ridge within it. A large fumarolic area, Volcan de Azufre, lies between this ridge and the western caldera wall. This fumarolic area is one of the locations where terrestrial sulfur flows have been identified, this is associated with the melting of sulfur deposits.