Sierra Negra (Galápagos)

Geology

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.

The volcano is one of the most active in the Galapagos, with the most recent eruption beginning on 26 June 2018, only ten days after a nearby Volcano, La Cumbre, also began erupting. A group of scientists predicted this eruption through supercomputer models five months before the eruption actually occurred. The previous eruption began on 22 October 2005 and ended on 30 October 2005. That eruption was estimated to have produced 1.5x108 m3 of lava.

Other eruptions in the historical record are 1911, 1948, 1953, 1954, 1957, 1963, 1979 and 2005. Eruptions in earlier years are recorded by dating lava flows but their location on the volcano and the dates of the eruption are not known precisely.

Ecology

The majority of the 2200 population on Isabela live in the town of Puerto Villamil on the southern shore of the volcano. Consequently, the eastern side of the volcano is partly used for agriculture, with fishing the initial activities of the island's population. Those involved in farming were one of the sources of non native species including cattle and goats, some of which escaped and became feral, other sources were sailors who released goats to provide a source of fresh meat on future trips. Eradication efforts of feral goats were undertaken in Northern Isabela island, north of the Perry isthmus between Sierra Negra and Alcedo volcanoes but not in the southern portion of the island.

While the southern and eastern side of the volcano is covered with lush tropical forest vegetation and fertile soil at moderate altitudes, the higher reaches are more barren with bushes and grasses of modest height of 2–3 meters. The northern side of the volcano is almost entirely devoid of vegetation (with the exception of some cacti) as it has been resurfaced in more recent times by lava flows.

The habitat for the Sierra Negra giant tortoise, Chelonoidis nigra guentheri, which has a saddleback shell, is on the southern and south eastern slopes of the volcano. This species is endangered due to population pressure brought on by hunting, habitat alteration, and predation by feral mammals. C. n. guentheri is part of an ecosystem restoration program in order to recover the population of the tortoises in their natural habitat.

References

References

1. (1996). "Slopes of western Galapagos volcanoes from airborne interferometric radar". American Geophysical Union.
2. (May 2002). "Petrology and Geochemistry of the Volcán Cerro Azul: Petrologic Diversity among the Western Galápagos Volcanoes". Oxford University Press.
3. "Sierra Negra".
4. This fumarolic area is one of the locations where terrestrial sulfur flows have been identified, this is associated with the melting of sulfur deposits.[https://web.archive.org/web/20100514034320/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19820025583_1982025583.pdf NASA A Primer on Sulfur]
5. "Great timing, supercomputer upgrade lead to successful forecast of volcanic eruption".
6. (2008). "The 2005 eruption of Sierra Negra volcano, Galápagos, Ecuador". Springer-Verlag.
7. (October 26, 2000). "Widespread uplift and 'trapdoor' faulting on Galápagos volcanoes observed with radar interferometry". Nature Publishing Group.
8. [http://www.darwinfoundation.org/files/species/pdf/goats-en.pdf Darwin Foundation]
9. (1989). "The Conservation Biology of Tortoises". International Union for Conservation of Nature and Natural Resources.
10. (1996). "Tortoise & Freshwater Turtle Specialist Group. Chelonoidis nigra spp. guentheri". International Union for Conservation of Nature and Natural Resources.
11. "Ecosystem Restoration: Giant Tortoise Recovery". Galapagos Conservancy.
12. (April 10, 2013). "The Galapagos National Park Service will repatriate 150 juvenile tortoises from the Isabela Island Breeding Center". Galapagos Conservancy.