Alcedo Volcano
Encyclopedia
Alcedo Volcano is one of the six coalescing shield volcanoes that make up Isabela Island
in the Galapagos. The remote location of the volcano has meant that even the most recent eruption in 1993 was not recorded until two years later. It is also the only volcano in the Galapagos to have erupted rhyolite
and basalt
ic lava.
The volcano has the largest number of wild tortoises of any of the volcanoes in the Galapagos, though their genetic diversity is amongst the lowest of any of the breeds in the archipelago. The habitat of the tortoises was threatened when feral goats crossed from southern Isabela Island in the 1970’s and then reproduced rapidly.
which also have shallow slopes for the majority of their height. It is also similar to these two in that the depth of the caldera is less than 25% of the height of the volcano. This contrasts with the other major shield volcanoes, Cerro Azul
, Wolf
on Isabela and Fernandina
that have much steeper slopes and have calderas that have a depth which is 40-60% of the sub-ariel height.
Alcedo, like the other volcanoes in the Galapagos, has been formed as part of the Galapagos hotspot
which is a mantle plume
resulting in a hotspot
. The volcano is not directly on the hotspot, which is believed to be under the neighbouring island of Fernandina to the west. However, it is still active with the last eruption in 1993. Due to the remoteness of Alcedo historical eruptions are not well recorded. There was an eruption between 1946 and 1960, as determined from variation in photographs. Another eruption in 1954 is now believed to have taken place on neighbouring Sierra Negra. Even the 1993 eruption was not observed at the time and only recorded after an expedition discovered new craters in 1995
It is the only volcano in the Galapagos to have erupted Rhyolite lavas. Approximately 1 km3 of Rhyolite was erupted in total, a small amount compared to its sub ariel volume. The eruption of this began about 120,000 years ago in a series of eruptions, the largest of these has resulted in a tephra
covering on the eastern slopes. Following the rhyolite eruption there has been a return to basaltic lava eruptions. It is theorised that the change in volcanic output is due to the movement of Alcedo away from the hotspot, which has resulted in the changing chemical makeup of the magma being supplied to the magma chamber
.
There is a hydrothermal system within Alcedo, the temperature of the water released is less than 97 degrees Celsius, though chemical analysis has shown that the reservoir temperature is over 260 degrees Celsius. The analysis has also shown that the residence time, the time that water remains in the hydrothermal system, is approximately 400 years and that the source of the water is from rainfall.
of Galapagos tortoise
on Alcedo, Geochelone nigra vandenburghi the genetic diversity was shown to be much lower than would be expected in a population of its size, currently estimated to be approximately 4000. The Alcedo tortoise has the largest wild populations in the Galapagos Islands, however, the genetic diversity was 3-5 times less than that of other sub species of Galapagos tortoise. This low diversity is believed to be caused by a massive reduction in the population approximately 100,000 years ago. The reduction in population to only a few individuals is believed to have been caused by a large eruption of Alcedo volcano. The Alcedo tortoise has managed to rebuild its population since that time. Since the islands discovery by man it has come under new pressure, as have other Galapagos tortoises. They were hunted by early sailors to the islands as tortoises provided a food source on subsequent voyages. The introduction of non native species has also impacted the habitat of the tortoise and other fauna and flora on Alcedo.
For many years it was believed that feral goats would not be able to cross the Perry Isthmus, a 12 km wide lava field, from southern Isabela to northern Isabela Island. However, goats were recorded as having crossed this in the early 1970’s and by 1998 it was estimated that the population was between 75,000 and 125,000. The goats had an impact on the environment that was detrimental to the native tortoises, eating plants that were the food source for them. The removal of the plants also meant that watering holes dried up. To combat this Project Isabela was undertaken to eradicate the goats from northern Isabela island by the National Park and the Darwin Foundation. The project was completed in 2006 with all goats on northern Isabela Island being removed, following the project there has been a rapid recovery of small trees and shrubs that are associated with the native habitat.
Isabela Island
Isabela Island may refer to:*Isabela Island *Isabela Island...
in the Galapagos. The remote location of the volcano has meant that even the most recent eruption in 1993 was not recorded until two years later. It is also the only volcano in the Galapagos to have erupted rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
and basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic lava.
The volcano has the largest number of wild tortoises of any of the volcanoes in the Galapagos, though their genetic diversity is amongst the lowest of any of the breeds in the archipelago. The habitat of the tortoises was threatened when feral goats crossed from southern Isabela Island in the 1970’s and then reproduced rapidly.
Geology
It is believed that Alcedo volcano emerged from the sea approximately 313,000, based on its sub ariel volume of 234 km3 and average eruption rate of 1x106 m3 per year. This has produced a volcano that is 1130m high with a caldera that measures 6.1 x 7.4 km. Alcedo volcano is similar to its neighbours to the north, Darwin and to the south, Sierra NegraSierra Negra (Galápagos)
Sierra Negra 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...
which also have shallow slopes for the majority of their height. It is also similar to these two in that the depth of the caldera is less than 25% of the height of the volcano. This contrasts with the other major shield volcanoes, Cerro Azul
Cerro Azul (Ecuador volcano)
Cerro Azul is a shield volcano on the south western part of Isabela Island in the Galápagos Islands, the name translates from Spanish as blue mountain...
, Wolf
Volcán Wolf
Volcán Wolf is the highest peak in the Galapagos Islands and is situated on Isabela Island and reaches and is the highest point in the Galapagos Islands...
on Isabela and Fernandina
Fernandina
Fernandina may refer to:*Fernandina Beach, Florida*Fernandina Island, Galapagos Islands*Fernandina , a citrus fruit...
that have much steeper slopes and have calderas that have a depth which is 40-60% of the sub-ariel height.
Alcedo, like the other volcanoes in the Galapagos, has been formed as part of the Galapagos hotspot
Galápagos hotspot
The Galápagos hotspot is a volcanic hotspot in the East Pacific Ocean responsible for the creation of the Galapagos Islands as well as three major aseismic ridge systems, Carnegie, Cocos and Malpelso which are on two tectonic plates. The hotspot is located near the Equator on the Nazca Plate not...
which is a mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
resulting in a hotspot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
. The volcano is not directly on the hotspot, which is believed to be under the neighbouring island of Fernandina to the west. However, it is still active with the last eruption in 1993. Due to the remoteness of Alcedo historical eruptions are not well recorded. There was an eruption between 1946 and 1960, as determined from variation in photographs. Another eruption in 1954 is now believed to have taken place on neighbouring Sierra Negra. Even the 1993 eruption was not observed at the time and only recorded after an expedition discovered new craters in 1995
It is the only volcano in the Galapagos to have erupted Rhyolite lavas. Approximately 1 km3 of Rhyolite was erupted in total, a small amount compared to its sub ariel volume. The eruption of this began about 120,000 years ago in a series of eruptions, the largest of these has resulted in a tephra
Tephra
200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....
covering on the eastern slopes. Following the rhyolite eruption there has been a return to basaltic lava eruptions. It is theorised that the change in volcanic output is due to the movement of Alcedo away from the hotspot, which has resulted in the changing chemical makeup of the magma being supplied to the magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
.
There is a hydrothermal system within Alcedo, the temperature of the water released is less than 97 degrees Celsius, though chemical analysis has shown that the reservoir temperature is over 260 degrees Celsius. The analysis has also shown that the residence time, the time that water remains in the hydrothermal system, is approximately 400 years and that the source of the water is from rainfall.
Wildlife
In a 2003 study of the subspeciesSubspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
of Galapagos tortoise
Galápagos tortoise
The Galápagos tortoise or Galápagos giant tortoise is the largest living species of tortoise, reaching weights of over and lengths of over . With life spans in the wild of over 100 years, it is one of the longest-lived vertebrates...
on Alcedo, Geochelone nigra vandenburghi the genetic diversity was shown to be much lower than would be expected in a population of its size, currently estimated to be approximately 4000. The Alcedo tortoise has the largest wild populations in the Galapagos Islands, however, the genetic diversity was 3-5 times less than that of other sub species of Galapagos tortoise. This low diversity is believed to be caused by a massive reduction in the population approximately 100,000 years ago. The reduction in population to only a few individuals is believed to have been caused by a large eruption of Alcedo volcano. The Alcedo tortoise has managed to rebuild its population since that time. Since the islands discovery by man it has come under new pressure, as have other Galapagos tortoises. They were hunted by early sailors to the islands as tortoises provided a food source on subsequent voyages. The introduction of non native species has also impacted the habitat of the tortoise and other fauna and flora on Alcedo.
For many years it was believed that feral goats would not be able to cross the Perry Isthmus, a 12 km wide lava field, from southern Isabela to northern Isabela Island. However, goats were recorded as having crossed this in the early 1970’s and by 1998 it was estimated that the population was between 75,000 and 125,000. The goats had an impact on the environment that was detrimental to the native tortoises, eating plants that were the food source for them. The removal of the plants also meant that watering holes dried up. To combat this Project Isabela was undertaken to eradicate the goats from northern Isabela island by the National Park and the Darwin Foundation. The project was completed in 2006 with all goats on northern Isabela Island being removed, following the project there has been a rapid recovery of small trees and shrubs that are associated with the native habitat.