Galapagos Volcanoes

The Galapagos Volcanoes

Conservation efforts and institutions that protect the Galapagos Islands

Mid-ocean islands like the Galapagos are formed from basalt, the most basic of all types of lava. Basalt has a very different chemical composition from the lavas that erupt from continental volcanoes and is much more fluid. Consequently, as the lava flows build up to produce a volcanic cone, the island cones have a much shallower slope than those on the mainland. These shallow-sloped volcanoes are called shield volcanoes and in the Galapagos, they are often compared to over-turned soup bowls. Such shield volcanoes can clearly be seen in the younger western islands of Isabela and Fernandina. To the east, the volcanoes are lower and more eroded.

Many volcanoes are topped by a caldera, a large circular depression derived from the original crater (sometimes this is subsequently filled in by new lava). During an eruption, the crater is fed from a magma chamber, but as activity dies down, the magma withdraws, leaving a large, open cavity. The ceiling periodically collapses, lowering the crater floor and widening the diameter. There was a major caldera collapse on Fernandina in June 1968, when the floor dropped 300 meters! The largest caldera in the islands is that of Volcan Sierra Negra, Isabela, which is 7 by 10 km.

Isabela is not only the largest island but is also the Island with most volcanoes in the Galapagos archipelago with 6 volcanoes.

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Two distinct types of volcanoes occur in the Galapagos. In the west, on the islands of Isabela and Fernandina, large volcanoes with an “inverted soup-bowl” morphology and deep calderas occur. In the east, smaller shield volcanos with gentler slopes occur. The difference between these two volcanic morphologies appears to be due to the difference in lithospheric thickness. The fracture zone at 91° W separates oceanic crust and lithosphere of distinctly different age. West of the fracture zone at 91°W, the lithosphere is older and thicker, and therefore able to support the load imposed on it by a large volcano. East of the fracture zone, the lithosphere is too young and weak to support large volcanic edifices.

The “inverted soup-bowl” morphology of the large western volcanoes is quite unusual (though not entirely unique) and its origin is not entirely certain. The Hawaiian volcanoes, which are the largest on Earth and much larger than the largest of the Galapagos volcanoes, are more similar to the shields of the eastern Galapagos volcanoes. According to one theory, this morphology results from the way in which eruptive vents are distributed on the volcanoes. Most of the vents occur either on circumferential fissures near the flat summits or on radial fissures on the lower flanks and aprons of the volcanos. Relatively few vents occur on the steep upper flanks of the volcanos. Thus the volcano grows outward at the bottom and upward at the top, resulting in this distinctive morphology. The location of vents and fissures primarily reflects the stresses within the volcano. Why stresses in Galapagos volcanoes should differ from those of other volcanos and lead to this distribution of vents remains unclear. An alternative hypothesis for the morphology of Galapagos volcanos is that it reflects the pattern of intrusion of magma within the volcano. In essence, magma intruded into the volcano inflates the central part, pushing the summit region upward and steepening the slopes on the upper flanks.

Another unusual characteristic of the western Galapagos volcanoes is the large size of their calderas, particularly in comparison to the size of the volcano. The presence of a caldera is responsible for the volcano’s flat top; this flat top is well illustrated by Alcedo. Calderas form as a result of a collapse of an underlying magma chamber. Magma within a magma chamber contributes to the support of the overlying edifice; when magma is withdrawn, the surrounding rock may not be able to bear the load and collapse results. Almost certainly, none of the calderas formed in a single event; instead they are the result of numerous episodes of collapse, as is evidenced by the uneven floors of some and benches and the walls of others. A partial collapse of the caldera on Fernandina occurred in 1968 when the northern part of the caldera floor dropped 200 meters. The collapse occurred several weeks after a brief eruption. It was observed from a distance and scientists arrived shortly after the event, so that this is one of the best-documented examples of a caldera collapse. Once formed, caldera may broaden as parts of caldera wall collapse. This occurred on Fernandina in 1988. Calderas may also occasionally fill entirely with lava, then reform. Marchena, in the northeast, has a caldera that has been very nearly filled with lava. The floor of Genovesa’s caldera is below sea level and broke on the south side, forming Darwin Bay.

Historic eruptions have occurred on many of the Galapagos volcanoes, including Fernandina, Volcan Wolf, Alcedo, Sierra Negra, Cerro Azul, Santiago, Pinta, Floreana, and Marchena. Eruptions in the recent geologic past (the last ten thousand years or so) have also occurred on Volcan Darwin, Volcan Ecuador, Genovesa, San Cristobal, and Santa Cruz. A number of submarine volcanoes many have also been active at this time. It is quite unusual for a mantle plume to produce so many simultaneous active volcanoes. In Hawaii, for example, only 6 volcanos (including the seamount Loihi) have erupted in this same time, and most of the activity in Hawaiian is located on just 3 volcanos). In Reunion, only a single volcano has been active. It should be noted, however, that the magma output of Mauna Loa, the largest of the Hawaiian volcanos, has probably exceeded the output of all the Galapagos volcanos combined.

The islands of Espanola and Santa Fe are remnants of extinct volcanoes. In both cases, only part of the volcanic structure has been preserved, the remaining parts having been faulted away. Espanola and Santa Fe have been extinct for several million years. Pinzon and Rabida are both small extinct shield volcanos that have not been active for about 1 million years. Though Santa Cruz and San Cristobal remain active volcanoes, parts of their edifices are much older, more than a million years in the case of Santa Cruz (including its small neighbors of Baltra, Seymour, and Las Plazas), and nearly two and a half million years in the case of San Cristobal.

The Volcanoes

Galapagos Volcanoes

Sierra Negra Volcano has the second largest crater in the world

Sierra Negra rises to an elevation of nearly 1500 meters (4890 feet) and is located in the southeast of Isla Isabela, (or Albermarle Island). It is the largest and one of most active volcanos in the archipelago, with 10 known eruptions since 1813, the most recent in 1979. Over ninety percent of the surface of the volcano is younger than 4500 yrs, judging from cosmic ray exposure ages of lavas. Though Sierra Negra shares a number of characteristics with the other large western Galapagos volcanos, such as steep upper slopes that give it an “inverted soup-bowl” shape, it does differ in several respects. First, its slopes are somewhat gentler, and it has a broader coastal apron than those of its neighbors; second, there is a prominent rift system that runs from northeast to southwest which deflects around the caldera. In this respect, it is similar to the Hawaiian volcanos, which also have prominent rift systems. As in Hawaii, many of the eruptive vents on Sierra Negra are located along the rift. The 1979 lava flow erupted from a vent called Volcan Chico, located along the rift just to the northeast of the caldera.

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.

The composition of Sierra Negra’s lavas are quite uniform, yet they differ markedly, particularly in trace element abundances from it that of its neighbor to the north, Volcan Alcedo. Thus they neighboring volcanos derive there magma from distinct areas of the mantle through different plumbing systems. Sierra Negra is, however, compositionally similar to it western neighbor, Cerro Azul, which is currently erupting. It is conceivable that magma feeding these two volcanos is derived from a common melting zone in the mantle.

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 originally established to mine sulfur from the fumaroles in the area. Some cattle and goat ranching occurs on the southern slopes, and there are many feral cattle and goats as well.

Galapagos Volcanoes

Cerro Azul Volcano

Cerro Azul volcano occupies the southwest corner of Isla Isabela (Albermarle Island). Although smaller than its neighbor to the west, Sierra Negra, at 1690 m (5541 feet) its summit is higher. Cerro Azul is quite active, with 9 known historic eruptions, the most recent of which began in September 1998 and has continued into October. That eruption was somewhat unusual because lava erupted both from a vent near the eastern base and from a vent within the caldera. Because of its remoteness, it is likely that many eruptions over the last several hundred years have gone unnoticed. The composition of Cerro Azul’s lavas is quite uniform and rather similar to that of Sierra Negra’s. Both Cerro Azul and Sierra Negra have strong enrichment in incompatible elements, indications that these melts are derived primarily from the Galapagos mantle plume rather than entrained asthenosphere.

Cerro Azul’s caldera is elongate in an ENE-WMW direction and is among the smallest in the western Galapagos, measuring roughly 4 km by 3 km. Nevertheless, at 650 m (2000 ft), it is among the deepest of the calderas in the western Galapagos. The caldera is quite complex, with numerous embayments and benches, indicating it formed not in a single event, but through multiple collapses. A large cinder cone may be seen on the caldera floor. Numerous young flows erupted from an arcuate vent system on the south rim of the caldera and cascaded down the caldera wall to pond in the caldera floor. A lake has developed in the last decade or so and now occupies much of the northeast half of the caldera floor. Caldera lakes are not uncommon in the Galapagos, but they are rarely permanent.

Galapagos Volcanoes

The crater of Alcedo volcano is home to thousands of Galapago tortoises

Volcan Alcedo is located in the center of Isabela, (also known as Albermarle Island) located between Volcan Darwin to the north and Sierra Negra to the south. The composition of its lavas differs from that of both its neighbors, indicating it has an independent magma plumbing system. It quite symmetrical and rises to an elevation of 1100 m (3650 feet). Relative to its size and height, Alcedo’s caldera is large (7 to 8 km in diameter) and shallow (270 m deep). A steam vent, or fumerole, is located on the western side of the caldera. Only one historic eruption is known, a basaltic lava that erupted the southeast flank sometime between 1946 and 1960.

Volcan Alcedo is unusual in that its flanks are mantled by rhyolite pumice. Rhyolite is a light-colored volcanic rock that is produced by fractional crystallization. This occurs when basaltic magma intrudes into the crust and cools. As it does, it crystallizes, the crystals settling out. As a result, the composition of the magma steadily changes, becoming poorer in magnesium and iron and richer in silicon. The low iron content accounts for its light color compared to basalt. Alcedo is probably the oldest volcano on Isabela and may be in the final phase of its evolution. This may account for its unusual lava chemistry and morphology.

Rhyolite is highly viscous, so unlike basalt, it does not flow easily. Furthermore, rhyolite magma is often rich in water. These two factors combine to produce explosive eruptions, which generate ash rather than lava flows. At least two such explosive eruptions have occurred on Alcedo, the most recent of which was 100,000 years ago, producing the pumice that mantles its flanks. The eruption was of “Plinian” style (similar to Pinatubo or Mt. St. Helens), with the eruption column reaching heights of 20 to 30 km). Such eruptions are extremely unusual on oceanic islands. Once the gas pressure in the magma was released somewhat, a viscous rhyolite magma flowed out on the caldera floor, freezing as obsidian. The white of the rhyolite pumice stands in stark contrast to the black basalt.

The pumice covering the flanks makes the ascent to the summit comparatively easy, and Alcedo, along with Sierra Negra, are the only calderas that tourists may visit. Alcedo is home to the largest population of giant tortoises in the islands. Up until a few years ago, the caldera floor was lushly vegetated and giant tortoises not threatened. The tortoises were often found wallowing in temporary rain pools. However, in the last 10 years, the population of feral goats on the volcano has absolutely exploded. In 1983 there were no goats. It is estimated that there are now between 50,000 and 100,000 goats living on the volcano. The goats have now virtually defoliated the volcano, threatening one of the best tortoise habitats. Alcedo is also excellent habitat for land iguanas, as they can easily burrow in the rhyolite ash that mantles to the volcano.

Galapagos Volcanoes

Named after the famous scientist visit

Volcan Darwin is the second volcano south on Isabela, located between Volcan Wolf to the north and Volcan Alcedo to the south. It is nearly perfectly symmetrical and rises to an elevation of 1325 m (4350 feet) and is the fifth highest volcano in the Galapagos. Like the other western volcanos, it has a large central caldera, nearly 5 kilometers (3 miles) in diameter and 200 m deep (700 feet). A number of young lava flows may be seen within the caldera, as well as on the flanks of the volcano. The primary eruption patterns seem to be from circumferential fissures surrounding the summit caldera, though radial fissures are also apparent. There are no confirmed historic eruptions of Volcan Darwin, but an 1801 eruption in “central Isabela” witnessed by Amasa Delano from anchorage in James Bay on Santiago could have been Volcan Darwin (Alcedo is the other possibility). Nevertheless, many of the lavas are quite young and no more than a few hundred years old, thus Volcan Darwin certainly remains an active volcano.

Two young pyroclastic cones, Tagus Cone and Beagle Cone, are located at the shore on the west flank of the volcano. Tagus Cone is partially breached and forms an excellent anchorage known as Tagus Cove, which has been a favorite stopping place for centuries. In the last century, whaling ships would anchor there to supplement their provisions with tortoises. Tortoises were particularly favored because they could be kept alive in the holds of ships for months without water or food. The Beagle anchored there in 1835 and Darwin spent considerable time exploring the area. Today, Tagus Cove is one of the common tourist stops in the western Galapagos.

Pyroclastic cones located near the shore, called littoral cones, such as Tagus and Beagle Cone, are common in the Galapagos. Darwin was the first to recognize that these cones form when basaltic magma comes in contact with seawater. When this occurs, the water is flashed to steam, and the magma fragments into small ash particles that react with the steam to form palagonite, the light colored material of which the cones are composed. The entire process is quite explosive, and the ash is thrown into the air. It settles back to build a cone around the vent.

Like the other large western volcanos, much of the surface of Volcan Darwin is covered by aa lavas. Aa lava surfaces are extremely rough and much more difficult to walk on than the other type of lava surface, pahoehoe. Some lavas flows begin as pahoehoe but convert to aa when flowing down steep slopes. The characteristic steep slopes of the western Galapagos volcanos is why aa lavas predominate.

Tagus Cove is one of the many excellent localities to see Galapagos wildlife, particularly the abundant seabirds. Among those that may be seen here are the brown pelican and the blue-footed boobie. The vegetation in the area is typical of that in the dry lower elevations in the Galapagos, and include the unusual Palo Santo trees. These white-barked dwarf trees are leafless and all but dead most of the year. They leaf out and spring back to life in the short wet season, which usually begins in March or April. The Darwin race of giant tortoises is usually found in the moister highlands, but females sometimes come down to the Tagus Cove area to nest.

Volcan Darwin was investigated in 1986 by a team of geologists from the University of Oregon and Cornell University.

Galapagos Volcanoes

An active volcano on Isabela Island

Volcan Wolf, located at the northeast end of the island of Isabela (also known as Albemarle Island) is the highest volcano in the Galapagos, reaching an elevation of over 1700 meters (5600 feet). Like the other western volcanoes, it has a large caldera. Volcan Wolf’s caldera is slightly elongated in an NW-SE direction, measuring 5 by 7 km, and, with a depth of 600 m, it is the second deepest in the Galapagos. Volcan Wolf is among the most active volcanos in the Galapagos. The caldera floor is covered with young lava and many young flows may be seen on its flanks, particularly on the eastern side. Ten historic eruptions have been documented between 1797 and 1982. The 1982 eruption was particularly spectacular: fountains of lava emanating from vents in the caldera floor rose above the caldera rim. Because of its steep slopes, most of the lavas on Wolf are of the aa type, making the climb to the summit a difficult one.

The distribution of vents on Volcan Wolf is typical of that of the western volcanoes. Vents are aligned along circumferential fissures on the summit plateau. On the flanks, however, most vents are aligned along radial fissures. A particularly active ESE-WNW aligned fissure zone connects Volcan Wolf to its western neighbor, Volcan Ecuador, which is much smaller. Although they abut each other, Volcan Wolf and Volcan Ecuador differ markedly in the composition of their lavas, particularly in trace element and isotopic composition. There is also a clear distinction in lava chemistry between Wolf and its southern neighbor, Volcan Darwin (although less marked than that between Wolf and Ecuador). In terms of isotopic composition, Volcan Wolf is quite unusual because isotope ratios are more similar to lavas erupted at mid-ocean ridges than those erupted at other oceanic islands. It shares this trait with several other Galapagos volcanoes, most notably Santa Cruz and Genovesa.

Until very recently, the remoteness and steepness of Volcan Wolf had kept its biota relatively insulated from the effects of man and the animals he brought. However, the area is now threatened by the invasion of feral goats that is affecting most of northern Isabela and is most severe on Alcedo. Goats were first reported on Wolf in 1995. Goats devastate the natural flora, thereby threatening the native herbivores such as the land iguana and giant tortoise.

Ecuador Volcanoes

This volcano is located at the Equator line on Isabela Island

Out of 6 volcanoes on Isabela Island – Volcan Ecuador is the only one that is not active anymore (recent studies reveal that it may still be an active volcano).

It has an asymmetrical shape, it does not have a western half, perhaps due to erosion caused by the tides or most likely a strong tidal wave.

It takes its name from being located exactly on the Equator and because the Galapagos Islands are part of Ecuador. Together with Wolf volcano, they are the only volcanos in the Galapagos archipelago at the Equator.

By many Geologists its seen as the youngest of all volcanoes on Isabela Island, its caldera holds lava that is still considered fresh.