Volcan Darwin is located on Isabela Island in the Galapagos. It nestles comfortably between Wolf to the north and Alcedo to the south on the northern part of Isabela Island. Close by to the west is Fernandia Island and Fernandia volcano. Just a little farther to the east is Santiago Island and volcano. The final nearby volcano is Ecuador on the far NW tip of Isabela Island.
We took a look at the 2015 eruption of Wolf in a 2016 post.
Isabela was built by the Galapagos hot spot with six coalescing eruptive centers. Neighboring Fernandia and Santiago are not connected with Isabela. All the main volcanoes on Isabela are basaltic shields, topped with large subsidence calderas. Pyroclastic tuff cones are not uncommon on the flanks of these volcanoes. A couple of these are breached by the ocean and serve as protected anchorages for visitors.
The Galapagos Islands are currently a province of Ecuador, located some 926 km W of Ecuador. They were known as the home for multiple unique species studied by Charles Darwin during the second voyage of the HMS Beagle in 1835 leading to his theory of evolution.
The archipelago has 7,880 km2 spread over 45,000 km2 of Pacific Ocean. It has a total population of 25,000 mostly Mestizo. There are 18 major islands, 3 smaller islands, and 107 rocks and islets. The largest of these is Isabela, some 2,250 km2. It is this island that Darwin landed, with the Beagle’s anchorage at a breached tuff cone on the western flank of the volcano eventually named Darwin.
Climate on the islands is controlled by their location on the Equator and the presence of the Humboldt Current, which brings cold water. Like the Hawaiian Islands, there is a mixture of arid conditions, tropical savanna, and tropical rainforests depending on the location of the rain shadow from the prevailing winds. The cool current brings frequent drizzle throughout the year and dense fogs June – Nov. Average temperature ranges 28 – 22° C over the course of the year. About 58 cm/year average rainfall.
The islands are administered as a provincial government of Ecuador. The islands are a national park with a limited and tightly controlled tourism operating. There have been violent confrontations between parts of the local population and park service staff over the years. Surrounding ocean is rich and has been designated over the years as a marine reserve, whale sanctuary, World Heritage Site, and a biosphere reserve, mostly to protect species of plants and animals not found anywhere else. Plants and animals introduced by newly arrived residents are a continuing threat to the indigenous wildlife and biosphere. A Chinese fishing fleet of over 400 ships has been chasing squid in and around the Galapagos for some years, leading to conflict with the navies of Chile and Peru.
The HMS Beagle visited the Galapagos in 1835 and surveyed multiple islands. Isabela was the third island visited. Darwin described the island as the most deserted and volcanically active island of those visited. The Beagle anchored on the western side in the breached tuff ring Tagus Cove. Darwin explored the volcanic landscape and observed land and marine iguanas. Tortoises were also present on Isabela and neighboring Santiago to the east, the last island visited before returning home.
Note that the names of the islands and volcanoes have changed over the years depending on most recent ownership. British nomenclature and names were applied for at least a couple centuries. Present / past (British) island names follow: Fernandia – Narborough; Isabela – Albermarle; Santiago – James or San Salvador; Santa Cruz – Indefatigable; San Cristobal – Chatham; Floreanna – Charles or Santa Maria; Espanola – Hood; Pinta – Abington; Marchena – Bindlow; and Genovesa – Tower. This post will use current names for the volcanoes and islands.
Volcanic activity has been underway in and around the Galapagos for at least 20 Ma. The oldest island remnant is 8 – 9 Ma. Erupted lavas built a 3 km thick platform under the islands. As the islands migrated SE, continuing magma supply from the Galapagos Spreading Center (GSC) built the Northern Galapagos Volcanic Province between the Galapagos and the GSC.
The western volcanoes are the youngest of the islands. Fernandia is the westernmost and youngest island with a single shield. It emerged some 32,000 years ago and was resurfaced as recently as 4,300 years ago. Neighboring Isabels Island to the immediate E is constructed of 6 coalesced shields, Cerro Azul, Sierra Negra, Alcedo, Darwin, Wolf and Ecuador. All are connected by land bridges formed by lava flows. Most lavas from the western Galapagos are too young to be dated by radioactive argon methods. All are normally polarized. And the oldest lavas are estimated at less than 500,000 years.
The major islands of eastern Galapagos are Santiago, Pinzon, Rabida, Santa Cruz, Floreana, San Cristobal, Santa Fe, and Espanola. The oldest of these is San Cristobal (2.34 Ma) and Espanola (2.66 Ma). Santa Cruz lavas range from 1.6 Ma – 30,000 years old. Floreana lavas are similar in age to Santa Cruz. Santa Fe, Rabida and Pinzon all have lavas ranging around 2.76 – 1.03 Ma to 0.92 – 0.09 Ma. Santiago is the youngest of the eastern islands with lavas dated between 770,000 years ago and the most recent eruption in 1906.
Western volcanoes have shallow lower and upper flank slopes with steeper slopes in between. They have circumferential fissures around their summit calderas which shift to radial down the flanks of the volcanoes. Eruptions appear to begin with short eruptions from the circumferential dikes / fissures and then transition down to the radial dikes / fissures lower down the flanks. The flanks of the volcanoes below sea level have flat sheet flows stacked in terraces and rifts. There are relatively few seamounts and no Lohi-like new volcano growing to the W of Fernandia.
Eastern volcanoes are much more variable in size and shape than the western volcanoes, with Santa Cruz the largest at 40 km in diameter. Santa Fe is only 7 km. There are structural alignments of eruptive cones, pit craters, fissures and faults on each island. There are no circumferential or radial fissure structures observed on the western volcanoes. Instead, scoria cones and pit craters seem to be aligned with local faulting across the islands. The submarine environment has not been as extensively mapped as that of the west, but there are clusters of monogenetic seamounts off the coasts of three islands. There are no large terraces of sheet lava flows or large submarine rift zones in the E.
Eruptions from the western volcanoes are generally effusive, beginning with lava fountaining. This can be followed with a Strombolian phase once the eruption focuses on a few active vents. Observed eruptions have been relatively short from days to weeks long. There is phreatic activity within the calderas as magma interacts with caldera aquifers. There are numerous tuff rings along the shorelines or just offshore. There was a single Plinian-style eruption, from Alcedo that ejected around 1 km3 DRE of rhyolite some 100,000 years ago,
Eastern volcanoes had extended eruptive activity (over 1 Ma) on several islands. Much of the more recent activity on Santiago, Floreana, Santa Cruz, and San Cristobal was more explosive than western volcanoes. All the eastern islands have numerous monogenetic scoria and tuff cones. Effusive recent eruptions take place from fissures. There is a rhyodacitic ignimbrite on the NE coast of Rabida, the only explosive silicic volcanism found on eastern islands. Espanola may be the remnant of a larger structure that either suffered a flank collapse or extensive wave erosion of its southern half.
The large calderas of the western shields were caused by subsidence from large, robust, long-lived, shallow magma reservoirs. The calderas underwent many cycles of collapse and refilling. Eastern volcanoes have no calderas and do not appear to have significant accumulation of magma under the volcanoes, meaning they were supplied by a less robust magma flow.
These islands also subside. The 2.3 Ma Santa Cruz has subsided some 7 m since the last glacial maximum some 20,000 years ago. Islands to the west (Isabela and Fernandia) are thought to have subsided some 10 m over the last 20,000 years.
Volcan Darwin is nestled between Wolf to the N and Alcedo to the S. It is a symmetrical shield, rising to 1,325 m, the fifth highest in the Galapagos. Neighboring Wolf is the tallest at 1,707 m. Darwin is topped with a circular caldera nearly 5 km in diameter. The floor is 200 m deep and has multiple a’a lava flows in it. There are also multiple a’a flows on the flanks. Primary eruption appears to be the circumferential fissures below the caldera rim on the flanks. There are also radial fissures farther down the flanks below the caldera.
There are no known historic eruptions from Darwin, though the a’a lava flows in the caldera on the flanks appear to be quite fresh, perhaps a few hundred years old. There was an observed eruption from central Isabela in 1801 viewed from a ship anchored at Santiago to the E that could have been either from Darwin or Alcedo to the S.
Unlike its neighbors, Darwin has erupted a range of lavas from basalts to andesites. Some of these lavas are much more highly evolved than Wolf and appear to be due to crystal fractionization within the shallow magma chamber. Unlike neighboring volcanoes which erupt relatively well-mixed batches of magma, the structure and longevity of Darwin’s storage and transport system have allowed some magmas to evolve and cool much more than most other Galapagos magmas, leading to the eruption of andesites.
An uplift episode at Urvina Bay on western Isabela Island exposed lavas erupted from a shallow magma chamber onto the SW flanks of Volcan Darwin some 1,200 years ago. These lavas appear to be as a result of primitive magma injected into a magma chamber and mixing with more evolved crystal mush. The injection of hotter, more basic magma triggered the eruption.
The two main tuff cones on the western shoreline of Darwin are Tagus to the N and Beagle to the S. While they do not overlap, their cones do meet. Both have multiple young lava flows on the land side. Tagus consists of at least four nested craters, the youngest of which has a small salt lake, Darwin Lake. This lake is about 3 m above sea level and 3 m deep. It is unclear how salt water managed to fill the lake, though tsunami action is suspected. Salinity of the lake is about three times that of the surrounding ocean. Both cones are breached to the S. with the Tagus breach opening to the sea forming Tagus Cove. The Beagle breach is closed by lava flows from Darwin.
Life cycle of the two tuff rings appear to have built the tuff rings on the shoreline (Beagle) or right offshore (Tagus). Subsequent volcanic activity built smaller craters within and on the flanks of the first two. Volcanic activity then breached the southern walls of both cones. Final activity was lava flows from the flanks of Volcan Darwin against the eastern and southern sides of the tuff rings.
Beagle Crater (Cone) is located 1.5 km S of Tagus. Beagle is nearly circular and rises some 300 m above sea level. It has a shallow saltwater lake with several islets. The southern wall is breached, leading to a conclusion that the lake was cut off from the sea by the extension of southern lava fields from Darwin. Beagle also has a smallish satellite tuff cone at the 4:00 position.
Tagus Cove was named for the British naval vessel that first moored there in 1814. The ship was looking for tortoises to use as food. It has been used historically as an anchorage for pirates and whalers. Today, it is one of the most common stops for tourists. There is graffiti from visitors over the last 300 years, something visitors are no longer allowed to do. The channel between Isabela and Fernandia Islands has some of the coldest and most productive waters in the Galapagos.
Tagus (Bank’s) Cove is a narrow inlet about a kilometer wide extending 1.5 km to the N. The outer tuff ring of Tagus is similar to Beagle, though the western rim is perhaps 100 higher. There are some dry gullies on the crater. There is a small, breached scoria crater preceding the lava fields to the N.
There is a trail out of Tagus Cove up the flanks of Darwin volcano. Palo Santo trees line both sides of the trail. These are aromatic trees that only sprout leaves during the brief wet season. The name means ‘holy stick’ as the branches were burned for incense in churches. Tagus Cove has abundant wildlife, particularly birds, tortoises and iguana. Tortoises generally spend most of their time in the moister highlands, though some females do come down to the sea to lay eggs.
Our 2016 post on the most recent eruption of Wolf is a good starting place for deep historic tectonics associated with the Galapagos. This section will take a look at more recent tectonics over the last 5 Ma.
The Galapagos are tied to the Nazca Plate traveling over the head of a mantle plume located some 1,000 km off the west coast of Ecuador. Rate of travel is around 5 cm/yr. The Galapagos Spreading Center, the boundary between the Nazca Plate to the S and the Cocos Plate to the N, lies some 250 km N of the Galapagos. Most eruptible magma is available at the plume head underneath the spreading center. The farther you get from that ridge, the less eruptible magma there is.
The Hawaiian model of birth, subsidence, and death of a string of volcanic islands created as a plate travels over a hot spot or plume head works in many parts of the world. But it does not work for the Galapagos, likely due to structural differences in the underlying plate which allow differing amounts of magma to reach the surface.
Before 8 Ma, the archipelago was in a different tectonic setting, that of a ridge-centered hotspot. Before then, the hotspot was on the neighboring Cocos Plate to the North.
Differences between eastern (older) and western (younger) volcanic provinces appear to be driven by differences in structural features, fractionation of magma before erupting, and melt generation. The western volcanoes host large calderas on shields. The eastern volcanoes do not have calderas and appear to be built by a different construction mechanism controlled by proximity to the Galapagos Spreading Center some 3 – 1 Ma.
The Nazca Plate is currently moving some 5 cm/year relative to the plume head / hot spot. The Galapagos Spreading Center (GSC) is located some 250 km N of Fernandia Island. There is a neighboring transform fault to the S. The GSC is currently migrating from the Galapagos toward the NE at 4.7 cm/year. The GSC jumped southward at least twice 3.5 – 2.5 Ma and around 1 Ma.
Evolution of the western Galapagos volcanoes appear to take place in three phases. The Juvenile Transient Phase began with volcanoes growing at the leading edge of the hotspot. Batches of hot, relatively primitive magma cool and crystalize in the crust. Magma reservoirs are relatively small, hot, isolated and deep. The volcanic plumbing system is established by a growing magma supply. Crust thickens from 8 – 15 km during this phase.
The Mature Steady State Phase increases magma supply, with individual magma reservoirs combining into a several km-thick much zone. Magmas partly crystalize within the plumbing system, creating a shallow sill beneath the caldera. Erupted magmas are moderately evolved and homogeneous. Crust thickens to around 18 km in this phase.
The Dying Cooling Phase takes please as the magma supply wanes, being carried farther from the plume head by tectonic motion. The reservoir cools, crystalizes more, and erupts cooler lavas. Varying crystallization leads to eruption of a wide range of erupted lavas all the way to rhyolites. Alcedo and Sierra Negra are in this waning stage.
Eastern volcanoes have not undergone much evolution, as they emerged on pre-existing oceanic crust above the hotspot. They always erupted more primitive and heterogeneous basalts from tectonically controlled vents. There was a discrete shift in local conditions between 1 Ma and the present. This was due to the closer proximity of the islands to the plume 3 Ma and the long transform fault NW of the Galapagos opening due to the northward motion of the GSC relative to the plume and two southward jumps of the GSC. The eastern volcanoes had a smaller magma flux than the western ones. This accounts for the absence of a crustal mush column. It is possible that the plume has become more robust over the past 1 Ma, delivering a larger flux through a thicker crustal lid.
Volcan Darwin is a great example of a recently active hot spot / mantle plume oceanic volcano. Interestingly enough, it erupts varying types of products from basalts to andesites. Lava flows on its flanks and in its caldera are mostly very rough and seemingly new a’a lavas. While there are no known historic eruptions from Darwin, given the activity of other volcanoes making up Isabela Island, future activity from Darwin is not out of the question.