Sierra Negra volcano in the Galápagos Islands has piqued our attention once again. Reports of renewed activity are getting more convincing by the month. Here we are again – as advertised – at a “second largest caldera” or, more often even “second largest crater” in the world”… NOT! It seems that everyone accepts Yellowstone with its over 3000* km² as the largest, but every sizeable caldera seems to come next.
Sierra Negra is not a “large crater” at all but a medium sized caldera. Even in Central and South America there are several larger calderas like Atitlán (GT, ~285 km²) or Caldera del Atuel (AR, ~120 km²) that easily beat the 82 km² of the Sierra Negra (sizes just roughly calculated as an ellipse of the caldera floor, using the stated lengths and widths of the calderas).
Sierra Negra is a large shield volcano that rises to an altitude of 1124 m at the southern end of Isla Isabela, just S of the equator and near the assumed location of the Galápagos hotspot. Approximately 1 million years old, the island was formed by the joining of 6 shield volcanoes – Alcedo, Cerro Azul, Darwin, Ecuador, Sierra Negra, and Wolf.
The island, as part of the Galápagos National Park, is noted not only for its well preserved wildlife of ancient animals that are nowhere else to be found. It has also excellent examples of the geologic history that created the Galápagos Islands. Desolate lava fields can be seen in many places, and Chico volcano displays a burst of red and rusty colors. Special points of interest include the lava tubes on the southern shore, uplifts at Urvina Bay and the Bolivar Channel, tuff cones at Tagus Cove, and pumice cones on Alcedo and Sierra Negra.
For the tectonic setting see agimarc’s post about Wolf volcano from 2016.
A VOLCANO WITH A TRAPDOOR
Of the 12 eruptions listed by the GVP in recorded history there are only six explicitly assigned to the Sierra Negra caldera. The rest were *somewhere* on southern Isabela. The last two (2005 and 1979) and the 1953 eruption were labeled as VEI 3, all others 2 or 0.Like with all the other Galápagos hotspot volcanoes, the Sierra Negra normally erupts in Hawaiian style. The volcano is built up by sill, radial, and circumferential dike intrusions. Outside the caldera rim there are lots of flank vents, including cinder and spatter cones, concentrated along an ENE-trending rift system. Tuff cones rise along the coast and form offshore islands. In 1979 an eruption happened from Volcán Chico, a parasitic cone on the ring fissure zone 1km N of Sierra Negra’s caldera rim. This was also the site of the previous two eruptions from Sierra Negra, in 1963 and 1953. The 1979 eruption lasted for (more than) two months and produced ~1 km³ of lava.
The summit caldera of Sierra Negra volcano is with 100 m depth relatively shallow in comparison to other western Galápagos calderas. It has near vertical ring faults on all sides, except in the north. Inside the caldera, in the S and W part, there is a 14km long and up to 100m high wavy ridge. This is caused by the intra-caldera fault system. The southern sector of the ridge accommodates inflation of a shallow sill in trapdoor* fashion. A large fumarolic area, Volcan de Azufre, lies within a graben between this ridge and the western caldera wall. There are rich sulfur deposits and also molten sulfur flows have been found. Temperatures of the fumaroles here reach more than 250°C.
Long-term deformation records at Sierra Negra indicate a flat-roofed shallow magma reservoir (perhaps a sill), roughly 5×3×0.7 km in size, with its top being about 2.1 km below surface. This would be fed by a deeper reservoir, a crystal mush zone, imaged in crustal tomography at ca. 8km b.s.l. This way the magma has time to evolve, mainly by fractional crystallization. The Sierra Negra volcano magmas are tholeiitic basalts produced steadily from a homogeneous source of mantle derived material.
A geophysical study 2009-2011 allowed recording an average of 100 earthquakes per month, of less than 3.4 Ml (local magnitude). The count included a seismic swarm under the volcano’s SE flank related to an intrusion of a magmatic dam. The great majority of earthquakes in Sierra Negra are located at depths of less than 8 km, since they generally originate in the interior of the oceanic crust.
*About Trapdoors and Safety Valves
This seems to be a good thing to know about some calderas: Scientists found out that the Sierra Negra fault system produced something like a safety valve: earthquakes ruptured one part of the caldera “lid” – leaving the other side of that part “hinged”. Remember the strong inflation between the 1979 and 2005 eruption? They say that the lifting of the “trapdoor” in the 1990s and early 2000s relieved much pressure inside the magma system. Without that an eruption might have happened years earlier. It might be possible to forecast an eruption in the future if this trapdoor faulting and lifting can be worked into a forecasting model.
From spring 2017 on earthquakes below the volcano have been increasing steadily. Same goes for an exceptionally fast inflation of the caldera floor.
From IGEPN reports:
Nº-2-2017 (24/11/2017): During the last month at least five EQs of M>3.0 have been recorded below the caldera. Most were volcano-tectonic (VT) in nature, with some long-period (LP) and hybrid (HB) events as well. Daily numbers rose from below ten in 2016 to up to 48 in the last months. The largest was a M3.8, located 16 km below Chico volcano on the NE edge. This was classified as a DLP (Deep Long Period) earthquake at the lower limit of the continental crust (Moho). The caldera floor continued to inflate at a rate of 70 cm per year. This rate surpasses the speed it had shown before the eruption of 2005. This level of volcanic deformation is one of the highest recorded in the world.
Nº-3-2017 (22/12/2017): Number of daily EQs further increasing. Since Dec. 1, 564 earthquakes have been detected. Many of them were LP quakes, indicating movement of volcanic fluids (gases or liquids) in cracks and fractures inside the volcano. At the moment, a reactivation of the volcano seems very probable.
Nº-2-2018 (12/01/2018): Further increase in activity is interpreted as a state of internal agitation associated with intrusion of magma inside the volcano, at relatively shallow depths (few km below the caldera). Numbers reach more than 100 quakes daily. 98 cm of inflation has been measured since the beginning of 2017. The total since the 2005 eruption is now at 6 m permanent rise of the caldera floor. The rate of uplift is now higher than it was when the 2005 eruption occurred. The chances of a short-term eruption in the volcano (days to weeks) have increased. Possible scenarios of future activities are outlined in the report. They are:
– SMALL TO MODERATE ERUPTION IN THE NORTH FLANK (most probable) or
– MODERATE TO LARGE ERUPTION IN THE NORTH FLANK (less likely) or
– ERUPTION IN THE SOUTH FLANK (very unlikely).
We can expect a new report within the next few days.
THE 2005 ERUPTION
Scientists had reported roughly 5 m of total uplift in Sierra Negra since 1992. This is the largest precursory inflation ever recorded at a basaltic caldera. On 16 April 2005 the active fault system below the caldera had experienced an mb 4.6 earthquake and ~84 cm of uplift. This earthquake marked another trapdoor-faulting event, a process presumably driven by a shallow (<2 km) intrusion of magma.
Sierra Negra volcano began a week-long eruption on 22 October 2005. It was preceded by several earthquakes earlier that month and an M 5.5 three hours earlier that same day. At 17:30 an explosion was heard by many people in the town of Villamil, 20 km SE of the volcano. 15 minutes later, the Washington VAAC detected an ash cloud of ~15.2 km on satellite imagery. The column may have reached a height of 20 km later. Extensive lava fountains were seen rising to heights of 200-300 m along a segment of the caldera rim. Incandescent lava flowed several km down the outer NW flank, and tourists reported seeing two lava flows descending the N flank.
Wonderful 16-min. video of the eruption by ‘Colgate University’ on YT, with people who seem to know what they are talking about (must see!):
Fortunately, scientists had a chance to be there to watch, and measure as many parameters as possible. They described the style of the Sierra Negra 2005 eruption as follows:
Evolution through four stages:
1. An explosive vent-opening phase that produced the high plume of steam, gases, and ash.
2. The Curtain of fire phase: Within an hour, the length of the dike had opened a fissure, erupting as a 2-km-long curtain of fire. The fountains fed lava flows that quickly progressed down the north side of the volcano. They also supplied a voluminous ‘a’a lava flow that advanced 5 km south across the caldera floor.
3. A phase of fire fountains, now from isolated vents. The lava flow on the caldera floor was now fed by a perched lava pond, which in turn fed the interior of the main flow. This ‘a’a main flow inflated and continued in an unusual way: After the first day, it advanced slowly by emitting lobes of pahoehoe around its fronts. – Except for the opening blast, so far the activity was Hawaiian in style.
4. Strombolian phase: On the last day of the eruption, the eruptive style changed to Strombolian – the bursting of large bubbles near the surface and pulsating fire fountains from a single vent. The transition occurred when the ascent rate of the magma in the conduit decreased below a threshold value of ~25 m³/s.
The eruption ended in the early hours of 30 October 2005, when the cooling (solidifying) rate within the dike exceeded the critical flow rate. It had produced 0.12 km³ of lava. The magma reservoir began inflating again the next day. This relatively rapid re-inflation of Sierra Negra suggests that the shallow sill that fed the eruption is a persistent feature.
The M 5.5 earthquake just hours before the eruption probably initiated dike injection from the magma reservoir. However, the dike did not propagate along the active fault: instead, it followed a path on the other side of the volcano, opposite the active faults. This had been the same in the earlier eruptive patterns of Sierra Negra.
WHAT ELSE ON SOUTHERN ISLA ISABELA?
There is lots to do and to see on the island, especially the animals seem to be very trustful of tourists. Diving and hiking would be the two most rewarding activities here. Reading through many travel reports, these two places struck me as particularly impressive:
This is the site of a historical lava flow apron that has been eroded by the sea. A maze of lava tubes, caves, arches, pillars, platforms… you name it… with crystal clear waters flowing through black rocks covered in cacti. The lava features are home to a great variety of marine life like sea horses, tortoises, sea urchins, starfish, sharks and tropical fish, a paradise for snorkel divers!
Wall of Tears
Isabela’s main town, Puerto Villamil is a small port village, with sandy streets, a few locally run restaurants, and hotels. It wasn’t always that peaceful. In 1944, the Ecuadorian government established a brutal penal colony on the island of Isabela. In 1946, 300 prisoners were transferred to Isabela. They were forced to build a wall, purely as a way to punish them and break them physically and spiritually….
The prisoners had to walk long distances to the quarry, cut out large volcanic rocks and then carry them back to the site. It became about 5-6 m tall, 3 m wide and more than 100 m long. Many prisoners died in suffering. Finally, in 1958 the prisoners revolted leading to the guards being killed and many prisoners dying. The government closed the Isabela penal colony a year later. This historical site is now known as El Muro de las Lágrimas in Spanish. Some locals in town believe that the wall is a haunted place, and that at night they can still hear cries of those that died at the wall during its construction.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP, Sierra Negra
– IGEPN, Islas Galápagos (incl. reports)
– IGEPN, Sierra Negra (PDF)
– 2005 Eruption of Sierra Negra Volcano (2007, paywalled)
– On trapdoor faulting at Sierra Negra (2005, paywalled)
– A volcano bursting at the seams […] (2006, PDF)
– Stress interaction between magma accumulation and trapdoor faulting (2009, paywalled)
– Galápagos National Park
– Imaging rapidly deforming ocean island volcanoes in the western Galápagos archipelago (2014)
– Seismicity patterns during a period of inflation at Sierra Negra (2016)
Thank you! What a fascinating article! It will be interesting to see if the upcoming eruption will fit the trapdoor model.
I’m also curious to know if that model could fit some of the larger Icelandic volcanoes, or if the trapdoor best suits the large shield volcano.
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Kusatsu-Shirane (JP) has erupted… At least one dead and 15 injured by avalanche in ski resort… Alert level “3” January 23, 2018, 11:50 JST. Webcam is here: http://www.ktr.mlit.go.jp/tonesui/livecam/kusatu.jpg
Historical eruptions came from Yugama which is intensively monitored and people are prepared for. This time, it is erupting from Moto-shirane which has had almost no activity & is less understood. Apparently its last eruption was ~1500 years ago.
Hi Agimarc, hope you are on higher ground! It took 26 minutes for the Tsunami warning to appear on the TWC website. That is a hell of a long time for a relatively shallow major earthquake on a known subduction zone. It is the middle of the night I guess, but still. Hope this quake hasn’t generated any major tsunami. fingers crossed.
Howdy Bruce –
Thanks for thinking of us. Woke last night to dresser drawer pulls rattling. Went on for a while before surface wave hit. House rolled a while. Was not gentle but not as violent as the 7.1 a few years ago. Did not feel any aftershocks. Did not move anything in the house or otherwise rearrange the furniture or items on the shelves. Based on the delta between the arrival of P / S waves and the surface wave I figured it was large and a long ways away. Surprised this morning that it was as far away as it was – 580 km south. No tsunami warning here. We are on top of a 20 m bluff and whatever is generated has to rattle up Cook Inlet to get to us. If tides are low, no problem. If they are high, things may be exciting. A 10 m total tidal throw is not uncommon, though they are not all that large. West Coast Tsunami office is online but the page doesn’t load due to internet traffic. Looks like there were at least 10 aftershocks in the M 5+ range. Interested in seeing what caused the quake as there is not a lot out of there and perhaps 100 km south of where the plate starts down. Cheers –
My last info was 17 aftershocks in the 4-5 range abt an hour ago. The quake was a strike-slip, so not much up-down motion:
Local news had Kodiak with perhaps a half a meter wave. Warnings from Kodiak to Yakutat. Was an exciting evening in Kodiak, sounds like. Scrambled Coast Guard helos and planes to get them off the ground. Link to local article follows. Cheers –
Interesting thread by seismologist S. Hicks on the AK M7.9 (begins just after quake happened). Click on it and scroll down:
Mount Motoshirane (the main peak of Mt. Kusatsu-Shirane) from a helicopter after the eruption this morning:
Mount Motoshirane – red pin shows location of eruption, south of main crater
thanks for showing us some glimpse of the Galapagos
about the phreatic eruption in Japan some footage from La culture Volcan blog. The guys filming were very near.
wow, must have been pretty exciting. It looks like a dirty geyser.
Article on the phantom 32′ wave reported out of a single buoy near the quake epicenter earlier this week. Cheers –
Yes, they say it was a phreatic eruption similar to Ontakesan in… when, 2015?… because the analysis says ash is 80% base rock material. What about the other 20%? And there was no avalanche, it was the eruption cloud that hit the people. The one who died was hit in the chest by a rock, while together with others trying to shield a family. Flying rocks were up to 1 m big.
Photo taken by the Kyodo News Agency helicopter about two hours after the eruption. One plus 4 craters in a row. The craters are near the ropeway summit station. There a number of people were stuck for half an hour when the power failed during the eruption. (They must have soiled their pants, waiting, waiting, – at least I would have, I guess)
The craters extend about 220 meters on a line that runs almost east to west. Each depression is between 10 and 15 meters in diameter. A professor from Tokyo University, Toshitsugu Fujii, said that a surveillance of this area is necessary, because these new craters could result from different eruptive mechanisms. – Soo… not phreatic, perhaps?
I would keep it Phreatic. If you look at the shape of the plume it is cypressoidal. Also the color of the ash is whitish, so a lot of steam in this. Also there was no forewarning I suppose this volcano is well equiped as it is in Japan.
Yes, of course you are right, and I should have said: not *just* phreatic. My idea behind it was that it could perhaps have been phreato-magmatic. As the volcano had been restless for a long time, there might not have been a distinct rise of magma necessary. A prolonged presence of magma near the surface could have caused cracks to open, by heat and pressure, that eventually allowed hydrothermal fluids to get deeper and explode. The crater row speaks for that hypothesis, imho: a purely phreatic eruption would happen too fast and in a more chaotic manner. There might not be time to create a neat row of craters. I think the fissure might have been “prefabricated”.
Just like what happened on the Pinatubo in 1991 before the big VEI 6 eruption, a phreatic eruption along a fissure caused a row of small craters.
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well, I guess phreatic eruptions like this should just be taken as one of a suite of possible signs of “restlessness” that includes fumaroles, gas emissions, seismic unrest, inflation, etc. It appears that in the vast majority of cases the period of unrest dies away again quietly without anything more happening. Occasionally it doesn’t and the activity builds up to an actual eruption of fresh magma. But even in this case most eruptions are small affairs like we have seen recently and only a few make it to the big league like Pinatubo – thankfully!! I am quite happy if eruptions stay in the small category – although even these can be fatal, as we see here, sadly.
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