A VOLCANO WITH A TRAPDOOR – SIERRA NEGRA CALDERA (Galápagos, EC)

Volcán Chico, cinder cone on the north-eastern rim of the caldera Sierra Negra (© Michael R Perry, via wikimedia).

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.

Map of Isabela (Instituto Geografico Militar del Ecuador)

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.

Not an easy feat for Giant Tortoises! (© BilleBalle1958, via wikimedia)

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

Caldera of the Sierra Negra volcano. The floor of the caldera is covered by the dark lava flows from the 2005 eruption with flows emanating from the slightly elevated fissure plateau on the far right of the picture. (© Ofbarea, via wikipedia) Click!

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.

Topography map showing locations of eruptive centers (purple dots) and the seismometers (red triangles).

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 outer flanks of Sierra Negra. The caldera floor is just visible in the upper right corner. (© Kami, via masrutasymenosrutinas.com)

Chico volcano, Sierra Negra, in vibrant red. (©Sandra&Michael, via One Million Places)

Map of Sierra Negra caldera showing the intra-caldera fault system. The approximate location of the initial eruptive fissure (red line) and the lava coverage of the October 2005 eruption are shown as well (Jónsson et al., 2005; Geist et al., 2007).

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.

Conceptual model of the magmatic system below Sierra Negra. (Tepp et al., 2014)

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

Uplift of Sierra Negra volcano recorded by InSAR and GPS from 1992 until the eruption in October 2005 (adopted from Chadwick et al., 2006) . GV04 is a GPS site near the center of Sierra Negra Caldera. (from S. Jónsson et al., 2009)

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.

Trapdoor faulting in Sierra Negra caldera. Click for larger image and explanation. (from Jónsson et al., 2005)

Sierra Negra Caldera. I’m not quite sure, but I think the ridge in the middle ground is the “trapdoor” fault in the SW part. (© Kami, via masrutasymenosrutinas.com)

 

PRESENT ACTIVITY

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:

Seismograms of the deep LP earthquake recorded on 2017-10-20 in the Sierra Negra volcano by the seismic network of the IGEPN.

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

The Sierra Negra eruption  viewed from the east caldera rim on the night of October 24, the 3rd day of the eruption. Vigorous lava fountains rise from several locations along the fissure vent, feeding several anastomosing (uniting) lava flows that poured into the caldera. The scattered glow in the foreground was due to ponded lava covering the caldera’s eastern floor. (© Gregg Estes, 2005)

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!):

2005, on October 25, MODIS flying onboard the Terra satellite captured this image, three days into the last eruption of Sierra Negra. (© NASA image by Jesse Allen)

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.

This composite view looks west from the NE rim (right) of Sierra Negra’s caldera on October 23, 2005. The caldera floor is to the left. Four active vents are superimposed in this photo, aligned along the E-W fracture that lies at the base of the inner caldera wall. Numerous lava flows descended southwards to the left where they joined to form one single flow of ‘a’a lava ~ 1 km wide and 7 km long that had already reached the southern inner wall of the caldera on the 23rd. (© Minard Hall, 2005, Escuela Politecnica Nacional, Quito).

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?

Blue-footed boobies, a strange but common sight on Isla Isabela. (© schokoflieger, via tripadvisor.com)

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:

Los Tuneles
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!

A drones’ view of Los Tuneles, Isla Isabela. Dec 2015. (© Xavier P, via tripadvisor.com)

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….

Wall of Tears, built by prisoners 1944-1959, near Puerto Villamil on Isabela Island. (© Steven Bedard, via Flickr)

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.

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Marine iguana on Isla Isabela. These seem to be still plenty there, while land iguanas have apparently been decimated a lot since Darwin’s times. He reportedly had difficulties finding a spot to pitch up his tent as the place was teeming with iguanas. (© Daniela&Christian, via Von einer Reise)

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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)

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