Here comes the conclusion to the series, the 3 volcanoes that I considered the likeliest to produce a VEI-8 eruption.
3. Calabozos and neighbours (Chile)
This volcano is located in Chile. It forms part of a little known, little studied, silicic flare-up of the Southern Andes Volcanic Zone. Steepening of the subducting Payenia Slab gave rise to massive amounts of basalt lava flows and shields in the back-arc, while large caldera systems erupted from the arc. Calderas as big as the 20×30 km Varvarco volcano, of which almost nothing is known about. The Payenia flare-up is located to the north of another group of calderas that “died” at around 1.6 Ma, and to the south of a new flare-up area that is just starting to pick-up some strength. This is as if the subduction steepening was propagating northwards sequentially.
The Calabozos system became active at around 2 Ma, creating a 300-500 meter thick plateau of andesitic lava flows, a volcanic field in all likelihood. It’s possible that this reflects the formation of a large zone of molten andesite, similar to the Altiplano-Puna Magma Body, but much smaller. The Calabozos Caldera collapsed into a 26×14 km depression during large eruptions at 0.8, 0.3 and 0.15 Ma, each around 500 km3 in volume, but these should be considered minimum estimates that do not include distal ashfall. At the same time monogenetic vents and 2 stratovolcanoes, Cerro Azul and the imposing Descabezado Grande, erupted andesites over an area to the west. This westward migration would be attributed to the ongoing slab rollback (steepening of subduction).
Only recently the area to the west of Calabozos has shifted from andesitic to rhyodacitic volcanism, seemingly during the Holocene. Apart from some very small volume eroded flows, silicic activity is formed by 2 late prehistoric lava flows that look youthful, and eruptions of 1760 and 1846-1932. This rhyodacite magma body is 5-7 km deep (from thermobarometry studies), which is a similar depth to the magma bodies of most large calderas. It must have coalesced with the immediately adjacent Calabozos, resulting in a 30×25 km magma-mush body that is twice as big as the older Calabozos system which produced previous collapses.
It was 1846, an arriero was riding through the pass between Cerro Azul and Descabezado Grande mountains. He noticed nothing strange, but a few hours later, with not the smallest earthquake whatsoever, lava started flowing out effusively from the pass, and would come to erupt 5 km3, inundating nearby valleys. When a local was asked about the name of the new volcano, he said QUIen SAbe PUes, which means: who knows? And hence it was given the name Quizapú. The conduit of Quizapú remained open, and starting in 1907 it was having small gas and ash explosions, but with not enough pressure to overflow. However explosions grew gradually more severe and a gas jet effect in 1932 allowed it to eject another 5 km3 of dense magma in a plinian VEI 5-6 borderline event. Two months later the north flank of Descabezado Grande blew up, literally; it formed a hole a km across (Respiradero Crater). This explosion I suspect could be related to heating of the hydrothermal system following pressure loss of the volcano after the plinian eruption.
The size of the Calabozos Complex is 40×30 km, more than large enough to host a VEI-8. Much of it is probably molten already, as shown by large effusive eruptions like Quizapù or the Mondaca lava flow. Young basaltic-andesite eruptions next to Cerro Azul show that hot magma keeps rising into the system and making it grow further. On the other hand it lacks a ring of vents typical of more mature large calderas like nearby Laguna del Maule and eruptions are not as large as those of other volcanoes considered.
I think this Calabozos has got a lot of potential but it’s not ready just yet. And thankfully, its remoteness means that eruptions are unlikely to harm anyone.
2. Serdán-Oriental (Mexico)
Volcanoes evolve over time, and on a much grander scale whole volcanic arcs also transform over time. Among them the Trans-Mexican Volcanic Belt has one the most fascinating stories of them all, and I can only but scratch its surface here. México is the best place in the world to find the processes that lead into major silicic flare-ups taking place right before our eyes.
In the past México was the scenario to one of the greatest episodes of silicic volcanism the Earth has ever seen, the Sierra Madre Occidental silicic igneous province. After 20 million years ago the northern portion of it became the Gulf of California, while the southern became the Trans-Mexican Volcanic Belt and entered a period of flat-slab subduction.
But things have changed, the flat-slab is plunging down into the mantle (rollback), exposing a hydrated crust to the hot upwelling mantle. Extension opens grabens like Tepic-Zacoalco or Chapala. The crust has thickened already to 50 km below the eastern part of the belt. All those geologic processes that are inferred to have caused other active or long dead flare-ups are taking place right now below México.
The dazzling snow-covered stratovolcanoes like Popocatepetl (meaning smoking mountain) or Citlaltépetl (the highest volcano of North America) get most of the attention, and why wouldn’t they? They are prominent so that they dominate the landscape. But that would be missing the most unique volcanoes of Mexico, which are none other than its gigantic andesitic volcanic fields. Anywhere you go along the arc, they are there, volcanoes, visible as small conical hills to voluminous shields that rise a kilometre above the surrounding valleys. It is no wonder that some of the youngest new volcanoes of the world formed here in 1759 and 1943 (Paricutín).
The volcanic fields reveal that vast portions of the crust are melting. This creates the ideal conditions for large calderas to pop up. Enormous amounts of silica-rich melts are created within these melt zones by diverse differentiation mechanisms and supplied upwards to growing magma reservoirs, the nascent large calderas. But a batholith is not created in a day (nor in a million years) most of them are still immature, however there is one that has gone ahead. The first supereruption of México since the Sierra Madre Occidental might well come from the Serdán-Oriental Volcanic Field, also called Oriental Basin.
This vast volcanic field is big enough to host multiple large calderas, and actually it has got one already, Los Humeros, which collapsed in the 115 km3 DRE, Xaltipán Ignimbrite. A much bigger silicic volcano could be emerging further south, below the basin. A series of 6 large rhyolitic eruptions less than 25,000 years old seem have taken place along a curved line, delineating a melt body that must be, at least, 20×30 km across, putting it in the VEI-8 range.
Most calderas behave like trapdoors where one side of the ring-fault opens up and erupts while the other closes. Taupo for example usually erupts from its east side, while Yellowstone prefers to erupt from the west. Serdán-Oriental shows, perhaps, a nascent trapdoor caldera structure, although it is poorly defined because of the small number of eruptions so far.
The last major eruption took place around 20 AD. The ground was lifted up from within by 2 enormous lava domes, this rock carapace started to collapse into debris avalanches together with pyroclastic surges and lahars. This is thought to have caused the pre-hispanic populations, who lived in nearby villages, to flee to the city of Cantona further north. The population of Cantona may have increased from 28,000 to 52,000 people as a result. The volume of rhyolite extruded during the eruption was 10-11 km3 and formed the twin domes of Las Derrumbadas.
Another way volcanic activity has impacted local population is through obsidian trading which was what the economy of Cantona was mostly based on.
Action has just begun in this corner of Mexico. Continuing rollback should provide a powerful melting source for a few million years to come. Rifting could also take place like it did during the Sierra Madre Occidental event. The very big rhyolite magma body of Serdán-Oriental should, if anything, grow even more. How long will it take? It cold be tens of thousands, or hundreds of thousand of years (remember we are talking of geologic timescales here), but I do think it will get the VEI-8 eventually, and it might just be the first of many similarly huge calderas to emerge across México.
And in the meantime beware of maars. Those, often scenic craters, that can show up in a cornfield or the middle of a town, are highly explosive and destructive eruptions. The Oriental Basin has got a number of them, including rare examples of rhyolite maars, like Cerro Pinto, which produced pyroclastic currents reaching 18 km from its 2 craters and must be one of the most violent eruptions of its kind.
1. Okataina (New Zealand)
My number 1 choice brings us back to New Zealand. Okataina volcano is located at the northern end of the Central Taupo Volcanic Zone, which is known for its many calderas. Together with Taupo it has been the other major player during a resurgence of activity that has taken place during the last ~55,000 years. Okataina is located in a rifting arc, spreading at rates of 1-1.5 cm/year, which is somewhat faster than the East African Rift to give a reference. This is really important for how the activity of Okataina functions. Its last eruption in 1886, although much smaller than its usual eruptions, illustrates these processes.
Maori people used to build their villages next to hydrothermal fields. Hot springs are good for cooking food and pools of water provide relaxing baths. They were not aware though of the volcanic powers within the Earth sitting below their villages like a ticking bomb, they did not know their land as the powerful volcano it actually was.
At 12:15 AM on 10 June 1886, minor earthquakes started to be felt and ramp gradually in strength awakening people from their sleep. By 2 AM a black plume was erupting from the top of Tarawera Mountain. Soon after the mountain was torn open by a propagating fissure above a dyke intrusion and fiery fountains of basalt lava shot up to ~500 meters. Henry Roche was viewing the eruption from a high point:
“We then beheld the striking spectacle of a dark, flat-topped mountain more than a mile long, red hot along its crest, and surmounted by a wall of fire 1,500 feet high. Over this hung an immense volume of dense black smoke clouds through which the forked lightning flashed without ceasing.”
The eruption had so far been harmless. The dyke however kept propagating to the southwest and intruded below Lake Rotomahana, an already hot and fractured hydrothermal system, was turned to steam by the hot basalt from below, while new fractures formed as the ground was pulled apart, decompressing the system. At 3:30 AM Lake Rotomahana uncorked in a series of explosions that sent pyroclastic currents over a radius of 6 km destroying a few Maori villages, leaving no survivors in most of them. 120 people died as a result of the unforeseen Rotomahana explosion. Even now with our knowledge and monitoring something like this would be very hard to foresee. Back then many people may not have understood what was happening.
Henry Burt gives a description of the event:
“As soon as Ruawahia stopped sending forth its terrible balls of flame a huge white cloud issued from the cup of Rotomahana, and heavy booming was heard, followed by dense volumes of white compressed steam from Rotomahana. It rose with terrific velocity, and seemed to be going towards Okaro lake… and the appearance it presented was something like a huge boiling cauldron, bubbling in all directions.”
By 6 AM the eruption was over. Smaller steam explosions had opened the Waimangu Valley, today occupied by large hot springs. The ground was fractured all the way to Waiotapu (indicates the subterranean path of the magma intrusion). In total a dyke intrusion of over 25 km long must have intruded along the tectonic rift.
The explosion at Rotomahana excavated a crater complex 2.5×5.5 km across and unfortunately destroyed the beautiful Pink and White terraces, and various other terraces and hot springs that existed here prior to the eruption.
An article I was reading from Ronald F. Keam explains how the Pink and White terraces formed and why they were unique in the world. An eruption 5700 years ago had dammed Tarawera River with rhyolite lava flows which caused a great lake to form east of Mount Tarawera. Several geysers were created during this high-stand, and later, as the river slowly eroded the lava barrier, the lake level gradually fell, geysers found themselves perched on steep slopes above the water. Eventually Rotomahana became separate from the rest of the lake. By this time the geysers were 30 meters above the water level and the waters they discharged precipitated silica to form silica terrace formations unique in world (they are different from the more common travertine terraces that are made from carbonates). The many resulting bluish pools of water were also found at adequate temperatures to have a relaxing warm bath in them.
Known to the Maori as Te Tarata (meaning “the tattooed rock”), the White Terraces, were particularly impressive. The top formed a crater 20-30 meters across filled by a turquoise-blue pool of boiling water and surrounded by high steep walls, probably excavated in the hydrothermal explosions that gave it birth. Te Tarata was a geyser that erupted frequently in diverse ways but sometimes as powerful jets, like this description I encountered:
“By 4 p.m. the action had become vigorous, and dense clouds of steam were given off; and at 5 p.m. the basin was half full, the water violently agitated. Suddenly a deep boom was heard and felt on the upper terrace, like an underground explosion, and the water in the basin was instantly lifted into a huge dome, from out of which shot vertically with enormous velocity a glistening fountain, the top of which was lost to sight in the dense mass of accompanying vapour, the broken waters falling in heavy showers around.”
However the magma-water interaction that created them would also bring their destruction. People who visited the location after the eruption found that the location of the terraces had become a massive crater. While there are still those who have been searching in hope of discovering any buried or submerged remnant of the terraces, I think that this search is hopeless. Early witnesses to the site agree that the terraces had been in the location that blew up.
The 1886 eruption shows many aspects of how volcanism in the Taupo Volcanic Zone may work: with sudden rifting events, steam-driven explosions and fissure eruptions. But it doesn’t represent Okataina well. Most previous eruptions from Okataina have erupted rhyolite from long fissures along either the Tarawera “Fissure Swarm” (I will borrow the term from Iceland) or the neighbouring Haroharo Fissure Swarm. Nine such eruptions are known in the last 25,000 years, they all start with VEI-5+ plinian or phreatomagmatic eruptions followed by the extrusion of the voluminous rhyolite lava flows and domes of up to 12 km3 for an 8000 years old eruption.
The last major event was the Kaharoa eruption around 1300 AD. It started with a fissure plinian eruption from Mount Tarawera that produced at least 15 km3 of ash together with steam (hydrothermal) explosions through a line to Waiotapu, forming a 25 km long stretch of erupting vents. It was followed by the formation of 3 lava domes that form the flat top of Mount Tarawera.
Other smaller eruptions like the one in 1886 are produced by dykes of basalt, previous events may have been hidden by the larger rhyolite flows and ashes. 2 odd examples are known that are outside the usual fissure swarms. A basaltic eruption 3400 years ago formed the Rotokawau line of craters. Another 10,000 years old eruption created a line of steam explosion craters south of Mount Tarawera at Lake Rerewhakaaitu, this one is an example of a dyke that didn’t breach the surface since the ejecta consists only of old rock.
Okataina is large enough to host a VEI-8 caldera and the voluminous rhyolite eruptions reveal the existence of a sizable magma chamber. We know a lot about how the magma was stored over time and can place the moment this magma chamber was created. Interestingly it happened at a time Taupo was also undergoing great changes. Looking further the history of Taupo and Okataina appears to be connected to some degree.
Taupo, Okataina, and the entire group of calderas had been very quiet until around 55,000 or 45,000 years ago depending on the controversial estimates of the Rotoiti eruption at Okataina. The Rotoiti eruption was a caldera collapse with a volume of 370 km3, and no more than a few thousand years later, known from stratigraphy, Taupo started to produce plinian eruptions (and presumably lava flows that were later destroyed in the collapse). Magma from these plinian eruptions had been stored at shallow depths of around 4 km.
Studies of zircon crystals in Taupo magmas reveal that the volcano got a very large influx of melt that rapidly triggered the ~25,300 years old Oruanui VEI-8 eruption, which destroyed the storage of Taupo. Interestingly Okataina formed a shallow 4-5 km magma chamber around that time. An eruption ~25,200 years ago (basically contemporaneous) was the first to tap shallow storage, and initiated the series of voluminous fissure eruptions that continues to present without much change. There are some changes that took place separately in each volcano, like the collapse at Taupo in 200 AD. However the seemingly simultaneous change that both volcanoes experienced around 25,300 years suggests it was something more than a coincidence.
The details of how Taupo and Okataina seem to have received magma at the same time remain speculative. Storage at Okataina is right now very similar to storage at Taupo before Oruanui, could it be a pre-caldera stage? If it goes caldera it could very well be a VEI-8. Perhaps it’s a matter of when the next major regional influx of magma will reach Taupo and Okataina. A 3rd large influx could send Okataina into collapse like it happened with Oruanui, and with Rotoiti before that. It is impossible to know if or when that could happen but the possibility it could take place somewhere in the next several thousand years makes Okataina the volcano with the best cards to produce Earth’s next VEI-8 eruption. New Zealand blows again!
Information and research
On the Payenia flare-up and plateau basalts:
Okataina and Taupo:
Photographs and description of Lake Rotomahana geysers: