We continue with a list of volcanoes capable of producing a VEI-8 eruption from least to most likely and taking into account the factors that I explained in the previous post, here.
Next volcano capable of a VEI-8 is…
6. Long Valley-Mono (USA)
The volcanism of the western United States can give you a headache in the attempt to understand it: subduction, rifting, a flood basalt, a likely hotspot and volcanoes everywhere, at places where they defy the 3 classical volcanism explanations (including Long Valley). The “weird” volcanoes that are located randomly over the country could mostly be explained by extension and a hot mantle, which are conditions prevailing over much of this area and that create zones of melting. Long Valley Caldera can be grouped with the rest of random volcanoes, it is located along a rift-like feature, Owens Valley, that also includes Big Pine Volcanic Field and the not-yet-caldera Coso Volcanic Field. It’s also a setting with tectonic shearing as shown by the 2019 Ridgecrest Earthquake.
At a smaller scale the Long Valley area is also very complex, with Long Valley, Mammoth Mountain, the Inyo Craters, the Mono Craters and Mono Lake volcanoes.
Long Valley’s history starts as a broad basalt-andesite field of voluminous plateau flows and shields, 4-2 million years ago, with dacitic eruptions showing up towards the end. This follows a common pattern for most caldera complexes. At 2-1 Ma rhyolite eruptions took place from Glass Mountain at the future east rim of the caldera, these included lava domes and plinian eruptions with some reaching VEI 6. The caldera-formation took place at 760 ka and erupted around 790 km3, falling short of a VEI-8.
Since the caldera, eruptions have taken place from Long Valley but also from the new eruption centres of Mammoth, Inyo, Mono and Mono Lake. First I should say that Mammoth and Long Valley are likely parts of the same system, they are so close to each other it would practically be impossible that their extensive mush systems were not joined at depth, plus their recent unrest also argues in favour of this.
Unrest started in 1978 after a tectonic swarm that included M6 earthquakes. Mammoth Mountain and Long Valley started to inflate at about the same time while long period earthquakes and spasmodic tremors, 10-25 km deep, took place under Mammoth. Together with strong emissions of CO2 this suggested that magma was rising below Mammoth Mountain. There have been some fatalities due to people falling into CO2 filled snow-pits and large areas of forest has been killed. Long Valley has kept inflating ever since. Basaltic eruptions are spread around the central dacites of Mammoth Mountain, this also suggests deep magma rises upwards through here as a feeder conduit and it could be responsible for supplying, heating and inflating the centre of Long Valley. So it would be the Long Valley-Mammoth system.
Further north the Mono Craters form an arcuate structure of domes and tuff rings, possibly representing the ring fault of an independent rhyolite magma chamber that would be in the low-VEI7 size range. Some eruptions from Mono Lake, even further north, may be from a new system, the youngest and northernmost of the group. Eruptions of the Inyo craters suggest that there may be a connection or bodies of melt growing between Long Valley-Mammoth and Mono so that the two could coalesce in the future.
Eruptions and cryptodome uplift took place 200-100 years ago at Mono Lake, while phreatic explosions took place in Mammoth Mountain at 500 years and eruptions from Mono and Inyo around 600 years ago. The last eruption of Mono was around 0.5 km3. With the ongoing uplift at Long Valley the entire group can be considered to be very active.
So regarding the actual potential for a supereruption, what are the chances? Separately Mono and Long Valley have a size where they would most likely result in VEI-7 eruptions if they collapsed. They would be capable of a VEI-8 only if they coalesced into a single melt body. The eruptions taking place at Inyo in between them shows this is possible and perhaps starting. However in their current state it is out of their reach, with the small volume of recent eruptions showing that the amounts of melt assembled are certainly below that needed for a VEI-8. Major changes in their internal structure and a lot of further melting is going to be needed if Long Valley wants to go for the 8.
5. Yellowstone (USA)
This one is going to be controversial, I can already feel it. Some are maybe surprised to find it here while other would argue it should be the number 1 (though most of the latter may have been scared away by previous VC posts on this subject). Either way I hope to show that none of this 2 extremes are very accurate.
First of all, plume or no plume? To me Yellowstone shows the characteristics you would expect from a deep-seated mantle plume. A head that impacted against North America in the youngest flood basalt of our Planet, the 17-16 Ma Columbia River Basalt Group. A plume tail that has created a perfectly obvious trail of calderas as the overlying plate moved over it, including at least 11 supereruptions. It is usually active over one location for a certain period then shifts forth to its next location, like oceanic shield volcano trails but with calderas.
Lavas of Yellowstone have a high 3He isotopic ratio, rocks with this chemistry are thought to originate from the deep mantle. All of this makes the strongest evidence, I believe, for a continental deep-origin hotspot. But… tomography hasn’t found any deep origin for Yellowstone yet. This opens a window for alternate theories to show up regarding how it formed. There are also a lot of theories as to why tomography hasn’t found it, like resolution being too bad, the Farallon Slab messing with the plume, or maybe it is waning and the deeper parts have already dissipated… I personally think that the techniques used to image the mantle are not precise enough to image narrow plume tails, so I will go with Yellowstone being a deep origin hotspot.
Perhaps as a result of its distinctive origin, Yellowstone has an internal structure that is unusual for large caldera systems. Yellowstone has a Lower Crust body with ~46,000 km3 of basalt crystal mush at only 2% melt, then a second Upper Crust body that contains ~4,000 km3 of rhyolite at an estimated 30% melt. This makes Yellowstone strongly bimodal, it produces basaltic and rhyolitic eruptions but not the stuff in between, excluding a very few exceptions. This contrasts with the Central Andes (and other subduction zone calderas) which typically have a very large andesite mush body and produce a more varied suite of magmas including basalt, andesite, dacite and rhyolite.
There is something else that is very distinctive of Yellowstone, apart from the bisons and the bears: there is an abundance of geysers. Here is found the greatest concentration of geysers, mud pots, hot springs and fumaroles on Earth. There are also hydrothermal explosion craters that have ejected rock kilometres away, making them one of the greatest hazards that Yellowstone currently poses. The enormous heat flow (estimated at ~6.5 GW) that is being released by the hydrothermal system is too great to come from its shallow magma reservoir, so it is actually thought to originate from deeper convecting basaltic magmas that also result in a high emission of CO2. Instead rhyolite gives the building material for geysers, the silica precipitates that form conduits which then transport heated water to the surface. So the combination of basalts as the heat source plus rhyolite seems to be what gives Yellowstone its famous geysers.
This caldera is really interesting for study, that’s for sure, but what about its future? Some have argued that as the thick North American Craton moves over the hotspot it will be the end of it, and I guess that could be true. The thick and cold cratons are practically devoid of volcanism. The thing is that regardless what happens when activity shifts eastwards, now at its current location, Yellowstone can, and probably will, supererupt again.
The caldera last collapsed in the 1000 km3 Lava Creek eruption at 0.6 Ma, but the story doesn’t end there. Lava domes have kept being erupted mostly in 3 pulses: 0.52-0.48, 0.16-0.15 and 0.11-0.07 Ma. The last pulse has involved lava flows erupting along a western ring fault that delineates the Lava Creek eruption caldera. These were HUGE rhyolite eruptions, for example the youngest one, the Pitchstone Plateau flow, had a volume of 70-80 km3. It’s so big that the shallow reservoir of Yellowstone seems to fall short in size to explain them. Consider this, even the largest Icelandic fissure eruptions, 1-25 km3 (like Holuraun or Laki) are only able to reach this size because a caldera collapse takes places that keeps re-pressurizing the magma chamber with each collapse event and keeps the eruption going. Kilauea started to collapse in 2018 when less than 0.1 km3 of magma had been removed from its summit. Yellowstone did these enormous flows without the help of any caldera collapse. I am not aware of any other silicic volcano having done any flow of similar size, the largest lava flows of the Altiplano-Puna only reach to 1/3 the size of Yellowstone’s most voluminous.
These large effusive eruptions are another oddity of Yellowstone that hasn’t been given much attention by scientists. I think however the explanation may lie in the basalt once again. If the shallow rhyolitic melt bodies are well connected to deeper bodies of molten basalt that are also very voluminous then these could be backing up the shallow chambers with their pressure and making eruptions bigger. This would be my best guess to an unresolved question.
So summing up, Yellowstone has great volumes of melt, including rhyolite, a long history of supereruptions, a large heat flux and a ring-fault full of lava domes. It is looking as if Yellowstone plans to do another supereruption. Its weak spot is that this volcano likes to sleep, in fact it has been dormant for 70,000 years! And for all we know it could go dormant for another 70,000. Paleo-deformation also reveals that the caldera has been predominantly deflating for the last 15,000 years. That is why Yellowstone is not going to erupt any time soon and it will need a great deal of pressurization and re-melting before it does so.
4. Taupo-Whakamaru (New Zealand)
The North Island of New Zealand is the scenario to an extraordinary group of calderas (the Taupo Volcanic Zone or TVZ). Its formation is only relatively recent but is in the context of longer changes that have been taking place in this area for the past 16 million years. An ancient arc has migrated towards the trench creating a number of now forgotten calderas and volcanoes. This arc migration must be due to a steepening of subduction and is still ongoing at the southwest end of the island. This is demonstrated by Taranaki where a chain of 3 stratovolcanoes become younger towards the trench. The Central Taupo Zone where the most interesting action is placed became settled at its location around 2 million years ago. However rollback continued to stretch the island and create a rifting arc.
The first large caldera system considered as part of the TVZ was Mangakino which became active around 1.6 million years ago. It produced many large eruptions, including 4 VEI-8 events. However the location of most current calderas was at this time still dominated by andesite volcanism. It wasn’t until 0.9-0.7 Ma that the volcanism became predominantly rhyolitic while rifting also was accelerating.
Most silicic flare-ups are known to be episodic, in a series of steps, and one of the best researched cases is the 350,000-280,000 years old sequence at the TVZ. No less than 7 caldera forming events took place during this period and within 90 km of each other!
The majestic opening was the >2200 km3 eruption of Whakamaru Caldera. A series of VEI-7 eruptions followed. First came 3 collapses at Kaingaroa and 1 at Okataina. At around 300,000 years voluminous lava domes erupting through the west ring fault of Whakamaru Caldera created a natural dam to the Waikato River and turned most of the Central Taupo Zone into a lake (which has been called Lake Huka). Ohakuri and Rotorua calderas then collapsed simultaneously in a rare event. Finally the collapse of Reporoa around 280,000 years ago closed this period of activity and the Taupo Zone sank into a time of relative quiet in which nothing very remarkable happened.
What makes flare-ups be episodic is not exactly known. One possible explanation is that this is due to rifting. Melting of the crust weakens it and makes it easier for rifts to develop. Rifting in turn draws more magma from the mantle and generates more melting, tectonic and magmatic activity are closely related to each other in these settings and are a feedback to each other. Whatever the causes, past activity has been highly variable and evidence suggests that the TVZ may have entered a new period of high activity 55,000 years ago (the best of which could still be to come). This last resurgence has seen very high activity from Okataina volcano to the north and the emergence of a new centre, Taupo, to the south.
Although eruptions had been happening in the Taupo area for a long time, the first ones that can be certainly attributed to Taupo as a separate volcano date back to only 50,000 years. They consisted of several VEI-5 eruptions with a magma chemistry very similar to the later caldera-forming event. There would have also been lava domes and flows, but this is hard to know for sure since they must have been engulfed by the later collapse. At 25,000 years, Taupo produced the youngest VEI-8 of the planet, the Oruanui eruption, with a volume of 1200 km3. Some research has been trying to figure out how long it took for the magma body that fed this eruption to form. Some argue that it took less than 3000 years, after its next oldest eruption. This is because a younger group of zircons (a type of mineral that can be found in magmas) shows in the Oruanui ejecta that wasn’t in the older VEI-5 events. However there were also many zircons similar in age to those of previous eruptions so I don’t think it was so simple. Part of its growth was probably rejuvenation of partially molten mushes belonging to the old Whakamaru system, while new melt intrusions must have been supplied over some tens of thousands of years and finally some last powerful deliveries of melt may have entered Taupo just before Oruanui that introduced the new zircons and expanded the pre-existing melt body.
The speed at which Taupo grew was spectacular and so has been its recovery after the eruption. Studies reveal that Oruanui destroyed the entire shallow magma chamber of Taupo. Yet it has regrown new shallow magma bodies, erupted 25 times in the last 12,000 years and collapsed once again in its ~200 AD eruption.
Even though the volume “only” amounts to a bulk of 45 km3 ,the 200 AD eruption is remarkable for its violence. During one of its phases it produced a 50 km high plume into the atmosphere, inspiring the term ultraplinian. It was immediately followed by an ignimbrite that would have blasted into oblivion anything less than 80 km from the vent (you can imagine all vegetation and animals within this radius perishing under a pyroclastic current, a burning hurricane). The ignimbrite was as far reaching as those of Oruanui. Even though this eruption was technically a VEI-6 it seems to have reached an intensity more becoming of a VEI-8, if it was for a brief time only.
I do wonder if the 200 AD eruption destroyed all of the melt body that had been assembled after Oruanui or not. Once a caldera collapse starts it sustains itself because each collapse event rises the magma chamber pressure so that it often stops only once the entire chamber is destroyed. The 200 AD eruption could have well brought Taupo back to the starting line, to build itself from the mushes. But its small size also opens the possibility that it only evacuated a fraction of the melt down there. Taupo still should need some time to recover before another VEI-8, particularly if the 200 AD eruption evacuated most or all of the magma, but it is also a volcano that can evolve very fast. Melt bodies could form within the neighbouring Whakamaru system as well, but there is nothing to indicate this is happening now.
Taupo remains dangerous for smaller eruptions in the VEI 4-6 range, that are very frequent. I have also wondered about the possibility of floods of water running down Waikato River which drains Lake Taupo. It is densely populated downstream and I don’t know what would happen to the dams in between, whether they could hold or not against potential releases of water or pyroclastic currents.
There has always been an aura of mystery surrounding Taupo. Early volcanologists were perplexed as to why Taupo seemed to be a reverse volcano, a depression that deepened inward into its dark waters, instead of a conical mountains like most other volcanoes. Now we now that volcanoes come in various form, not just cones, but other mysteries remain. Why did the strange 200 AD eruption happened and why was it so powerful? or when and how did Taupo form and how it related to the wider tectonic processes? A lot remains to be known.
To be continued…
Information and research
On Long Valley:
Taupo recovers from Oruanui, zircon chronology:
The Taupo Volcanic Zone:
Overview on Taupo: