Batholiths and flare-ups

An eruption that ejects more than 1000 km3 of material (ash, pumice, rock…) is considered a super-eruption, a VEI-8. These represent the greatest volcanic events that have taken place during human existence. Such apocalyptic phenomena attract a lot of attention, from scientists, volcanoholics and doomsayers.

The term supervolcano has become increasingly popular but also increasingly misused. Why is the low VEI-7 Campi Flegrei being called a supervolcano? The term has the problem that it seems to imply that a so-called “supervolcano” will blow gigantic again and as pointed out by some scientists this is not true. We have volcanoes that have done super-eruptions but won’t do them again and others that have not, but will at some point in the future. So regardless of a volcano having done one of these, how do we know what actual potential a volcano can have of producing them?

The Fish Canyon Tuff that resulted from the 27.8 MY 5,000+ km3 eruption of the La Garita Caldera (Wiki/Wheeler)

Problem, as always with volcanology, is that things happen deep below where we cannot see them. A meteorologist can watch a thunderstorm being born, but the birth of a volcanic eruption takes place within the crust, often many kilometres down. Indirect geophysical and geochemical methods are all we have to study these processes. Knowledge on how volcanic caldera systems grow and collapse has been evolving recently. In light of these developments I am here to answer a personal question that some of you may have asked yourselves. Where will the next VEI-8 super-eruption of the planet take place? And since he who asks a question cannot avoid the answer (particularly if you ask it compulsively), I shall give my personal view based on the new paradigms that are emerging in volcanology.

The rollback model

Before I can answer that question we should know how caldera systems grow, in general. They do not show up randomly over the world but actually burst into clusters of systems that erupt over a short period on time. Such upticks of calderas, ignimbrites (giant pyroclastic flows), and lava domes are called silicic flare-ups. Understanding them is the first step. The vast majority of flare-ups occur in subduction zones and are related to the same process, this being the transition from flat slab subduction to steep slab subduction through slab roll-back which results in the steepening of the subduction angle. These may be, to you, a lot of new terms, but important terms when talking calderas.

Sierra Madre Occidental with more than 400,000 km3 is the mother of all flare-ups (if we ignore Australia) and can be used as a model. Western United States and México used to be in a period of flat-slab subduction called the Laramide Orogeny/Magmatism during which volcanic activity shifted away from the trench and was relatively low. The Laramide slab detached around 50 million years ago triggering a chain of events. At 45-30 million years the subducting slab underwent rollback towards the trench. Mainly andesitic volcanism and some ignimbrites retreated trenchward together with the slab and preceded the spectacular silicic flare-up at 30-20 Ma that covered much of western México in up to a kilometre of ignimbrites. This happened together with extension that eventually culminated in rifting of the Gulf of California at 18 Ma.

This is the Tran-Mexican Volcanic Belt. Note the change from steep to flat subduction 20-10 Ma, and then how the flat slab is plunging back into the mantle through slab rollback at present times. Drawn by Molear3, Wikimedia.

Let’s look more closely at this cascading sequence of events, because it seems to repeat in a remarkably similar way in most flare-ups. Where does all the magma erupted in ignimbrites and lava domes come from? The first element is water and it comes from the subducting slab. During flat-slab subduction volcanism often stops completely but water keeps seeping from the oceanic subducted crust and sediments into the overlying lithosphere mantle and continental crust, the crust becomes hydrated which lowers the melting temperature of the rock and makes it easier to turn it into magma later on.

A slab detachment usually leads to the flat-slab plunging into the mantle backwards towards the trench. This is called rollback. The growing mantle wedge creates upwelling of the hot asthenosphere, which provides heat, and decompresses as it ascends. The overlying volcanic arc is stretched which creates widespread extension. Melting results from water, heat and decompression; this is the perfect cocktail. It is interesting to note that for the Sierra Madre Occidental the proximity to the East Pacific Rise seems to play as a factor. North America is moving over the East Pacific Rise, this perhaps promotes the mantle being hotter or helps create extension. The result is that basaltic melt intrudes into the lower crust and thickens it while the lithospheric mantle melts or delaminates and is replaced by the hotter asthenospheric mantle (<1300°C). The crust in turn melts as well. This generates enormous batholiths reaching up to hundreds of kilometres across where clusters of large calderas develop, and hence flare-ups and super-eruptions. Widespread extension provides pathways for the magma to go up and can climax into the formation of a proper focused rift. The most intense flare-ups seem to be those in which the extension succeeds to progress into a rift, like the Taupo Volcanic Zone, Sierra Madre Occidental, and if we go further back in time, Australia. [caption id="attachment_11068" align="alignnone" width="681"] The subduction angle is varied. Also note the 2 prominent flat slab areas in the Andes. These are the Peruvian and Pampean flat-slabs and are areas where volcanic activity has died off. At some point they will steepen and unleash flare-ups. By Gavin P. Hayes, USGS.[/caption]


Earth’s distribution of large caldera systems is not even. Most tend to be concentrated in those areas where the transition from flat slab to steep slab has taken place. An example is the Central Andes. The Altiplano flat slab started to disappear around 30 Ma in southern Peru. The resulting flare-up is already over but was followed by steepening of the Altiplano flat slab further south at 12 Ma. This second episode created the remarkable Altiplano-Puna batholith and related calderas, among others. The flare-up is still active although in apparent decline.

The next largest active flare-up is taking place in New Zealand, here the Taupo Volcanic Zone is related to a rollback affecting the entire Tonga-Kermadec arc that has resulted in a 3000 km long back-arc rift (the Lau Basin). The Taupo Rift is just its southern continental end. At its northern end the Lau Basin is spreading at 15 cm/year as the Tonga Arc moves over the Pacific Plate in the fastest subduction of the planet (24 cm/year). However only the continental crust of New Zealand has resulted in significant ignimbrite activity, here the volcanic arc migrated east, towards the trench, over the last 12 million years, presumably in a transition from flat slab to steep slab subduction, though I haven’t found this mentioned anywhere. Andesitic volcanism was followed by ignimbrites and calderas starting at 1.6 Ma concentrated over a 120 km segment of rift. Despite the relatively small area and short duration, the Central Taupo Rift has already produced a volume of around 20,000 km3. It can be considered to be at its peak strength.

Note how an ancient arc was split in two by rifting of the Lau Basin. At the Tonga Arc the crust is too thin to sustain large caldera complexes, but numerous VEI 6-7 size calderas exist.

While these are the 2 major flare-ups of our times there are also a number of smaller ones active throughout the world. Southern Kyushu, Japan, is another good example, where a brief steepening of the subduction at 5-3 Ma has resulted in a trenchward migration of the volcanic arc and extension. A chain of calderas (Kikai, Ata, Aira and Kakuto) line up with the Kagoshima Graben.

Aira Caldera and Sakurajima volcano. It is located within an extensional structure, the Kagoshima Graben which can be seen running down to the lower right on the image. NASA.

Back to the Andes, but further south, we find the Payenia flat slab, which stopped being flat some time 5-2 Ma ago. This one is quite remarkable because it has resulted in the formation of a mini flood basalt. Over the past 2 million years 8400 km3 of basalt have been erupted in the back-arc through shield volcanoes and volcanic fields while large calderas and ignimbrites erupted through a series of grabens in the volcanic arc. The basaltic volcanism is thought to be a result of the asthenosphere upwelling in response to the slab rollback and a shallow mantle plume has been imaged below Payún Matrú shield volcano. The rollback process seems complete except perhaps in the northern parts of the slab. Here the Calabozos and Laguna del Maule large caldera systems remain very active. The latter is currently inflating at more than 20 cm/year, faster than any other silicic system on Earth.

Massive Payún Matrú shield volcano is a result of mantle upwelling behind slab rollback.

Continental rifts and hotspots

Almost all major silicic volcanic provinces can be related to the steepening of subduction and ensuing extension/rifting. This probably applies, I suspect, to areas of intense ignimbrite activity in Hokkaido (Japan) and the Eastern Volcanic Zone of Kamchatka, although I couldn’t find scientific literature on this subject. However there are other settings in which calderas form. The key element as we will later see is melting of large volumes of the crust and the formation of batholiths.

The Yellowstone hotspot has produced at least 11 super-eruptions throughout its history. Here the main element is heat. Yellowstone shows that a powerful hotspot can manage to melt the crust and generate super-eruptions. Then why is it such a rare case? The reason probably is that Yellowstone is the only strong deep-seated mantle plume located under continental crust, the others are located in the ocean where they form shield volcanoes instead.

A third situation that can result in large calderas is continental rifting. This is shown by the East African Rift where many VEI-7 sized calderas and at least 1 VEI-8, Awasa Caldera, occur. In East Africa melting is due to decompression, but also to an anomalously hot mantle related to the Afar Plume, which dramatically showed up 30 million years ago kick-starting rifting.

And then we find the Tibesti Mountains which form a cluster of sizable calderas (VEI-7) in the middle of the Sahara, far from rifts, hotspots or subduction zones. So how do we explain this oddball? My guess is probably extension and asthenosphere mantle upwelling which often results in basaltic volcanic fields but in extreme cases may evolve into something more exciting than that.

A volcanic range in the Sahara that is the size of Iceland! The Tibesti Mountains. Large calderas and ignimbrites are visible. NASA.

A final interesting kind of silicic flare-ups that I see are those which happen in small localized rifts within subduction zones. These include the Macolod Corridor in the Philippines, the Managua Graben in Nicaragua, the San Salvador Graben in El Salvador and the Bay of Naples in Italy. They have some things in common that may be coincidental or not. Each has pair of one or two silicic caldera systems next to a mafic caldera system: Laguna de Bay-Taal, Apoyeque-Masaya, Ilopango-San Salvador and Campi Flegrei-Vesuvius. Each happens to have a city of more than 2 million inhabitants in it, which makes all 4 of these volcanic areas among the most hazardous on Earth.

Masaya and Apoyeque dangerously flank Managua from each side. They have contrasting magma compositions where Masaya is basaltic while Apoyeque is andesitic-rhyodacitic


So we have seen the main settings and locations where large calderas form but now it is time to dissect a particular system and see how it works in the inside.

Both observations and models are showing that the old magma chamber idea falls short to capture the complexity of a volcano’s internal plumbing. Instead it turns out that volcanic systems are formed by extensive networks of melt lenses and areas of crystal mush that extend from the mantle to the surface in most cases. What is a crystal mush? This term refers to rock that is only partially molten, with less than 50 % of melt, and the rest is solid crystals. You can picture it like a sponge of crystals that is soaked with melt. Most of the volume stored within a volcano is probably in this state, and mush is not eruptible. The actual magma is ponding in layers or lenses. Multiple such layers can exist from the MOHO all the way to the surface and containing magma of different chemistries.

From Wikimedia, by Julien.leuthold.

Different kinds of volcanoes have different internal structures. A spreading ridge and a large caldera are going to have completely different insides! A spreading ridge for example has a mush of basaltic composition (gabbro) that extends vertically only the few kilometres from the MOHO to the brittle crust. The mush will be narrow but long, extending below the axis of the spreading ridge for up to hundreds of kilometres. Eruptible magmas will form only very small lenses at the top of the mush that feed dikes and small eruptions. On the other hand a large caldera system will have mush bodies that extend vertically over tens of kilometres, horizontally tens or even hundreds of kilometres, both long and wide, and which can host many magma lenses of various chemical compositions, with some containing very large volumes. We can use the Altiplano-Puna of the Andes as the model for a large caldera plumbing system.

The Altiplano-Puna system has all the rights to be called a batholith, the largest of intrusions. It connects about 10 large caldera systems and many tens of stratovolcanoes down to a large single mush body that is about 350×180 km horizontally. Geophysical and geochemical observations reveal that the inner structure is layered like a cake, with 3 main levels of storage.

The first is formed by melting of the mantle and its intrusion and ponding in the lower crust, which forms a mush zone at 85-40 km depth. The scientific term for these zones is MASH, which stands for Melting, Assimilation, Storage and Homogenization. This first level is basaltic, mafic and ultramafic in composition. The second level is the famous Altiplano-Puna Magma Body (APMB) which is a 500,000 km3 body of andesite with up to 25 % melt, a roof at a depth of 15 km bsl and a temperature of 1000 °C.  The active stratovolcanoes feed from this body. The third level is formed by a series of shallow reservoirs that sort of branch upwards from the AMPB. These are located 3-8 km deep and contain dacites and rhyolites that fed past super-eruptions, ignimbrites and lava domes. The shallow reservoirs were active mostly 8-4 million years ago during a series of activity pulses, however mantle melting seems to have declined afterwards and these reservoirs are mostly frozen. So much for the Altiplano-Puna!

Models show that these systems probably develop from the bottom up, and the eruptive history of many large caldera systems, like Taupo, seems to support this, because andesitic volcanism precedes rhyolitic, or in other words a deeper mush zone that feeds andesite eruptions forms before shallow reservoirs that feed the rhyolite eruptions. Models predict that the lower crust needs to melt first in order to maintain a higher heat flux into the upper crust that would allow reservoirs to form there. So it seems reasonable to believe that caldera systems in general grow upwards from the mantle.

How magma travels upwards is also important. Magma can intrude through cracking, the formation of sills and dykes. Another way is porous flow where melt travels up through the mush using the spaces in between crystals, but this is very slow and only works over long timescales. More intriguing are overturning events in which an amount of melt builds up in a certain level and becomes unstable due to buoyancy so that it moves up to an overlying level through a diapir/channel/dyke. These events could explain the evidence for magma chambers being generated very quickly before an eruption.  Overturning events could deliver large intrusions of melt proportional to the size of the body/MASH that the melt originates from, this could rapidly assemble melt at the top of the system, increase temperature, turn rock and mush into new melt and create overpressure due to decompression of gasses.

This is how the Altiplano-Puna Complex might have looked at its peak, some million of years ago. Multiple melt bodies exist and there may be many modes of upward magma migration, the crust thickens and melt evolves in its ascent towards silica rich granite-like compositions. Own creation based on tomography slices of the region.

Volcanic fields

The size of the shallow reservoir is what determines if a volcano is capable of producing VEI-7 or VEI-8 super-eruptions. There needs to be a sufficient volume of eruptible dacite or rhyolite magma built up. The calderas that result from super-eruptions are very wide but with a downdrop that is not too different from smaller calderas. This shows reservoirs are relatively flat, sill-like, and that to build up enough volume they must be laterally extensive. For example stratovolcanoes have mush systems that are vertically extensive because they reach to the surface, but are often very narrow so they tend collapse into VEI-6 to low VEI-7 calderas. Instead the largest calderas would be expected to form from volcanic fields and broad stratovolcano complexes that are wider in area.

Caldera systems are in fact closely related to volcanic fields, this is why they themselves are often called volcanic fields. Melting of the lower crust or uppermost mantle over a wide area would reflect at the surface as a basaltic volcanic field, Mayotte is an example of such a system. Here a dyke in 2018 intruded directly from the MOHO (mantle) and resulted in the formation of a new vent, a monogenetic volcano that only erupts once. Melting of the upper crust instead results in the formation of andesitic, dacitic or rhyolitic volcanic fields which are much more rare. Los Humeros in México is an example of a system that seems to have evolved from an andesite VF, to a rhyolite VF and later collapsing into a large caldera.

Andesitic volcanic fields are a relatively rare occurrence in the world and may show the areas where magma bodies similar to the Altiplano-Puna are forming which gives an idea where new flare-ups are emerging. BY FAR the most spectacular ones are located in Mexico.

In 1943 a new volcano emerged from a cornfield in the State of Michoacán, México, this volcano would come to be known as Paricutín. Early this year, 2020, a volcano-tectonic swarm took place in this area which has been later been interpreted as a sill intrusion, reminding how active this location is. If we go back to 1250 AD a 9 km3 monogenetic shield (El Metate) formed close to the current location of Paricutin. This is just one part of the Trans-Mexican Volcanic Belt that is everywhere covered in volcanic fields of dominantly andesite lava. In some areas monogenetic shields cover the entire landscape so tightly that they overap on each other, what does this mean? It means that the upper crust is melting almost everywhere along the 1000 km of the Trans-Mexican Belt.

Paricutin and other nearby monogenetic volcanoes.

If we look at Mexico it has all the ingredients. The lithospheric mantle has almost disappeared (considered to have melted) below the active volcanic arc so that hotter than 1300 degrees asthenosphere lies directly below the crust. Rollback of a flat slab is taking place and will still take some millions of years to be completed. Volcanoes are running southward as the slab retreats. Chains of stratovolcanoes such as Tlaloc-Telapon-Iztacciuatl-Popocatepetl, are being created as the stratovolcano chases the sweet spot of water release at which the slab sinks to 110 km; this leaves curious mini hotspot-like trails. Additionally México is under widespread extension with grabens popping all over the place, and there is the threat of the East Pacific Rise jumping into México like it did at the Gulf of California, perhaps bringing a Sierra Madre Occidental all over again!

The Trans Mexican Volcanic Belt is shown in grey. The flat slab is being eaten in a clockwise rotation of the arc. White lines mark trails of stratovolcanoes formed by southward migration of fluid plumes that come from the subducting plate. Colima, Popocatepetl and Pico de Orizaba are at the leading edge of 3 such trails. A wider flat slab to the east creates the unusual oblique orientation of the Mexican arc plus a relative gap in activity between Pico de Orizaba and the CAVA. (Adapted from Macías 2007)

Another illustration showing the evolution of the Trans-Mexican Belt. The Miocene-Pliocene mafic pulse is thought to have been triggered by an eastward propagating tear/detachment of the subducted slab. Subsequent silicic volcanism was due to the opening of the Tepic-Zacoalco and Chapala Rifts. Ever since the mafic pulse the arc is rotating clockwise towards the trench. Andesitic volcanism predominates now. Drawn by Molear3, Wikimedia.

Another promising area is the western United States. Rollback has been taking place over the past 10 million years in Oregon and Washington with a westward migration in volcanism. Some sizable volcanic fields dot the area with monogenetic shields. The nearby East Pacific Rise could try to jump eastward onto the continent and long Iceland-like fissures around Three Sisters hint at rifting. Newberry, Shasta, Medicine Lake, Lassen and Three Sisters all look like potential VEI-7 volcanoes to me and each could pull off a Crater Lake. The hot mantle and widespread extension also give birth to silicic systems at unexpected locations, like Coso or Clear Lake. So this is another emerging flare-up.

Cascadia Subduction Zone. From USGS,

The very largest

By this point I must have mentioned nearly all flare-ups of the world. Those that were, those that are and even those that will come. It is from these areas that many VEI-7 eruptions will originate in the future, but also the even greater almighty super-eruptions will come from them. But which volcano shall be the next VEI-8. This question will be answered in upcoming articles!

Research and further reading

Mike Poland wants to end the supervolcano word:

Rollback and back-arc basin opening recreated in an animated model:

Sierra Madre Occidental was followed by opening of the Gulf of California:

Andes likes to alternate flat slab with steep slab:

Meet the Payenian flare-up and flood basalt:

Mushes, magmas, and even more mushes:

Bottom-up growth of magmatic systems:

The extraordinary history of the Trans-Mexican Belt:

Rotating the Cascades:

61 thoughts on “Batholiths and flare-ups

  1. Thank you Hector for this masterpiece
    You are also very good at drawing!
    Time for you to do volcanic art and geological art

    • Yes time for you to draw all kinds of volcanoes, like flood basalts, volcanoes on IO, Kilaueas magma system, plumbing systems, calderas

      The magma systems of large sillic calderas in subduction continetal belts, are one of the most complex of all magma systems. Magma must rise through a rather thick sillicous crust. So lots of evolution and storage, crystalizing

  2. Thank you.
    Interesting convergence with recent posts in ‘Cafe’ about part of Hawaii’s missing plume-head found deeep under Siberia, and the oft-lively consequences of subducting slabs splitting and providing a ‘window’ for ascending magma.

    I must wonder if a slab roll-back may contribute such.

    If part of a roll-back hinge fails, it would seem to offer an elongated window rather than a mere ‘hotspot’ plume. Would this be related to ‘Back-Arc’ spreading ??

    Also, thank you for mentioning both the ‘Bay of Naples’ and the Tibesti Massif. Until the latter’s troubles settle down, it will remain less instrumented than ruddy Mars !!

    • I am not sure of the exact role the slab detaching plays, it is maybe what starts the rollback. Sierra Madre Occidental started after the “Big Break” of the Farallon Slab. Perhaps it creates the window for a mantle plume. But ultimately rollback would be what pulls the mantle up by growing the mantle wedge and rifting.

  3. Ref: “Mike Poland wants to end the supervolcano word” EXCELLENT. It is BS hype used to push interest in a video presentation, nothing more. If you really want to be more precise, “Large Caldera Event” is much more accurate. That would entail anything in the upper half of caldera eruptions and would at least have SOME sort of meaning.

    • From what I remember of a ‘New Scientist’ interview with one of the UK Volcanologists invited to study Paektu, the many years without adequate instrumentation meant it was a bit of an anomaly.

      Being on the Chinese border and ‘just too close’ to Kim’s nuclear testing site meant a triple dose of paranoia. Hence a lot of UK/US geo-phys instruments were refused export licences as their tech could have been reverse-engineered and re-purposed for eg sub-hunting…

      IIRC, Paektu is in the wrong place. Really the wrong place. Really, really the wrong place. Did I mention it is in the wrong place ?? Like Tibesti Massif, itsa ‘Who Ordered That’ Volcano…

      Finding more chunks of delaminated slabs may go some way to explaining their workings…

      Distantly tangential, what’s driving Lake Baikal rifting ? Is it simply far-field effects of Indian plate whupping and skewing Asian plate, or is there something more profound ??

      • Just watched the video. Wow. ‘Horizontal Slabs’ !! Very well made, wondrously well presented. My dear wife (RIP) would have loved it, too…

        • I really like the presentation as well. And now I see Hector here presenting slab rollback while the video suggest actually quite the opposite for some volcanoes so even not that much offtopic!

          Hector, beautiful article. thank you.

  4. Are you challenging me!? You’re intruding on MY territory. Calderas are my lovely partner and everlasting love?! I will take this as a declaration of war and prepare MY series accordingly!

    Nice article by the way.

  5. Hector, big fan of your articles. The way you approach the question where the next VEI 7/8s will be is both structured and reasonable. Looking forward to your next article, thanks for the effort and time!

  6. Im not even sure I should submit my article now, a month ago it would have been fine but now I dont think I can compare with this… 🙂

      • Sorry to say but neither of my articles in the making is in your neck of the woods, both concern effusive basaltic volcanoes probably to no-ones surprise. One is perhaps well known by now after what happened in 2018 but looks can be deceptive, the other is on the only active volcano in Australia and not the one you think.


  7. Hi Folks – Good morning.

    From reading online and as i understand it the magma supply for Vesuvius in Italy enters that volcanic system through what is proposed to be a ‘window’ or break in the sub-ducting slab.

    Is the source of the Campi Flegrei the same? Is it diferent? If different, (considering that Campi Flegrei is about 30km away) what/why are there tw odifferent process driving different volcanoes relatively close together?



    • They are close enough to share a same magmatic reservoir at depth, I don’t know of any particular evidence though. The way they erupt and the magma composition are so different, not sure why, but as mentioned in the post similar striking contrasts exist between Masaya-Apoyeque or San Salvador-Ilopango, where much like in the Bay of Naples each pair is located within some sort of extensional basin.

    • They are both produced by the same process – subduction of the African Plate (Apulian Plate) under the Eurasian Plate. Vesuvius is a cone of Mt Somma and sits on the caldera, whereas Campi Flegrei is the volcanic system (caldera plus cones). They have similar lavas.

      The most obvious reason for any differences is that much of Campi Flegrei is underwater in the Bay of Naples.

      • Not sure whether the water is deep enough to have a significant impact on the constraining pressure but the water would make any eruption more explosive.

        • The whole area looks like its had extensive volcanism in the past. The lago di agnano looks particularly scary. Actually looking at google earth, much of italy seems to be littered with volcanic artefacts.

          • Not forgetting the myriad of middling calderas reaching up the coast and surrounding Rome, as well as the huge edifices in the main extension basin (Tyrrhenian?) – Marsili and Vavilov.

      • If the water is enough to significantly increase the containing pressure, the magma would spend longer in the crust and become more evolved.

        I would need geolurking or someone to advise on how the water does affect the containing pressure.

          • True. The 1538 eruption that produced Monte Nuevo was on land. But much of the caldera is submarine. Richard was asking about differences.

          • What are the lavas associated with calderas at the Gakkel ridge?

            In general, pyroclastics could come from the Gakkel Ridge from: magma evolution or crustal mixing – the ridge is slow spreading.

            There is also evidence for an old subduction zone in the west Gakkel Ridge.

          • The Gakkel caldera is located at the end of the ridge. Suspect that some of the caldera was formed from tectonic activity rather than solely eruptive activity.

        • “I would need geolurking or someone to advise on how the water does affect the containing pressure.”

          Not much of an addition, but I’ll try. Sea water density is about 1030 kg/m³. Below about 2.5 km depth the pressure is high enough that water can not flash to steam no matter how hot you make it even if there is an exposed pathway to the surface filled with water. That 2.5 km value is not exact, but is in that realm.

          Non Basalt crust is around 2700kg/m³. Basalt is approximately 3100kg/m³.

          So any “overburden” made up of water is easily 1670 kg/m³ LESS than a pure rock covering. The main difference being that the water can be more quickly removed by an incipient eruption if it is not too deep. Personally, I am not aware of any caldera lakes that are deeper than that. It’s not impossible, but I just don’t know of any.

          Where this comes into play. Grimsvotn is known for going full-on in short order. My guess is that any Jökulhlaup that occurs rapidly lowers the confining pressure and this enables the quicker nucleation of volcanic gasses entrained in the magma. once it reaches the lower extent of the lake, the remaining water quickly flashes to steam and an energetic phreatomagmatic phase starts the festivities.

          • Think the Bay of Pozzuoli and Naples are shallow from what I can make out; the former reaches a depth of 110m; and, the latter 140m(?).

          • IIRC, the vast ‘Gulf of Naples’, arcing from Ischia via Naples, Vesuvius and Sorrento to Capri, which bears an, um, unsettling resemblance to a flooded mega-caldera, seems to be basement-limestone to a ridiculous depth. No volcanic plume apparent…

      • Which is quietly, quietly extruding its old lava plug…

        ( I’ve excluded some ‘leakage’ around edge of plug causing local historical venting…)

  8. This sounds a bit serious. Icelandic Coast Guard aircraft mechanics go on strike. This was posted today on facebook by Magnús Tumi Guðmundsson (Jarðvísindastofnun Háskólans). Automatic translation from Icelandic:

    “Serious situation due to strike at the Coast Guard
    The text that follows is in no way an official opinion of the Institute of Earth Sciences or the University, as the person writing this has no authority to give such an opinion. What follows is based on my experience over the past 25 years in monitoring and researching eruptions and other natural hazards.
    The grave situation of the Coast Guard’s fleet is now coming to a standstill due to a strike by aircraft mechanics. The Isavia plane, which also plays an important role, is also locked inside due to the strike. No position can be taken in this dispute and the important work that aircraft mechanics do should be valued. But it is a completely unacceptable situation that the organization of affairs is such that the basic elements of the country’s security system are stopped due to a strike.
    Natural disasters often strike at short notice. The avalanches and storms this winter did not have many days’ notice as it was foreseen what was going on. The warning period for eruptions is usually not long, often a few hours. In the eruptions in Gjálp 1996, Grímsvötn in 1998, 2004 and 2011, Hekla 2000, Eyjafjallajökull 2010 and Holuhraun 2014-15, the Coast Guard’s helicopters and aircraft and the Ísavía aircraft played a key role in assessing the situation so that decisions about evacuations and responses could be made. If there is news in the next few days, the situation could arise that the reaction would be very damaged because the aircraft are not airworthy.
    The epidemic has shown us how important it is to prepare for possible shocks. How important it is to have good plans and a strong team of responders who know how to respond. But if the group does not have the tools and equipment needed to get used to the door, it can greatly increase the damage caused by disasters. Therefore, an immediate solution must be found to the current dispute between stopping the Coast Guard and Isavia’s flights and ensuring that this situation does not recur.
    Magnús Tumi Guðmundsson”

    I hope Iceland can put any eruptions on hold until the dispute can be settled.

    • Guess that if you live near an active volcano, an investment in a drone and seismometer might be advisable.

      Don’t know what Icelandic law is in respect of drone usage.

  9. Completely unrelated, but does anyone here know why Mt. Baekdu (Changbai mountain on the Chinese side) exists, as a large volcano in that particular location?

    The best ginseng in the world comes from it’s slopes. Might be the minerals in the soil.

    • I have posted this just above. completely unsure as for relevance, esp. as Hector has slabs rolling back and this video has slabs going flat.

      Why China’s Largest Volcano Is So Unusual

  10. Wow, this was a really good article. You effortlessly wrote what I’ve been trying to say for a very long time, but added in additional stuff on the dynamics of slab rollback.

    For what it’s worth, there seems to be a similar cycle in Italy if I remember correctly from some article that I read a few years back. This is why Italy is littered with large caldera systems, but precious few active volcanoes on the land itself.

    Also, Toba lies in a rather significant rift system as well, although I’m not sure it is a product of slab gab dynamics.

    Separately, I’ve long thought that Kyushu in southern Japan resembles a young taupo ignimbrite flareup, and I’ve been aware of a sinking slab / opening rift there. Do you think that the slab steepening there is potentially still in its infancy, or do you think it’s largely done at this point? Not sure if you know of any information on this.

    • Toba does not obviously fit into this scheme. It is sitting on continental crust, on a big fault, and in an area where rotation causes some extension. Do we need any more?

    • There is rollback at Italy creating rifts (all over the Tyrrhenian and at Naples), also maybe responsible for the mantle upwelling that feeds Etna. I don’t recall there being any flat slab subduction there previously, but I may have missed some of those examples.

      Kyushu resembles Taupo in that there has been a slab steepening that may or may not be over. What makes Taupo special is that the transition from a shallower to a steeper subduction has been followed by continental rifting, 5-19 mm/year, at places faster than the East African Rift. Kyushu is under some extension but there is nothing to indicate a rift is really going to form.

      Most flare-ups do not culminate in rifting, Taupo and Sierra Madre Occidental are rare cases. The Taupo Volcanic Zone is very small in length compared to other major flare-ups so that is why the total volume it has erupted is not that impressive but it’s an intense one.

  11. Sentinel image: image Lewotolo
    The Indonesian Bahasa Wikipedia page says: “On Sunday (29/11), at 9:45 a.m. local time an eruption occurred and the eruption column reached 4,000 meters above the peak. The BNPB Pusdalops is still coordinating the impact of this eruption. Based on the Center for Volcanology and Geological Disaster Mitigation (PVMBG), it was observed that a thick gray ash column tilted to the east and west. This eruption earthquake was recorded on a seismogram with a maximum amplitude of 35 mm and a duration of about 10 minutes. Currently, Mount Iile Lewotolok is still in the status of level III or “standby.”

    That is quite an impressive ash plume for 10 minutes of actively eruption!

  12. I don’t think so, it is a small volcano and has erupted relatively frequently throughout history, and it doesn’t seem to have access to evolved magma. So I don’t think it can produce anything more then a VEI 4 or a low-end VEI 5

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