Big basalt blasts I. The trigger

Rhyolite has more silica, this makes it more viscous, more explosive and in turn more dangerous. Basalt is the opposite, fluid, well-behaved, safe. This could be a phrase out of any geology textbook, I can almost feel some readers getting ahead of me and thinking what I am obliged to say. But there are exceptions!

Exceptions don’t make the rule unimportant, because it is a really important rule. As you turn up the silica, andesite, dacite, rhyolite it becomes increasingly violent until you get to the pinnacle of volcanic explosivity, Taupo, Toba. Yet for a volcanologist, or anyone who wants to dive deep into volcanic knowledge, being aware and understand all the possible situations is key. This is why this series is about the exceptions.

I bring to light an eruption mechanism that has so far been in the shadows. I don’t know of anyone who has described it before separately, which it deserves to be, or explained the way it works, this is possibly because they are rather unusual events, I have only been able to confirm 3 cases since 1900. It is possibly the pinnacle of basaltic explosivity, nothing compared to what a silicic caldera can do, but still quite dangerous, deadly eruptions.

For this kind of eruption it is first important to understand well how caldera collapses happen. Volcanic calderas form in mighty events where a magma reservoir empties and the roof comes falling down over it. This is not exactly slow or fast, it is best described as episodic, the roof falls in series of steps known as collapse-events, each collapse-event generates a peculiar earthquake with very long period waves (VLP) and a focal mechanism called CLVD. The focal mechanism is created by the ring shape of the caldera fault that generates the earthquake. With each time, the roof drops on the top of the reservoir and repressurises it so that it will result in a surge on the ongoing eruption/intrusion.

Image from Brian Shiro, USGS.

Kilauea demonstrated dramatically how this works in 2018 with a total of 62 collapse events, each a M 5.3 earthquake. But the step-like nature of a caldera formation was well know before, Bardarbunga in 2014, Piton de la Fournaise in 2007, every single caldera collapse observed has taken place this same way, even in silicic volcanoes. With Katmai in 1912 it formed in 4 earthquakes of M 6.5-7.0.

There are 2 main ways a caldera can collapse. The first is by explosive eruption which drains the reservoir upwards, this is reserved for silicic volcanoes with high viscosity magmas, dacite, rhyolite and trachyte, once the collapse is ongoing more primitive stuff can get flushed out. The other is when a reservoir drains laterally to feed a dyke intrusion or a lateral eruption, this is far more common in basaltic volcanoes but silicic systems can do it as well.

With that part explained we are off to Galapagos to make a shield volcano blow up.

Fernandina. 1968.

Fernandina rises above the waves of the Pacific Ocean as a classic Galapagos shield, with that overturned soup bowl shape topped by a big caldera that is so popular in Martian volcanoes. This is the fiery heart of the archipelago, the most frequent eruptor and closest volcano to the hotspot. As a good shield volcano its eruptions are effusive, gentle, from fissures, and any other day Fernandina would have just done that. But the 11th of June of 1968, it decided to throw an enormous 0.6-1.9 km3 explosive event, a VEI 4-5, and also resulted in a caldera 1.5 km3 in volume. How does a volcano go from effusive eruptions to this? There is more than meets the surface, lets see.

Looking at the volumes is a good way to start, the caldera is 1.5 km3, it collapsed like a trapdoor which is no surprise since Galapagos is all about trapdoor calderas. There is an additional excavated crater at the eruption vent with a volume of 0.3 km3. The explosion volume is estimated in 0.6-1.9 km3, however we need to consider that only 5-10 % of the eruption was juvenile, this means a very small amount was fresh magma, the rest was old rock, called lithics (gabbro, fragments of lava flows…), at most there was only 0.046 km3 DRE of magma used in the eruption, then where did the caldera come from, and all the lithics?

Fernandina after its collapse in 1968. Note dust from continuing rockfalls. Photo by U.S. Air Force, published in Simkin and Howard, 1970

The lithics seem to come from the crater that was excavated in the caldera wall because when 0.3 km3 of rock are grounded to ash it falls within the range of tephra volume. Almost the entire eruption material came from blasting away the caldera wall.

There is just about one explanation I can see for the 1.5 km3 caldera, a submarine flank eruption. Fernandina is just the tip of the iceberg, it is an enormous mountain that rises 5 kilometers above the ocean floor, its submarine flanks are made of countless craters and fissures. There is enough information to picture how this likely went, first there was a subaerial effusive eruption sighted by a ship 20 days before the explosion, that is May 21, at this point a dyke had reached the surface and formed a fissure. This same dike probably fed the submarine eruption as it kept propagating down the flank, evidence is that the first week of June has intense seismic unrest and earthquakes up to M 4.6. After June 7 three days of seismic quiet follow, sounds like the start of the eruption, the dyke releases pressure and stops producing earthquakes. The volume of lava erupted may well be greater than that of Kilauea 2018 or Holuhraun. Judging from the caldera it formed we are talking of one of the largest effusive eruptions of the post-Laki era.

Fernandina in 1947, note outside of the caldera the concentric ring fissures feeding lava flows, Galapagos volcanoes often erupt this way. Photo by U.S. Air Force, published in Simkin and Howard, 1971.

Now back to the explosion, what caused it? My initial theory (before I started researching for the series) was that some of the most powerful eruptions of basaltic volcanoes were due to a laterally-triggered caldera collapse which causes water to make its way into the magma reservoir and blow up, relatively simplistic, and as it turned out relatively wrong.

First, there was a lake inside the caldera before 1968, everything below the lake must be filled with groundwater, the lake was also there after the events, so yes the water survived the caldera collapse and and a VEI4-5 explosive eruption, water is more resilient than it seems is it? There are few eyewitnesses of the events, Fernandina is remote and uninhabited, the ash was also distributed over the ocean away from the islands. We do know when it started, on the morning of June 11 an enormous pillar of steam rose 22 km into the atmosphere and started to spread out, at this point there was still no ash but this is a huge amount of steam, it starts to become obvious that the eruption was a gigantic steam blast.

The climactic phase starts in the afternoon, a loud powerful boom is heard all over the islands as far as 220 km. A strong earthquake is also felt at that same time, an author estimates it to have been close to M 6 based on intensity reports, however it is missing from the seismic records that should have been more than sensitive enough. I don’t know what to make out of this phantom earthquake. Pyroclastic surges sweep over the west flank of the volcano amidst ceaseless lightning and red flashes, ashfall fell on a ship 350 km away.

At this time the plume is 175 km across, just as the climactic phase starts. Photo by J. Harten, published in Simkin and Howard, 1970.

While it is not known when the eruption ended the strongest phase appears to be over by morning June 12, now problem is, problem with my initial theory, that the first collapse event doesn’t happen until later that day, an M 5.1, from here on the caldera forms in periodic large earthquakes every 6 hours for 2 days followed by more days of decreasing magnitude earthquakes as the collapse gradually slows down. The theory is off somewhere, if collapse was the eruption trigger then the blast would have waited for the first M 5.1.

By this point I had an improved model on how the eruption worked but it wouldn’t be until I inspected my last case of study that I found the strong enough evidence my search was aimed at. The enlightenment came from a japanese volcano.

Miyakejima. 2000.

Miyakejima is an island 150 km to the south of Tokyo, it is best known for gas masks. If you look it up on google a lot of black and white photos of people wearing gas masks will show up, I don’t know why, because actually those images are not from Miyakejima. But it is true that residents were required to carry gas masks at all times when allowed to return to the island in 2005, after a years-long evacuation, they would have to put it on if the volcanic gas concentration rose high enough. The reason for this? After Miyakejima underwent a collapse in 2000 it was followed by more than 2 years of high SO2 emissions. At its peak in late 2000 Miyakejima was releasing 54 kt of sulphur dioxide per day, without a reference that is nothing more than a number, but it happens that this number is greater the estimated degassing of all other non-erupting volcanoes of Earth together, it is higher than the rate of degassing of Kilauea back during its 2018 eruption, and by 2003 the amount of SO2 emitted was similar to that of the 1991 eruption of Pinatubo!

Big question is how Miyakejima managed to produce such massive gas emissions while not erupting. It was suggested that the caldera collapse had been responsible, however Kilauea dropped to less than 1 kt/day after its collapse ended so it can’t be that simple. So far this was a mystery, but no more.

Miyakejima in 2004

In 2000 a 1.2-2.0 km3 dyke and 0.6 km3 caldera collapse hits this corner of the world. You may have noticed the difference between the 2 volumes, the dyke intruded westward for 30 km below the ocean towards nearby Kozushima island, it has been suggested that the volcano of Kozushima started supplying the dyke as well. It is also possible that deep levels of storage at Miyakejima provided more magma.

The rifting event, as it might be called given the scale of the intrusion, has initiated caldera formation at Miyakejima by July 8, a small explosion and caldera collapse take place together. Conditions for the first month of caldera deepening were dry, DRY, that is important. A later lake that formed around September-October at the bottom shows the water table was roughly 450 meters below the ground, enough it seems to inhibit phreatomagmatic eruptions.

There were just a few explosions, probably driven by magmatic gasses, like at Kilauea in 2018, SO2 separates from the magma due to the dropping pressure, when the roof comes falling, it pushes the gas out in a smallish explosion.

Collapse turns for much wetter conditions on August 10, a stronger explosion to 9 km produces mudflows on the flank of the island, at noon a tall white steam plume rises. Inside the caldera a little crater next to the wall ejects small explosions and base surges from a mud pool and mud flows down towards the center of the caldera. At this point the water table/hydrothermal system has surfaced but only partially in one corner, the rest of the crater floor is dry.

August 18 is the climax, the last collapse-event and largest explosion. The ash plume rises up to 16 km, the timing of the events is highly interesting. Explosion started at 17:05 and peaked at about 18:15 when the last collapse-event of the sequence struck, however the explosion was well underway an hour before the roof failed again proving a problem to the initial theory. Some have argued the explosion was magmatic because it contained a higher proportion of juveniles compared to other phases, but it also contains accretionary lapilli, this kind of volcanic product only shows up in steam-rich plumes.

One last explosion takes place on August 29 and sends a pyroclastic surge into a residential area, luckily it was cold and diluted so there was no one’s death to grieve. It is really surprising that the residents stayed there in the island during the whole eruption only to go on an evacuation more than 4 years long. It also shows the eruption was relatively small, a low-end VEI 3, it was more dangerous than a pure dry collapse like that of Kilauea in 2018, but much smaller than a pure wet collapse like that of Fernandina 1968.

Unraveling the mechanism

But if Miyakejima stands out for something that is the massive SO2 emissions that followed the event, this is something that didn’t happen after Kilauea, Bardarbunga or Piton de la Fournaise had their caldera collapses, and they were well monitored. It needs an explanation and a good one because it is highly abnormal, almost unbelievable. There is one fundamental difference with Miyakejima that I can see, the hydrothermal system was at the surface.

After a caldera collapses the magma reservoir is at a very low pressure until it reinflates, at this low pressure the SO2 can escape the magma easily so it creates the condition for strong degassing, but that is not enough, the hydrothermal system of Miyakejima must have played a role in transporting the available SO2 to the surface. In contrast Kilauea had extremely low emissions, why? As SO2 rises it encounters a layer of water and it is lost by reacting into sulfuric acid, this a reason why HVO thinks degassing in currently so low, dissolved into the lake…

The hydrothermal system of Miyakejima managed to transport gas to the surface where those of Kilauea, Piton de la Fournaise and Bardarbunga failed. And here is my theory, by being at the surface the hydrothermal system can flash into steam, this creates a suction force on the underlying system and a rapid plume of gas, vapor and water forms that transports SO2 rapidly to the surface without being lost. The thick glacier of Bardarbunga or the few upper hundred meters of rock at Kilauea confined the water and did not allow it to flash into steam, and by the time Kilauea had formed a lake it had reinflated enough. But there is more, I think there is yet a 3rd element, a really important one, lets go back to Fernandina.

Before 1968 there was a lake at Fernandina, a normal looking lake with no important hydrothermal phenomena going on. But that June 11 of 1968 the people of a fishboat would be looking at massive 22 km high column of steam, this is no longer the cold hydrothermal system of earlier years, something had changed. What? Well by June 11 the volcano would have been through 3-4 days of flank eruption and the pressure of the magma chamber would be VERY LOW, well that’s it! The low pressure of a magma chamber, the deflation before a collapse event allows a better contact between water and the magma reservoir providing the strong heat flux capable of powering a VEI 4-5 steam blast or a violent plume of fluids that transports SO2 to the surface.

The exact physics mechanism is speculative but the way I think it goes is that water simply runs into the magma reservoir in the low pressure before a caldera collapse. The pressure from magma and volcanic gasses keeps the water away, but as magma drains it reaches a point where it loses the “protection”, a possible way I picture it happening is a cavity at the roof of the reservoir filling with supercritical water and volcanic gases then convecting upwards and heating up the shallow hydrothermal system, water becomes superheated. Fernandina was a ticking bomb on June 11.

The big moment arrives. In the same way as Miyakejima it starts from the surface where water can flash into steam, a big enough steam explosion may suddenly reduce the pressure in the hydrothermal system at depth, superheated water here exists at temperatures of 300 °C, in liquid state, but if pressure would drop… The hydrothermal system literally blows up, flashes into steam blasting rock into ash and blocks, projected into the atmosphere. This is what must have happened at Fernandina the afternoon of June 11, it must have also been involved at a smaller scale in the eruptions of Miyajima of August 10, 18 and 29. Smaller probably due to the lesser amount of water available. Kilauea or Bardarbunga couldn’t because the hydrothermal system was not close enough to the surface for it to flash into steam.

Example of a small hydrothermal explosion at Yellowstone. Courtesy of USGS

The implications of this are actually amazing, it is a exception to the rule volcanoes need pressure to erupt, it is an important rule, a very useful one, almost always true but here it is the opposite. Extreme deflation and depressurization of a volcanic system can lead to eruptions as well.

New questions arise, how do we predict these events? Which volcanoes are susceptible of producing them? Which are the main hazards? These and others aspects we will look into in upcoming posts.


Relevant links

How calderas form:

On the 1968 eruption of Fernandina, by Tom Simkin and Keith A. Howard.

Chronology and images of the 2000 Miyakejima events:


89 thoughts on “Big basalt blasts I. The trigger

  1. Fascinating! Thank you. This is going to be interesting to consider over the upcoming years when new events occur. Thorough work, Hector.

    • Thank you, I do hope this will prove useful next time such an event occurs.

  2. What a wonderful article! A thoroughly enjoyable read. Chapeau, Hector. Thank you very much for this.

  3. Un artículo extraordinario, especialmente para los que somos profanos en la materia. Es muy didáctico y clarificador.

    Enhorabuena Héctor por el artículo y por acercarnos a este fascinante mundo de los volcanes.

    VC is a multicultural and multinational environment and we welcome people from all backgrounds and cultures who are fascinated by the world of volcanoes. The comments and posts should be understandable for everyone, and we strongly recommend to use English. For many people here English is not their first language, and they may feel that they are far from fluent and make mistakes. Everyone understands. Welcome! -admin

  4. Wonderful article and writing – thanks for sharing.

    Believe it or not, basalt eruptions can get bigger… actually, they can get a LOT bigger even. One of the single most fascinating items I’ve come across in the last 10 years of being a volcanophile is that there is historic precedent for a VEI-8 eruption that was primarily basaltic… from Iceland (in the very very distant past when it was just rising out of the ocean).

    • Thank you, that is amazing. A basalt VEI-8… the realm of possibility is much broader than we often assume. I will read that article, sounds fascinating.

    • 1000+km3 basalt flows are sometimes used as a criterion to define a flood basalt province, though places like Hawaii and Iceland lack these so probably not anymore. Anyway just have one of those erupt in an abundance of water… 🙂

      • I think it’s at least a little more complex than this. Extremely large basalt eruptions occur all the time in water, but do not produce explosive VEI-8 sized eruptions.

        Water adds explosivity for sure – but the real key is when water vapor is sealed within the magma itself, which allows for massive sudden expansion. Either that, or there must have been a very sudden exposure of massive amounts of water into a very very large compressed environment.

        Think of it this way:

        We know that water expands massively when it turns to steam. Yet we convert water to steam all the time… Everyone here has boiled water on the stove without getting an explosive reaction.

        The key is that when boiling water off of a stove, the pressure is not contained within an enclosed environment. When that pressure gets contained and stored up however, we get a very different dynamic, where there is potential for the pressure to get released in one sudden movement.

        See this wonderful Mythbusters video for an example of this (classic exploding water heater)

        The same dynamic happens with Volcanoes. Magma erupting into a water environment will increase explosivity a bit, but the pressure is not bottled up and contained, and thus we only get the sudden expansion of pressure from where the magma is contacting the water and boiling it. Thus, there isn’t the sudden massive force we would see if that water vapor were all contained within the magma chamber itself and then released all at once.

        This is why water vapor and other volcanic gases that are locked within volcanic crystals is probably more important than water exposure surrounding a volcano.

        • of course phreatic explosions are quite explosive. You are right, this is underground water. But not necessarily water inside the magma.

        • a pressure cooker went apeshit, oh boy did it ever, re decorated the kitchen, and no it wasn’t me

  5. Mauna loa in 1877 I think erupted in this way too, nowhere near so big but I remember reading (and forgetting where I read it…) about a report of a sudden explosive eruption that year there that showered the entire upper 2 km of the mountain in glowing bombs. HVO has some Volcano Watch articles on Mauna Loa being explosive but they dont answer how and suggest it is the same as for Kilauea which is unlikely, it sounds very much the same as this article though as 1877 had a very voluminous submarine flank eruption. No hydrothermal system there though, not close to the surface so maybe this doesnt work so much.

    I guess if Kilauea has another large flank eruption immediately upon waking up you get something like this though now and possibly bigger, Bardarbunga the same again if it has got a lake. It does seem like the residents of Ambrym could have had a very narrow escape in 2018 too.

    • Mauna Loa has had dry summit collapses probably, and that is enough to explain the blocks strewn a short distance around the caldera. Small explosions like those of Kilauea in 2018 or 1924. I would not rule out bigger explosions are possible on Mauna Loa, but no evidence exists of them.

      There may have been some suggestion that 1877 is related to the block deposits but I think this is unlikely, I have read the original narratives of the 1877 event, it was ten days of a glowing plume from the top of Mauna Loa before the flank eruption, sounds like vigorous fountaining but not explosive. Maybe I read that in “Hawaii and its volcanoes” by Charles Hitchcock, not sure.

      • Maybe in the ice age, when it was glaciated, there would have been bigger eruptions. I guess it is possible Mauna Loa was too active to be extensively glaciated though, Grimsvotn is glaciated because it is under an icecap that is way bigger than it so no matter how big it goes there is just way too much ice elsewhere, but the icecap on Mauna Kea was not very big and a similar cap on Mauna Loa would be largely destroyed by a caldera collapse or decent summit eruption which are a few times a millennium type events.

  6. A great read. Thank you, Héctor.

    Does the water for the Fernandina eruption have to come from the lake? It could have come from aquifers? They were worried during the Kilauea eruption that magma might meet groundwater that way.

    • There was no groundwater in the caldera in 2018 because of the lava lake. This wasnt known at the time apparently, but if it was the case the lake would have appeared immediately, but it took over a year in reality. If the 2018 eruption had taken place in 2007 before the summit was active and a few decades since the last eruption there I suspect there would have been a very similar sort of eruption as at Fernandina or Miyakejima.

    • The water driving the eruption is superheated (at temperatures much higher than the boiling point), so that if pressure drops it flashes instantly into steam. So it needs to be subterranean. I think it is water that has touched the magma reservoir, that would be 1-2 km underground, and then circulated up in a column, a plume of hot fluids.

      The lake is not very important, but it does show that water is at the surface which is fundamental. I think water flashing into steam at the surface creates a suction force that draws up water from deep and creates a concentrated plume in the hydrothermal system/aquifer. Miyakejima shows it is necessary for water to exist at or near the surface to do any of this, a thick layer of rock or ice makes the water confined under pressure and suppresses the explosions or rapid convection/hot plume.

      For both Miyakejima and Fernandina hydrothermal activity was focused at a particular fixed spot of the caldera, probably marks the location where a concentrated hot plume of fluids formed. This is highly unstable, water expands many times in volume when flashing into steam, you have a 1-2 km tall cylinder of rock, water, gas, that can burst into a pressurized jet of steam perhaps all the way down to the reservoir and incorporate some fresh magma. It is much more violent than magma running into water at shallow depths.

    • The Kilauea summit did have groundwater (aquifers?) triggering steam explosions once the lava lake had sufficiently drained late in the 2018 eruptive cycle. Eventually the conduit cooled and the explosions stopped….with a new lake now accumulating water from both rainfall/runoff plus continued groundwater seepage.

      • No that was the old theory applied in 1924 but now disregarded. The explosions were actually caused by extensive degassing of the summit which was caused by the lack of pressure as it drained which is explained in the article. Early on the collapse would cause the gas to blow out of the drained overlook vent, the 2018 summit eruptions were vulcanian like at Sakurajima. The lake today is over 80 C at the source springs and took a year to show up all the while the caldera was steaming profusely, sounds pretty hot to me…

  7. Galapagos haves very hot thoelitic magmas and a large yearly magma supply. But the lack of summit lava lakes supprise me. All the Galapagos calderas haves rootless lava lakes formed from violent fissure eruptions.
    But there seems to be NO historical activity of open conduit lava lakes.
    There are no pahoehoe fillings in any of Galapagos calderas. The large deep calderas does indicate shallow good sized magma resovairs.

    • Of course none of the Galapagos calderas formed in a single event, they are the products of numerous episodes of collapse, as is seen by the uneven floors and terrace benches and the walls of others. But the volcanoes haves difficult to fill their calderas with lava fillings.

    • Trapdoor calderas, when the pressure rises and then the magma erupts out of the caldera fault, usually at huge pressure driving a violent effusive eruption not unlike a flood basalt. Then the pressure is reduced and it stops. The roof is not structurally strong enough to keep an open conduit given its great width to depth ratio and weak edges.

  8. Is a jökulhlaup from Grímsvötn starting? The tremor plot for GRF shows an increased signal, that could also be weather, but is not present in the plots of nearby stations. GRVC gps looks like it might be slowing down, but more samples are needed before that can be said with any confidence.

    • Husbondi and Jokulheimar are showing about the same tremor. Drumplots suggest those are weather related I think. Grf drumplot has some weird signals tonight, as a vehicle passing by or so. The signature is not familiar …
      No gravel paths or bridges around there! 😏

  9. This video might be interesting for you Hector, includes the USGS 2018 eruption final details, as of some point in the last month 🙂

    • 1,2 km3 indeed very similar to Holuhraun these two eruptions where very similar. The eruption vents also almost looks the same… Baugur and Fissure 8, with Baugur just being longer. But both haves spillways and nice channel.

      But Bardarbunga was more of a deep plug piston collapse than a shallow collapse. The Bardarbunga plug coud be as large as the Chicxlulub Asteorid itself.

    • To be honest when Holuhraun started I belived that we woud get a centruy long lived shield eruption… basicaly a that ”Bardardyngja” woud form there. And specialy so when after 2 months the eruption showed No signs of weakening.

      But it was not the case.
      Holuhraun never feed from the deeper massive resovairs thats required for a long lived shield eruption in these Vatnajökull volcanoes. Instead Holuhraun drained itself dry.

      Fissure 8 was also a bummer when it stopped

      • Fissure 8 was in my mind never going to be a shield even though it was presented as a possibility by HVO, there are no other shields in that area in recent time (which they also said), but many large pyroclastic cones. Pyroclastic cones are rare further up, probably most of them on Kilauea evolve into shields above a certain size and can only avoid that fate if the eruption is beyond the stable rift conduit.

      • I dont know what is required to create shields at Vatnajokull, but I can only see one northeast of Bardarbunga so probably they arent that common. They could become a lot more common as the glacier melts though. It would be quite nice to have a volcano in Iceland that behaves like Pu’u O’o.

        Grimsdyngja 🙂

    • Good catch, thanks.

      I find the LIDAR and deformation maps insightful, like a window into the Halema’uma’u reservoir. As the caldera collapse advances new areas of the caldera initiate collapse. First a small pit around the old lava lake, then a section of 1×1.7 km starts sagging, followed by the 1.5×2.5 km area that was still collapsing by the end of the eruption. The deformation maps show that it was about to jump into an unstable area broader to the north and south, and later roughly the entire caldera, a 3.3×4 km rectangle was probably going to drop. This is as if deeper broader levels of the Halema’uma’u reservoir were being drained gradually from top to bottom.

      • I found it interesting that the volumetric rate of the caldera collapse when averaged out was 6 m3/s lower than the fissure 8 eruption rate average. It is possible for a big margin of error, but that number is pretty close to the eruption rate of Pu’u O’o, it looks as though 2018 really had no effect at all on the rate of magma influx, actually could have increased it a bit even.

        If that rate is still ongoing I dont know but after 2 years it adds up to 0.4 km3 of magma, and it would have covered for the difference between the eruption volume and collapse volume in about June, which is just when earthquakes on the rift became more common. Coincidence I think not 🙂

  10. Once more some interesting signatures from Grimsvotn (GRF Drumplot). The weather is fairly calm in the area, so it is not weather. But those tremor pulses perhaps could be steam in the system? The slow awakening continues. My bet is for December 2020!

  11. It’s interesting that Hawaii & Iceland have a constant state of rumbling just below the surface/eruption/lava lakes whereas most other hotspot shield areas like Galapagos & Piton do not. Is the downtime between eruptions at these places just down to the lower magma influx or is it crustal differences allowing longer build-up times?

    • Iceland haves a large magma supply.. But most of the magma never comes up to surface, goes into passive rifting filling the ridge gaps.
      Thats why Iceland generaly does not do shield building type eruptions. But the general historical output remains quite large

    • Iceland is a plate boundary, and Hawaii is sliding under its own mass to achieve a superficially similar effect. Piton is possibly not big enough to slide the same way or its submarine slopes are not as steep, and only some of the Galapagos volcanoes have rift structures that are poorly developed compated to Hawaii.

      The most likely reason though is that the Galapagos are not monitored publically in real time, there is no ‘GVO’. English is also not the local language, where it is in Hawaii and is widespread in Iceland, so those of us in English speaking places (probably most people here I dont know) are likely to miss the background level.

      • What woud Iceland look like If it was a isolated hotspot without the spreading ridge? lets say place Iceland plume in the pacific where cook Islands are… what woud happen.. explain.

      • It grow and form an Island group not very diffrent from the Galapagos Islands.
        Perhaps smaller Islands, as the sea is way deeper at that location, far from the speedy spreading boundary close to Galapagos.

        • Hi friend! It woud likley be a bit larger than Galapagos. Because Iceland is a very powerful hotspot. Even if I knows that Galapagos is a strong mantle thermal source too of course.

  12. Grimsvötn is a completely different volcano now than it was as an example before 2011. Since 2018, it has received a lot of new fresh magma. This year there have been very elevated levels of geothermal activity and glacial melting. The next outbreak will probably be a large and perhaps long-lasting one compared to previous eruption outbreaks.

    • I actually have a question on its name, i have bare minimum understanding of Icelandic mostly from reading this site 🙂 but wouldnt ‘Grims lake’ be Grimsvatn instead of Grimsvotn? Also if it was named as a lake that means some point in the last 1200 years it was not subglacial?

      I agree its next eruption will probably break its schedule and erupt a large volume, after 2011 apparently filling most of the caldera it is probably to the point of becoming subaerial and effusive. I dont think we get another 2011 scale eruption though, similar volume but much longer duration and lower intensity.

      • Purportedly, Grimur’s wife cursed the lakes after he killed her dad.

        Really interested in knowing more of the story, but haven’t run across it yet.

      • Um …….Grímsvötn & Katla both restless … Remind me, again, what is in between them 😉

        OTH Katla appeared to be restless before activity ramped up at Bárdarbunga & Holuhraun in 2014.

        • A surge of new magma arriving via the plume could explain this. As it reaches the base of Iceland it pushes fingers of magma up into all of the volcanic systems that have downward-opening conduits to the deep feeder region. Those systems all pressurize and start groaning and creaking. Eventually, one of them fails, and when that happens, the fresh magma (and stale magma from that particular volcano) pushes itself out there, and the pressure on the others returns to normal. So, the activity quiets down at the other volcanoes again, while a single one of them erupts. In 2014, Katla quieted down again once the Holuhraun dyke was propagating well. If Grimsvotn is the closest to failure this time, then many Icelandic volcanoes will groan and creak for a while right before Grimsvotn pops, and then the others will settle down again, Katla included.

          The same seems to happen at Hawaii, with Kilauea and Mauna Loa both creaking and groaning shortly before one of them goes pop, and the other quieting back down at that point. This behavior can be expected at any site with two or more volcanoes sharing a common mantle source and deep feeder system.

          (And on the topic of Grimsvotn, what is the votn-vs.-vatn distinction? Simply plural vs. singular?)

          • Given the amount of rifting going on in both the Reykjanes Peninsula and Tjörnes Fracture Zone, anything could give. And it may not be just the one?

          • The mantle is not liquid like that. Look at the Pahala swarm, thousands of earthquakes happening in the mantle and underneath Hawaii no less where the mantle plume is at Archean temperatures of over 1500 C, it is still mostly solid. The magma composition also differs. Probably most of the quiet elsewhere while an eruption happens is simply the whole signal being drowned out as well as people not noticing.

          • Its still the problem of different magma, vatnajokull has plume tholeiite, katla has more alkaline basalt, reykjanes and myvatn area is MORB composition (maybe got some of that wrong but close enough). I think it was also said a few articles ago that the so called cyclic volcanism of Iceland is almost entirely from Grimsvotn, so the above idea probably works there but the rest is just tectonic stress rather than an actual deep magmatic connection. Its the same in Hawaii, many studies for decades have consistently failed to find a hydraulic connection between Kilauea and Mauna Loa, which should be obvious given their summits are not at the same elevation but both erupt there sometimes simultaneously. Their connection is through compression of each other when they inflate which is generally somewhat one sided.

          • I don’t know about Iceland but what happens in Hawaii is that Mauna Loa and Kilauea are connected about 30 km deep, probably by a large sill.

            Magma from the sill flows preferentially to whichever volcano “pulls” supply the strongest, this is the volcano that has its rifts in a more active state, this makes one of them get most of the supply for decades or centuries.

            In the short term it looks like the sill gets deliveries of magma from the hotspot and when this happens both volcanoes get their supply increased. But as the current Pahala Swarm shows, the connection is very sluggish, a strong magma surge, maybe in response to the 2018 eruption, started around 2 years ago, should have reached the sill about 8 months ago but there is still no sign of unusual activity below the volcanoes, so it is taking its time. And a volcano can certainly not reduce the pressure from the other, the connection is not so good, magma seems to behave very different down there than close to the surface, not sure of the reasons.

          • I think I have mentioned this before probably but there are now two Pahala swarms, the original one and now a newer one that is 1/3 of the way to Kilauea, so it looks pretty cleat of the ultimate destination. That is in about a year of activity, so at this rate its going to take a few more years but we get quite the massive show when it does 🙂

            There has also been a lot more very deep quakes around Kilauea than Mauna Loa several a week vs maybe 1 in a month, these are not part of the swarm but same depth. This I read also happened in the late 50s before 1959 but has not really happened since then so much. It looks like maybe the Pahala swarm is a bigger than average resupply to begin a new phase of activity, and that is having to force its way through more mantle so taking longer than a smaller batch. Probably the mantle there is like slushy ice, not liquid in total but with liquid in it, so no hydraulic connection but still lots of available magma. I guess we might never know the answer for sure.

  13. Fascinating !!
    Rather than ‘merely’ phreato-whatever, that would blow a ‘maar’, this sound like the ultimate geyser…

    ( Warily making distinction between such, which may repeat reasonably soon, and sea-breaching which blows entire edifice apart and ring- fault collapses what’s left… )

  14. What in tarnation is that signature?

    I don’t think I’ve ever seen a pattern like that.

    • Don’t know. Beginning or working day and end of working day? Someone flying over to take a look – but seems a bit short for that.

      • Working day has no impact whatsoever. Grímsvötn is far too remote for that. Someone flying over to take a look is more likely, but I think there have been a few too many of these waveforms lately. I might be wrong, but I don’t think they go there that often to take a look.

      • I also wondered if it was anything to do with ice-melt and the subglacial lake. Apparently there is a seismic signal during a jökulhlaup but not sure about what happens in the run up to one – so this may or may not be relevant.

        Link for the paper “Seismic ground vibrations give advanced early-warning of subglacial floods”, Eva P. S. Eibl, Christopher J. Bean, Bergur Einarsson, Finnur Pàlsson & Kristin S. Vogfjörd, in Nature Communications.

    • The one at 09:00 looks like people to me. It is fairly symmetric with a a peak in the middle, i.e. like passing traffic. People may have been out: I think mid-october is about the last chance for a service to get the instruments ready for winter. Perhaps.

  15. Piton de la Fournaise keeps inflating very fast, can’t be very long before it snaps, and with the recent flank slip it is likely to be an extraordinary eruption.

    • Does certainly look like the makings of a significant eruption. I definitely found it intriguing when inflation began so fast after the swarm ended, looks very much like a lot of magma pushed the flank out of the way and that leaves the possibility of a large distal eruption. I think there is also the possibility of a gas rich high fountaining eruption on the upper part of the volcano but of much larger volume than recent examples, something like what made those big cones on the plaines de sables west of the active complex. Recent eruptions in several cases have been high fountaining over 100 meters and possibly over 200 meters for the eruption in April so this is plausible I think. Probably not much more than a few weeks before we find out.

    • Hello,

      Though there is certainly something happening here, I wouldn’t be so sure about the “flank slip” interpretation, at least if you associate it to a gravitational movement. If you look at the location and depths of earthquakes (e.g. and, they are rather deep and localized under a small portion of the east flank. If they were associated to a deep gravitational movement, I would expect them to be deep and more spreaded along the flank. And if they were associated to a small gravitational movement of a small portion of the volcano slope, they would be shallower.

      So of course there may be a little bit of both, but I think the predominant process at the origin of the displacements was a lateral dike, rather than a gravitational slope movement. Well, that’s my guess, but I’m not an expert 🙂 .

      • Yes, I am not sure, the crisis might have been a dyke, or a dyke + flank slip, or a sill + flank slip, the signals are strange. What is certain is that there has been spreading of the rifts, shown by the lengthening of E-W lines during the crisis and this creates space for dyke intrusions to take place.

        The summit is now inflating at a significant rate for Piton or at least that is what OVPF says, and they probably have tiltmeter data to back up that conclusion. So a new lateral intrusion could happen, and be longer or wider than usual given the space created, with whatever consequences that brings.

  16. Hector S .. have you ever heard of Theistareykjarbunga? Its one of Icelands very largest effusive shields.
    It was formed mostly through one prolonged Puu Oo like eruption, that coud have lasted over 150 years. Its volume is around 50 km3 of lava likley even more. Theistareykjarbunga Is built of almost whole pahoehoes, hallmarks of long lived slow eruptions. This lava shield resembles very much an overgrown Puu Oo, but alot larger. The 1975 – 1984 Krafla flows have overunned the Northen areras of Theistareykjarbungas pahoehoes.
    This lava shield appears as a flat boulge in Northen Iceland.

    Theistareykjarbunga was likley mostly one single prolonged eruption. A lava lake connected to lava tubes resided in the summit for decades likley. There is also a nice round deep collapse pit there. Just North of the summit of Theistareykjarbunga there is a fantastic lava channel system. Its a snake of pits and open channels ( massive collapsed tubes ) looks like marsian tube channels. One of these Icelandic tube – channels of Theistareykjarbungais 6 km long and must have feed Thoelite lava at crazy rates. Look at it in Google Earth.

    Overall Theistareykjarbunga is one of Icelands largest effusive events and a fantastic but remote place.
    If a new one happened today, it woud be excellent for tourists in Iceland, because lava coud flow for 200 years non stop. I wonder what kind of mechanism creates these rare large Icelandic eruptions.

    • The mechanism is getting a molten pipe of magma and keeping it that way while new magma keeps being supplied. But I don’t know what specific conditions are required to produce such an eruption. It has probably something to do with very large magma chambers or a huge supply. Many volcanoes in Iceland were able to do them when they deglaciated (with the ensuing surge of melt), now probably only a few remain that can produce them.

    • Mammoth large lava channel for soure peering at it in Google Earth! kind of seems like a tube – channel mix with pit chains. The feeder vent to channels are 650 m long.
      Cheers … brother

    • Maybe only Grimsvötn, Bardarbunga and Askja may have that capability now.

      Katla also haves a large magma influx, the 20 km3 Eldgja flood basalt must been an amazing sight, but it have not done any slow lava shields what I knows in holocene.

      The shields are often blamed at decompression melting after the enormous Pleistocene Island covering icesheet melted away.

      • I think it is quite a plausible situation for lava shields to form where Vatnajokull is now when it melts, or even before full melting. There arent shields where the rifts are to the southwest but there are also no central volcanoes so this makes sense. In my mind when the glacier melts probably there will be shields at Grimsvotn and trapdoor fault eruptions at Bardarbunga, rift eruptions at either volcano probably will be still rare like now and generally at the closer part to the central volcano than the really distant parts active historically but maybe not. Grimsvotn may begin its shield building with its upcoming eruption the way things are going.

    • Theistareykjarbungas lava tube channel mix system is awsome! two branches. One of the planets largest most spectacular holocene lava channel systems, and it seemed to began to evolve into a lava tube, despite huge eruptive rates. It must have been flowing for many many months, possible years since the tube channel roof is very thick. Just north of the shields caldera summit.

    • Theistareykjarbungas lava tube channel is very similar overall to the marsian lava channels that can be traced to snaking lines of small pits.
      Perhaps Theistareykjarbungas lava channel is a monster tube ( one of earths widest ) that have simply collapsed in places forming pit chains

  17. New indigestion near Pahala. If the two quakes in the time frame of the tremor are associated with the tremor, then it is getting shallower.

    2020-10-13 13:25:41 2.7 31.9
    2020-10-13 13:24:01 2.2 31.3

  18. I actually overlooked this entirely as did I think everyone else. In the article it says there are 3 examples of explosive basaltic caldera formation since 1900 where the hydrothermal system blows up at nuke level power, you mention Fernandina and Miyakejima, but what is the 3rd?

    • Hope so! I have a (no cash) bet on late December for Grimsvotn!
      Albert and Carl will disagree I am sure. 🙂

      • Fine by me .. I already won my bet. Carl owes me a beer! Of course, if Grimsvotn decide to postpone until 2022, Carl may well argue that we both owe each other a beer.

        • Well that sound a deal on both sides. About £1000 in airfares for a free beer!
          Mind you I suspect one each will be far from sufficient.

          • And Grimsvotn will blow during the 4th beer. Can’t trust volcanoes.

  19. Overall Theistareykjarbunga is a much much larger pahoehoe shield and field than Puu Oo is. At its summit a large calm lava lake bubbled and sloshed for 100 of years feeding active buried lava tubes. All way to the atlantic ocean downslope. Theistareykjarbungas pahoehoe field is enormous must have lasted 100 s of years nonstop. If this happens again in Iceland it woud be wonderful for the tourism and active lava for the rest of our lives

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