Guest post by Tallis.
I have one slight disagreement. I’m not necessarily sure that the signs before a super eruption would be particularly obvious, at least for a volcano that isn’t being actively monitored. Would there be signs? Definitely, but determining the difference between normal inflation and inflation that precedes a super-eruption may not be particularly easy.
The issue is that we don’t have a long enough time to measure influx into a volcanic system. We only can see the magmatic input into a system that occurred in the last few years, not the 400 years before now. So a volcano that saw Iwo-Jima style magmatic input for 200 years before the magma inflation stopped may still be straddling the threshold between erupting a big eruption and not doing anything even 200 years later. In this type of instance, it may not take a large injection of magma to create a big eruption.
Another point worth making is that Rhyolitic eruptions seem to erupt extremely fast and violently. We know the violent part, but the “fast” part is what I think would catch a lot of people off guard. Chaiten was a great example of this, as it erupted extremely quick due to the explosive nature of the rhyolitic magma and the deep chamber. From what we know about the Taupo Volcanic Zone calderas, they also seem to exhibit this trait, which is frightening considering how large and explosive they are. The Tarawera eruption seemingly occurred in a flash from historical accounts.
The more pressure there is that is held back, the more likely it is that you get an eruption that accelerates extremely quick. Adding to this, it should be noted that in crystallized form, magma won’t exert tons of upward pressure. But if the magma reheats or is rapidly depressurized, that can change in a flash.
This comment is something I had casually responded to but my interest in the idea of a large subtle eruption has greatly interested me. Thanks, cbus05. This point has been strengthened by some volcanic systems. No system is the same; every large eruption will produce an abundant amount of noise and the coming event would be obvious at a certain point in the process leading to the eruption, but there doesn’t have to be a lot of time once that point is reached. The build up phase of a large caldera system will have abundant noise but not necessarily a long warning. The uplift, seismic activity and geothermal convection do not have to reflect the size of a future eruption.
Laguna del Maule is the fastest known uplifting volcanic system on the planet at the moment, it possesses a large magma chamber and is experiencing an intrusion capable of destabilizing the chamber. This very dangerous volcano has some of the most benign seismic activity you will ever see in a system of such potential. Micro seismicity is the only thing of note that you will see from this system in terms of earthquakes.
The system has not reached the point where it is causing earthquake activity through the stressing of regional faults. This system has had periods of unrest like this frequently in the past 100,000 years. Any rocks holding back the intrusion has been rendered inert. The pathway to the system has cleared like Agung recently. The only type of noise you might see from the system now are harmonic tremors and small micro quakes.
A system that has been multiple intrusive periods will not produce consistent seismic activity, that is not to say that there would not earthquake swarms but the swarm doesn’t have to accurately reflect the unrest within the system. This situation is frequent within volcanoes like Laguna del Maule, and makes the fact that there might be significant intrusion at a system unknown to the surrounding area is scary to say the least but what about the other signs?
The Volcano doesn’t need to be uplifting at a particularly rapid rate for the entirety of the unrest, past cyclical periods of intense uplift could easily be missed from an unknown volcano that seems to be rising 20-30 mm a year. In fact, it is so easy improperly gauge the past that some eruptions that have taken place just a few hundred years ago were not properly scaled until recently. Past unrest would be even harder to pinpoint conclusively and once another severe period of uplift start, most would think that volcano is just now starting to prepare for dinner when the chicken has already cooked, and on its way to the table.
A slow intrusion of mafic magma could destabilize the system over a period of 1000s of years while causing minor uplift, and while looking benign in terms of activity.. Frequent intrusive episodes can maintain a certain level of heat in the chamber. The separation of the eruptible magma from the useless mush in the system could be some of the quietest experiences you could ever see. When the severe uplift kicks in it could just be a few months before the main eruption.
It is important to remember that not all volcanoes prepare for a large eruption the same way, when considering the size of a future eruption, the history of the volcano should be established. The uplift for a VEI 8 eruption doesn’t have to be extreme for most of the build up phase. If the chamber is molten enough, the volcano would just need a smaller intrusion to get things moving
The accumulation of eruptible magma would likely cause an increase in heat within the system but that also doesn’t have to represent the danger of the system. Ironically the cooling of the magma chamber could also represent the geothermal activity. Geothermal activity is not a usual sign of a large eruption and would be taken seriously be no one who is in a position of authority, The Los Humeros system is an example of this, despite the heat within the system being possibly caused by magmatic intrusion and the large chamber and scary history, it still is not being monitored properly.
The heat may not even be transmitted to the surface properly, and even then there might not even be any good ways of measuring the activity without a large backing. Rhyolite is the coolest magma, in a way this magma may not even have the ability to generate abundant heat.
It is kind of scary to think about how little warning you could have before an event in excess of 1×10^22 J event. An eruption like this will still create an abundant noise at some point but that point might be at the end of the activity, a few weeks or months before the eruption. It is even possible that something like this already happening with certain systems like Campi Flegrei or Aso but that would be more speculative and unlikely. For monitored volcanoes, this idea could be used to monitor more subtle details and perhaps find out what other signs a volcano makes before erupting.