Some things are so easy to miss and once they are noticed, we either crawl into a fetal position and cry ourselves to sleep or become very excited over the new prospect. I have done both recently and formulated my own hypothesis, concerning silicic systems and their dynamics. After looking at the previously mentioned supercomplexes, I think I have found the answer on why some intrusions cause no eruptions, small eruptions, or big eruptions, as well as the movement of magma within the reservoir without external variables, and at the end, I will explain why I believe that the 40% percent threshold for eruptions is not valid for these silicic systems.
Everyone thinks of these systems as one volcano because it’s true.The long valley caldera is huge but it’s one volcano. However, once things start getting big, the rules begin to change. After all, the altiplano puna volcanic complex comprises one huge magma body but not a lot of people refer to it as one volcano. So in this article, we are not going to consider these massive silicic systems as one volcano, we are going to consider them as multiple volcanoes. As we have established before the melt within the system takes the form as connected or unconnected dykes and sills within the un molten, semisolid magma body.
First, let’s answer why some intrusions don’t cause any eruptions, All intrusions would rejuvenate a portion of magma within the system but as we all know this doesn’t always produce an eruption. Let’s use a 1,200 km3 chamber for an example, with only 24 km3 or 120 km3 of natural melt magma mixing between rejuvenated and natural melt will be unlikely. This is part of the missing piece, everyone thinks of these systems as a whole structure but these volcanoes are so big that the same rules that apply to smaller volcanoes won’t always apply here. What I am saying is that these pockets of melt acts as “mini-volcanoes”
The intrusion rate is usually 1/100,000 of the total volume of chamber per year, sometimes a little less sometimes a little more but ultimately this such an insignificant percentage it’s hardly worth comparing in terms of volume but how does the magma chamber react? Instead of reacting with the entire chamber, the intrusion produces a melt pocket that takes the form of a sill or dyke. The pocket of melt, being hotter than the surrounding chamber makes an effort to rise to the top, it won’t always be successful, the pressure at the bottom at the magma chamber could keep the melt from ascending and it might stagnate at the bottom.
I believe these melt pockets drive polygenetic and monogenetic volcanism, and as these pockets have multiple compositions, a mostly silicic system could produce abundant mafic and intermediate products. I have briefly touched on this in one of my previous articles and I will continue this idea.
Instead of causing major destabilization of the chamber as a whole, the intrusion simply makes another pocket of melt within the system and more minor instability across the chamber as a whole. The melt within these systems would have to last longer under this idea, as the intrusions would not rejuvenate the same areas of melt which would lead to the second part of my idea but we’ll get into that later. This idea would explain why silicic systems such as Corbetti produces plenty of mafic products, as mafic melt pockets produce eruptions without a lot of commingling with the rest of the felsic or intermediate magma. More evidence for this idea lies in the fact that magma-mixing has been speculated to be an important factor for caldera-forming eruptions and is not a regular occurrence in these volcanoes. In order for this to be true that would mean that there is some level of segregation between the fresh and evolved magma.
Another important factor in caldera-forming eruptions is magma motion, and since thermal rejuvenation is not needed for the magma to move, how does this happen? Even if you were to assume the melt in the chamber was homogeneous in terms of viscosity, the melt will always be hotter then the surrounding chamber and as such it will have a degree of buoyancy. As all the magma slowly moves in a common direction, this magma will start to co-mingle, becoming interconnected before turning into one large melt body.
Another factor in magma motion is likely the difference in gas content, the gas-rich magma is likely to ascend more quickly than the gas-poor magma. It likely that less viscous magma degasses within the chamber and enriches the magma surrounding it while losing it’s own eruptibilty. As these bodies become more connected, the system could maintain a healthy transfer of gases between different portions of melt. How active and healthy these systems are would have a massive effect on the mechanics of the melt within the system. Using this idea I’ll break down why some intrusions, using this idea, cause no eruptions, small eruptions, massive eruptions, and finally medium eruptions.
Why do some intrusions cause no eruptions? This is the easiest thing to explain. Even though calderas may not need any thermal rejuvenation to erupt big, it’s still the most common cause for eruptions, and without it, a large eruption is less likely. The first reason is that the melt pocket is not strong enough to reach the surface of the magma chamber. Even though magma may be buoyant, the pressure at the bottom of the reservoir would put the ocean to shame, and it would still take time to reach the surface and as such weaker intrusions in an unmolten chamber would not do much for the system.
The second reason is that there is a significant amount of useless mush within the system and due to the more local effect of the intrusion under this idea, the intrusion may not even react directly with the eruptible magma. Instead of the intrusion directly rejuvenating the good magma, the heated bad magma activates the system. This is what I believe is happening at volcanoes like Laguna del Maule. This could easily explain why some volcanoes go through heavy resurgence and produce no eruptions.
Smaller eruptions are produced by melt pockets that reach the surface of the chamber. However, the intrusions that produce these eruptions do not activate a lot of the volatile magma within the system. For mafic and intermediate eruptions, the intrusion was intense and instead of being bogged down inside the chamber, the magma directly rises. Since the more youthful, previously intruded material is likely to be towards the bottom of the system, an intense intrusion would reactivate this magma and not the other more volatile magma towards the top. Once at the surface the melting pocket will trigger some phreatic eruptions while the magma escapes the reservoir. The scale and type of the eruption would depend on the strength of the crust, the quality of magma, in terms of gases and temperature. Interaction with the hydrothermal system would likely lead to more explosive eruptions and interactions with the tectonic faults could lead to more effusive eruptions although this might not always be the case.
The eruptions could last as long as the intrusion does and even linger after it ends, as long as the melt pocket is strong. This idea works with calderas with more rigorous magma supply. The same concept would drive recurrent eruptions at certain vents, examples of this could be Satsuma-iwo jima and Soputan.
The same idea could work for silicic intrusions. Even without an intrusion, isolated melt pockets reaching the surface could also trigger these eruptions. The aforementioned “Bad magma activating good magma” (We’ll call this Magmatic Aid or M.A in this article) could drive small pockets and this magma could rise to the surface. This idea explains both polygenic and monogenic eruptions at calderas and provides another answer to why active systems don’t produce large eruptions like dormant systems.
This idea came from the Cerro Bravo- Cerro Machin system. In order to drive that much lava and tephra over huge distances from a single source, the yearly supply must be massive, and with that supply, it created massive melt pockets that drive the volcanoes of this system, and with a more interconnected system, the magma that goes into the complex will be inclined to move into certain areas and I think it’s clear what that area is <coughs> Nevado Del Ruiz <coughs>. A supercomplex as vigorous as this will probably not produce a massive eruption because it’s too active, but once it evolves, I think this system will produce a HUGE eruption. I don’t know how much but still, I think it’s going to be massive. We’d have to wait tens of thousands of years at the minimum so don’t hold your breath.
What about large eruptions? How does an intrusion cause massive eruptions when it seems they aren’t needed? The first thing that is needed is that the volcano has not erupted in a long time. In that way there isn’t an aids to enhance the initial ascension of the intruded material. It begins to rejuvenate the products at the bottom of the reservoir, and some of the silicic magma, through M.A. Towards the top, some of the pre-existing melt has become connected or even formed one body. Once the intruded and/or M.A born products reach the older melt, it gives it a massive boost in terms of mobility and now a large portion of magma is in motion at once. This would also set the stage for more substantial magma mixing and better transfer of gases
How much magma has been activated and put into motion with these chain of events will decide the pressure of the magma chamber and the pressure decides the size of the eruption, not the amount of melt directly. If pressure preceding the Wah Wah springs caused a cool system to produce a massive eruption through depressurization or pressurization, then it would stand to reason the same idea would work with other caldera-forming eruptions just on a smaller scale. Even then more melt in this stage would mean more pressure and a bigger eruption.
With really big systems a similar thing could happen without magma intrusion. As previously mentioned the pressure at the bottom of a magma chamber would be far greater than the pressure at the bottom of the ocean. Let us use our 1,200 km3 chamber as an example and assume it’s an almost perfect cylinder with an area of 120 km2 and a depth of 10 km. What would be the pressure at the bottom? Within an area of 1 inch2, there would be 6.45 cubic meters of magma and since we’ll assume the whole chamber is silicic, there would be pressure levels of around 31,000 to 32,000 psi. Rounded off of course! I don’t think this enough to rejuvenate a large amount of magma and would have to shave the number because there is no magma at the very bottom of the chamber anyways and you would have to factor in the shape of the magma chamber as well. But with deeper bodies, these number could easily double or triple. This number doesn’t factor in the crust, gases, and compressibility of the chamber. So take this number with a grain of salt but this gives us a picture of what life is like at the bottom.
A “cool” magma chamber still has a temperature of around 400-600 C so I believe the pressure at the bottom of a very large magma chamber could activate a portion of magma and this magma would make its journey to the surface. The pressure would need to add 400-200 C to the material at the bottom. Once this level is reached it would be almost impossible to stop it since the only way to end the machinations producing the self melting volcano, would be an eruption. The chamber could be loaded with gases due to past intrusions not producing eruptions This would not be a quick process, the machinations of this would take 10s, 100s, or maybe even 1000s of years.
This is how I believe the Wah Wah springs erupted. The volcano was so big, it activated itself. This would explain why thermal rejuvenation-less eruptions are not common because a lot of volcanoes just aren’t big enough for it. This could be the machinations driving the activity of Uturuncu. While it is no guarantee, if there was an existing system big enough for this process, it would be the Altiplano-puna volcanic complex. If this is what’s happening at Uturuncu, then this is what likely drives eruptions at this complex and makes things scarier and more interesting for Bolivia. Unfortunately, this @#$! Pandemic is stealing all the doomsday attention from both scientists and the media and making international volcano investigations more cumbersome so we might not be able to verify this idea.
We’ve got an explanation of smaller eruptions and large eruptions but what about medium eruptions i.e VEI 5s, VEI 6s, and low-end VEI 7s? Similar mechanics would cause these eruptions just on different scales. VEI 5 and low-end VEI 6 eruptions are likely driven by the same mechanics that cause smaller eruptions. Instead of just being predominantly composed of mafic or intermediate magma, the intrusion picks up a relatively large portion of felsic magma which forms a diversely composed magma pocket. On the other end on the intensity scale, the processes would be identical to the large eruption machinations just without as much magma.
Now here is the hard part, how can we, using our modern instruments, find out what exactly is happening when a caldera goes through unrest? There could be some basic hints depending on the levels and progression of unrest. A protracted period of seemingly static unrest likely means that the volcano is going through M.A with swarms taking place as the M.A activated magma interacts with existing melt. When a smaller eruption is coming, the unrest would be intense but gradually become more localized as the magma rises. The build-up for the eruption would vary but it could be very quick.
For large eruptions, it would be much more subtle. There would be the initial earthquake swarm caused by the intrusion, a swarm for when the magma reaches the surface and chamber start to pressurize, and the swarm preceding the eruption with smaller mini swarms caused by smaller mechanics and interaction within the system. There would be constant long period earthquakes growing more and more visible on instruments as the magma rises. With intrusion-less eruptions, there would be no initial massive earthquake swarm. There would only be small levels of activity before the eruption as the process would be slower. The best way to discern what is going is to study the movement of magma within the system and to find the cause of seemingly innocuous activity.
You’ve known me to be a person who never trusts anything mainstream and who loves to challenge himself, other and pre-existing ideas. As an example, I once tried to publish an article discussing conspiracy theories, the differences between dangerous and non-dangerous conspiracy theories, combating those dangerous ones, and what leads to them in the first place. So the point of this series is to challenge the 40% melt threshold, I believe that this idea is outdated. The first issue is that if caldera-forming eruptions are driven by buoyant magma, under the 40% melt threshold there would be more than enough pressure to trigger a caldera formation eruption. In my opinion, there would be no conceivable way that a magma chamber that molten shouldn’t produce a large eruption and only trigger small eruptions based on the idea of buoyant magma as a trigger. What would produce a large eruption with the 40% melt threshold idea devolves in confusion and a big mess. Why would a 40% molten chamber trigger a puny VEI 3 eruption and not a VEI 7+? The chamber isn’t going to lose all of its heat in a few months, it would take years! So the idea of magma buoyancy triggered eruptions and the 40% melt threshold doesn’t go hand in hand unless the pressure and melt needed for a large eruption would be far greater than this. Thankfully, the existence of the Wah Wah Springs eruption tips the odds in my favor for my argument that the 40% threshold is invalid. We can’t assume that we’re right about anything we can’t prove and I can’t prove my hypothesis so I could be wrong. Volcanoes operate on their own rules, this world, this universe, and God (if you’re religious like me) operate on their own rules and it’s up to us to find out what they are.