Big intrusions = Big bangs?
With mafic systems hogging all the attention, as a felsic guy, I feel compelled to represent my magma type. Large felsic systems are a slow burn, they spend plenty of time accumulating magma and give frequent small eruptions before releasing huge amounts of material in one eruption. These systems usually take 10s or 100s of thousands of years to recover from their massive blasts. Felsic systems usually don’t have a large supply. Intrusions do add to the volume but normally at a range smaller than 0.01 km3 a year, nothing compared to caldera-forming eruptions. It is natural to think that bigger intrusions would destabilize silic systems more than smaller ones but looking at one of the largest explosive eruptions in geological history has led me to question this.
Just because a system is experiencing a huge intrusion doesn’t mean you’ll see inflation that’ll represent the scale of the intrusion. Corbetti caldera is a good example of this and is the volcano that brought this idea into my head. This volcano is rising at a rate of 6 cm/year, not bad but something that has been observed before. The magma, chamber, and set up for this volcano, however, is compressible. This hides the scale of the intrusion and after a while, the seismic activity won’t serve as a good representation of the intrusion due to a more open pathway for magma to move through; 1×1011 kg of mafic magma enters this system yearly.
Corbetti is a very close contender to being the number 1 most dangerous volcano on the planet in my opinion. It has a strong chance of producing a caldera-forming eruption, I think it has the potential to produce some extremely intense eruptions, It is surrounded by people, and hosts a myriad of threats. This is just another reason why this volcano needs permanent monitoring.
The volcano is on the East African rift, where Africa is splitting apart, this volcano is somewhat unusual as it is a sillic system on a very healthy and expanding rift. But upon further investigation, this is not the case both in terms of the compressive setup and the size of the intrusion.
A compressive volcanic background is not well studied but I do know of another volcano that has a compressive setup this time on a subduction zone. You’ve heard the name before, Chiles-Cerro negro. During the seismic crisis in 2014-2015 deformation was observed of around 20 cm however it was found that the deformation was tectonic, not volcanic. As such there was no known volcanic deformation during this period of unrest that was caused by magmatic intrusion. However seismic analysis has shown during this period around 0.035 km3 entered the chamber in less than two years. Enough to trigger substantial deformation, as the intrusion at Laguna del Maule is around that volume per year. The compressive setup here is because of regional tectonics and less of the type of rock.
Deformation is not enough to figure out the scale of an intrusion but I don’t have access to technology to find out a past intrusion’s size so I’ll have to rely on deformation. The question is where has an intrusion like this happened before? This is a difficult thing to figure out. As past non-eruptive unrest is a very hard thing to conclusively model. I think the answer to this lies within the phenomena known as resurgent domes. A common feature at calderas, this is the result of magma going to the chamber and displacing rock, this isn’t a sign of an imminent eruption but is fun to look at and a perfect source for scary headlines. There are such domes at the long valley and Yellowstone calderas. These domes are over 500 meters high but only after 600,000 years so deformation of around 0.1 cm/ year is enough to result in these structures.
So let us look at another “Super Volcano”. Plenty enough come to mind but for some reason, my mind is on Kikai. This caldera located in Japan, to the south of the Kyushu island, is famous for producing a VEI 7 eruption around 7.3-6.3 thousand years ago, consisting of 500 km3 DRE of rhyolite magma. It is currently erupting at a subaerial vent known as Satsuma-Iwo Jima.
The caldera is 19 km by 20 km so it’s pretty big, it has a dome rising 600 meters off the caldera floor. After only 7,300 years at the most, it has produced a dome that large?! Yes. Assuming the post-eruption deformation started right after the eruption and didn’t end until now. That means this volcano has risen 8.5 cm/year. At the minimum. The earlier date yields 9.5 cm a year. That’s high but not the rate of Laguna del Maule’s current unrest. The value goes up depending on how long you want to say high levels of unrest last for.
This does not account for the magma erupting through subaerial vents or potential horizontal expansion of the chamber. If the setup at Kikai is compressive like that of Corbetti then you can expect even more magma. The fact that this is a submerged caldera gives it an edge over other volcanoes in terms of compression as this dome rises 600 meters below water. Initially, the dome started using some basic math, with 18 trillion kg of water over it or 385 kg/in2.
Kuchinoerabu-Jima, within the same region, experienced a respectable 0.01 cubic km3/year intrusion for at least 10 years before erupting and only experienced modest deformation.
Now, here’s a question, why are these systems getting these large intrusions and don’t look any closer to erupting? Corbetti may be receiving a large intrusion but everything seems relatively stable in this area. Different volcanoes react to the same things differently but if large magma intrusions cause significant destabilization why is Corbetti only moderately unstable despite this large and protracted intrusion? In fact, why do some intrusions cause smaller eruptions and not bigger ones if the overall processes within the magma chamber are the same? Do volcanoes even need thermal rejuvenation to erupt big? Apparently not. The Wah Wah springs eruption released over 5,000 cubic km DRE of magma so one would expect to find some thermal rejuvenation right? Well not at this volcano and in fact the magma seems to have been in a cool state before erupting. Thermal rejuvenation for sillic systems seems like it’s the most common but not the only cause behind eruptions.
So how in the world could a near solidus magma chamber produce one of the largest known explosive eruptions? The current models of Caldera forming eruptions have an intrusion invigorating the dormant system for the volcano, this phase is universal among countless models. The fact that an intrusion might not be needed for the largest eruptions is such a crazy and outlandish idea that a lot of people shy away from but if the scientists analyzing this eruption are right and I am in no position to assume that they’re wrong, we should investigate this proposition.
All eruptions begin with the movement of magma but what could cause the mobilization of the magma however is a different story, with no thermal rejuvenation something else would have to get things moving. Regional faults and magma chambers have an interesting poorly explained relationship. The local faults of a volcanic system have historically focused magma accumulation into a specific location. The Wah Wah springs eruptions came from highly evolved sources and likely had an extensive regional tectonic setup.
It is a well-known fact that large earthquakes can mobilize and destabilize magma. It would seem crazy however to assume that a tectonic earthquake could mobilize 5,000 km3 of magma and in fact, if the magma chamber wasn’t molten, where’d all that melt come from anyway?
The natural assumption is to think that the erupted products were the 2-10% of melt within the chamber. It would also seem unlikely that all of this melt would find itself in exactly the right spot to erupt within a massive magma chamber. Melt with silicic systems usually take the form of sills within the larger system but in my opinion, it would be too dismissive to write this off as a possibility
It would seem more likely that 1 or more tectonic earthquakes mobilized a significant but relatively small amount of magma compared to the magma released in the eruption. Whereas the force of billions of tons of rising magma against the roof of the magma chamber could generate a lot of pressure that still doesn’t explain two things. If we are to assume that the all of the already small percentage of melt didn’t coalesce into one area then how did they get into the right area for eruption and more importantly where did the heat come from? 100s of cubic km of magma pressing up against a vulnerable setting may be enough to trigger an eruption but the products of the Wah Wah springs eruptions were HOTTER than the Fish Canyon tuff eruption in which the latter received thermal rejuvenation.
Assuming all of the erupted melt didn’t pool into one area, then we have only one other option, the eruption itself rejuvenated the system. That is, without a doubt, one of the craziest ideas I have ever come up with but let’s investigate. How in the world could an eruption make turn mush into magma? This where things get even foggier because in order to really delve into this idea we would need to know the specifics of this eruption which we don’t have. Just like everything else I find interesting! Chiles-Cerro negro, volcanic winter, Tatun Volcanic group, and The other thing I am going to write about!
But I am going to stop whining and do my best to explain this conundrum. An eruption is the result of a system releasing the pressure within itself, the pressure needed to trigger this caldera-forming eruption would be orders of magnitude larger than whatever we have seen historically. This eruption released over 100 times the material of the Mount Tambora eruption. The Tambora eruption released 45 km3 DRE of magma while the Wah Wah Springs eruption released over 5,500 cubic km3 DRE of magma. (I guess this makes Wah Wah springs a VEI 9 in bulk!) So since we don’t have any specifics on the nature of the eruption, let’s use crude math and say that there was a similar difference in pre-eruptive pressure. Which leads to a chamber pressure of 55,000-68,750 Mpa or 7,975,000-10,037,500 psi. I don’t think the chamber pressure rises linearly with volume but it’s the best we can do for now. Funny things could happen with this pressure, the compression or the decompression could bring substantial amounts of heat to the system.
The magma at Wah Wah springs was crystal-rich and water-poor so I think the magma was teetering on the edge of being useless mush but was at its most volatile stage before this transition. Now just like all my other propositions I can’t actually prove it because I don’t have any resources but this does show one thing. We’re missing something on caldera-forming eruptions. If there doesn’t need to be an intrusion or thermal rejuvenation then the 40% molten threshold may be invalid for these eruptions.