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So. There's a group of nutcases in my setting; they are an ages-old, all-powerful secret society, yadda yadda yadda, control massive amounts of the world behind the scenes, have agents everywhere of every ethnicity, religion (or pretending to be that religion), nationality, etc.

The thing is, however, that they're basically a cult of Hephaestus/Vulcan that worships volcanoes. Their goal is to cause a supervolcanic eruption. To do this, they're going for a multi-step process:

  • Drill 10 kilometers (it's possible) underground in the Phlegraean Fields to drill into the cap over the magma reservoir under the guise of attempting to monitor the state of the volcano. Advancements in ground-penetrating radar have made the detection of these reservoirs possible. This stage is basically "finding a filled and sealed soda bottle underground".

  • Set off a long, skinny (it's got to fit down the borehole) 1-gigaton multi-staged Tellar-Ulam device to "take the cork out", as it were - to negate the lithostatic pressure that has so far prevented a second eruption in the vein of the Campanian Ignimbrite eruption, which this group sees as an attempted but failed manifestation of their god. This stage is basically "digging down and blowing the cap off of the soda bottle with a firecracker".

  • Pump water into the large resultant salt dome cavity with repurposed hydraulic fracturing technology in order to destabilize the volcano. This stage is basically "drop a Mentos in the bottle for shits and giggles".

  • ????

  • Profit/die.

The actual end result of this is that the cult/conspiracy/thing miscalculated, and they accidentally cause a supervolcanic eruption across the entire Campanian volcanic superstructure, resulting in a semi-apocalyptic event and a multi-year-long nuclear winter across the entire planet.

Now, I recognize that, provided that you portray things believably enough, willing suspension of disbelief is easy for the reader, but I'd also like this to be somehow grounded in some kind of realism. So, first question, out of many I have about this scenario:

Is it even possible (hypothetically, of course) to make a supervolcano erupt by nuking its top off and then pumping water into it?

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  • $\begingroup$ Yes you can drill 10km deep. NO you cannot do so into a magma reservoir. You would need to resort to trickery, as per LSerni's oblique drilling approaches, get under the reservoir without actually penetrating it. $\endgroup$
    – PcMan
    Commented Nov 4, 2021 at 22:22
  • $\begingroup$ @PcMan I think the more specific way I should have put it was "into the cap that's keeping the magma reservoir in". $\endgroup$
    – KEY_ABRADE
    Commented Nov 4, 2021 at 22:23
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    $\begingroup$ Make sure you read the Richter 10 before engaging in excessive handwaving. $\endgroup$ Commented Nov 5, 2021 at 0:37
  • $\begingroup$ The Tsar Bomba nuclear test explosion of the Soviet RDS-220 hydrogen bomb was ONLY 50 megatons. It had a 100% destructive blast radius of 35Km (22 miles). That's the radius, not the diameter. A 1-Gt bomb would have a 100% destructive blast radius covering most of Canada and the U.S. Granted, it's not long-and-skinny... but you might have overestimated the size of bomb you need. $\endgroup$
    – JBH
    Commented Nov 5, 2021 at 5:56
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    $\begingroup$ @JoinJBHonCodidact Start with this little factoid. Tsar Bomba was detonated at 4000m above surface. Do you see a 31km deep hole at the blast site? No. How about a 1 km deep hole? No. Ok, how about 100m deep? No. The blast site lost some 5-10m of altitude following the blast. This question deals with the deep underground detonation of a nuclear bomb. You cannot just apply the "100% destruction of built houses" range on it. and you know that $\endgroup$
    – PcMan
    Commented Nov 5, 2021 at 6:54

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Usually, the objection to this kind of scheme is that the energy of a VEI7-level supervolcano is orders of magnitude beyond that of even a gigaton fusion device.

However, the energy inside the soda can might be way beyond what is needed to weaken the lid enough. So, the possibility can't be ruled out that an eruption might be triggered this way.

Triggering a supervolcanic eruption in the Campi Flegrei, though, is in all likelihood not possible due to the magmatic chamber geometry.

enter image description here

The supervolcanic chamber might be actually beneath a high-temperature magmatic reservoir at about 3.5 km; drilling there is difficult since it's out at sea, and being able to drill past that is unlikely in the extreme. Furthermmore, while there is the potential for a disastrous eruption, it would almost certainly not be the envisioned super-eruption.

Trying to make it work

Start drilling from the coast, obliquely, getting almost under the upper magma chamber. Assume there's another one below. The excavation tunnel will have to be cooled down substantially, but should reach almost inside the chamber. At that point, the super-Tsar Bomba is detonated so that the blast goes horizontally -- against the magma chamber. This will (waves hands rapidly) destabilize the cryptodome between the larger lower chamber and the upper chamber. The mixing of the two different kinds of magma leads to a massive increase in pressure, and the whole lower chamber blows up.

I still think we're short of supervolcanicity (the whole volume interested by the event is less than five kilometers in radius and less than three in depth; assuming half of that goes up, which is really unlikely, we're looking at about 120 cubic kilometers of mass, which while being comfortably in the VEI-7 range still falls short of Mount Tambora.

On the other hand, the Campi Flegrei undergo periods of activity when magma flows from beneath, gets stocked in the upper chamber and slowly outgasses towards Solfatara and Monte Nuovo. If the explosion happened at the point of maximum flow, that could further increase the scale of the disaster.

Regarding which, a significant portion of the ejecta would have been massively irradiated by a fusion bomb. Lots of high-activity, short-lived radioactive isotopes in the fallout.

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    $\begingroup$ +1. If you could, somehow, set off the supervolcanic chamber, however, would it result in a super-eruption? $\endgroup$
    – KEY_ABRADE
    Commented Nov 4, 2021 at 21:32
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    $\begingroup$ @KEY_ABRADE I'm afraid nobody really knows. The "volcanicity" of an eruption depends on the area interested by the eruption, the magma composition and the eruption dynamics. You are correct that an eruption has much in common with a Mentos event. The Campanian magma can lend itself to an explosive, catastrophic outgassing (the Plinian eruption of 79 d.C. proves it conclusively), but usually this happens when the pressure ruptures the "seal" above the chamber. Rupturing it from the outside might be like blowing the lid of a pot that is just simmering - vapor blows out, but no explosion. $\endgroup$
    – LSerni
    Commented Nov 4, 2021 at 21:46
  • $\begingroup$ Would it be possible to drill into the seal from the side to destabilize it with a nuke/frack it afterwards, provided that you set it off towards the "magma" side of the seal? $\endgroup$
    – KEY_ABRADE
    Commented Nov 4, 2021 at 21:49
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    $\begingroup$ @KEY_ABRADE ...possibly. Added to answer. $\endgroup$
    – LSerni
    Commented Nov 4, 2021 at 22:06
  • $\begingroup$ Great, radioactive volcanic ash. That's fun. I can only give you one upvote, though, and I'm thinking that I got ahead of myself in the OP regarding the location of the eruption. I'm going to ask a later couple of questions about what supervolcano would have a magma reservoir shape capable of making this easier. I'm thinking Yellowstone, since it still seems to be active. $\endgroup$
    – KEY_ABRADE
    Commented Nov 4, 2021 at 22:11

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