Stopping a Volcano

I've seen in multiple forms of fiction the sort of joking notion of stopping a volcano from erupting by just... plugging up the top with something? Like, a large and sufficiently heavy rock?

While the idea is obviously silly and extremely impractical, the prevalence of this trope got me thinking: is it actually impossible?

If you were to somehow fill in or "plug" the main crater of an active volcano, either with a single large object or maybe by filling it in with dirt or concrete or something, would that actually temporarily repress the eruption? Or would the pressure just blast the obstruction out again, or open a new vent out the side, or do something like Mt. Saint Helens did and blow out the whole side of the mountain? Just a fun thought, I know it probably wouldn't work but I'd wondered if anyone had actually done any sort of testing or research on the subject.

• Many volcanoes are naturally plugged. It may only get worse. – Alexander Jan 24 '19 at 22:56
• It looks like you almost answered it already ;-) – NofP Jan 24 '19 at 22:57
• You seem to have answered your own question. Pressure will increase until something ruptures, whether that be the plug, or the flank of the volcano. At some point, it WILL explode. – Arkenstein XII Jan 24 '19 at 22:58
• And for the record, yours is nowhere near the silliest question this site has seen. Heck, it might not even rank. – JBH Jan 24 '19 at 23:57
• Perhaps instead of the "plug it" solution which is combatting a symptom, could someone answer if you could relieve the pressure? Perhaps a large-scale pumping effort? For stakes lets say its a supervulcano near large population centers and you have to get it quiet. – Demigan Jan 25 '19 at 0:13

Yes... and no...

A volcano explodes (very simplistically, this is the problem) with a force. Apply the equation F=mA. We know the force of the volcanic explosion, we know the acceleration of Earth's gravity. Voilà! We know the mass of the rock we need to plug the eruption.

Sounds simple, doesn't it...

Outrageously ignoring the fact that volcanic eruptions occur for a number of reasons, we'll simplify the discussion to assume Mt. St. Helens is indicative of the complexities of the problem. And we can sum up a lot of that complication with one word...

Earthquakes

The eruption of Mt. St. Helens was preceded by a number of earthquakes of various sizes, ranging from itty-bitty to magnitude 5.1. Earthquakes have the nasty habit of softening up and shifting around rocks, faults, vents, and everything else associated with volcanism.

But the fact of earthquakes isn't all, where they take place is also a problem. Some are deep in the earth, others are shallow. Some are miles away, some are right under the honking volcano. And they all add up.

And we're not done

Just to make things ugly (remember, I'm already simplifying by ignoring types of volcanism), you have the geological makeup of the surrounding area. Do you have a really tall mountain with thin walls? Or a really low one with thick walls? Do you have a big throat or a small one? Is the Earth desperate to pop this proverbial zit? Or is it just trying to clear a little mucus?

Conclusion

Not to put too fine a point on it, but you're trying to fix the Earth's "grumbly tummy" by putting a billiard ball in it's mouth. The billiard ball is certainly more mass than whatever might come out of the stomach,1 but if you tried it yourself, you'd likely be surprised to discover that it didn't stop much — in fact, it usually makes it a lot worse.

So, yes, for some (few) eruptions dropping a big ol' rock on top will stop it. For the rest... you'd better have evacuated the area, first.

But it'd sure look cool, wouldn't it? Kinda like when they tried to get rid of the beached whale by blowing it up.

Oh, and just a reminder, musket rifles use the same basic premise of putting a billiard ball atop a grumbly tummy. And what goes up....

1In my metaphor. It's my metaphor, after all.

• Yeah, seemed unlikely to be a long-term solution XD – BonnetBee Jan 25 '19 at 0:11
• That only applies to eruptions that explode. Only some types of volcanoes do that. – TheBlackCat Jan 25 '19 at 19:19
• @TheBlackCat, please note from my answer the phrase, Outrageously ignoring the fact that volcanic eruptions occur for a number of reasons.... – JBH Jan 25 '19 at 19:30

Every volcano that isn't actively erupting is actually just like this

All volcanoes that are not erupting are currently plugged up by something. Usually it's rock or cooled magma. The better the volcano is plugged the more pressure and heat it will need to erupt. It's also possible for it to never erupt, but then you might say that it really isn't a volcano anymore, because, after all, if you dig deep enough anywhere on the earth you can get down to molten rock, and then you have a volcano. The volcano under your house is very well plugged right now (please don't start digging).

• The sentiment here is pretty much correct. However, I'd live to point out that the idea that "if you dig deep enough anywhere on the earth you can get down to molten rock" is a misconception. Unless you live in a volcanic zone, it is likely that there's nothing but solids beneath your feet all the way down to the core-mantle boundary. – Arkenstein XII Jan 24 '19 at 23:29
• At which point you reach the earth's mantle which is essentially molten rock. – Mathaddict Jan 24 '19 at 23:30
• Nope! The Earth's Mantle is solid. Mainly composed of the mineral forsterite, and various kinds of pyroxenes. – Arkenstein XII Jan 24 '19 at 23:35
• @Mathaddict that is not correct. The mantle is mostly solid, with very small spots where melting occurs (which happen to below volcanoes) – Gimelist Jan 25 '19 at 2:14
• Also, this answer is incorrect. Volcanoes that are not erupting are not erupting mostly because there is no new influx of magma, or fluids have not been saturated, etc. The overpressure of a "plug" is usually insignificant for determining whether it is important or not. – Gimelist Jan 25 '19 at 2:15

No

You cannot plug a volcano.

Volcanoes erupt because:

1. The magma is very fluid, and there is a new influx of magma coming from the mantle. This magma is very efficient in carving a way through solid rock at extreme pressures. When it gets close to the surface it will just make make new dykes and sills to flow through. These type of magmas will be very fluid, something like Hawaiian magmas. Your attempt to plug it would be like putting a piece of paper in front of a bulldozer.
2. The magma reached gas saturation, and it is rapidly nucleating bubbles of gas (mostly H2O), and it's going to explode. Your plug might only make it worse, because you're blocking the escape path of the gas, allowing overpressure to build up. Plugging it only delays the inevitable. Again, it's like putting a small piece of paper on top of a mentos-coke combination, hoping that it will stop it exploding. It will not.

Submarine Volcanoes Suggest NO

You are asking whether we can plug volcanoes by putting something heavy on them. Well, consider the volcanoes that erupt at the bottom of the ocean: https://en.m.wikipedia.org/wiki/Submarine_volcano There are tons of these volcanoes, and many sit at depths where massive weight of water is pushing down on them. That’s a pretty effective plug. But they still erupt regularly, and some throw magma quite a distance vertically. The pressure below is more that the oceanic pressure. You can get some pretty freakin’ heavy rocks and plug specific volcanoes for a time, but the pressure to erupt is going to win eventually.

There are two solutions to high pressures.

1. Plug it, if you can. If the rock walls of the volcano are sufficiently hard and stable, a massive plug should be possible. However, it must be securely and safely anchored to the rock walls without damaging their ability to bear load.

2. Vent the pressure and flow elsewhere. This creates more of a managed volcano than a non-existent one, however.

In practical terms, if you chose the second option, you still would have to implement the first as well. It would also have a higher chance of success. Successfully managed "volcanic boreholes", if they are possible at all, could diminish pressure and flow to create stable areas suitable for life. If the vented areas are lower in elevation than the area that needs protection then spillover is less likely. But, for consideration, remember that the Earth is really massive and generates subterranean pressures in similar or greater force than a nuclear explosion. The materials that could withstand such forces are rare, highly engineered, and may have other limitations which prevent their use. Are materials of this nature capable of withstanding volcanic heat for instance? For the sake of argument, let's suppose that a carbon fiber reinforced ceramic metal composite of high quality could be produced in sufficient quantity to plug a volcano. Now the rock wall must bear the pressure of the eruption without giving way. There is a lot of uncertainty there. Perhaps more of a dome is needed, with ventilation tubes and other features. The more you consider it, the more futuristic the technology should be. Cooling the lava also could lower the pressure, but doing it wrong will make it worse. Water turning into steam expands to 1000 times the volume. Freezing it with liquid nitrogen could cause thermal shock, causing high energy explosions. Cooling it with supercritical CO2 is viable enough that early experiments have been done with this technology as an enhanced geothermal system. One noted side effect is that some earthquakes could have been linked with this technology. There is always a side effect to working with intense forces on geological scales. So, in short, managing a volcano will take intensely futuristic technology and will still cause earthquakes. This still isn't quite the same as stopping it, but managing high energies sometimes just implies slowing them rather than stopping them.