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(Please read the text thoroughly before giving an answer) A big explosive eruption goes off with the energy of hundreds or thousands of nuclear weapons combined. It would be expected that the massive expanding bunch of gas (most vividly seen in the photographs of Mt Saint Helen’s’ blast cloud expanding) would create a large pressure wave.

Often it does make a pressure wave, with in many large explosive eruptions audible sound being heard from hundreds of km off. Sometimes it even rattles windows (on Mount Etna) and/or pops ears (as in the preliminary phase of the 1991 Pinatubo eruption), and that’s for very much smaller (compared to a VEI 5-6) explosive eruptions (there is, on YouTube, footage of Mount Tarvurvur exploding, and the pressure wave can be seen rippling outwards, with the eruption cloud in the distance). But oddly, even in much larger (VEI 5-7) events, the pressure wave does not seem to cause damage (there are a few exceptions to this rule). Not even the pressure wave generated by the likes of Tambora or Rinjani was enough to do significant damage (at least no records have survived, or I haven’t found them). This makes little sense, because of the fact that smaller explosive eruptions can produce pressure waves on the scale I have mentioned, and also seeing as the power of the eruptions can be so great that they do such things as:

-leave holes in the earth km across (actually the ground just collapses into the empty void left by erupted magma),

-throw rocks and other material several kilometres (so that homeowners at these distances had to watch out for huge boulders a few metres across totaling their homes) to tens of kilometres, and maybe more. Often what happens is the aftermath will have boulders littered across the landscape.

-launch tremendous ash clouds into the air, as well as an assortment of other material, often totaling cubic kilometres

-create truly tremendous blast clouds and/or jets/plumes, that expand at hundreds of metres per second (A nuclear fireball pushes air out of its way as it expands very fast, creating a shock wave. A fast expanding volcanic blast cloud should do the same.) which can be seen if you look at any picture of a large volcanic eruption and try to compare the size of the cloud to its surroundings or other things in the picture. Since the Mount Saint Helens blast cloud seems to have grown to the size of the mountain, it was probably several km across at its widest, although I’m just guessing. It’s hard to make sense of this dilemma, because the blast cloud expands at very fast speed (hundreds of kmph or meters per second).

In some cases this would be explained partially by the eruption plume shooting upwards, but in a large portion of such events the eruption cloud would not be just a straight vertical jet. So we would then expect a more intense pressure wave (from the perspective of someone on the ground close to the eruption).

But as I have said, the pressure wave very rarely causes much or any damage. And it’s even stranger because of the things I have said above. Anybody know the answer?

Below are some images that will help to illustrate my point.

This footage is the explosion sequence of the Beirut blast. As you can see, the expanding gas cloud pushes out a pressure wave.

The power of the 1980 Mount Saint Helens blast. The blast cloud at this point has expanded to close to the size of the volcanic peak itself.

(Usually it’s not the explosion that kills. There are an assortment of other volcanic hazards that are apparently more dangerous, like volcanic bombs, lahars, pyroclastic flows, and tephra fall many of which aren’t directly related to the explosion.)

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    $\begingroup$ I think it's just a matter of timescale. It is true that large explosive eruptions (VEI 5 and above) release tens or hundreds of time the energy of a nuclear bomb, but it takes hours to do so. Plinian eruptions usually last a few hours to a day or two, during which all the mass (hence energy) is released quite progressively. I don't know much about nuclear bombs, but I think their energy release is almost instantaneous, in a matter of seconds. $\endgroup$ May 14 at 15:06
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    $\begingroup$ Even so, the massive volcanic blast clouds move outwards and hundreds of km per hour, and smaller eruptions somehow make intense pressure waves that larger individual eruptions somehow dont build upon as much. $\endgroup$
    – A. N Asker
    May 15 at 8:44
  • $\begingroup$ I mean that the pressure waves of larger eruptions arent proportionally as intense as those of smaller explosions, And, whats more, they arent even close to being so. $\endgroup$
    – A. N Asker
    May 15 at 9:08
  • $\begingroup$ From what I’ve gathered, a nuclear explosion releases its energy in a tiny fraction of a second. Only after that the energy spreads out. The fireball pushes air out of its way, making a shockwave. A fast expanding, huge volcanic blast clpud should do something similiar. $\endgroup$ May 15 at 9:57
  • $\begingroup$ If you have a huge several km blast cloud expand at hundreds of km per hour, in some cases at supersonic speed, it will produce a large pressure wave. See footage of the Beirut blast of 2020 or the Tarvurvur footage I mentioned (in fact there’s multiple videos that show a volcano’s pressure/shock wave passing the camera, sometimes shaking it) to get some idea of the pressure wave a big, fast expanding blast cloud makes. So that’s not a very good answer at all. $\endgroup$ May 15 at 10:00

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Volcanoes are not bombs, they don't explode through a high explosive reactions, it more about dissolved gases being released from pressure. More like shaking up a coke can and popping it, spraying stuff over a large area than it desintigrating itself and spreading a cloud of gas instantly. A jet is a far better analogy than a bomb.

This means

  1. It is a slower release, with little in the way of shock wave power. Energy needs to be released faster than the speed of sound to get a shock wave. Volcanoes just do not release most of their energy in this way, at best just the initial breach which is only a fraction of the energy released. The eruptive power is not released all at once but slowly over time. It is similar to the difference between a low explosive and a high explosive in many ways.

  2. The stuff released by such a volcano is a far far bigger threat than the relatively tiny shock wave. Ash, gases and pyroclastic flows are very deadly, very destructive and travel a lot farther so they kill far more people. If you are close enough for the shock wave to matter you were going to die no matter what; likely by being incinerated. As an example, two cities were buried under the Vesuvius eruption outflow but the shock wave likely did not even make it off the cone of the volcano.

A really big volcano produces a greater shockwave for the same reason fifty pounds of gunpowder produces a bigger shockwave then five pounds of gunpowder, even though they have exactly the same explosive mechanism. The largest volcanoes release thousands of times more energy than small ones. Volcano energy scales logarithmically just like the richter scale. Soa really big volcano's even though the shockwave makes up only a tiny fraction of its power still has enough energy to do some minor damage at a distance, although the accompining earthquake is ussually far worse.

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  • $\begingroup$ Maybe not a shock wave, but still a very intense air wave. These can still do damage, as the Beirut blast showed. And this doesn’t explain why a large eruption doesn’t usually create a massive pressure wave when small explosions already can create a quite intense pressure wave. $\endgroup$ May 16 at 6:26
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    $\begingroup$ @TitaniumVCarbon mybe you should look inot why some small explosions can produce strong shockwaves. is has to do with the speed of the explosion, it is literally the diffrence between low and high explosives. some Volcanoes do produce very power blasts of gas (volcanoes don't emit air), they are called explosive eruptions and they can level towns. Also the Beirut blast was a shockwave. You may be better off asking how exposions work first. $\endgroup$
    – John
    May 16 at 20:32
  • $\begingroup$ Putting eruptions like Krakatoa (for some reason the pressure wave was so intense walls cracked a 100 miles away) aside for a moment (again, these mere pressure waves are not benign), a normal big eruption should be expected to create an intense, maybe damaging pressure wave or perhaps a shock wave, judging from those made by smaller preliminary explosions alone, because whatever the explosion speed is, it seems enough to make an air wave that pops ears in small preliminary explosions. Why then, do the main, more intense, phases of large eruptions not make an actually damaging pressure wave? $\endgroup$ May 17 at 10:03
  • $\begingroup$ Is it that the smaller eruptions somehow have a higher expansion speed than the large (VEI 5-7) explosions? $\endgroup$ yesterday
  • $\begingroup$ Again you are misunderstand what a pressure wave is .from a volcano, it is a wall of ash and gas being ejected from the volcano. research how shockwaves work becasue you seem to be very confused about the diffrence between shockwaves, sound waves, and pressure waves. Diffrent volcanoes can have vastly diffrent eruption types, it is not about size but the chemistry of the margma, some volcanoes are explosive some are not. St helens was a high felsic volcano so it exploded like a bottle of coke you dropped mentos into then sealed up. It has nonthing to di wtih the size. $\endgroup$
    – John
    16 hours ago

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