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On May 18, 1980, Mount Saint Helens made American history with an eruption that took 57 human lives and killed thousands of animals. It has released only a quarter of a cubic mile of ash, but it is the influence of the prevailing winds that allows us to measure the area and thickness of the ash cloud. The prevailing winds of 1980 made the Mount Saint Helens ash cloud a bit like this:

enter image description here

The reason I brought it up is that it inspired this particular alternate scenario. In an alternate Earth, there is a volcanic island six miles wide and 8,157 feet above sea level. It is located 42°54′59″N but 50 miles west of the coast. Let us say that this fictional volcanic island, which I'd christened "Wizard Island", was the alternate volcano that erupted in 1980, not Mount Saint Helens. It's not just the different location of the eruption that would affect the outcome of the question, but also the different volume of the eruption. Wizard Island erupted a dozen cubic miles of ash, 48 times bigger than Mount Saint Helens in OTL. Using the provided information, how big and how thick would the resulting ash cloud be?

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch May 24 at 4:22
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First you have a problem 50 miles off the coast is solidly on oceanic crust. volcanoes that pop out of the oceanic crust are not releasing a lot of ash, As oceanic crust is mostly basalt so is the laval. thus they tend toward basaltic lava type volcanoes instead. You need a more rhyolitic composition to get a lot of ash. This means you are going to need to do some serious changes to the underlying tectonics so no exact match is possible.

for your ejecta volume look as similar volcanoes on the eruption scale.

The more basaltic the lava the less of the ejecta will be ash. A basaltic volcanic eruption will be one of the largest in earths history to generate that much ash, and will be heard across the entire planet and cause massive tsunami. looking at 1815 eruption of Mount Tambora sizes, which created the year without a summer and will seriously screw over human civilization. And that's on the low end, on hte high end something closer to the deccan traps in size. so expect the end of human civilization and the one of hte largest mass extinctions in history.

But honestly things like how the eruption occurs and over what time scale it occurs can change this greatly so even this is just a ballpark. If you somehow get a motre rhyolitic volcano you are looking at something closer to the The Mount Pinatubo eruption which will still be quite massive. A complete review of the eruption ashfall can be found here.

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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There is a very close real-life example. The climactic eruption of Mt. Mazama that created Oregon's Crater Lake (which perhaps coincidentally contains Wizard Island) ejected an estimated 12-14 cubic miles of material: https://en.wikipedia.org/wiki/Mount_Mazama#Climactic_eruption The distribution of that material is well-mapped - the layer is even used to date prehistoric objects, since the date of the eruption is fairly accurately known.

Here's a link to a map of the ash fall: https://www.researchgate.net/figure/Estimated-spatial-distribution-of-Mt-Mazama-volcanic-ash-across-the-western-United_fig1_265168566 Just offset it to the location of your "Wizard Island", and you'll have something realistic. (But note what others have said about the realism of this kind of volcano being found off the Pacific Coast.) You could also change the wind direction somewhat. Mazama's winds seem to have been to the northeast, while Mt. St. Helens' were a bit southerly.

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  • $\begingroup$ Mazama ejected 12-14 miles of LAVA (mostly pumice) not ash that is very different type of eruption. $\endgroup$ – John May 23 at 22:48
  • $\begingroup$ @John: Not according to the articles I've read, for instance the Wikipedia link. From what I've read and know of the geology of the area, Mazama was a typical Cascade volcano, very similar to St. Helens. $\endgroup$ – jamesqf May 24 at 16:40
  • $\begingroup$ Cascade volcano has nothing to do with the type of eruption. Cascade is a reference to the layout of the volcano and the surrounding terrain not the composition of the volcano. It being a cascade volcano means it is part of a subduction mountain arc. $\endgroup$ – John May 24 at 16:50
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    $\begingroup$ Mazama erupted volcanic ash as well as pumice and other typical material. That it is why it is called Mazama Ash. And it's called a "Cascade Volcano" because it is in the Cascade Range and has nothing to do with the "layout of the volcano" whatever that means. The statement that it's typical Cascade volcano means it is typical of volcanoes found in the Cascades, like St. Helens. Which it was. pubs.usgs.gov/fs/2002/fs092-02 $\endgroup$ – Keith Morrison May 24 at 19:29
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    $\begingroup$ @John: 1) When I wrote "typical Cascade volcano", all I meant was that it was/is typical of the other volcanos in the Cascade range, nothing more. I am not a geologist, and was not using it as a geological term. 2) And likewise with "ash" (which of course is a misnomer anyway, as it's not the product of burning). I meant stuff that gets blasted into the air and carried by wind. I don't care if you want to call it "pumice", and really don't see that there's a non-technical difference. $\endgroup$ – jamesqf May 26 at 19:13

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