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I posted this question: How would the United States survive and adapt in the decades following a Yellowstone eruption? What things would need to change as the world recovers? and it was closed due to it being too broad of a question, which was a fair decision. After some thought and a few more people commenting on the question, I decided I needed to take a step back and figure out what I actually need answered.

The existence of this eruption having happened in the past is crucial to the main settings I'm working with, one of which is a rural town that was settled inside the crater that the Yellowstone eruption left behind. It would be very hard at this point, having very nearly finished my first draft of the final rewriting of this book, I don't want to have to go back through and restructure everything. I don't want to have to remake this setting.

So. Some things about this event that have to happen:

  • The Eruption itself has to happen. I've already determined how it happens, which I'll go more into detail about if needed, but it's not super related to the aftermath of the event.
  • The Eruption is large enough and powerful enough to leave behind a really large crater. Big enough for a full forest, a lake, a small city, and a handful of tiny rural towns.
  • The Crater itself is turned into that environment by a similar type of magic to the one that makes the volcano blow.
  • The United States still exists as an independent country after everything recovers.

Before I can ask questions about what that recovery would look like, I need to ask how much damage such an eruption would cause in the first place. The other similar questions on this site covering this topic all include the assumption that scientists would see the eruption coming and there would be some amount of forewarning time to start evacuating people. I want to know what that big of an eruption would do to the country if there wasn't any forewarning, because it is magic that artificially causes that eruption, and nothing is really too different from our real world regarding humans in this fictional version of earth, because they don't know magic exists.

To sum up: If Yellowstone had an inexplicably sudden and large eruption, with little to no forewarning time, what would be the scale of that damage, and how would it effect the US as a country?

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    $\begingroup$ Supervolcano is a pseudo-documentary film made by Tony Mitchell for the BBC in 2005. It is a BBC effort, so it is more in the nature of a dramatisation of a scientific paper. Highly recommended. Long story short, if Yellowstone erupts then the USA would be better advised to use its military might to conquer Central America and move there, because mid-latitudes North America will be quite uninhabitable for a looooong time. $\endgroup$
    – AlexP
    Nov 9 '21 at 20:45
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    $\begingroup$ The earth shaking is not the problem. the kaboom is not the problem. the crater is not the problem. The problem is that about 1/3 of the country will be under 6 feet or more of dead,crispy-fried and pulverized rockflakes. Its hard to grow stuff under volcanic ash, and if you drop your carkeys in the stuff, you will never find them again. Or your car. Or the highway. $\endgroup$
    – PcMan
    Nov 9 '21 at 21:18
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    $\begingroup$ Harry Turtledove has a thriller/SF trilogy in which Yellowstone erupts. Also Baxter and Pratchett's Long Earth has Yellowstone erupt (everyone escapes to parallel universes though). I propose shifting the earth's mantle underneath the crust such that the hotspot is underneath France. The cost would be significant, but you should ask what are your tax dollars really for if not for that? $\endgroup$
    – John O
    Nov 9 '21 at 22:04
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    $\begingroup$ Note that if you want forest to grow inside the crater and make it in general a habitable nice place to be, that will require at least a few centuries to a few millenia after the eruption. This is a very long time for humanity. $\endgroup$
    – quarague
    Nov 10 '21 at 7:09
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    $\begingroup$ For comparison purposes consider the effects) of the 1883 eruption of Krakatoa which would be a tiny little bang by comparison with Yellowstone. $\endgroup$
    – StephenG
    Nov 10 '21 at 17:59
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EDIT: damn, this got me my first gold badge

Here is a useful source, which is itself based off of this. Here is an ancillary source.

WTL;DR: all the damage

TL;DR:

  • everything within 80 kilometers utterly deleted to an extent I am incapable of describing
  • everything within 125 kilometers is the surface of Mars, unbreathable atmosphere included
  • everything within 200 kilometers sees everything from buildings to biospheres collapse
  • everything within 300 kilometers is post-apocalyptic
  • everything within 800 kilometers is basically post-WW2 Germany
  • everything outside of 800 kilometers is basically COVID-19 but ten times worse

ZONE 1: HELL

ESTIMATED ASHFALL: 3 Metres

DISTANCE FROM BLAST: up to 80 KM

MAJOR TOWNS: West Yellowstone, Mammoth

POPULATION: 70,000

POPULATION DISPLACED: 100% (guess why)

COST (2012 DOLLARS): 7.1 billion

Zone 1 is called the 'pyroclastic zone'. Surges of scorching hot gas and ash (over 400 centigrade) exceeding the speed of sound would spill from the side of the volano [sic] and could extend up to 100 kilometres out. It is not survivable.

There used to be an entire building here.

There used to be an entire building here. Note that its rebar - this is reinforced concrete, i.e. what they build support columns for big buildings and the bulk of the structure of nuclear bunkers out of - has been bent in the direction of the pyroclastic flow, rocks have been embedded in the concrete, it has been sandblasted, and it has been scorched black.

There are various ways that that bending of rebar should impress you. Either:

  • the flow blew hard enough to bend rebar
  • it got hit with so many rocks, pebbles, and the like that it was slowly "hammered" into that shape over the course of a few minutes
  • it was heated so heavily that it could be bent like taffy
  • a combination of the above

What's more - if this is were the product of a Yellowstone-derived pyroclastic flow, this would be the second story, as the first story would have been buried in 10 feet of ultra-hot ash.

It's a nuclear blast wave from hell - yes, I mean worse than a normal one - without the radiation. It is unsurvivable. It is a soup of ultraheated gases and rocks moving faster than a jet aircraft. Anyone in this zone is irrevocably dead, do not pass GO, do not collect $200, no saving throw, etc.; if you are not in a far-underground, well-reinforced nuclear bunker with air recyclers and zero connections to the surface, you are fucked, you are dead, and the heat might still cook that bunker anyhow.

It's like Hell throwing up on you. A nuclear weapon is child's play compared to this. Extremophile bacteria might survive this. You went outside with a gas mask well haha dumbass it melted onto your face. Life is impossible.

I cannot overstate how incredibly destructive and lethal this is.

ZONE 2: MOON

ESTIMATED ASHFALL: 1.8 Metres

DISTANCE FROM BLAST: 80-125 KM

MAJOR TOWNS: Bozeman, Cody

POPULATION: 350,000

POPULATION DISPLACED: 95%

COST (2012 DOLLARS): 28 billion

Zone 2 would have to be evacuated completely prior to the eruption. The very heavy ashfall would collapse all structures. Vegatation [sic], livestock and aquatic life would die. Power and telephone lines would break and roads would become completely unusable.

Aside from the folks with easy access to fast transportation, nobody is getting out before ~6 feet of ash destroys everything; if you are not in a nuclear bunker (albeit a less-tough one than for Zone 1), you are dead.

Pretty much paved flat.

Pretty much paved flat. Everything has been crushed by ash fallout. Ash has a density of, on average, 1,000 kg/m^3, which ups to about 1,500 kg/m^3 as it gets rained on. Breathing outside will be very difficult, due to massive levels of airborne sulfuric acids. A gas mask is mandatory.

The dry bulk density of newly fallen and slightly compacted deposits ranges from 500 to 1500 kg/m3, whereas the bulk density of wet ash ranges from 1,000 to 2,000 kg/m3. Distal ashfall deposits most commonly show slight decreases in bulk density with distance from the volcanic vent source.

In other words, there is between 1.8 and 2.7 metric tons of ash per square meter in the average portion of this area. That's like having two Mini Coopers dropped onto a space the size of a bath towel. Not really survivable for most forms of life; it's just too thick and there's just too much of it.

Roads and reinforced infrastructure would still exist, but that's about the only thing that wouldn't collapse, and they'd still be buried. A lot of buildings are going down under that much weight. Power lines might survive, especially if undergrounded. Microbes will survive in the long run, as well as the hardiest of insect colonies.

ZONE 3: VERDUN

ESTIMATED ASHFALL: 1 Metre

DISTANCE FROM BLAST: 125-200 KM

MAJOR TOWNS: Idaho Falls

POPULATION: 640,000

POPULATION DISPLACED: 90%

COST (2012 DOLLARS): 225 billion

Zone 3 would have to be evacuated completely prior to the eruption. The very heavy ashfall would collapse all structures. Vegatation [sic], livestock and aquatic life would die. Power and telephone lines would break and roads would become completely unusable.

An irritating copy of the text for Zone 2, to be sure, but an accurate one.

People will have time to flee, but they'd have to flee to survive; if you are not in a reinforced, sealed building with long-term supplies, you are fleeing or dead but don't know it yet. Breathing outside will be very hard. A respirator is mandatory.

This is what ~1 meter of ash looks like.

This is what ~1 meter of ash looks like.

The collapse of the car and building was likely averted by shoveling ash off of both; in the event of a Yellowstone eruption, imagine this but with 3 feet of ash on top of everything - not just the ground - and the car and the building crushed. I recognize that that this doesn't look as bad as the image below, but I guarantee you it's worse; it just doesn't look that way because it's in a more built-up area. It's the best image I could find.

As you can see, cars are going to have a bad time. It won't be as bad as Zone 2, but it's still going to mess people's shit up. Many things will survive the initial ashfall, but then the rain will come along and double the weight, and they'll be crushed. Biological life and supply chains will die right off the bat. The ecological devastation will still be extreme. Bugs might survive in the long run, and small underground animals - like, the smallest of mice.

ZONE 4: POST-BOMB HIROSHIMA

ESTIMATED ASHFALL: 60 centimeters

DISTANCE FROM BLAST: 200-300 KM

MAJOR TOWNS: Salt Lake City, Boise

POPULATION: 11 million

POPULATION DISPLACED: 80%

COST (2012 DOLLARS): 225 billion

Zone 4 would still see substantial amounts of ashfull [sic]. Any areas with more than 30 centimetres of ash are under severe risk. The primary cause of death would be from roofs collapsing due to heavy ash load, potentially 1 in 3 people would be killed this way. Breathing outside would be very difficult. The water would be contaminated with sulphuric acid. The ash would enter air filtration systems causing air quality problems in structures still standing. There would likely be no power and very limited transporation [sic], if any at all.

Basically a post-nuclear wasteland, which says a lot about the past three zones; if you aren't a prepper here, you're either dead or fleeing. It's still survivable by biological life, though - the ash is thin enough that burrowers, VERY small land wildlife, and creatures in deep lakes might survive. Breathing outside without mechanical filtration is indeed "very difficult" - but, unlike the previous zones, also possible. A respirator is heavily advised.

60 centimeters of ash.

60-ish centimeters of ash.

Among the first challenges they faced as they sloshed ashore from a motorized canoe was the two feet of ash.

As you can see, it's not completely collapsing that shack - note that it's a one-story house; it's just that the nearby trees have collapsed under the weight of ash, giving the illusion that those are treetops. On the other hand, ash is about twice as dense when wet, so rain might actually be what takes out buildings that the ash doesn't.

Life will go on in this part, if incredibly differently. Ash levels are low enough that human habitation might be possible in the long run, and certainly in the short run - it won't take down the vital infrastructure right off the bat, so you'll have the time and resources to shovel yourself out and get ready to dig in for when things eventually start failing.

If you want a post-apocalyptic story, this is the place - the right blend of "survivable" and "catastrophic".

ZONE 5: POST-WW2 GERMANY

ESTIMATED ASHFALL: 15 centimeters

DISTANCE FROM BLAST: 300-800 KM

MAJOR TOWNS: Denver, Portland, Calgary, Seattle, Regina

POPULATION: 34 million

POPULATION DISPLACED: 10%

COST (2012 DOLLARS): 234 billion

Buildings in zone 5 are still at risk of collapse if the roofs are not immediately cleared of ash. Trees would be severely damaged due to breaking of branches. Road transport would be halted due to build up of ash on roads and cars would stop working as air-filters become clogged. Rail transport and electricity may be cut as wet ash short circuits signaling systems and sub-stations.

Much more survivable, but there will be serious supply chain and minor societal collapses.

This.

A light blanket of ash; oh, sure, the sky isn't black, but look at the ground.. A respirator is advised, as is a snow shovel. This level of ashfall is more dangerous because it's better at killing crops, not because it's better at killing people (at least, not directly).

Imagine this part as Europe during Zero Hour. Society is technically still around, but things are breaking down. The ecosystem is in bad shape, but it'll bounce back.

ZONE 6: INTRA-COVID-19 USA

ESTIMATED ASHFALL: 1-6 centimeters

DISTANCE FROM BLAST: Beyond 800 KM

MAJOR TOWNS: LA, Dallas, New York, Toronto, Chicago

POPULATION: 190 million

POPULATION DISPLACED: 1%

COST (2012 DOLLARS): 213 billion

Minor damage to buildings in Zone 6 will occur as ash enters, soils interiors and blocks air conditioning filters. Electricity may be cut as wet ash causes shorting at sub-stations. This would cause water supplies to be cut. Roads would need to be cleared to prevent ash from blocking storm-water systems. Crop damage is possible and livestock may be affected by lack of feed and contamination of water supplies. Damage to electrical equipment and machinery is likely.

Bad, but basically normal life with massive inconveniences. Imagine the COVID-19 supply chain shortages blown up 10x.

Doesn't look that bad, does it?

It's nothing in comparison to Z̵͖̤̜͍̼̐̒̇͊̚ô̴̱̹̆̏͒n̶̞̲͌̓͊̓̚ẻ̷̢͎̝̙ ̶̯̳̂̊̀ͅ1̵̥͝. Wear a facemask, change your filters on a bi-daily basis, and listen to your local disaster control authorities. Ration. Store water in your bathtub. Shrink-wrap electronics if you're not using them.

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    $\begingroup$ forget the sulfuric acid the ash makes breathing deadly, breathing in a heavy ashfall just cakes the lungs with ash which turns into volcanic mud. in zone 3 lots of small animals will survive, just due to luck and microclimate conditions. Also don't forget the volcanic winter that follows, the northern hemisphere ill experience at least a year or two of unending cold. $\endgroup$
    – John
    Nov 10 '21 at 0:47
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    $\begingroup$ Depending on chemistry 1mm of ash fall will kill crops, livestock, and anyone outside without a respirator. $\endgroup$
    – Ash
    Nov 10 '21 at 6:39
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    $\begingroup$ Part of the chemistry problem is fluorine which poisoned crops in England after the 1783 Iceland Laki eruption. fluoridealert.org/news/… So, even a small dusting of ash can cause significant problems with crops and livestock. $\endgroup$
    – David R
    Nov 10 '21 at 16:51
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    $\begingroup$ @Michael It's hard to make 2020 worse when 2020 has passed, yes. Also, it would have made 2020 a lot worse. $\endgroup$
    – KEY_ABRADE
    Nov 10 '21 at 23:34
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    $\begingroup$ @Mathemats To stop dust from getting into the nooks and crannies. $\endgroup$
    – KEY_ABRADE
    Nov 11 '21 at 2:51
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I assume that by "large eruption" you mean VEI8, that being the case up to two thirds of the continental US is buried in volcanic ash that hits the ground still glowing red, or hotter, and welds together to form at least several feet of solid rock. This new tuff layer takes centuries to completely cool but life starts to colonise the surface after only a few years in many areas.

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  • $\begingroup$ "Several feet" (even by a loose definition of "several" is only out to 300 kilometers from the blast site. That's a lot, but definitely not two thirds of the US. The problem isn't parts of it getting "rocked over"; the problem is the ash killing crops. The ash. Hah-ha. $\endgroup$
    – KEY_ABRADE
    Nov 10 '21 at 7:23
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    $\begingroup$ @KEY_ABRADE Yeah-no, the one metre isopach for the Huckleberry Ridge Tuff covers roughly two thirds of the continental US. I don't know what projection you're looking at I'm looking at the historical eruption series. $\endgroup$
    – Ash
    Nov 11 '21 at 5:52
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    $\begingroup$ @Ash, where are you getting your isopatch from? The one in Wikipedia covers less than half the US for the entire deposit. $\endgroup$
    – Mark
    Nov 12 '21 at 0:49
  • $\begingroup$ @Mark That looks like 2/3 to me and that is the first time I've seen that map described as the "ash bed", that is the same image I'm used to seeing labelled as the Huckleberry Ridge Tuff deposit and reading into the articles have found that they choose the 1m isopach as their reference field. $\endgroup$
    – Ash
    Nov 12 '21 at 8:45
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    $\begingroup$ @Ash please link those articles. I think that Wikipedia image is the 1 millimetre, not 1 meter ash deposit: agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014GC005469 $\endgroup$
    – Tim
    Nov 12 '21 at 13:14
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and how would it effect the US as a country?

I read all the technical data in both KEY_ABRADE answer and the links too. Lots of information.

What it looks like to me is there is some kind of hope in those numbers that USA could keep existing like a nation after a massive Yellowstone eruption.

After all recent nation wide problems caused by Covid19 virus and the political/social catastrophic change of government (the invasion of the Capitol Hill, in instance) does any one with a small sized imagination can believe USA will keep stand up after a total clash of all administrative system?

If the eruption was today Republican party governors could immediately start to work a way out of the Union. Texas probably could be the first.

I doubt USA could be "refurbished". And I doubt that the less affected states will accept to pay the bill to help the 80% worst affected. It is a scorched earth situation. Any international help will be symbolic because, of course, a mega vulcano eruption is a global catastrophe, sooner or later the ashes of the wrath will hit any country in this world, be in the form of a two years winter or in the form of fully three years of lost crops. Famine, in a global scale, never seen before. Does not do exist a survival courses that can prepare you to such wrecked panorama.

If USA citizens intend to survive like a nation after a massive Yellowstone eruption and keep the world safe to any other country the only way is start to prepare an WORLD WIDE plan to deal with what looks like is (like Thanos) inevitable. To act after the scientists given the warning will be too late.

Do I need make a diagram?

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  • $\begingroup$ It's possible for the US to survive, but not probable. $\endgroup$
    – KEY_ABRADE
    Nov 11 '21 at 4:10
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    $\begingroup$ I wouldn't bet my money on anything resembling the US to survive. With the expected MASSIVE stream of refugees and the complete collapse of infrastructure and supply chains for literally hundreds of millions of people there is bound to be a degeneration into countless civil wars, plundering, raiding, ... $\endgroup$
    – fgysin
    Nov 11 '21 at 9:29
  • $\begingroup$ Ok, I hurt the feelings of several (three at least) Dr. Strange Love lover guys who think USA could romantically survive a total apocalypse. And I am not off topic. The technical data of the above answers are too optimistic. I only added the real world hard to bear idea that does not matter how deep is your prep bunker, you gonna die. Period. You can not destroy a large part of the world and think you could have a romantic love story full of hope of a new world. These movies are hard, but close of the reality: Miracle_Mile - On_the_Beach_(1959_film) - -These_Final_Hours. $\endgroup$ Nov 12 '21 at 3:32
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    $\begingroup$ @techsystem2000 I don't really your answer a "hard to bear idea" moreso than what I wrote about, considering that I live on the Zone 2/3 border and I'd be boned regardless. Just because your answer has more people dead and more devastation doesn't mean that it's accurate. $\endgroup$
    – KEY_ABRADE
    Nov 12 '21 at 9:00
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    $\begingroup$ @techsystem2000 I will add here, as OP, that yes I did ask about how it would effect the US as a country. However, your answer feels A) politically biased, B) more like pessimism than realism, and C) lacks supporting facts and research to help prove your argument. It also did not answer the question of "how would it effect the US as a country" it tried to answer the unasked question of "how would the US respond". Whereas KEY_ABRADE's answer Did actually tell me some of how it would effect the US. Supply chain collapse, uninhabitable land, crop loss, population loss, travel, utilities, etc. $\endgroup$
    – WordyBirdy
    Nov 15 '21 at 5:15

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