Size of the volcano that ends the world
One of the theories behind the extinction of the dinosaurs is the eruption of one or a series of volcanoes. Supposedly the ash expelled by these eruptions blocked out the sun, killing plants and thus disrupting the entire food chain. A more recent example of the power of volcanoes on the environment would be the Little Ice Age, where the whole world felt colder for a while because of a volcano.
What I'm wondering, though, is if, some time in the near future, there was a volcano that ended most if not all life on this planet (humans can survive, but only because they're so smart), how big would this volcano have to be, and what would the eruption be like? Is there perhaps a place where a volcano would have an easier time of it? Is there maybe a volcano out there that fulfills all these criteria?
This post was sourced from https://worldbuilding.stackexchange.com/q/18037. It is licensed under CC BY-SA 3.0.
1 answer
It all depends on how obliterated you want the Earth to be.
If you want nothing left, you need to completely destroy Earth and accelerate all the remaining fragments so that they clear the area pretty quickly. Using some base numbers from this answer:
The gravitational binding energy of Earth, $E_{GB} = 2.4 \times 10^{32} \text{J} $.
The energy released by a VEI 5 event is 24 megatons. 1 megaton = 4.184PJ = $ 4.184 \times 10^{15} \text{J}$. Going on the exponential VEI scale of "ten times more destructive", we can estimate (this is not exact) the energy of a VEI 18 (as ShemSeger suggests) to be $ 4.184 \times 10^{28} \text{J} $. I shall call this number $E_{V18}$.
Let's see: if $E_{V18} = E_{GB} $, the explosion merely destroys the Earth and separates the fragments enough that they don't rebind gravitationally. If however $ E_{V18} \gg E_{GB} $, the planet explodes and is never seen again.
In this case, $E_{V18} \ll E_{GB} $ - so we need more energy. Approximately a VEI 22 event has enough energy to simply gravitationally unbind the planet; to completely splinter it off into space you want more.
Note that this is theoretical: firstly, it relies on perfect energy efficiency and distribution, which are not going to happen; and secondly, the Earth doesn't hold enough energy in total to cause an eruption this size.
(Looking at some collateral effects of that, fragments of this explosion are going to go shooting off at something like 12,000 metres per second... don't put anything in the way.)
Using the scale of 10 times more or less destructive as you move up or down the VEI scale, you can also work out roughly what level of destruction you'll get for a given VEI event. For comparison, a VEI 10 is pretty much guaranteed to cause mass extinction of almost everything.
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