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So there's a flying island about 500 to 600 metres in diameter and roughly 300 metres in height in it's highest point. It is mostly basalt and other volcanic rocks, with a layer of dirt on top. The center of it's topside is directly impacted by a nuke that is pretty much an exact equivalent of Little Boy, with the yield of 15 kilotons.

The question is: will the island break apart or stay intact with just a big crater in it? Also, what do you think is the maximum yield the island could survive? I'm asking this since unlike a piece of the landscape that is supported by what's around and under it, this is a separate structure that's taking all the force alone and I'm not sure if it can take it without fracturing.

The island is just normal rock, and doesn't fall only because it is supported by magic levitating fungus within that's driving it's roots all throughout the island's interior (note: this fungus didn't bore it's way through the rock but grew for decades in molten magma and made the rock gradually solidify around it). There's also Maginot line style fortifications build throughout it's volume with bunkers and gun turrets on top (the question is still primarily focused on the island itself though).

(each square in the images represents 100 metres. The image on the left shows the radii of effects as shown by good old NUKEMAP: black is the inner radius and lip radius of an average crater, red, orange and blue show pressure (red: 3000 psi, orange: 200 psi, blue: 20 psi), and the yellow circle shows the radius of the fireball in air)

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    $\begingroup$ Not a full answer: the magic flying fungus makes the island so light (it flies) that the explosion just pushes it away with very little damage. Your enemies have just provided you with a nuclear powered jet engine. $\endgroup$
    – fdomn-m
    Commented Jun 12 at 13:34
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    $\begingroup$ Can the fungus die in the explosion? Or is it invulnerable for this purpose. $\endgroup$
    – Bubbles
    Commented Jun 12 at 19:56
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    $\begingroup$ unless the thrust is uneven, Why blow up the island when you can flip it around multiple times, throwing everyone off it, and turning everyone else into a sort of beaten meat paste... $\endgroup$ Commented Jun 13 at 8:29
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    $\begingroup$ @fdomn-m: I don't know how magically lifting fungi works, but I would assume that they provide some kind of lifting force counteracting gravity. Not that they decrease the actual mass of the rock. If the rock would have very little inert mass, it would blow away with the wind. $\endgroup$ Commented Jun 13 at 11:01
  • $\begingroup$ @AkselBergfeldt indeed, it depends a lot on the magic. When I added that comment I was also thinking that if the explosion/expulsion was at the top, it would just push the island downward into the ground. $\endgroup$
    – fdomn-m
    Commented Jun 13 at 12:40

4 Answers 4

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As this is a surface burst, the airborne shockwave won't even remotely have time to expand to a width equalling that of the island before it makes contact the island, meaning it won't transmit much energy. However, the fireball, — which'll be starting at essentially ground level — will vaporize all it contacts. The resulting ultra-hot gas will violently blast away from the island in the manner of a rocket engine. A similar but lesser effect will be caused by various forms of radiation, thermal, visible, and otherwise, which still have effects outside the fireball. In actuality not all of the nuke's energy will be applied but as it'll be getting applied very quickly the rock will have issues getting out of the way of the shockwave. As such, the island doesn't need to worry about getting propelled into the ground; it needs to worry about — essentially — getting whacked incredibly hard, albeit the force is from vaporized rock rather than from a physical object.

The shockwave being transmitted through rock — rock, being far less compressible than air, is better at transmitting shockwaves — will damage the tunnels; even for ones on the underside of the island, it's the equivalent of being almost exactly atop the epicenter of a moment magnitude 4.34 earthquake. However, solid rock will hold up better than most residential buildings; the damage will be notable but the rock shockwave likely won't be enough to collapse the tunnels. Unless the topside fortifications are overpressure-proof to the standard of a nuclear silo, the airborne shockwave following right after the rockborne shockwave will get into the tunnel network and obliterate anything inside that isn't overpressure-proofed to the same standard — crushing people, tearing fixtures off the walls, ripping up flooring, throwing everything into everything else, etc. It'll have dampened somewhat by the time it reaches tunnels in the island's lowermost parts, but my bet is it'll still be well over 20 PSI, meaning Earthly life within the tunnels and not also within a blast shelter will die. This air won't be hot enough to ignite combustibles but it will be enough to scald to death any Earth-like life inside — the reason it's shooting through the tunnels in the first place is it made contact with the fireball and is converting the thermal energy it gained during that into kinetic energy.

As for non-shock effects, the entire network of surface fortifications on top will be destroyed. Most energy will, like with all surface bursts, radiate off into the atmosphere and form a spherical shockwave, but this fireball is large enough to encompass essentially the entire topside; I expect enough thermal energy would be pumped into it to vaporize somewhere on the order of a hundred to a thousand tons of rock, perhaps leaning somewhere towards the low hundreds. The topside will, up close, look like someone held it under a giant blowtorch for a while and then hit it with a hammer, and from a distance look blasted and fused.

Any gaps, vents, etc. in the rock will violently shoot out dust cloud as the airborne shockwave rams its way out through them, and a titanic curtain of dust will fall from the entire island as loose stuff is shaken free by the rockborne one. If the magic fungus's area of effect is large enough it'll probably hold onto a good deal of this dust, shrouding the island in a funeral pall of its own remains.

Long story short, it won't destroy the island, but it'll utterly destroy the surface fortifications, wreck but not collapse the tunnels/rooms inside, kill anyone on the island not inside a blast shelter, and potentially surround the island in a cloud of partially radioactive dust depending on how, exactly, the magic fungus works.

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Underground nuclear explosions vaporize a sphere of rock and cause severe fractures in the rocks surrounding the vaporized volume, which then collapses when the pressure decreases. The radius of damage is a function of the nuke yield, as one would expect.

Surface explosions, on the other hand, are "less" dramatic, in that the expanding fireball has means to escape toward the atmosphere and is also forced to bounce up by the shockwave reflected by the ground.

There will be some ablation, but I doubt that there will be any major damage like fracturing of the whole body. For a reference, Trinity test, at 25 kT did this damage

The explosion created a crater approximately 4.7 feet (1.4 m) deep and 88 yards (80 m) wide.

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    $\begingroup$ Of course, it should not be misunderstood that a 5-foot-by-264-foot crater is small, but it's small enough that the island should not suffer major damage. $\endgroup$ Commented Jun 11 at 16:32
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    $\begingroup$ @controlgroup I’d be less sure. The same body of mass, set into the surface of the earth, will not fracture and break into shards because it’s held together by the surrounding matrix and supported from underneath. Punch flat sand, and you get a small crater. Punch a sand castle, it comes apart. $\endgroup$
    – Daniel B
    Commented Jun 11 at 20:49
  • $\begingroup$ While I agree with this answer in general, one needs to consider where the nuke hits. Nowdays, we think of them fired from missiles... a floating island might well catch a nuke on the bottomside. Basalt can have faults throughout it depending on how it cooled, and if these were weak (as if but not quite just "floating side-by-side"), there is a possibility of it fracturing into one or more pieces. Otherwise, this just knocks pictures off the walls on the homes up top. Topside, it wipes out any habitation and causes a shitload of fallout. $\endgroup$
    – John O
    Commented Jun 12 at 14:36
  • $\begingroup$ Daniel B is right though, this is literaly the reason for my question. As for where the nuke hits, the pictures show it (the highest point of the topside) $\endgroup$
    – Kugelblitz
    Commented Jun 12 at 17:08
  • $\begingroup$ Makes me think about anti-flying-island nukes, maybe combined arms: a few missiles to dig a hole, a nuke to detonate inside a hole. $\endgroup$ Commented Jun 13 at 18:12
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Is the nuclear explosion underwater?

The general opinion was that the Operation Crossroads 'Baker' test was that atomic weapons were not that effective at disabling WWII ships, and it would be possible to redesign ships so they would survive. The fallout would be nasty, though.

A floating rock structure would be brittle. It would probably not survive the rarefaction following a shockwave.

Reply to comments: the text says 'floating' but the title says 'flying'. I hadn't seen that. The overpressure from an airburst nuke can flatten a city, but the momentum transfer to solid rock would be inefficient, and most of the shockwave would be reflected due to the jump in density. Except that this is magic flying rock, which I know nothing about.

The solution is probably similar to using nukes to destroy asteroids: hit the target with a penetrator first, then detonate the nuke in the hole, so it is surrounded by rock.

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    $\begingroup$ I think, given the materials stated, that this island is magically floating in air, rather than in water, since basalt sinks pretty effectively. $\endgroup$
    – jdunlop
    Commented Jun 11 at 18:10
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    $\begingroup$ @jdunlop in principle, I agree with you, but I’m forced to point out that basalt also notably sinks in air $\endgroup$
    – Daniel B
    Commented Jun 11 at 20:45
  • $\begingroup$ The floating/flying thing isn't really your mistake though, I actually edited the title and later the text as well after I realized ''floating'' isn't the best word for it $\endgroup$
    – Kugelblitz
    Commented Jun 13 at 17:31
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If we ignore the spherical pressure waves and fracture propagation, the high temperatures would ablate and melt the rock, which would quickly cool to something resembling obsidian. This would lead to the formation of a crater, but wouldn't do much damage to more distant parts of the island.

However, one factor that the other answers to date don't address sufficiently is fracture propagation through the basalt rock. The atomic explosion is exerting a pressure of 3000+ PSI over a circular area around 100 metres across, and even discounting the thermal shock, the blast-wave pressure exerted over that area won't be parallel, but will be exerted radially from the point of detonation.

This will result in forces pushing at the rock in different directions, and if there are any fractures in the rock or even just concavities, there is a significant probability that the radial pressures may result in one or more fractures propagating away from the blast centre.

Since large forces will be applied over the entire floating island, there is a chance that any flaws in the stone of the island, especially near the point of detonation, will result in fracture propagation over a considerable distance. Unlike a normal ground burst, there are fewer forces pushing the rock of the island together than might exist in a normal rocky hill of this height.

Basalt is a strong type of rock with an irregular crystalline structure. However, if the island has been subjected to cycles of freezing and cooling, such as might occur in climates with freezing winters, the expansion of water as it freezes would pre-stress the rock and cause fractures that could weaken its structure.

So, I estimate that a 15kt blast at close range would have a 5% to 25% probability of suffering a significant fracture, depending upon its exposure to periodic wet and freezing conditions, and the presence or absence of any relatively large concavities near the blast.

The higher the pressures applied to the rock and the larger the area over which they are applied, commensurate with higher yield detonations, the higher the probability that the floating island will crack or even crumble.

However, without knowing the exact geology of the island, this answer can be nothing more than a guess, and the OP will have to make a personal decision as to the degree that fracture propagation threatens the island's integrity.

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  • $\begingroup$ Why 5 to 25%, specifically? $\endgroup$
    – KEY_ABRADE
    Commented Jun 14 at 1:26
  • $\begingroup$ @KEY_ABRADE It's a guesstimate. $\endgroup$
    – Monty Wild
    Commented Jun 14 at 11:56

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