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I was playing a table-top rpg with a friend. It was set... well it was pretty weird, so I won't get into it. Suffice it to say the bad guy had a stick of dynamite and we were on a volcano. I hit the dynamite with my slingshot, knocking it into the lava directly behind the villain. My DM said the dynamite melts away. I said it should have blown him up. Needless to say, as a couple nerds (we were, after all, playing a fantasy game and then arguing about the physics of it) we were pretty upset with each other.

What would have happened in this... totally realistic scenario? Would the stick of dynamite exploded? Or simply melt away?

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it would probably fizzle a little.. – lurscher Jun 21 '11 at 17:11
I'm not quite sure if this is on topic or not... it might be more chemistry than physics. We'll see what the community thinks. – David Z Jun 21 '11 at 17:28
i think this depends if the fumes from the lava will displace oxygen density enough to interrupt quick combustion - which is critical for an explosion.. I don't know what sort of measurements of oxygen density near hot lava exists, but probably there should be enough to enlighten the discussion – lurscher Jun 21 '11 at 17:48
I seem to recall having once read that without a blasting cap dynamite will burn rather than explode. I think an explosion requires a pretty strong shockwave to initiate, so a detonation, as opposed to some slower exothermal reaction seems unlikely. – Omega Centauri Jun 21 '11 at 18:42
My son, age 9, agrees that this is a job for Mythbusters! :) – user34786 Nov 26 '13 at 19:11
up vote 9 down vote accepted

The most likely answer is that the nitroglycerin portion of the dynamite would deflagrate (burn) and the diatomaceous earth would melt. Neither constituent would detonate. Dynamite needs a shockwave to detonate.

Now, this is not to say that the deflagration of the nitro would be a tame thing. Bullets are propelled out of guns by deflagration of materials behind said bullets. It appears as an explosion because it is confined.

The dynamite would burn, most likely very rapidly. Rapidly enough to make it appear as an explosion? Probably not, since it is not confined; but I would not try viewing the experiment unprotected from anywhere closer than 30 yards.

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i think this is a job for the mythbusters – lurscher Jun 21 '11 at 22:05
So it would seem my DM could have been justified in his assessment of the situation, but it likely would have been dangerous for both of us (because playing with dynamite on an active, erupting volcano wouldn't have been dangerous otherwise.) – corsiKa Jun 21 '11 at 22:54
@lurscher --> Definitely! :-) – Vintage Jun 22 '11 at 22:28

Dynamite may be detonated by heat using a fuse and doesn't require another high explosive to detonate unlike TNT for example. It also doesn't require confinement in order to explode.

Dynamite will not melt away and the lava will definitely cause it to explode.

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Reed, I could not find a temperature at which dynamite would spontaneously explode. Detonation method given by various sources is "shockwave" from a detonator cap. I did find reference that one of the differences between TNT and dynamite is that the former could be melted and poured into forms, which implies that dynamite is not typically melted; but modern dynamite is a gel, which could be pushed into forms at low temperature. – Vintage Jun 22 '11 at 22:23

Whether an explosive just burns or detonates when it comes in contact with heat is what defines an explosive as a "primary" or "secondary" explosive. Nitroglycerin and lead azide are examples of primaries. They would likely detonate if thrown into lava. Dynamite, being nitroglycerin stabilized by diatomaceous earth or sawdust, is a bit tricky to classify. TNT and C-4 (RDX) are examples of secondary explosives. They would likely not detonate if thrown into lava and would just burn, unless they were confined in a metal pipe.

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protected by Qmechanic Nov 26 '13 at 19:16

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