Mythbusters thermite and ice
 

So last night on Mythbusters, they proved that you can get a big explosion by igniting a 10lb bucket of thermite resting on 10lbs of ice blocks. They had no explanation as to why this happens. What do you think?

I'd guess it has to do with the extreme difference of temperature. The thermite would be RAPIDLY heating the ice.

Silly infidel, it happens because God says so! Just like the napalm assault beetle or whatever...

On the show, one of them theorized that it was water decomposing to hydrogen and oxygen, then igniting, but that doesn't make sense to me, since that's basicly no change. The energy gained by ignition was already spent cooling the whole thing down earlier. You'd think that if the thermite was going to ignite such gasses, that it would happen pretty much coincident with their creation...

What he said.

It's the logical explanation.

Was it like a big fire ball? If not then super-heated steam can pack a punch.

I watched that, it was pretty cool...

I thought that maybe the ice that the thermite came into contact with went so quickly from solid to gas that it forced the solids around it to burst in order to make room for itself.

Very much a fireball
http://www.youtube.com/watch?v=412EaepSAlE
Which is why I don't think it's just flash-steam.

you know how ice cubes crack when you put them in water


i think its something like that but much bigger and faster

no wait i think i figured it out:

The thermite melts into the ice until its about at the center of the blocks, at which point it very quickly superheats a pocket of water. Superheated water will indeed explode upon agitation or contact with a nucleation site, which there are plenty in this situation. Normally the water wouldn't have a chance to superheat without starting to boil but thermite burns so hot and so quickly that all of this can probably happen in half a second or something. The pocket of water will flash to steam and blow apart the ice and thermite (making it look like a fiery explosion in the process).

That's exactly what I said only more detailed. :T

Yea, that's what I was thinking. The steam just blew out the thermire in all directions ...I could tell quite a bit of that was steam. I would assume blowing the thermite out would expose even more of its surface area to oxygen.

But the thermite doesn't need to have a surface with oxygen in order to burn. It's self-oxidizing; it would burn in space.

I'm sure more oxygen wouldn't hurt.

The idea conditions for explosion are equal parts oxidant and feul. If well planned, the thermite already provides that, which is another reason I don't buy most of the theories.

Your idea is feul.

God did it.

What I meant to say was that the ideal conditions for explosion are balanced oxygen providers and consumers, but it didn't come out that way...

This is the thermite reaction aka aluminothermic reaction:
Fe2O3 + 2 Al → 2 Fe + Al2O3

As someone said it would happen in space or in a total vacuum, so oxygen is not needed at all. Regarding your point of perfect balance, well it's the thermite mix itself, if you respect these proportions it will burn completely.
One possible source of explosion would be dihydrogen which is highly explosive (it's what pretty much destroyed the Chernobyl nuclear reactor), I just don't see how it could get created in these conditions. Sublimination of water or even ice is a much more likely.