Burning down a mountain

Question

In the lore of the world I'm building, the Dwarves used to have a very large territory surrounded by more or less impassable mountain ranges. This territory shrunk significantly when the elves got four dragons (the only dragons in existence and the only dragons to have ever been seen) and quite simply burned down enough of the mountains to force the Dwarves out of a lot of their land.

My questions are as follows:

1. Is this feasible to have happened? I know the melting point of igneous rocks is about 1200°C. Do you think four dragons could achieve that heat?
2. How long would they have to use their fire to burn down significant passages into the mountains?
3. What would the burned parts of the mountains look like now that they've been melted down and had time to cool?

EDIT

Since @NeilSlater pointed out that to properly answer this question I need to better define how dragons in my world work, I present you with the following:

1. I suppose I mostly want to know what it would take to make this feasible, as opposed to if it is feasible. What temperatures for how long would be needed to achieve this?
2. Keep in mind that the entire mountain doesn't necessarily need to be melted down, just enough to bring it from impassable to passable. This way the Elves can plan future raids on the Dwarves.
3. I'm more than fine with assuming that the mountains are tunnelled out for the purposes of this. (thanks @DaaaahWhoosh)

Answer 1

You will require an extremely large amount of energy to do this.

The heat capacity of granite is $0.79~\text{kJ}/\text{kg}/\text{K}$ and the latent heat of fusion is $400~\text{kJ}/\text{kg}$. Granite weighs approximately $3000~\text{kg}/\text{m}^3$.

To create a valley $20~\text{m}$ wide across a $3~\text{km}$ long by $1~\text{km}$ deep mountain pass for invasion, the dragons will have to output

$$ \begin{align} &3~\text{km}\times 1~\text{km}\times 20~\text{m}\times 3000~\text{kg}/\text{m}^3\times\left(1200~\text{K}\times 0.79~\text{kJ}/\text{kg}/\text{K}+400~\text{kJ}/\text{kg}\right)\\ &\approx 2.4\times 10^{17}~\text{J}=240~\text{PJ}=58~\text{Megatons of TNT} \end{align} $$

alone, in order to melt all the rock in the mountain pass. This assumes the heat transfer is perfect, and no losses occur during the melting process.

This is greater than the entire energy output of the Tsar Bomba (the largest atom bomb ever detonated), at $2.1\times10^{17}~\text{J}$.

The elves will be much better served if they used the dragons as troop carriers, much like this question.

Answer 2

Mountain Happens to be made of Gunpowder

I don't know where your story sits on the Sci-Fi / Fantasy spectrum, so this answer may not mesh with the feeling of your world. However, I think the most fun way to burn down a mountain would be for there to be large naturally occurring deposits of niter and coal throughout it. Both of these minerals naturally occur in large deposits in our world. They are stable on their own, but if a dragon got enough burning for a chain reaction to begin mixing the deposits, the explosion could be self sustaining until all deposits were burnt up. This mix would have approximately the energy density of gunpowder, or 3 MJ per kg, and the mountain would require about .4 MJ per kg to melt, so to melt the entire mountain, it would have to be 1/6 to 1/3 coal and niter to begin with.

Answer 3

There are quite a lot of factors that make this scenario unlikely:

1- Air gets less dense the higher up you go, meaning that any fire your dragons produce is going to lose potency at higher altitudes, like the ones you'll find on mountains.

2 - Mountains are very large, and all that mass has to go somewhere. You may be able to melt the rock, but vaporizing it is pretty much out of the question, so now you've got molten rock that has to stay molten as it rolls down an entire mountain. Basically, your dragons are going to have to put out more energy than the average volcano to pull this off, and they'll have to do it for a very long time. For peak efficiency, probably two dragons would moltenize the mountain, while the other two maintained a river of molten rock, keeping it hot enough to flow all the way to some nearby valley. This process would actually get harder the further along you went, as you lose the benefit of gravity.

So, since I've said some reasons why this would be hard to do, I think it's time I gave some potential solutions. For instance, suppose the mountain was already somewhat volcanic. Perhaps there was magma swirling around, and hot gases building up in caves, searching for a way out. The dragons would exploit this volcanic activity, using the gases to blow holes in the mountain and the heat of the magma to enhance the heat of their own fires. EDIT- If you give your dragons demigod status, perhaps part of their power comes from their ability to bend natural forces to their will. Thus, perhaps by their power the earth below the mountain opened up and fire came forth, and this fire they channeled through their own bodies until the even the hardiest stones of the mountain could not help but yield, et cetera.

Another possibility would be that since it's a Dwarven mountain, it's already been mostly hollowed out. The dragons, which I assume are large, could use their fire to weaken the rock, then simply knock the mountain down. At that point, they could probably melt some remaining boulders and such, but the bulk of the work will be over.

As for how it would look, I'd suggest looking up pictures of volcanic islands.

Answer 4

No, it's not feasible for them to burn down the mountains, given that mountains don't really burn, and are enormous.

Usually legends are metaphorical rather than literal.

Often stories have other truths behind them.

Perhaps the dragons used magic to somehow set off large volcanic eruptions which resulted in passable terrain. Or perhaps the dragons somehow knew where there would be a huge eruption, and told the elves they were using magic to cause it, to increase their standing with the elves. Maybe they even told them they were burning down the mountains themselves, taking credit for an eruption they already knew about, but the elves had no way to know any better, or something.

The Mount St. Helens eruption shows an example of a mountain reducing substantially in size through eruption. A variation in shape and scale could result in a similar but even more passable aftermath.

Answer 5

I think you'll need to include some ritualistic element for this to be feasible. The amount of energy required here is stupid huge as other answers pointed out and if you want the dragons to be responsible for it, you need to put those dragons to an incredibly high power level (screw fire-breathing, I spew Tsar Bomba's out my ***). If you want these dragons involved again at any point in the story, you now have to deal with the incredible power level you put them at...how could any character contend with a being that melts mountains when asked? (you do need to wonder what the elves possibly could have offered 4 beings that could melt giant segments of the globe they live on).

As an alternative...assuming you are going with magic of some degree here...a ritual that needed to be preformed by all 4 dragons that isn't easily repeatable that melts the mountain might be a bit more feasible for your plot line in that the power level of each individual dragon can be greatly reduced.Perhaps it's a summoning ritual where the 4 dragons were capable on a combined level of summoning a gigantic fire elemental the just sat in the mountain (superimposed?) bored out of it's mind and heating all area's of the mountain at a constant rate. Mountain melted, 4 dragons capable of doing it, but not a single dragon capable of launching fire with the heat potential of fusion bombs.

Answer 6

This question: "Do you think four dragons could achieve that heat?" is going to depend on how dragons work in your world. And answering that is going to answer most of the rest of your questions, one way or another.

You could simply say it happened, how long it took etc, and this sets the bar for how powerful dragons are in your setting. There are no reality checks here, unless you want to set specific measurable traits on your dragons, such as declaring the temperature and area covered by a dragon's flaming breath. Perhaps you need to do that for other purposes, in which case you could comment or update what your premises (or rules if this is for an RPG) are for dragons in general.

The story could also be set in a mythological/heroic age with respect to your current timeline. It is very common in mythologies to have more potent forces acting in the world, the further you go back in time. After all, something needs to explain how the sun and moon behave, and they are far larger than a few mountains.

Answer 7

Instead of trying to melt an entire mountain, take a look at how quarries break apart rock.

They don't drill every square inch with dynamite and pulverise everything in one go - the most common approach is to get to a position where you have a "cliff face" and then using carefully placed explosive charges, sheer off a layer from the cliff face.

Focused heat can crack rocks. If you are smart with where you focus the heat from your dragons, you could possibly get a similar effect to sheering off a layer in a quarry. And if your mountains are impassable, I assume it is because of a high amount of vertical cliff faces. If your world is similar to ours in terms of geography, those vertical cliff faces will eventually drop down to streams or rivers in each of the valleys.

Assuming this is the case, sheer off enough rock from higher up, you'll block off the valleys and create dams/lakes. To then traverse the valleys, you simply drag some light weight boats up your new "dam wall" and sail/row to where you need to get. As a bonus, the flooding of the valley floors will also force the dwarves out of those areas - not fun being in a cave when it floods!

Answer 8

(Sorry in advance for my weird english)

Well, you have many answers about the feasibility of your dragon attack, and it doesn't go very likely in your way. But here are some alternatives :

• S##t up, it's magic ! : Your dragon don't just spit fire, they can spit magic matter that can melt whatever your dragons want to melt, just like acid or cotton candy in water...
• They're dwarves ! : You don't have to melt an entire mountain. Draves are "known" (maybe not in your universe) to dig tunnels. Maybe you could use your dragon to "dig/melt" a hole to these tunnels and then make your elven army walk through them. For the story, it could make a good backstory/legend/whispers about abandonned mines and tunnels which could be located thanks to spying missions with "elite" elvens or whatever you see fit your universe ...
• Delete the problem ! : If the mountain is still a problem, then replace it by a deep black and dangerous forest / noxious infested swamps ! Your dragon could entirely burn the forest / dry the swamps, which would still keep the "powerfulness" of the dragons, passing dangereous and unexplored lands ...
• F##k it. Badass Dragons ! : as last possibility, and depending if you want your dragons to be immensely powerful or "realistic", then you can just make them do melt the mountain like it was nothing. Like this, your dragons look impressive and can just be feared even more. Narratively, you could drescribe the melting mountain with images like "tides of lava" / "sea of fire", as the melted and fluid matter flows down the other side of the mountain toward the dwarves' lands.

However, the situation raises anoher problem, which is that 4 "mountain melter" dragons are using their power pretty uselessly... i mean they can melt a mountain, why not just destroy dwarves' armies and villages instead ?

Anyway, if you found what you were looking for, good for you. Your story made me think and imagine stories about and around your universe, and it's always funny to do so. Thank you :)

Answer 9

Maybe we should change the way that the dragon fire is produced. A normal household dragon is more or less a napalm bomb on legs. It produces fire by spraying some burning liquid or gas and ignites it afterwards.

BUT: There's another way:

Stage free for the Hydroxy Dragon!

Say what?

If the dragon can produce some electrical arc in his mouth it can spit pure water through the arc. The water will be split into hydrogen and oxygen and recombine afterwards (releasing a large amount of energy that will literaly melt the stony mountain).

A more practical use is the dragon named Multiplaz 3500:

https://www.youtube.com/watch?v=VML-uRXyxHI&list=PLFF1285C2AC469A6E

https://en.wikipedia.org/wiki/Oxyhydrogen#Brown.27s_Gas

I know it's no common approach to dragons, but it'll surely melt down parts of the mountain in decent time.

(I know the arc needs a tiny bit of handwaveium, but on the other hand, eels can produce electricty too)

BR Alex

Answer 10

You could use the dragons to meerly burn through or weaken the structure around the magma resovoir of a dormant volcano.

Mt. Saint Helens erupted after a huge landslide reduced the weight of the "lid" and maybe shook things around deep down.

Ski resorts use explosives to trigger avalanches while closed rather than waiting for it to happen.

So what would happen if a missle hit Mt. Rainier?

---

Another idea: the residents of the mountain use it for mining I presume. What if they are mining something like a magnesium deposit?

Answer 11

What if the mountain contained Thermite? It doesn't have to be made completely of thermite, just some above critical amount in dirt and rocks in a form of veins, ... that will sustain the reaction. The rocks would crumble or explode under the heat and fly apart.

Answer 12

Other people have been discussing how to actually remove the mountains themselves, but there is another take on this to consider.

Scorched earth is a type of military tactic of destroying everything that might be of use to an enemy in an area, in order to render the area completely unusable for farming or industry, by polluting previously arable land with the soot, causing large-scale erosion problems (as there are no trees to hold the land together anymore), and burning bridges, demolishing roads, and razing cities to the ground. In your case that would mean collapsing tunnels, too.

Assuming these are near-surface dwarves (as opposed to rock-eating, lava-drinking, "what's this 'surface' you speak of?" dwarves), such measures would force the dwarves to leave. Even if they were not farmers themselves but only traded for food, there would still be no food available as nobody else could grow it either. The destruction of their tunnels and roads would make it difficult to get food to the scorched regions from elsewhere. Combined with continued military pressure, this would easily be able to exterminate or force the dwarves to abandon the area.

So while geologically there would still be a mountain, it would no longer be a dwarf mountain. The tales and stories told of this great burning wouldn't bother to distinguish between physically destroying the mountain or "merely" emptying it and destroying its former inhabitants. No need to literally destroy it.

Answer 13

You want to melt down the mountains, ok, but... Where will they flow to?
This isn't a little bit of rock we are talking about, melting that amount of material will make rivers and/or lakes of lava that will take from weeks to years to cool off unless aided by powerful ice magic.
Maybe have the dragons melt part of the mountains, making them flow into the dwarven lands. That way they did melt the mountains, the dwarves get a horrible situation to deal with, and meanwhile the elves march from wherever they want unimpeded by dwarves that will be too occupied with putting down fires and digging trenches to route the lava away from crops and buildings.

If you absolutely must melt a mountain...
Make sure it's the smallest and/or most dug out of them all to mitigate the problems, unless waiting years for it all to cool off is reasonable for both parties, remember that the new pass can be used by the dwarves as well as the elves.

Answer 14

If you have Granite or Quartzite as the primary rocks that form your mountain range then you don't need to melt the mountains, just heat them up a reasonable amount, 50 celsius or so, at night, especially if you can also get large quantities of water onto them as well. Granite and Quartzite are especially susceptible to thermal fracturing and break up quickly under heating/cooling cycles, if you can add water then frost wedging also takes place accelerating the weathering process even further. You should be able to cut many exfoliation layers lose every night, fewer, deeper layers if using ice and fire instead of just fire, from a large area. Each layer of heat only exfoliation is a few centimeters deep so you could break off up to a metre of material a night, possibly double that if you can water the cracks and use freezing as well. If the days are cold enough you can in fact keep this up all day as well as all night, it won't take that much forced erosion to make vertical walls into passable scree slopes.