Worldbuilding and biology have always been two huge passions of mine, especially when it comes to dragons. So well, at the moment I'm working on a little project in which I'm attempting to make different types of dragons more biologically plausible, and one type in particular really frustrates me. I've seen dragons and other creatures depicted swallowing rocks and then spitting out molten lava, an easy example that comes to mind is the Gronckle from the How to Train your Dragon franchise.

I was wondering, is there any way to make such an ability plausible? Like having the creature's stomach digest specific minerals of the rock in a furnace-like stomach without suffering damage from the heat, and spit them out half molten. If not I was also trying to think of alternatives to such ability that would fool humans into believing the dragon is spitting out molten rock, when it's actually something else. The only thing coming to mind in that case is projectile vomiting, but honestly that's rather lame to me and not something I see sparking myths about beasts spitting lava.

Edit: After much brainstorming from the comments I think I came up with an possibility.

  • The dragon could store rocks in a pouch which produces an oily flammable liquid. Strong muscles would contract the pouch to projectile the rocks out of the mouth like a rain of debris.

  • There would be two orifices on the dragon's lower jaw which spray a catalyst substance as soon as the rocks come out. The catalyst reacts with the oily coating and lights it up on fire.

  • Since the flammable liquid is oily it won't mix with the thick, mucous coating of saliva protecting the insides of the mouth. That makes the saliva more effective as a protection from the flames and prevents it from making the rocks less flammable.

  • The catalyst spray would probably cause a small burst of flames in front of the dragon's mouth, which only adds to the look of erupting lava. The nostrils would also close so the fumes don't go into the respiratory system.

  • $\begingroup$ Hello Lilian. Let's brainstorm a moment. A silicon-based lifeform could believably use sulfuric acid as the basis of stomach acid, rather than the hydrochloric acid humans rely on. However, lava requires heat. A lot of heat. That's a serious body temp. Plausible has more to do with the story than it does the actual physiology - so can your story handle a really, really, hot (literally) serious acid indigestion critter? $\endgroup$
    – JBH
    Commented Jan 6, 2019 at 0:47
  • $\begingroup$ Yeah I'm aware, that's mainly why this frustrates me so much. I can't find a way around it and no good alternatives come to mind. This little project isn't really part of my headworld, however. It's mainly just something I've been playing with as a personal creature design challenge: making dragons more biologically plausible without relying on magic at all. Therefore I'm just looking for opinions in general on how this could work instead of a world specific setting. $\endgroup$ Commented Jan 6, 2019 at 0:52
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    $\begingroup$ Dragons? Dragons don't spew lava. I'm fond of the realistic explanation from the move Reign of Fire, the dragons have two or more chemicals that combine in their mouths to form a natural napalm. Another example is Anne McCaffery's Dragons of Pern series of books where the dragons chew a particular rock and they burp flame (that's also believable). $\endgroup$
    – JBH
    Commented Jan 6, 2019 at 2:14
  • $\begingroup$ Not really? That's actually a very common skill to give dragons as an alternative to fire breathing, just like poison, acid and ice breath. Dragons are very flexible mythical creatures and people enjoy playing with their abilities. I even mentioned an example in my post from HTTYD, and they have many other dragon species besides Gronckles that do the same. I'm aware of the ways firebreathing can be made believable, but at the moment I'm trying to brainstorm a way to make this particular take on them more plausible. I don't remember seeing it discussed before. $\endgroup$ Commented Jan 6, 2019 at 2:36
  • $\begingroup$ Yeah, but... The amount of sheer weight the little dove would have to haul around so that it could be spewed upon demand. Cool? Yes. Practical? You only get a short burst and you're chewing & melting again. But, it is fantasy, after all. $\endgroup$
    – JBH
    Commented Jan 6, 2019 at 6:55

2 Answers 2


I don't think you can have a stomach hot enough to melt stone without your entire body being hot enough to boil the water around it. I don't think there's a way to make a biology work at that temperature, even starting from scratch, and more importantly there's the matter of providing enough energy to keep a body that hot. And this temperature is only useful while it's processing fresh food.

If your dragon has a gizzard (a pouch filled with stones from breaking down food (it wouldn't have teeth, just a beak,)) then maybe it's tossing up pebbles and small stones coated with flaming liquid?

In which case, the process is as follows:

  1. Load pebbles from the gizzard into the mouth. These used to be larger stones, but they've been worn down by use.

  2. Coat the pebbles with the volatile fluid. This would be flushed from glands on the mouth

  3. Breath out, launching the pebbles.

  4. Shut mouth, then stop breathing out. Doing that in the opposite order would bring the fumes from the fuel into the lungs, which would be bad.

  • $\begingroup$ Any ideas for an alternative then? Because honestly I'm clueless. Maybe if it were to coat the rocks in a flammable substance and then spit it out as flaming debris.... however my explanations for firebreathing, whether it's gas based or liquid fire, always happen in a completely separate system from the digestive and respiratory ones, with the fire being projected from the sides of the mouth rather than inside. So I'm not sure how that would work either. $\endgroup$ Commented Jan 6, 2019 at 1:04
  • $\begingroup$ If coating the rocks in something flammable counts, I can work up a solution. Give me a minute to do some edits. $\endgroup$
    – ltmauve
    Commented Jan 6, 2019 at 1:42
  • $\begingroup$ Hmmm that sounds good. When it comes to gas based fire I like to have the inside of the dragon's mouth coated in a thick, mucous saliva that shields the soft tissue from the fire. Also there would be a glottis-like mechanism to close the trachea and prevent the fire from reaching the lungs. The saliva would probably interfere with the inflammable coating on the pebbles, though... $\endgroup$ Commented Jan 6, 2019 at 2:28
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    $\begingroup$ Alright, after much brainstorming over your suggestion I think I came up with an idea. The dragon could store rocks in a pouch which produces an oily flammable liquid. Strong muscles would contract the pouch to projectile the rocks out of the mouth like a rain of debris, at the same time two orifices on the dragon's lower jaw spray a catalyst substance. It will react with the oily coating and light it up like a spark right as the rocks leave the mouth. Since the flammable liquid is oily, it won't mix with the thick coating of saliva protecting the insides of the mouth either. $\endgroup$ Commented Jan 6, 2019 at 21:06
  • $\begingroup$ Yeah, that'd work. $\endgroup$
    – ltmauve
    Commented Jan 7, 2019 at 1:23

Perhaps the dragon has a sort of exothermic chemical reaction that occurs in it's stomach, or perhaps some reaction that occurs as it's spitting it out? An example that comes to mind is magnesium melting other rocks.

  • Consumes rocks, especially basalt
  • Dragon separates magnesium from basalt (basalt has relatively plentiful supply of magnesium, 15%ish by weight)
  • Excretes magnesium strip into mouth, while simultaneously regurgitating basalt gravel
  • Ignites magnesium. Magnesium burns at ~2200 C while basalt melts at ~1100 C.
  • Spits out lava (magma? eh)

It does involve some suspension of disbelief, but it's a dragon after all. Hope this helps.

  • $\begingroup$ Organic thermite... I would not want to be the first person to try gut one of these beasties. $\endgroup$
    – Joe Bloggs
    Commented Jan 7, 2019 at 14:51

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