A word of caution: Combining marshmallows and lava is a bad idea. Some lost soul on Twitter asked the U.S. Geological Survey if they should engage in this cookout practice, and this is one time we agree with centralized government on science and health.
Americans often make lemonade from lemons, it is part of our culture. Whereas when Europeans see a nice car, their culture often thinks in the vein of 'Why should they have a nice car? while in our culture, no matter your upbringing, you have the opportunity to declare 'I am going to get a nice car too.'
However, there are limits to such optimism - like cooking in the presence of a volcano. If you see someone roasting a marshmallow over molten lava, lean more toward European "precautionary principle" ideals. You should not declare you are going to roast a marshmallow also. Now, I do not want to curb your freedom when it comes to roasting marshmallows, any more than I want you to stop rockclimbing, skydiving or drinking Death Wish coffee. I just have to caution you that marshmallows and lava are bad. Someone on Twitter asked the U.S. Geological Survey if they should, and this is one time I agree with centralized government on science and health.
— Jay Furr (@jayfurr) May 29, 2018
Well, the dose makes the poison, but they are going to taste like nothing you want to eat.
The reason is one of the simpler, more fun chemistry lessons. Marshmallows are almost pure sugar, while a volcano's fog ("vog" has now become part of the lexicon thanks to Hawaii’s Kilauea volcanic eruption) contains sulfuric acid. Sugar is a carbohydrate which has water, just like it sounds. Sulfuric acid will cause an exothermic reaction in the sugar, which means the water will boil off as steam.
C12H22O11 (sugar) + H2SO4 (sulfuric acid) → 12 C (carbon) + 11 H2O (water) + mixture water and acid
Not so bad, the giant snake of carbon left over looks neat. But since you will not be in a fume hood, scraping burnt sugar and carbon off of your clothing will not be a great afternoon, as Dr. Anne Marie Helmenstine on ThoughtCo notes. And there is a reason why you don't find sulfur dioxide and hydrogen sulfide in recipes. It smells awful. Limburger cheese smells great by comparison.
And then there is the the problem of getting close enough to roast a marshmallow. USGS has issued a a red-level aviation code and while there is no equivalent for cuisine, there should be.
Second to "do not open the Ark of the Covenant if you are a Nazi" is "don't spit into the wind" but third on that list should be do not roast marshmallows over erupting volcanoes. Screen capture from "Raiders of the Lost Ark", copyright Paramount Pictures.