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Yellow phosphorous and air.

Then stand back.
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Oh, and it's packed in oil for a reason.
 
regular peroxide and iron shavings does the trick too

or if you want to be really morbid; peroxide and dried blood (fishing bait)
 
Brake fluid and chlorine (the kind you get for swimming pools). Best to maintain a distance after you mix it and not do it anywhere near anything you mind having razed.
 
use real magma.

I remember making a 'Mt. Etna' easter bonnet in primary school. I spent a long time in the burns ward.

(mentos and diet coke is a "good" idea)
 
If safety isn't a concern (meaning you have a method to ignite it, and can watch from a proper distance), I'd mix up a batch of thermite. Easy to make, and quite spectacular.
 
You can use potassium permanganate and glycerin.

But, I think one of the better chemical reactions is to use ammonium dichromate (add in some magnesium shavings) - then light using a match. It is not really dangerous but you want to be in a ventilated area because the fumes are not the most healthy for you. It does not do the flowing lava thing but instead does a slow burn with flame and ash. That is what I used years ago for my model volcano.
 
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