The bizarre phenomena happens at the Ijen volcano – in Indonesia – and you can go to see it for yourself. Sulphur lava creates blue flames on other valances but Ijen’s crater is the biggest in the world. Exposed to the oxygen present in air and sparked by lava, the sulfur burns readily, and its flames are bright blue. There’s so much sulfur, Grunewald says, that at times it flows down the rock face as it burns, making it seem as though blue lava is spilling down the mountainside. But because only the flames are blue, rather than the lava itself, the effect is only visible at night—during daytime, the volcano looks like roughly any other.
- The blue glow is caused by sulphur gas bursting from cracks in the volcano and igniting at 6,000C.
- The flames can shoot out 16ft high.