What's Actually Glowing Blue
The blue glow is not lava — it's burning sulfur gas. At Kawah Ijen, volcanic gas loaded with sulfur compounds escapes from cracks in the crater under high pressure and temperature — reports put the venting gas above 600 °C. The instant it meets the oxygen in the air it ignites, and burning sulfur produces a vivid electric-blue flame. Some of that sulfur cools just enough to condense into a molten, bright-red liquid that runs downhill while still on fire — and thatglowing, flowing, blue-lit stream is what the internet calls “blue lava” (ExplorersWeb).
The distinction matters because real lava glows by temperature — incandescent orange, red and yellow, the way a lava flow does everywhere else on Earth. Ijen's actual molten rock looks the same. The blue is a chemical flame colour, the fingerprint of combusting sulfur, not a hotter or stranger kind of lava. One nuance that trips people up: the escaping gas is blisteringly hot, but sulfur itself ignites at a much lower temperature — around 360 °C — so the flame is a combustion reaction, not a measure of how hot the lava is.
Where You Can See It — Kawah Ijen, East Java
Kawah Ijen (“Ijen crater”) is a stratovolcano in the Ijen complex of East Java, Indonesia. In our Smithsonian GVP record it stands 2,769 metres high with 11 recorded eruptions, the last confirmed in 1999 — a subduction-zone volcano on the Sunda arc, part of the Ring of Fire. The blue fire burns at vents inside the crater, and it's only visible in the dark — which is why visitors make the notorious pre-dawn climb. By sunrise the flames vanish into daylight and all you see is yellow sulfur and steam.
It is very nearly a one-of-a-kind sight. A few sulfur-rich volcanoes flicker faintly blue, but Ijen is the only place the phenomenon is reliable and dramatic. It belongs to a small club of volcanoes that don't glow the usual red — the other famous member is Dallol in Ethiopia, whose acid springs paint the ground neon yellow and green. Different chemistry, same lesson: volcanoes are far more colourful than the orange-lava cliché.
The Acid Lake and the Gas
Filling the crater is the reason Ijen is beautiful and deadly in equal measure: the world's largest highly acidic crater lake. It holds roughly 36 million cubic metres of water turned to acid — a brew of sulfuric and hydrochloric acid with a pH below 0.3, more corrosive than battery acid and able to dissolve metal (GVP / published reporting). Its unreal turquoise comes from those dissolved acids and minerals, not from life — almost nothing lives in it.
The same system that makes the blue fire makes the air hazardous. The vents pour out concentrated sulfur dioxide and hydrogen sulfide — the same sulfur compounds that build the bright yellow mineral crusts around the crater — and a sudden surge can be lethal. This is genuinely one of the more dangerous volcanoes tourists visit; people carry gas masks and move upwind, and the crater has killed before. The spectacle and the hazard are the same phenomenon.
The Miners of Kawah Ijen
The part of the story the chemistry blogs skip: people work in there. Every day, sulfur miners climb down into the crater, past the blue flames and through the toxic gas, to chip solidified sulfur off the rock with iron bars. They load it into baskets and haul it back up and out on their shoulders — 70 to 90 kilograms at a time, often twice a day, frequently with nothing more than a wet cloth over the mouth for protection (reporting from the crater). The sulfur ends up in sugar refining, matches, fertiliser and cosmetics; the miners earn a few dollars a load.
So the famous photo has a second subject the captions rarely mention. The blue fire is a chemistry curiosity. The men carrying 80-kilo baskets of burning-hot sulfur out of a crater full of acid and lethal gas, for a few dollars, are the reason it's worth looking twice — and the reason “blue lava” is a prettier name than the place deserves.