The blue fire is not lava. It is sulphuric gas igniting as it escapes the vents at over 350°C, and it is only visible in darkness, which is why the hike starts at 1am.
By sunrise the flames have faded and something else takes over: a milky turquoise lake, a kilometre across, acidic enough to dissolve metal.
Ijen is also a working mine. Miners carry 70 to 90 kilogram baskets of raw sulphur up the crater wall and down the mountain, twice a day. Meeting them changes how the photographs feel.
