Kawah Ijen is a composite volcano located on Java island in Indonesia, and it hosts the largest natural acidic lake in the world. The lake is turquoise-colored due the high concentration of sulfuric acid.
It’s also known for its blue lava. It’s called blue lava, but actually it’s not blue. The lava has a reddish color, just like with most other volcanoes.
The reason the lava has a blue glow is because high quantities of sulfuric gases emerge along with the lava. Once these gases are exposed to the oxygen in the air, it starts burning, sparked by the hot lava reaching temperatures to 600°C. The flames it creates are bright blue, and can be as high as 5 meters.
Sometimes there’s that much gas released, it starts flowing down the mountainside. Thus giving the illusion blue lava is flowing down.
Since it’s only the flames giving off a blue color, the effect is only visible at night. Overday the volcano looks just like any other volcano.