And once this heated sulfur comes in contact with oxygen, it bursts into flames – specifically, electric blue-colored ones. These cobalt fires, moreover, leap out of the vents to heights of up to 16 feet.
Even as it burns, though, some of the gas begins to condense. And as it condenses, it forms liquid, yet continues to burn. That’s why you get liquid rivers of blue flame sometimes flowing down Kawah Ijen – and why it is also known as the Blue Fire Crater.