A spectacle of chemistry
The electric-blue fire of Kawah Ijen isn't lava. The mesmerizing light show, visible only at night, results from the combustion of highly concentrated sulfuric gases. These gases escape from fumaroles—cracks in the volcano's surface at high pressure and temperatures reaching 600°C (1,112°F). When this superheated, sulfur-rich gas mixture meets the oxygen in the air, it ignites, creating flames that can reach up to 5 meters (16 feet) high.
Some of the gas rapidly condenses into liquid sulfur, which has a relatively low melting point of 115°C (239°F). This molten sulfur then flows down the crater's slopes, still burning with the same eerie blue flame, creating blue lava. The phenomenon is present day and night but can only be seen after dark. Kawah Ijen is the largest and most consistent area for this blue fire phenomenon in the world.
The world's largest acid lake
The crater of Kawah Ijen also holds the world's largest highly acidic lake. The one-kilometer-wide body of water is a turquoise color due to its extreme acidity and high concentration of dissolved metals. The water's pH is as low as 0.5, an acidity created by hydrothermal waters infused with gases like hydrogen chloride from the magma chamber below. This gas reacts with the lake water to form highly concentrated hydrochloric acid. The active crater is 722 meters in diameter and 200 meters deep, containing about 36 million cubic meters of acidic water.
This hyperacidic environment, combined with the continuous stream of sulfur-laden gases, makes the area a hazardous geological site. The volcano is part of the larger Ijen Caldera, which formed after a massive eruption around 50,000 years ago. While the last magmatic eruption was in 1817, frequent phreatic eruptions—steam-driven explosions—have occurred, posing a constant danger to those in the crater.