Stretching over 22 kilometers (14 miles) in East Java, Indonesia, is the Kawah Ijen volcanic complex. While this vast stretch of elaborate geological formations boasts striking views in the daylight, come nightfall the mountains spew electric blue streams of "lava".

Believed to be the only location on the planet to consistently exhibit these striking blue flames, the Kawah Ijen volcano’s beauty comes at a price, as the intense levels of sulfur make the air surrounding the complex toxic to anyone who breathes it.

Why is the Ijen lava blue?

At first glance, the Ijen volcano looks like your bog-standard 600–900°C (1,112–1,652°F) stream of red-hot bubbling molten rock, so what causes the mountain’s sunset transformation into something out of a sci-fi film?

The blue lava phenomenon is caused by an abundance of sulfur pockets in the rock. Sulfur is a chemical element that’s released as the rock liquefies, and while burning, it releases noxious gases like sulfur dioxide. It also creates a striking blue flame.

As this sulfur comes into contact with oxygen it ignites, causing the blue flame that gives the lava the appearance of being entirely blue, but it is in fact just the surface of the molten rock that’s covered in blue flames.

Despite only being visible at night, the blue flames are always ignited on the surface of the lava, they’re just harder to see in daylight. 

The Kawah Ijen volcanic complex

This vast complex is actually within a caldera – a crater that’s formed when a volcano erupts and collapses, often creating a large lake in its center. The Ijen complex contains around 22 eruption points, mostly around the rim of the caldera.

The water in the crater of the Ijen volcano is an inviting bright turquoise color, but once again, don’t let its beauty fool you as this is the largest acid lake in the world and has a pH of around zero. Swimming in the waters here could be life-threatening, or at least cause serious injury.

The abundance of sulfur in the area, referred to by locals as the “Devil’s gold”, means the complex is also home to one of the very few sulfur mining operations done by hand. After the sulfur-rich rock has cooled, and the blue flame has died out, it is collected for use in a number of products – in Java, the sulfur is used to make matches and to make sugar whiter.

While tourists wear gas masks to visit the area, many of the miners are unable to afford masks and instead use just a cloth to cover their mouths. Without the proper protective equipment, many of the sulfur miners suffer serious health conditions, with a third regularly experiencing respiratory diseases.

Despite the many dangers of the Ijen complex, its rare and (quite literally) breathtaking appearance makes the area a popular tourist attraction. Visits can be conducted safely if accompanied by knowledgeable tour guides and protective breathing apparatus.

Other instances of blue flames

While the Kawah Ijen volcano may be the only consistently burning blue flame, this striking phenomenon can technically appear anywhere with high levels of certain gases.

There are reports of blue flames appearing during a forest fire at Yellowstone National Park, US, where the sulfur surrounding the hydrothermal vents caught alight.

In 2018, the United States Geological Survey (USGS) released images of blue flames appearing to shoot out of the ground during an eruption of Kilauea in Hawaii. These flames were caused by a release of methane gas.

This gas was released as a result of lava from the eruption smothering vegetation and preventing oxygen from getting to it. This incomplete combustion process, called pyrolysis, causes methane to be released, which in turn causes a blue flame.

The Dallol volcano in the Danakil Depression, Ethiopia, has also been known to spew blue-appearing lava, but this happens rarely and will burn for a few days at a time. These flames are also caused by hydrothermal vents and sulfur springs inside the depression. 

Before you jet off to see some of these incredibly rare and beautiful flames, always be cautious that they usually come accompanied by some pretty toxic (and stinky) gases.