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Starts With A Bang

This Erupting Volcano’s Lava Appears Blue, And Science Knows Why

The one-of-a-kind blue color associated with the lava flows on Kawah Ijen volcano is not, as you might think, due to blue-colored lava. The scientific story is as fascinating as the images. (STÉPHANE DAMOUR / FLICKR)

Sometimes, chemistry can be more important for color than even physics.


All across the world, volcanic eruptions remind us of the destructive power residing beneath Earth’s surface.

You might be used to images of lava erupting from and flowing down active volcanoes, but that lava is normally dark, red, orange, or yellow. The blue that you see is unique to eastern Indonesia, and isn’t the color of the lava at all. (MICHAEL HEINIGER / FLICKR)

Molten rock, known as magma, erupts through fissures in Earth’s crust, becoming lava.

Lava erupts and flows from a Kilauea volcano fissure in Leilani Estates, on the Big Island, on May 23, 2018 in Pahoa, Hawai’i. (GETTY)

If that lava rises above 525 ˚C (977 ˚F), it glows red, with hotter lavas becoming orange or even yellow.

A slow shutter speed picture shows lava streaming down from Mount Anak Krakatau (Child of Krakatoa) during an eruption as seen from Rakata island on July 19, 2018 in Lampung, Indonesia. The orange-red color allows us to determine the temperature of the lava, which is hottest at the tip of the cinder cone and less hot as we view the lava flows farther down. (GETTY)

But one volcano, Indonesia’s Kawah Ijen, displays a spectacular blue color.

On many different parts of the mountains in the Ijen volcano complex, gas and liquid outflows can be seen. The blue color shown here is a true-color image, with an appearance unique among the known volcanoes of the world. (STÉPHANE DAMOUR / FLICKR)

This unique feature is practically invisible during the day, but is inescapable at night.

A flowing path of lava, shown here as photographed at night, displays a blue color to it, unique among lava flows. The presence of sulfur, which combusts when it interacts with air at sufficient temperatures, is the cause of this one-of-a-kind phenomenon. (MICHAEL HEINIGER / FLICKR)

Along the flowing lava path, this blue color is ubiquitous.

This image, taken by photographer Reuben Wu, showcases the flowing rivers of lava with the blue flame produced by the interaction of sulfur and oxygen at high temperatures. (REUBEN WU / REUBENWU.COM)

Bizarrely, the temperatures are insufficient to create a blue hue.

As lava flows down the mountainside at Kawah Ijen, what barely gives off a perceptible color in daylight glows a spectacularly bright blue at night. The low-luminance flames produced by a chemical interaction between sulfur and oxygen at high temperatures are the culprit. (GETTY)

Instead, the blue color is caused by the chemical composition of the environment surrounding the volcano.

The multicolored photo shown here is a result of greenish and yellowish fumes emitted by Indonesia’s highly acidic, volcanic lake, the Ijen volcano complex, and the blue flames which emerge from the combustion of sulfurous compounds. (GETTY)

The Ijen volcano complex contains the world’s largest lakes of acid, with an average pH of around 0.

The sulfur lake at Mount Kawah Ijien, Indonesia, is both a site of volcanic activity and noxious, sulfurous fumes. When the temperature is sufficiently high, the sulfur will combine with the oxygen in the atmosphere and ignite, resulting in a faint blue glow which is clearly visible at night, but difficult to detect during the day. (GETTY)

The cause of this acidity is widespread sulfur.

Sulfuric acid forms, creating such noxious conditions.

The yellow color we normally associate with sulfur is actually sulfur dioxide, which is the result of chemical combustion. The blue flame, visible in certain parts of this image, is the result of that very process of combustion. (REUBEN WU / REUBENWU.COM)

Sulfur also combusts when it contacts oxygen-containing air above a temperature of 360 ˚C (680 ˚F).

Molten materials will flow down the side of an erupting volcano, which includes sulfur for the Kawah Ijen Volcano in Java, Indonesia. Here, the liquid sulfur has caught fire, causing it to burn with a bright blue flame. This occurs even at temperatures low enough so that the lava itself has no intrinsically visible color. (REUBEN WU / REUBENWU.COM)

It’s this combustion that gives rise to the blue flames, not the intrinsic color of the lava itself.

The blue fire, shown burning here, isn’t due to the color of the lava itself, but rather is caused by the chemical process of combustion, where sulfurous compounds are ignited by sufficiently high temperatures and the presence of oxygen in the atmosphere. (GETTY)

As the burning sulfur flows down the mountain, miners work to collect this valuable material, even under these dangerous conditions.

Sulfur mining is a popular and profitable profession at the Ijen site in Indonesia, where miners brave long hours, toxic fumes, darkness, and the eerie conditions of a natural gas fire to collect this valuable mineral. (GETTY)

Mostly Mute Monday tells the scientific story of a physical phenomenon in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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