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The 20-kilometer Ijen caldera in Java, Indonesia is the site of not one, but two noteworthy geological phenomena: an active steam vent that spews out electric blue fire at night, and a lake full of turquoise-blue water that’s lovely yet lethal.

The caldera’s active opening, Kawah Ijen, is a solfatara (“sulfur place” in Italian), a steam vent full of sulfuric gases and hot vapor. These gases reach up to 1,112°F (600°C), expelled at high pressure and in tremendous amounts. When they come in contact with Earth’s oxygen-rich atmosphere, the gases ignite, creating vibrant flames rising up to 5 m high. When viewed at night, the flames burn bright blue. Some of these gases condense in the atmosphere, causing liquid sulfur to flow down the volcano. Due to the way the liquid sulfur flows, people have mistaken it for lava.

With a volume of 36 million cubic meters (1.3 billion cubic feet), Ijen’s kilometer-wide caldera lake is the world’s largest highly acidic lake. Its brilliant hue is the product of dissolved metals and extreme acidity (pH 0.5) from gas-charged hydrothermal inflow. While it attracts many tourists from all over the world, the lake presents a very real health hazard. It even affects the pH levels of the irrigation waters used by farmers living downstream. Meanwhile, miners have been capitalizing on the abundance of hardened sulfur near and around the volcano, wrapping cloth around their faces in a somewhat vain attempt to safeguard themselves from the volcano’s deadly gases.

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Cover photo: Oliver Grunewald via National Geographic


  • https://geology.com/volcanoes/kawah-ijen/
  • https://www.nationalgeographic.com/news/2014/1/140130-kawah-ijen-blue-flame-volcanoes-sulfur-indonesia-pictures/
  • https://www.smithsonianmag.com/science-nature/why-does-indonesian-volcano-burn-bright-blue-180949576/
  • https://www.volcanodiscovery.com/ijen.html

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Author: Mikael Angelo Francisco

Bitten by the science writing bug, Mikael has years of writing and editorial experience under his belt. As the editor-in-chief of FlipScience, Mikael has sworn to help make science more fun and interesting for geeky readers and casual audiences alike.