Electric-blue flames and glowing streams of burning sulfur inside the dark crater of Kawah Ijen at night, East Java, Indonesia

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Why Does This Volcano Burn Blue?

At night, the crater of Kawah Ijen in East Java fills with electric-blue flames. It looks like blue lava. It is not lava at all: it is sulfur on fire.

Photo: Thomas Fuhrmann, CC BY-SA 4.0

The short answer

The fire is blue. The lava is not.

Kawah Ijen releases huge amounts of hot sulfur gas through cracks in its crater. When that gas meets the air, it catches fire and burns with a blue flame. Molten rock here, like everywhere on Earth, glows red and orange.

What it looks like

Blue lava

Rivers of blue light running down the rock, as if the volcano were pouring out something no other volcano can.

What it is

Burning sulfur gas

Sulfur gas igniting in the air, and liquid sulfur that keeps burning as it runs downhill. That is what makes the "rivers".

A narrow blue river of burning sulfur running down a dark slope inside Kawah Ijen at night
A stream of burning liquid sulfur, photographed at 4:32 am. This is the image people call "blue lava".Photo: Jakub Hałun, CC BY-SA 4.0

How the blue fire works

From gas underground to blue light in the air.

  1. Up to 600 °C

    Hot gas escapes

    Gas rich in sulfur rises from the volcano and leaves through cracks in the crater floor, at high pressure and very hot.

  2. Above 360 °C

    It meets the air

    When this hot sulfur gas touches the oxygen in the air, it ignites and burns.

  3. Up to 5 m

    Blue flames rise

    The burning gas forms flames that can reach five metres high. Their colour comes from the sulfur itself.

  4. Liquid sulfur

    Some of it flows

    Part of the gas condenses into liquid sulfur. It keeps burning as it runs down the rock, which is what looks like blue lava.

Tall violet-blue flames and burning sulfur flowing over dark rock in the crater of Kawah Ijen
Blue flames above burning sulfur, before dawn.Photo: Thomas Fuhrmann, CC BY-SA 4.0

Why blue, and why only at night

Sulfur burns blue. Daylight hides it.

The colour of a flame depends on what is burning. Burning wood and molten rock glow orange. Burning sulfur gives off a blue light, the same blue you see when a little sulfur is lit in a laboratory.

The flames burn day and night, but they are faint. In sunlight they disappear against the bright crater. Only in darkness does the blue become visible.

Molten rockred to orange Burning sulfurblue Cooled sulfurbright yellow

The Ijen crater lake

The same crater holds a lake of acid.

Next to the blue fire lies a turquoise lake about one kilometre wide. It is described as the largest highly acidic crater lake in the world: its water is full of sulfuric acid from the gases rising through it.

Aerial view of the Kawah Ijen crater with its bright turquoise acid lake surrounded by grey crater walls and drifting clouds
Kawah Ijen from the air. The lake fills the floor of the active crater.Photo: FBilula, CC BY 4.0

How acidic is it?

pH 0 · most acidicpH 14

Measurements in 2008 found about pH 0.5 at the edges of the lake and 0.13 in the middle.

  • Width of the lake≈ 1 km
  • Depth of the crater200 m
  • Volume of the crater36 million m³
  • Lowest pH measured0.13

Where the blue fire ends

When the flames go out, the sulfur turns yellow.

Pipes channel the volcanic gas so it cools into molten sulfur, which hardens into bright yellow rock. Miners break it off and carry it out of the crater by hand, one of the hardest jobs on any volcano.

A sulfur miner carrying two baskets of yellow sulfur on a bamboo pole beside the steaming turquoise lake of Kawah Ijen
A miner carrying sulfur beside the lake, 2009.Photo: Jean-Marie Hullot, CC BY-SA 2.0
Woven baskets filled with chunks of bright yellow sulfur resting on the crater floor
Baskets of sulfur taken from the crater floor.Photo: CEphoto, Uwe Aranas, CC BY-SA 3.0
  • 75–90 kgCarried in each load
  • 300 mClimbed up the crater wall, then 3 km down the mountain
  • 14 tonnesOf sulfur taken out each day by about 200 miners

Where on Earth

At the eastern tip of Java.

The globe turned to Indonesia, showing Sumatra, Java, Borneo and Bali, with the Java Trench drawn as a dashed line south of Java and Kawah Ijen marked at the eastern end of Java Java Trench Kawah Ijen Java Jakarta Indian Ocean
The Java Trench, where one tectonic plate sinks beneath another, drawn by hand on the SeekGlobe globe.

Kawah Ijen sits inside the Ijen caldera, on the border of the Banyuwangi and Bondowoso regencies in East Java. South of Java, the Indo-Australian Plate slides under the Eurasian Plate along the Java Trench. That collision feeds the long chain of volcanoes running across Indonesia.

  • 8.06° SJust across the Bali Strait from the island of Bali
  • 2,769 mGunung Merapi, the highest point of the Ijen volcano complex
  • 20 kmWidth of the Ijen caldera that holds the crater
  • 2023The Ijen Geopark joined the UNESCO Global Geoparks network

Quick answers

Questions people ask about Kawah Ijen.

Does Kawah Ijen have blue lava?

No. Its lava is red and orange like any other. The blue comes from sulfur gas burning as it meets the air, and from liquid sulfur that keeps burning as it flows downhill.

Why is the fire at Ijen blue?

Hot sulfur gas escapes from cracks in the crater at up to 600 °C and ignites in the air. Burning sulfur gives off blue light, producing flames up to 5 metres high.

When can you see the blue fire?

Only at night. The flames burn all the time, but they are too faint to see in daylight.

How acidic is the Ijen crater lake?

Very. Its pH has been measured at about 0.5 at the edges and 0.13 in the middle, more acidic than battery acid. It is described as the largest highly acidic crater lake in the world.

Where is Kawah Ijen?

In the Ijen volcano complex in East Java, Indonesia, on the border of the Banyuwangi and Bondowoso regencies, just west of Bali.

The short version

Kawah Ijen does not erupt blue lava. Its sulfur burns blue at night, cools to yellow, and turns its crater lake into turquoise acid.

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