|Elevation||2,799 m (9,183 ft)|
It is inside a larger caldera Ijen, which is about 20 kilometres wide. The Gunung Merapi stratovolcano is the highest point of that complex. The name "Gunung Merapi" means "mountain of fire" in the Indonesian language (api being "fire"); Mount Merapi in central Java and Marapi in Sumatra have the same etymology.
West of Gunung Merapi is the Ijen volcano, which has a one-kilometre-wide turquoise-coloured acidic crater lake. The lake is the site of a labour-intensive sulfur mining operation, in which sulfur-laden baskets are carried by hand from the crater floor. The work is paid well considering the cost of living in the area, but is very onerous. Workers earn around Rp 50,000–75,000 ($5.50–$8.30) per day and once out of the crater, still need to carry their loads of sulfur chunks about three kilometers to the nearby Paltuding Valley to get paid.
Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of post-caldera cones run east-west across the southern side of the caldera. The active crater at Kawah Ijen has a diameter of 722 metres (2,369 ft) and a surface area of 0.41 square kilometres (0.16 sq mi). It is 200 metres (660 ft) deep and has a volume of 36 cubic hectometres (29,000 acre⋅ft).
The lake is recognised as the largest highly acidic crater lake in the world. It is also a source for the river Banyupahit, resulting in highly acidic and metal-enriched river water which has a significant detrimental effect on the downstream river ecosystem. On July 14–15, 2008, explorer George Kourounis took a small rubber boat out onto the acid lake to measure its acidity. The pH of the water in the lake's edges was measured to be 0.5 and in the middle of the lake 0.13 due to high sulfuric acid concentration.
Blue fire crater
Since National Geographic mentioned the electric-blue flame of Ijen, tourist numbers increased. The phenomenon has occurred for a long time, but beforehand there was no midnight hiking. A two-hour hike is required to reach the rim of the crater, followed by a 45-minute hike down to the bank of the crater. The blue fire is ignited sulfuric gas, which emerges from cracks at temperatures up to 600 °C (1,112 °F).
The flames can be up to five meters (16 feet) high; some of the gas condenses to liquid and is still ignited. It is the largest blue flame area in the world and local people refer to it as 'Blue Fire'.
Sulfur mining at Ijen
An active vent at the edge of the lake is a source of elemental sulfur, and supports a mining operation. Escaping volcanic gases are channeled through a network of ceramic pipes, resulting in condensation of molten sulfur.
The sulfur, which is deep red in colour when molten, pours slowly from the ends of these pipes and pools on the ground, turning bright yellow as it cools. The miners break the cooled material into large pieces and carry it away in baskets. Miners carry loads ranging from 75 kilograms (165 lb) to 90 kilograms (200 lb), up 300 metres (980 ft) to the crater rim, with a gradient of 45 to 60 degrees and then 3 kilometres (1.9 mi) down the mountain for weighing. Most miners make this journey twice a day.
A nearby sulfur refinery pays the miners by the weight of sulfur transported; as of September 2010, the typical daily earnings were equivalent to approximately $13 US. The miners often receive insufficient protection while working around the volcano  and complain of numerous respiratory afflictions. There are 200 miners, who extract 14 tons per day – about 20% of the continuous daily deposit.
In the documentary film War Photographer, journalist James Nachtwey visits Ijen and struggles with noxious fumes while trying to photograph workers. Michael Glawogger's film Workingman's Death is about sulfur workers.
Kawah Ijen volcano and crater lake, Java, viewed from Landsat 8
- "Ijen". Global Volcanism Program. Smithsonian Institution National Museum of Natural History. Retrieved 12 August 2014.
- "Photos from inside the volcanic sulphur mines of Indonesia". news.com.au. February 9, 2015. Archived from the original on November 11, 2016. Retrieved October 19, 2017.
- Lane, Megan (9 February 2011). "Sulphur mining in an active volcano". BBC News.
- Harsaputra, Indra (19 December 2011). "Kawah Ijen: Between potential and threat". The Jakarta Post.
- "University of Amsterdam, Faculty of Earth and Life Sciences, Natural Pollution Caused by the Extremely Acidic Crater Lake Kawah Ijen, East Java, Indonesia" (PDF). Retrieved 9 December 2013.
- Boating on acid
- "Wanderlust East Java Tourist Agency".
- Howard, Brian Clark. "Stunning Electric-Blue Flames Erupt From Volcanoes". Retrieved 13 June 2014.
- Schrader, Robert. "The Dark Secret of Indonesia's Blue-Fire Volcano". Retrieved 13 June 2014.
- "Think your work is hell? Thank your lucky stars you didn't have to mine sulphur for 12 hours in a volcano crater filled with toxic fumes that will probably kill you before you are 30". Daily Mail. 16 October 2012.
- Grunewald, Olivier (8 December 2010). "Kawah Ijen by night". Boston Globe. Retrieved 11 October 2013.
- "Kawah Ijen: Between potential & threat". The Jakarta Post. 19 December 2011.
|Wikimedia Commons has media related to Ijen.|
|Wikivoyage has a travel guide for Banyuwangi.|
- Ijen Gallery
- Volcanological Survey of Indonesia
- Official website of Indonesian volcanoes at USGS
- The Adventures Behind Filming Ring of Fire
- Large photogallery from Kawah Ijen
- Sulfur mining in Kawah Ijen (The Big Picture photo gallery at Boston.com)
- "Traditional sulfur mining in Kawah Ijen". Yahoo! News.
- More sulfur mining pictures at Ijen
- Spectacular Neon Blue Lava Pours From Indonesia's Kawah Ijen Volcano At Night (PHOTOS)