The Ijen volcano complex is a group of composite volcanoes located in East Java, Indonesia. It is inside an eponymous larger caldera Ijen, which is about 20 kilometres wide. The mount Merapi stratovolcano is the highest point of that complex. The name “Gunung Merapi” means ‘mountain of fire’ in the Indonesian language; 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 and a surface area of 0.41 square kilometres. It is 200 metres deep and has a volume of 36 cubic hectometres. 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 a high concentration of sulfuric acid.
Blue fire crater
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 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 Api Biru (Blue Fire). The other location at which the blue fire can be seen is in Dallol mountain, Ethiopia.
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 to 90 kilograms, up 300 metres to the crater rim, with a gradient of 45 to 60 degrees and then 3 kilometres 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.