Ijen, Stratovolcano crater lake in East Java, Indonesia
Ijen is a stratovolcano in Bondowoso, East Java, whose crater holds a turquoise lake roughly one kilometer across. The lake sits surrounded by steep volcanic walls at an elevation of 2769 meters above sea level.
Sulfur extraction at the volcano began over seven centuries ago and developed into a continuous mining tradition. The work has passed through generations and still supplies industries with volcanic sulfur today.
Workers mine sulfur at the crater edge using woven bamboo baskets carried on their shoulders up to the rim. The yellow chunks are broken by hand from the fumaroles and yield pure sulfur material after drying for industry and agriculture.
Visitors need gas masks and sturdy footwear for the roughly three-kilometer climb to the crater rim. The blue flames are best seen between midnight and sunrise when darkness highlights the glow.
Volcanic gases ignite on contact with air and produce natural blue flames visible at night. The phenomenon occurs because burning sulfur at high temperatures emits blue light instead of the usual yellow or orange.
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