Java is the Indonesian island with the highest concentration of active volcanoes. The island is almost entirely volcanic in origin and contains an east–west chain of about 38 mountains that have been active at some point, including roughly 35 recognized active volcanoes under commonly used classifications.
Java’s volcanoes formed because the Indo-Australian Plate is being forced beneath the Eurasian Plate along the Sunda Arc. This subduction zone fuels the island’s frequent earthquakes and eruptions, while volcanic ash has produced remarkably fertile soils. Those soils helped Java support intensive rice cultivation and one of the densest populations on Earth.
Among Java’s best-known volcanoes are Merapi, Semeru, Bromo, Kelud, and Krakatau, which lies in the Sunda Strait between Java and Sumatra. Mount Merapi, on the border of Central Java and Yogyakarta, is generally described as Indonesia’s most active volcano and has erupted repeatedly since 1548.
Sumatra, Bali, and the Lesser Sunda Islands also contain many active volcanoes, so Java does not contain Indonesia’s only volcanic belt. It is the concentration and continuity of its volcanic chain that make Java the standard answer.