The archipelago of Indonesia consists of more than 13,000 islands, spread over an area that is similar in size to that of the continental United States. It is the country with the greatest number and density of active volcanoes.
Most volcanoes in Indonesia belong to the Sunda Volcanic Arc, streching over 3,000 kilometers from NW Sumatra to the Banda Sea. This volcanic arc results from the subduction of Indian Ocean crust beneath the Asian Plate and includes 76% of the region's volcanoes. To the NNW, the basaltic volcanism of the Andaman Islands results from short spreading centers, and to the east the Banda Arc reflects Pacific Ocean crust subducted westward. North of this arc, the tectonic setting is much more complex: several fragments of plates are converging to form multiple subduction zones, mainly oriented N-S. These produce the Sulawesi-Sangihe volcanoes on the west and Haimahera on the east of the collision zone.
Indonesia leads the world in many volcano statistics. It has the largest number of historically active volcanoes (76), its total of 1,171 dated eruptions is only narrowly exceeded by Japan's 1,274, although not much is know about the volcanic activity in the time before European colonialists arrived from the 15th century on. Indonesia has suffered the highest numbers of eruptions producing fatalities, damage to arable land, mudflows, tsunamis, domes, and pyroclastic flows. Four-fifths of Indonesian volcanoes with dated eruptions have erupted in this century.
Two of the most devastating volcanic eruptinos in historic time took place in Indonesia: the enormous eruption of Tambora in 1815,- the largest known eruption of the world during historical times,- had such far-reaching effects on the climate that for instance Europe was to experience 1816 as the year without summer. In 1883, the disastrous eruption of Krakatau carved itself deeply into the collective memory of mankind. The eruption of Krakatau was followed by severe tsunamis that killed about 30-40,000 people.
In 1920, a volcano survey was established by the Dutch-led government, leading to much improved volcano monitoring and reporting. The Volcanological Survey of Indonesia (VSI) now operates a network of 64 volcano observatories continuously monitoring 59 volcanoes.
Freely adapted from: Simkin and Siebert, 1994, Volcanoes of the World: Smithsonian Institution and Geoscience Press, Inc., Tucson, Arizona, 349p.