Batur is an active statrovolcano on teh island of Bali, Indonesia, with a large, well-formed caldera whose formation is correlated with the erupstion about 23.700 years ago of a thick ignimbrite sheet. Our study of the volcanic statigraphy and geochemistry of Batur shows teh formation of the caldera was signalled by a change in teh composition of the erupting material from basaltic and andesiti…