Fluorine and chlorine have been determined for all the products of the 1783 activity of Asama volcano. The activity displayed various features that are characteristic of andesitic eruptions, i.e., pumice falls, two types of pyroclastic flow (the Agatsuma and Kambara pyroclastic flows), and the Oni-oshidashi lava flow. Their F-Cl variation patterns differ from each other according to the nature of the eruption. The pumice samples have very uniform and the highest Cl contents of all the eruptive products, despite their vesiculated texture. This is explained by the enrichment of volatiles in the uppermost part of the magma column at the beginning of the eruption. The Agatsuma pyroclastic deposit samples show the lowest Cl and F contents and the largest variations. An intense release of gas from the pyroclastic flow after the effusion is thought to have caused this. The F and Cl contents of essential blocks in the Kambara pyroclastic flow deposits, together with the field observations, suggest that the release of gas from the essential material contributed little to the fluidization of this pyroclastic flow. In conclusion, the transition from one mode of eruption to another observed in the 1783 activity of Asama must be intimately related to the variation of the volatile contents of the magma at the time of the effusion.

Key words:
fluorine, chlorine, volatile contents, pyroclastic flow, Asama volcan