Keywords:Asama volcano, Tenmei eruption, pyroclastic flow, clastogenic lava flow
The detailed process of eruptions of Agatsuma pyroclastic flow and Onioshidashi lava flow during the climactic stage of Tenmei eruption (1783AD) of Asama-Maekake volcano is reexamined, based on the occurrence of volcanic deposits, their stratigraphy and old documents. The Agatsuma pyroclastic flow deposit comprises four stages: the earliest, early, A-scoria flow and late stages. The pyroclastic flows of the earliest stage and A scoria flow were the column-collapsed type. Those of the early stage were the boilover type and those of late stage were the fountain-collapse type. The Onioshidashi lava flow is clastogenic and consists of three units: L1, L2 and L3. The L1 and L3 are the slope-collapsed (rootless) type; the formation of L1 was synchronous with the last stage of Tenmei pumice fall deposits (21p), which were the climactic sub-Plinian eruption. The L2 is the spatter-fed type, outpouring from the crater of Kamayama welded pyroclastic cone. The erliest stage of Agatsuma pyroclastic flow was small-scale and occurred during the eruption of Tenmei pumice fall deposit around 18:00 in August 3 (corresponding to 10a to 14a). The early stage of Agatsuma pyroclastic flow began during the dormant stage of eruption of Tenmei pumice fall deposit from 16:00 to 18:00 in August 4 (corresponding to 20a). The pyroclastic flow deposits of the early stage with low aspect ratio are abundant in matrix ash, the essential clasts of which are high in SiO2 (62 to 64wt. %). The A-scoria flow was small-scale and erupted during the eruption of Tenmei pumice fall deposit around 20:00 and 24:00 in August 4. The late stage of Agatsuma pyroclastic flow deposit erupted from 3:00 to 6:00 in August 5 just after the cessation of Tenmei pumice fall deposit. They show high aspect ratio and are relatively poor in matrix ash, the essential clasts of which are low in SiO2 (61 to 62wt. %). The L1 of Onioshidashi lava flow with relatively high SiO2 content (60.5 to 64wt. %) began to flow down around 18:00 in August 4 during the eruption of Tenmei pumice fall deposit and continued to 3:00 in August 5. The onset of effusion of L2 with relatively low SiO2 content (60 to 63wt. %) was around 3:00 in Augusut 5 concurrent with the eruption of late stage of Agatsuma pyroclastic flow. The flowing down of L3 (60.5 to 61.5wt. %) of the Onioshidashi lava formed by the collapse of slope of Kamayama pyroclastic cone was later than 3:00 in August 5.