3:00 PM - 3:15 PM
[SVC47-36] Petrographic study of juvenile blocks of the 1783 A.D. (Agatsuma) pyroclastic flow in Asama Volcano
Keywords:Asama Volcano, Agatsuma pyroclastic flow, intermediate type pyroclastic flow, opal, welding
On the surfaces of juvenile blocks ejected at the last stage of the 1783 A.D. eruption, we found the crusts impregnated with very fine (1–10 µm) ash particles and interstitial opal. The presence of opal shows that the juvenile blocks had been emplaced in a hydrothermal system after their rounded shape was formed. Since there is no geothermal area at the sampling location and its upper flank of the Maekake volcano, it is highly possible that silica precipitation from circulated geothermal water followed the formation of characteristic round shape of the juvenile blocks before generation of the APF. The weakly welded ash particles existed between the opal-bearing crust and the inner core; the welding process thus should have occurred before the crust formation. The surfaces of the juvenile blocks without opal-bearing crusts, which were collected from weakly-welded APF deposits, were covered instead with welded ash particles, which may have been formed at the higher temperature condition in the crater. These features indicate that the juvenile blocks were rounded probably via coagulation and welding and coated with volcanic ashes, and filled the crater before their eruption.