▁total ▁bulk ▁volume ▁from ▁this ▁is ▁ 1 2 0   km 3 . ▁Gu o ▁et ▁al ., ( 2 0 0 1 ) ▁used ▁the ▁exponential ▁method ▁estimate ▁that ▁volume ▁of ▁ash fall ▁is ▁ 1 3 5 . 2 ▁ ± ▁ 7 . 8   km 3 . ▁But ▁Gu o ▁assumed ▁the ▁geometry ▁of ▁ign im br ite ▁is ▁a ▁cone , ▁and ▁the ▁volume ▁of ▁ign im br ite ▁could ▁be ▁ 2 0 . 1   km 3 . ▁Gu o ▁also ▁calculated ▁the ▁volume ▁of ▁valley - ign im br ite , ▁because ▁in ▁a ▁valley ▁the ▁thick ness ▁of ▁ign im br ite ▁could ▁be ▁ 8 0 ▁m . ▁Then , ▁the ▁total ▁bulk ▁volume ▁is ▁ 1 6 1 . 6 ▁ ± ▁ 7 . 8   km 3 . ▁However , ▁ 1 0 0 – 1 2 0   km 3 ▁has ▁been ▁widely ▁used . ▁ ▁E ruption ▁dynamics ▁ ▁Base ▁on ▁the ▁sequence ▁of ▁py ro cl astic , ▁the ▁er u ption ▁began ▁with ▁p um ice ▁and ▁ash ▁falls , ▁and ▁then ▁an ▁er u ption ▁column ▁collapse ▁formed ▁ign im br ite . ▁The ▁column ▁collapse ▁probably ▁was ▁a ▁pul sing ▁collapse , ▁because ▁the ▁ign im br ite ▁and ▁p um ice ▁fall ▁depos its ▁are ▁inter le aved . ▁Mach ida ▁et ▁al ., ( 1 9 9 0 ) ▁divided ▁the ▁Mill en ni um ▁er u ption ▁into ▁ 4