Pyroclastic density current from the 1888 phreatic eruption of Bandai volcano, NE Japan

The great eruption of July 15, 1888 at Bandai volcano produced large phreatic explosions and a major debris avalanche toward the northern foot. This eruption is characterized as follows: (1) there was no significant precursor; (2) the eruption and collapse of the volcanic edifice were triggered by an earthquake with a magnitude of about 5; (3) the initial explosions were accompanied with a pyroclastic density current; (4) the main explosive eruption quickly ceased a few minutes after the eruption began; (5) a subsequent late-stage plume rose buoyantly to a height of 5000 m, falling with a heavy warm rain which generated a lahar; and (6) ejected materials were made up wholly of pre-existing rock fragments, with no juvenile glass shards. The main branch of the pyroclastic density current cascaded from the summit for a distance of 6 km along the Biwazawa Valley on the southeast flank. The Heim coefficient of the reconstructed energy line slope for the density current is 0.223 to 0.296; the estimated velocity using these values is less than 100 m/s. The density current deposit consists of normally-graded, parallel-bedded, or scour-fill cross-bedded thin layers of coarse ash and lapilli, except for coarse massive breccia near the vent. The sedimentary structures and grain size data suggest that the pyroclastic materials were transported in traction-saltation and true suspension; the estimated velocity is competent for transportation of the deposit. The time sequence suggests that the seismic shocks and subsequent collapse of the edifice broke the confined hydrothermal system and caused the phreatic explosions. The pressurized hydrothermal fluids had been presumably stored due to self-sealing of the system, because hydrothermal activities had been vigorous at the summit region until the July 15 seismic shocks. Collapse of the dry and dense initial eruption column generated the density current which spilled toward the opposite side of the crater along the topographic lows. The change from the collapse to the late-stage ascending plume results from a decrease in the rock-fragment contents, which made the plume light and wet. This is because that a decrease in the fraction of rock fragments causes an increase in the fraction of steam produced within the plume. The occurrence of the water-flushed ash fall and lahar during the late-stage eruption is consistent with this model.