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The eruption was caused by a phreatic explosion, i.e. the explosion of overheated hydrothermal fluids within the volcano, without any juvenile magma. The eruption, which was very short but violent, had little warning, except some precursory seismic activity. Small earthquakes were felt at the northern base of the volcano on 8-10 July, and moderate earthquakes on 13-14 July. There were no other changes observed in the volcanic activity before the eruption.
The eruption began on 15 July with 2 strong earthquakes. The second lasted for a minute, and the eruption began while the earthquake was occurring. Eyewitnesses report a large explosion with a huge thundering noise. The initial explosion was followed by about 20 more later on, occurring successively in 1 min. Within 10 min of the first explosions, a major debris avalanche was descended the northern flank.
The main explosive eruption lasted only for a few minutes, but thundering sounds continued for about 2 hours. A smaller debris avalanche flowed down through Biwazawa Valley, and buried approximately half of the houses in the village of Mine.

-- Excerpt from: Brantley and Glicken, 1986, Volcanic Debris Avalanches: Earthquakes & Volcanoes, v.18, n.6, p.195-206.
(CVO website)
"Nearly a century ago, the north flank of Bandai Volcano in Japan collapsed during an eruption quite similar to the May 18, 1980 eruption of Mount St. Helens. After a week of seismic activity, a large earthquake on July 15, 1888, was followed by a tremendous noise and a large explosion. Eyewitnesses hear about 15 to 20 additional explosions and observed that the last one was projected almost horizontally to the north.

S. Sekiya and Y. Kikuchi from the Imperial University of Tokyo visited Bandai Volcano within days of the eruption. After spending several months studying the new crater and area of devastation, they published a report (in English) that is a classic of volcanology. They described a "deluge of rock and earth" (debris avalanche) that descended the north side of the mountain and covered several villages and killed 461 people. They described conical hills and small cones up to 15m in height "standing out from the debris like so many miniature Fujiyamas." They also described "heated blasts of steam and air (lateral blasts) thickly mixed with dust and rock fragments fierce enough to crush the trees and strip them of not only of the branches but even of their bark." In a drainage basin on the east side of the mountain these blasts had felled a forest so that "trees with a diameter of more than a meter had been laid prostrate on the ground in thousands."

Although the Bandai eruption was similar in many ways to the recent eruption of Mount St. Helens, there were some significant differences. Seikiya and Kikuchi noted that the destructive agency was merely the "sudden expansion of imprisoned steam, unaccompanied by lava flows or pumice ejection." A study of the eruption by Yoichi Nakamura of Utsonomiya University in Japan in the 1970's confirmed the absence of juvenile material (new magma). The lateral blast and debris avalanche deposits at Mount St. Helens, however, contain about 100 million cubic meters of juvenile material. The debris-avalanche and blast-affected areas at Bandai are both considerably smaller than those areas at Mount St. Helens. Blast deposits were not documented on the north side of the volcano by Seikiya and Kikuchi, even though the lateral blast was observed to be directed primarily to the north; likely most of the blast deposits were lost amid the hummocks of the debris-avalanche deposit. They were found only where the blast explosions spilled over the south flank of the volcano.

Today Bandai Volcano and the area around it is a ski and vacation resort. The area is heavily vegetated, and the only signs of the catastrophic eruption are the horseshoe-shaped crater and the debris-avalanche hummocks. The trees "laid prostate on the ground in thousands" are nowhere to be found, and the blast deposit is not easily recognized."