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October 25, 2023

Dome Collapse and Pyroclastic Flow at Unzen Volcano, 1991

Mount Unzen is perhaps most famous for its destructive and fatal eruption on June 3, 1991 at 4:08 pm. This eruption caused the first large-scale pyroclastic flow, unprecedented at the time, which killed 43 people in the evacuation zone. Among these were French volcanologists Maurice and Katia Krafft, as well as American geologist Harry Glicken. The other forty fatalities consisted largely of those involved in the mass media, as well as firefighters, police officers, farmers and taxi drivers.

After 3:30 pm on June 3, small and medium-sized pyroclastic flows occurred frequently, and the first large-scale pyroclastic flow occurred at 3:57. Although this pyroclastic flow (and its resulting pyroclastic surge) did not reach the “fixed point”, visibility in the vicinity deteriorated significantly as volcanic ash from the dispersing cloud covered the surroundings, in addition to the rainfall that had continued from the morning.

At 4:08 pm, the second large pyroclastic flow occurred, descending the Mizunashi River Valley and reaching 3.2 km (2.0 mi) to the east from the lava dome. Although the main body of the flow continued to follow the river, the resulting pyroclastic surge spread out in a fan shape once it had exited the valley mouth and continued further, hitting Kita-Kamikoba town (4 km (2.5 mi) away from the lava dome) and finally stopping near the Tsutsuno bus stop (5 km (3.1 mi) away). The pyroclastic flow also flowed down the Akamatsu-dani River, but changed direction due to a gust of wind from the south, allowing the residents, firefighters, and the shooting staff (who left their camera behind to capture world-famous footage of the eruption) to immediately escape the eruption.

At the “fixed point”, members of the media initially left their chartered taxis and company cars on the road with their engine running, facing south so that they could escape immediately in case of an unforeseen event. However, the field of view was poor due to dispersing ash from the previous pyroclastic flow, as well as due to the presence of rain, and it was almost impossible to evacuate even from the windward side, as the pyroclastic surge that flowed from the Akamatsu-dani River would have cut them off. A fire brigade member of the agricultural training station, several hundred meters away from the “fixed point”, assumed incorrectly that the roaring of the pyroclastic flow was caused by a debris flow instead, and was engulfed by the pyroclastic surge when he left the training station to check the Mizunashi River. Many members escaped from the evacuation advisory area on their own, but suffered severe burns and airway damage. As a result, 16 members of the media (including part-time students), three volcanologists (the Kraffts and Harry Glicken), 12 firefighters who were on guard, four taxi drivers, two police officers who came by police car to conduct evacuation guidance, two workers who were removing the city council election poster bulletin board, and four residents who were working in agriculture were killed. The disaster resulted in 43 dead or missing, and nine injured. The photographer of the Yomiuri Shimbun newspaper died holding his Nikon F4. The camera recorded seven frames of the pyroclastic flow, although it was discolored by heat. All of the areas surrounding the “fixed point” where many casualties occurred, were within the advised evacuation zone.

A professional video camera used by an NTV cameraman who died in the pyroclastic flow was discovered in June 2005. The camera melted due to the high temperature generated by the pyroclastic flow and was severely damaged, but the tape it contained was playable, despite having been buried under volcanic deposits for almost 15 years. The video received from the camera shows the reporters observing the first pyroclastic flow, and then continuing to report after having moved their position to the “fixed point”. It also displays the police cars that announced evacuation, all unaware of the approach of the second large pyroclastic flow. The footage and sound were recorded and recovered, although heavily damaged and warped (the video ends when the cameraman notices a sound apparently associated with the generating pyroclastic flow, and asks “What’s that sound?”, whilst turning the camera in the direction of Mt. Fugen). This video was broadcast on October 16, 2005, and is now exhibited at the Unzendake Disaster Memorial Hall (Shimabara City) alongside the melted camera.

It is unclear as to what caused the unprecedented size of the second, large-scale pyroclastic flow on June 3. It has been suggested that the heavy ash-laden column that towered over the main flow, collapsed during its descent, causing the surge. It has also been pointed out that when the dome collapsed, 0.5 million cubic meters of hardened lava broke off, leaving a significant collapse scar afterwards, suggesting that the size of the dome collapse (rather than the column collapse) initiated the surge. Other explanations account the fact that previous lahars and pyroclastic flows filled the Mizunashi Valley, raising the level of the river-bed and making it more likely for the flow to “skip” ridges that would have once stopped it.

According to the 2011 documentary Face au Volcan Tueur (French for Facing the Killer Volcano), the unexpected surge could have been caused by phreatic explosions as the main body of the pyroclastic flow proceeded down two waterfalls at the headwaters of the river, fragmenting the ash-particles of the avalanche and expanding the energy of the overlaying gas and ash cloud.


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