People were eating lunch by the summit shrine when it went up, a few minutes before noon — a clear Saturday in late September, the busiest hour on a popular 3,067 m walk. Sixty-three died, most of them hikers near the top. This was 2014. Searchers went back through the ash the following summer; five people were never found.
The post-mortem findings were mostly blunt injury rather than burns or gas — rock thrown out of the vent at speed, some of it fist-sized, arriving with no time to take cover. That's why the mountain huts now keep helmets in stock, and why the shelters were rebuilt to take an impact from above.
Ontake is a pilgrimage mountain before it's a hiking one — people have been walking those trails in white robes for a couple of centuries. The routes came back only in stages, a section at a time, over years. Quiet work, that.