The ash cloud rose from Showa crater, located just beyond the summit as viewed from the lookout. Part of the ash cloud ascended into the atmosphere while some of the cloud flowed along the surface in a pyroclastic density current. A pyroclastic density current is a gravity-driven movement of hot gas and volcanic material. (See this interesting twitter conversation about the difference between the terms pyroclastic density current, pyroclastic flow, and pyroclastic surge). The video below shows the initial eruption cloud as it developed.
The eruption column continued to grow as multiple pulses of ash were emitted from the crater. It wasn’t long before the cloud was carried downwind and started depositing ash.
Up until this point, the eruption was relatively quiet. Most of the sounds were caused by rockfalls and explosively ejected bombs. Then the volcano made an eerie rumbling noise as the flank began to move downslope.
You can hear the collective oohs and aahs as the volcanologists observe the eruption with reverence and awe.
At this point, we thought the show was over.
Soon after the initial pulse, the deep rumbling sounds began again, and we were treated to a second, larger blast. The ash cloud was much darker and we could see large bombs raining down near the base.
The second column reached higher in the atmosphere, up to about 3 km is what I heard from one of the volcanologists on the field trip. The volcano continued emitting pulses of black pyroclastic material and was still erupting by the time I had to leave the lookout.