An analysis of satellite imagery of yesterday’s deadly eruptive events at the volcano and well-founded interpretations of the available data on the French blog Culture Volcan suggest that what most media now describe as a „massive eruption“ was actually not involving any significant explosive activity and casts strong doubts on the reported ash plume, which might have been a weather or other cloud not from Semeru, because it seems to originate 20 km away from the volcano and dissipated very quickly without much trace. Otherwise, the ash plume should have crossed the weather cloud layer and become much more visible. In fact, the images available only confirm that there were one or several massive pyroclastic flows.
If Culture Volcan’s suggestions are correct, it implies that – as catastrophic as the event for the affected area was – there was no new eruption or significant change in the eruption dynamics at the crater. Yesterday’s eruptive events were mostly a result of gravitational instability that had accumulated over time and triggered a larger collapse of lava material of the summit lava dome and/or of the largely lava-filled Koboan ravine on its southestern slope, into which a viscous lava flow from the dome had been flowing for years. In fact, the volcano has been in a continuous eruption for decades.