A massive eruption occurred from the volcano this afternoon at around 4.25 p.m. local time, producing a large pyroclastic flow (hot avalanche of ash and rock particles) that traveled down the southern slopes, as well as producing an ash column that rose to estimated 40,000 ft (12 km) altitude.
From videos and photos circulating on social media, the pyroclastic flow reached farmland but also inhabited areas, and it must be feared that it claimed victims and caused extensive damage.
We will update on this as soon as more precise information becomes available.
Judging from the imagery, the most likely scenario is that a large portion of the accumulated lava dome in the breached summit crater has collapsed into a violent avalanche, maybe triggered by or triggering a vertical explosion. The lava dome had been moderately active and slowly growing for years, at varying intensity. The event of today apparently came as a surprise, but such dome collapse events can usually not be predicted when they occur.
Mt. Semeru, Indonesia this afternoon. pic.twitter.com/dqGuOpApK4
— DARYONO BMKG (@DaryonoBMKG) December 4, 2021