On 4^th December 2021 at around 2pm local time, Semeru volcano, located in Eastern Java, Indonesia, generated lahars and large flows of volcanic rocks and ash. Badan Nasional Penanggulangan Bencana (BNPB), the Indonesia national disaster management authority, reported that the volcanic activity has killed more than ten people and injured dozens. The Center for Volcanology and Geological Hazard Mitigation (CVGHM), which monitors Semeru and all Indonesian volcanoes, maintains the volcanic alert level at 2 (advisory) on a scale of 1 to 4.


Map showing the location of the active and potentially active volcanoes of Southeast Asia (back triangles), including Semeru (red triangle) located in Eastern Java, Indonesia. Data from Whelley et al., 2015 (Source: Earth Observatory of Singapore)

Semeru is the tallest volcano in Java and one of the most active volcanoes in Indonesia. Its current eruptive period started in April 2014, with frequent ash plumes, pyroclastic flows, and rock avalanches. Some incandescence can often be seen at the summit crater where a lava dome has grown, forming lobes towards the south of the volcano.

This lava dome forms as the viscous lava accumulates at the summit instead of flowing down the slopes of the volcano. When the lava dome becomes unstable, it can collapse and generate hot and fast-flowing currents of rocks and ash called pyroclastic density currents (PDCs).

In a press release published on 5 December, CVGHM reported that the PDCs on 4 December were caused bythe instability of the lava lobes, and that they extended for about two kilometres (km) beyond the end of the lava flow to the southeast of the volcano, or four km from the crater. CVGHM added that the PDCs were preceded by lahars, or mudflows, which generated ground vibrations recorded by CVGHM’s monitoring network. Despite these events, the scientists do not report an increase in seismic activity characteristic of a supply of fresh magma.


Seismic activity recorded at Semeru from 6 September to 3 December 2021. The red bars represent eruption; purple, pyroclastic density current; light blue, rock fall; dark blue, degassing; grey blue; harmonic tremor; light green, shallow volcano tectonic; dark green, deep volcano tectonic; dark orange, local tectonic; yellow, felt earthquakes; light orange, regional tectonic; and brown, lahar. (Source: MAGMA Indonesia, PVMBG)

Even though the infrasound monitoring sensors located in Singapore can record eruptions thousands of kilometres away, the sensors did not record the activity of Semeru on 4 December. This could be due to the source mechanism of the activity and/or to the wind conditions.


Semeru volcano captured by one of the CVGHM’s monitoring cameras on 5 December 2021 (Source: Magma Indonesia, PVMBG)

For official updates on the activity of Semeru and the other Indonesian volcanoes, please follow the @PVMBG_ and @id_magma Twitter accounts.