PVMBG reported that the eruption at Semeru continued during 9-15 December. Incandescent material was ejected 50-100 m above the summit on 9 December and rock avalanches descended the Kobokan drainage 500 m on the SE flank. Explosions were recorded by the seismic network during 10-12 December but they could not be visually confirmed due to weather conditions. On 13 December a pyroclastic flow originating from the end of a lava flow traveled 1.5 km down the Kembar drainage (S flank), reaching a maximum distance of 3.5 km from the crater. The next day a pyroclastic flow traveled 3.5 km down the Kobokan drainage. Rock avalanches from the end of a lava flow traveled 300-1,500 m down both drainages during 14-15 December. The Alert Level remained at 2 (on a scale of 1-4), with a general exclusion zone of 1 km and extensions to 4 km in the SSE sector.

Geological Summary. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)