PVMBG reported that during 1-15 January incandescent avalanches of material from the Jonggring Seleko Crater at Semeru sometimes traveled 500-1,000 m down the Kobokan drainage on the SE flank. Incandescent material was ejected 10-30 m above the summit and white-and-gray plumes rose 200-300 m and drifted N. Weather conditions often prevented visual observations. A pyroclastic flow was detected at 1451 on 1 January, though weather clouds prevented visual confirmation. At 1724 on 16 January incandescent avalanches traveled as far as 1 km down the Kobokan drainage and a pyroclastic flow traveled about 4-4.5 km down the same drainage. A large ash cloud rose along the length of the pyroclastic flow to 2 km above the summit and drifted NE and N. Ashfall was reported in areas to the N. During 18-19 January dense gray-white plumes rose 300-500 m above the summit and drifted N. Rumbling was heard and incandescent material was ejected 30 m high. 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.

Sources: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM), Badan Nacional Penanggulangan Bencana (BNPB), The Jakarta Post