PVMBG reported that the eruption at Semeru continued during 12-17 January. White steam plumes that were sometimes dense rose as high as 1 km above the summit almost daily, and crater incandescence was visible nightly. Incandescent avalanches traveled as far as 500 m down the Kobokan drainage on the SE flank during 11-12 January. White-and-gray ash plumes rose 300 m during 14-15 January. At 1020 on 16 January a collapse from the end of the active lava flow in the Kobokan drainage produced a pyroclastic flow, and an ash plume that rose 1.5 km and drifted N. An eruptive event at 0534 on 17 January generated an ash plume that a ground observer reported rising 400 m. The Alert Level remained at 3 (on a scale of 1-4). The public was warned to stay at least 500 m away from Kobokan drainages within 17 km of the summit, along with other drainages originating on Semeru, including the Bang, Kembar, and Sat, due to lahar, avalanche, and pyroclastic flow hazards.

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)