PVMBG reported that the eruption at Semeru continued during 1-7 February. White-and-gray ash plumes that were somewhat dense rose 300-700 m above the summit and drifted N, NE, S, and SW. One of the ash plumes, recorded at 0729 on 1 February, was a dense white-and-gray plume that rose 700 m and drifted NE. At 0700 on 2 February an ash plume rose 600 m and drifted NE. On 3 February white-and-gray plumes rose 100-300 m and drifted S, SW, W, and N. An eruptive event at 1242 on 5 February produced a white, gray, and brown ash plume that rose around 1.5 km and drifted N. Webcam images showed a pyroclastic flow descending the SE flank and ash plumes rising its path. The Alert Level remained at 3 (on a scale of 1-4). The public was warned to stay at least 5 km away from the summit, and 500 m 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)