PVMBG reported that the eruption at Semeru continued during 18-25 April and frequent Volcano Observatory Notices for Aviation (VONAs) describing ash emissions were issued through the week. On 19 April at 0710 and 0829 dense white-and-gray ash plumes rose 800-1,000 m above the summit and drifted S. On 20 April at 0616, 0619, 0805, and 0902 white-and-gray variable density ash plumes rose 300-1,000 m and drifted N and NW. At 0534 on 21 April a white-to-brown ash plume rose 600 m and drifted NE and at 0640 a dense white-and-gray ash plume rose 700 m and drifted SW. On 23 April at 0448, 0553, 0643, and 0731 gray ash plumes of variable densities rose 400-1,000 m and drifted SE, S, SW, and W. On 25 April at 0519, 0710, and 0756 dense gray ash plumes rose 500-800 m and drifted NW, W, and SW. 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 in all directions, 13 km from the summit to the SE, 100 m from the banks of the Kobokan drainage as far as 17 km from the summit, and to avoid 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)