PVMBG reported that the eruption at Semeru continued during 12-18 April and daily Volcano Observatory Notices for Aviation (VONAs) describing ash emissions were issued through the week. At 0747 on 12 April a dense white-to-gray plume rose 600 m above the summit and drifted N. On 13 April at 1135 a gray-to-brown ash plume rose 400 m and drifted NE, at 1249 a dense gray ash plume rose 500 m and drifted N and NE, and at 1709 a dense white-to-gray ash plume rose 600 m and drifted E. At 0529 on 14 April a white-to-brown ash plume rose 500 m and drifted S. White-to-gray ash plumes at 0606 and 0731 on 15 April rose 600-700 m and drifted N and NE. At 0551 on 16 April a dense white-to-gray ash plume rose 1 km and drifted N. On 17 April at 0614 a medium-density white-to-brown ash plume rose 700 m and drifted S and at 0847 a white-to-gray ash plume rose 600 m and also drifted S. At 0540 and 0708 on 18 April dense white-to-gray ash plumes rose 500-600 m and drifted S and W. 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)