PVMBG reported that the eruption at Semeru continued during 15-21 March. Dense ash plumes were visible almost daily. At 0737 and 0748 on 15 March white-and-gray ash plumes rose 600-800 m above the summit and drifted S and SE. At 0601 on 16 March a white-and-gray ash plume rose 600 m and drifted S, and at 0748 a gray-to-brown ash plume rose 500 m and drifted SW and W. At 0534 on 18 March a white-and-gray ash plume rose 500 m and drifted SW. Just over an hour later, at 0655, a white-and-gray ash plume rose 1 km and drifted S. At 0713 on 20 March white-and-gray ash plume rose 600 m and drifted SW and W, and at 0811 a gray-to-brown ash plume rose 1 km and drifted S. 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 away 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)