PVMBG reported that the eruption at Semeru continued during 20-26 May, though sometimes foggy conditions prevented visual confirmation. Incandescent material from the ends of lava flows descended 400-700 m in the Kembar drainage (on the S flank) during 20-21 and 25-26 May, reaching a maximum distance of 1.2 km from the crater. Ash plumes rose 200-500 m above the crater and drifted S during 19-20 May and N during 24-25 May. White plumes rose 400 m and drifted NE, E, S, and W on 26 May. The Alert Level remained at 2 (on a scale of 1-4), and the public was reminded to stay outside of the general 1-km radius from the summit and 4 km on the SSE flank.

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)