CVGHM reported that pyroclastic flows and rockfall avalanches from Semeru detected by the seismic network declined in frequency during 22 May-3 June. On 22 May, four pyroclastic flows traveled a maximum distance of 2.5 km from the active crater. Visual observations of smaller rockfalls detected during the rest of the reporting period were inhibited by fog, but were observed to travel 200-300 m from the active crater. Based on visual observations and the decline in seismic activity, CVGHM lowered the Alert Level to 2 (on a scale of 1-4) on 5 June.

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)