PVMBG reported that on 25 October at 1004 an eruption at Rinjani generated an ash plume that rose 200 m above the Barujari crater, inside the caldera. Ash fell on the flanks, especially to the SW. The report noted that no seismic nor surficial precursory events were detected. The Alert Level was raised to 2 (on a scale of 1-4). Based on satellite observations, the Darwin VAAC reported that on 26 and 28 October ash plumes rose to altitudes of 3-4 km (10,000-13,000 ft) a.s.l. and drifted 45-75 km SW and WSW.

Geological Summary. Rinjani volcano on the island of Lombok rises to 3726 m, second in height among Indonesian volcanoes only to Sumatra's Kerinci volcano. Rinjani has a steep-sided conical profile when viewed from the east, but the west side of the compound volcano is truncated by the 6 x 8.5 km, oval-shaped Segara Anak (Samalas) caldera. The caldera formed during one of the largest Holocene eruptions globally in 1257 CE, which truncated Samalas stratovolcano. The western half of the caldera contains a 230-m-deep lake whose crescentic form results from growth of the post-caldera cone Barujari at the east end of the caldera. Historical eruptions dating back to 1847 have been restricted to Barujari cone and consist of moderate explosive activity and occasional lava flows that have entered Segara Anak lake.

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)