The VSI reported that several small explosions occurred at Bromo cone in the Tengger caldera. The explosions started on 29 November, increasing in intensity until 3 December. During this period ash was ejected up to 100-150 m above the crater rim. At 2130 on 4 December small-scale explosions began, sending ash up to 900 m above the crater rim to the NNE and depositing 1- to 3-cm-thick layers of ash up to 40 km from the volcano. As of 0600 on 5 November only small ash explosions were reported. There was no noted precursory activity; prior to 29 November daily activity at the volcano consisted of small ash plumes that rose up to 50 m above the crater. Seismic data were not available. On 29 November the local government recommended that no one climb the volcano. The Alert Level is at 2 (ranging from 1 to 4).

Geological Summary. The 16-km-wide Tengger caldera is located at the northern end of a volcanic massif extending from Semeru volcano. The massive volcanic complex dates back to about 820,000 years ago and consists of five overlapping stratovolcanoes, each truncated by a caldera. Lava domes, pyroclastic cones, and a maar occupy the flanks of the massif. The Ngadisari caldera at the NE end of the complex formed about 150,000 years ago and is now drained through the Sapikerep valley. The most recent of the calderas is the 9 x 10 km wide Sandsea caldera at the SW end of the complex, which formed incrementally during the late Pleistocene and early Holocene. An overlapping cluster of post-caldera cones was constructed on the floor of the Sandsea caldera within the past several thousand years. The youngest of these is Bromo, one of Java's most active and most frequently visited volcanoes.

Sources: Darwin Volcanic Ash Advisory Centre (VAAC), Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)