PVMBG reported that during 14-22 January brownish gray plumes from Tengger Caldera's Bromo cone rose as high as 1.8 km above the crater. Seismicity fluctuated; the number of explosion signals increased and some shallow and deep volcanic earthquakes were recorded. The Alert Level remained at 3 (on a scale of 1-4); residents and visitors were reminded not to approach the crater within a radius of 2.5 km.

Based on information from PVMBG and satellite images, the Darwin VAAC reported that during 23 and 25-26 January ash plumes rose to altitudes of 3.6-3.9 km (12,000-13,000 ft) a.s.l. and drifted ENE, E, and SE.

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.

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