CVGHM reported that during 22-23 January gray-to-brown plumes from Tengger Caldera's Bromo cone rose 400-800 m above the crater and drifted E. Incandescent material was ejected 200 m above the crater and landed as far as 500 m away on 22 January. The Alert Level remained at 3 (on a scale of 1-4). Residents and tourists were not permitted within a 2-km-radius of the active crater. Based on analysis of satellite imagery and information from CVGHM, the Darwin VAAC reported that on 24 January an ash plume rose to an altitude of 3.7 km (12,000 ft) a.s.l. and drifted more than 220 km E. A high concentration of sulfur dioxide in the area was also detected.

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)