PVMBG reported increased activity at Tengger Caldera’s Bromo cone in a 13 December press release. Emissions that day were white, gray, and brown, had variable densities, and rose as high as 900 m above the summit. The plumes drifted in multiple directions. The report noted that tremor was continuous and accompanied in December by three volcanic earthquakes. Deformation data indicated inflation in December. Daily white emissions that rose as high as 700 m above the summit and drifted in multiple directions were visible through 18 December. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to stay outside of a 1-km radius of the crater.

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)