On 11 December an air report to the Darwin VAAC stated that volcanic ash was observed near Semeru at an altitude of ~7.6 km a.s.l. The Darwin VAAC stated that the activity may have been associated with Semeru, which frequently erupts ash to ~4.5 km. Due to cloudy conditions, any low-level volcanic activity that was occurring at that time was not visible in satellite imagery.

Geological Summary. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Source: Darwin Volcanic Ash Advisory Centre (VAAC)