PVMBG reported that observers in Ndokum Siroga, about 8.5 km away, noted gray plumes rising 1 km above Sinabung on 6 December. Grayish-white plumes rose as high as 400 m on 7 December, and dense white plumes also rose as high as 400 m the next day. Dense grayish-to-white plumes rose 70-200 m on 9 December. White plumes rose 100-150 m above the crater during 10-13 December. Tremor during 6-13 December was recorded continuously, with varying amplitude. The number of low-frequency earthquakes significantly increased on 7 December, and the number of hybrid earthquakes increased the next day. RSAM values had steadily increased since 28 November. The Alert Level remained at 4 (on a scale of 1-4).

Geological Summary. Gunung Sinabung is a Pleistocene-to-Holocene stratovolcano with many lava flows on its flanks. The migration of summit vents along a N-S line gives the summit crater complex an elongated form. The youngest crater of this conical andesitic-to-dacitic edifice is at the southern end of the four overlapping summit craters. The youngest deposit is a SE-flank pyroclastic flow 14C dated by Hendrasto et al. (2012) at 740-880 CE. An unconfirmed eruption was noted in 1881, and solfataric activity was seen at the summit and upper flanks in 1912. No confirmed historical eruptions were recorded prior to explosive eruptions during August-September 2010 that produced ash plumes to 5 km above the summit.

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