BPPTKG reported that of Merapi’s two lava domes, the dome just below the SW rim was more active than the dome in the summit crater during 20-26 August. The SW dome grew and shed material down the flanks, increasing in height by just 3 m overall. On 25 August the volume of the SW dome was estimated at 1.4 million cubic meters and the summit lava dome was stable at an estimated 2.831 million cubic meters. A total of two pyroclastic flows descended the SW flank as far as 2 km; as many as 211 lava avalanches traveled a maximum of 2 km SW. Based on satellite images the Darwin VAAC noted that during 28-29 August ash plumes rose to 3-3.7 km (10,000-12,000 ft) a.s.l. and drifted SW, W, and NW. The Alert Level remained at 3 (on a scale of 1-4), and the public was warned to stay 3-5 km away from the summit based on location.

Geological Summary. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Sources: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Darwin Volcanic Ash Advisory Centre (VAAC)