BPPTKG reported that the “2021 lava dome” continued to effuse just below Merapi’s SW rim during 22-28 January, producing a total of 230 incandescent lava avalanches that traveled as far as 1.5 km down the Krasak and Boyong river drainages on the SW flank. Pyroclastic flows were recorded 71 times and descended the Boyong drainage as far as 3.5 km. A comparison of photos taken on 21 and 26 January showed that the morphological changes in the summit area were attributed to the emergence of lava domes. The 2021 dome volume was an estimated 157,000 cubic meters on 25 January, but avalanches and pyroclastic flows during 26-27 January reduced the volume to 62,000 cubic meters based on 28 January estimates. Deformation continued, though at a lower rate; Electronic Distance Measurement (EDM) data showed a distance shortening between points in the NW at a rate of 0.4 cm per day.

There were 36 pyroclastic flows recorded during 0000-1400 on 27 January; the longest pyroclastic flow traveled 3 km down the Krasak and Boyong drainages at 1253. The corresponding ash plume rose into low weather clouds that obscured the summit. At 1335 pyroclastic flows traveled 1.5 km SW. Sirens were triggered in Ngrangkah, Umbulharjo, Cangkringan, and Sleman (along the Boyong river drainage), alerting about 150 residents to temporarily evacuate. Ashfall was reported in several villages in the Tamansari and Musuk districts, Boyolali Regency, Boyolali City, and several locations in Klaten. The Alert Level remained at 3 (on a scale of 1-4), and the public were warned to stay 5 km away from the summit.

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), Detik News, Detik News, Detik News, Detik News, Detik News