BPPTKG reported that the lava dome just below Merapi's SW rim continued to grow and shed material down the flank during 19-25 March. The 2021 lava dome volume was an estimated 949,000 cubic meters on 25 March, with a growth rate of about 13,300 cubic meters per day. The Mount Merapi Selo Observation Post reported a white gas-and-steam plume rose 500 m above the crater on 25 March at 1430. Four pyroclastic flows traveled a maximum of 1.8 km down the SW flank, three of which occurred on 27 March at 0602, 0603, and 0631. An ash plume caused by a pyroclastic flow rose 3.5 km (12,000 ft) a.s.l. and drifted SW for about 1.3 km on 27 March at 1758 on, according to a ground observer. Incandescent avalanches, recorded 104 times, traveled as far as 1.2 km down the SW flank and twice down the SE flank as far as 400 m. The summit lava dome measured 65 m tall, similar to the previous week. On 30 March there were 33 incandescent avalanches observed traveling up to 1 km down the SW flank, one of which continued up to 1.5 km. One incandescent avalanche was observed from the lava dome. 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: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM), Badan Nacional Penanggulangan Bencana (BNPB), Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG)