PVMBG reported that the lava dome at Merapi slowly grew during 16-22 September and was an estimated 468,000 cubic meters, based on 19 September measurements from drone photos. Extruded lava fell into the upper parts of the SE flank, generating block-and-ash flows that traveled as far as 1.3 km down the Gendol drainage during 17 and 20-21 September. Diffuse white plumes rose as high as 100 m above the summit. At 1136 on 22 September the seismic network began recording signals indicating pyroclastic-flow generation, that lasted two minutes and five seconds; pyroclastic flows traveled 1.2 km down the Gendol drainage. An ash plume rose around 800 m above the summit and caused minor ashfall in areas as far as 15 km SW. Temperature increases at several points on the lava dome were recorded about one hour before the event. The Alert Level remained at 2 (on a scale of 1-4), and residents were warned to stay outside of the 3-km exclusion zone.

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.

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