PVMBG reported that Merapi observers at the Ngepos post noted white plumes rising 300 m above the lava dome on 15 April. Seismicity increased during 18-20 April. During 0426-0440 on 20 April an explosion occurred and rumbling was heard in areas as far as 8 km away. Ash plumes were observed from some observations posts, but foggy conditions prevented views from others. Based on satellite images and wind data, the Darwin VAAC reported that an ash plume rose to an altitude of 10.7 km (35,000 ft) a.s.l. and drifted 260 km WNW. PVMBG noted that ashfall was reported in areas within 15 km S, SW, and W. The Alert Level remained at 1 (on a scale of 1-4).

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), Darwin Volcanic Ash Advisory Centre (VAAC)