Report on Merapi (Indonesia) — May 1999
Bulletin of the Global Volcanism Network, vol. 24, no. 5 (May 1999)
Managing Editor: Richard Wunderman.
Merapi (Indonesia) Frequent lahars, lava avalanches, and pyroclastic flows during March-May
Please cite this report as:
Global Volcanism Program, 1999. Report on Merapi (Indonesia). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 24:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199905-263250.
7.54°S, 110.446°E; summit elev. 2910 m
All times are local (unless otherwise noted)
Merapi remained active throughout the reporting period of 9 March through 24 May. Although no deaths were reported, the volcano continually threatened surrounding populated areas with lahars, lava avalanches, and pyroclastic flows. Throughout the period, Merapi exhibited weakly pressured, thick, white, sulfur-tinted ash plumes extending 100-700 m above the summit.
During the week of 9-15 March a vigorous "white ash plume" was observed, weakly pressured, with 550 m maximum height above the summit. Observers additionally identified lava glowing along the SW flank, in the direction of the Blongkeng, Lamat, and Sat drainages (the maximum run-out distance was 0.8 km). On 11 March a small pyroclastic flow was noted traveling SW with 0.8 km of run-out distance. During this week multiphase events dominated seismic activity, presumably resulting from the emission of lava and glowing debris. A small lahar traveled down the Sat drainage on 13 March.
During 16-22 March, ash emissions and lava avalanches continued. Avalanches again traveled SW, with maximum run-out distances of 0.4 km. Glow at the lava dome was weak. Surface shocks (assumed to be from lava avalanches) dominated seismicity. From 23 to 29 March the lava avalanches continued in the direction of the SW-flank rivers with 1.8 km maximum run-out distances. These drainages glowed at night. On 24 March a small pyroclastic flow started at the edge of the lava dome, moved in the direction of the Sat River, and attained a 0.6 km run-out distance; no glow was observed at the dome. Seismicity was dominated by surface events mostly interpreted as lava avalanches.
During the week of 27 April to 3 May, sulfur-tinted ash plumes reached their highest point during the recording period, 700 m above the summit. Lava avalanches continued in the direction of Blongkeng, Sat, and Lamat with 0.9 km run-out distances; again night glow was observed in these drainages. In contrast, the body of the lava dome lacked glowing areas. A small pyroclastic flow from the edge of the 1998 lava moved downslope SW for a distance of 1.3 km. Surface events continued to dominate seismicity. A small lahar buried two trucks and a digging machine at Putih on 1 May; no injuries were reported.
Lava avalanches during 4-17 May continued towards the SW-flank drainages with 1-km maximum distances each week. There was a weak glow on the lava dome during the first week and none the second. There was one small pyroclastic flow each week from the edge of the 1998 lava, which again moved SW out to 1 km distance. From 18 to 24 May lava avalanches had 1.8 km run-out distances. Glow within cracks on the lava dome was weak. Seismicity during May continued to be dominated by multiphase shocks and surface events identified as lava avalanches.
Geologic Background. 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 2000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequently growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent eruptive 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 during historical time.
Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).