Report on Merapi (Indonesia) — December 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 12 (December 1981)
Managing Editor: Lindsay McClelland.
Merapi (Indonesia) Sixty percent of the summit dome destroyed; hot avalanches
Please cite this report as:
Global Volcanism Program, 1981. Report on Merapi (Indonesia) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 6:12. Smithsonian Institution.
7.54°S, 110.446°E; summit elev. 2910 m
All times are local (unless otherwise noted)
Landsliding on the summit lava dome 29 November was followed by a nuée ardente d'avalanche that flowed 3-4 km down the Batang River valley. Ash fell 15-20 km to the NW. An avalanche of incandescent lava from the dome set fire to 15 hectares of tropical mixed forest. On 1 December approximately 3/5 of the lava dome slid down the upper flank, flooding the Krasak and Batang Rivers and adding 1.2 x 106 m3 to the deposits from previous debris flows. As of early December, there were about 6.9 x 106 m3 of unstable material in the summit area. Authorities feared that monsoon rains could cause large cold lahars, and officials of Magelang District, W of Merapi, remained on constant alert in early December. AFP reported that the volcano continued to emit ash in mid-December.
A tripartite electro-seismometer at Babadan Observatory and two single-detector seismographs near the volcano recorded continuous trains of tremors and very shallow earthquakes that may have been landslide events. Continuous records of magnetic values showed no significant changes. No sharp variations in CO2 and H2S emissions were detected. Tilt measurements showed deformation of 10-20 µrad, within the normal variation.
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.
Information Contacts: A. Sudradjat, VSI; AFP; Jakarta DRS.