Activity increased in February compared to the previous month. The number of volcanic earthquakes increased, the lava dome grew higher, and red glares were observed at 2 points on the dome. The number of glowing avalanches decreased from January, however, and there was only one nuée ardente, on 4 February, that traveled about 750 m SW into the Batang River.

On 5 March at 1503, reddish white smoke emitted with strong gas pressure was observed to about 350 m above the summit. The volcano was hidden behind clouds. On 6 March, small avalanches took place continuously from 0135 until 0348, then the volcano was covered by clouds. At 1702, three avalanches were observed. From 1733 on, bigger nuées ardentes took place, interspersed with minor ones, until 13 March (table 1).

Table 1. Daily number of nuées ardentes and maximum distances traveled from the vent, 6-13 March 1976.

From the evening of 8 March until the evening of 9 March, when the volcano became visible from the Ngepos Volcano Observatory 11 km SW of the summit, it could be observed that the lower part of the lava dome, estimated to be about 400,000 m³, or almost 1/3 of its volume (1.4 x 10⁶ m³), had slid away. During the events on 6 and 7 March nuées ardentes moved SW, entering a tributary of the Batang River and continuing much further down stream into the rivers Sat, Blongkeng, and Bebeng. The prevailing wind blew E, depositing finer volcanic products on the E and SE flanks to a maximum distance of 37.5 km. Thickest ash falls were on the ESE flank. At 6 km from the summit the deposit apparently measured 5 mm and temporarily panicked the villagers.

Nuées ardentes caused forest fires on the upper SW flank near the tributaries of the rivers Sat, Blongkeng, and Bebeng. Roughly 580,000 m³ of ash were deposited on the E flank, and around 300,000 m³ of avalanche material were deposited on the upper SW flank.

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: D. Hadikusumo, Volcanology Division, GSI.