The slowly growing lava dome in the summit crater reached 6.6 x 10⁶ m³ by late 1989, a size described as "critical" for the collapse-prone dome. A white plume rose as much as 250 m from the crater's solfatara field, but under little pressure. COSPEC measurements showed an average SO₂ flux of 73 t/d, up slightly from August. No lahar occurred during September, but alert status has been increased with the coming rainy season. Seismicity generally increased in September from August values (table 3). No A-type events were recorded.

Table 3. Seismicity recorded at Merapi, January-February and August-September 1989.

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: VSI.