Eruptive activity increased markedly at Merapi during the period of 26 December 2000-22 January 2001. Instrumental monitoring first recorded a significant increase in seismicity, expressed in both shallow and deep volcanic earthquakes, during 26 December-1 January. Visual observations were hindered during this time because of hazy conditions, and VSI maintained a hazard status of 2 (on a scale of 1-4) for Merapi.

Activity continued to increase during 2-8 January. Atmospheric conditions were clearer, allowing observation of a 1,500-m-high plume above the summit. Lava avalanches flowed ~1 km from the summit down to the Sat River. Seismicity remained high, again with a significant number of shallow and deep volcanic earthquakes, and was dominated by multi-phase and avalanche earthquakes.

During 9-15 January, activity again increased with respect to the previous week. Accordingly, VSI elevated Merapi's hazard status to 3. Observers noted a light-colored, variable-density, low-pressure ash plume that rose 500 m above the summit. Glowing lava avalanches flowed into the headwaters of the Lamat, Sat, and Senowo Rivers, up to 2 km from the summit. On 14 January, 29 pyroclastic flows traveled down the volcano's flanks into the three above-mentioned rivers and reached up to 4 km from their source. During the week, lava avalanches and pyroclastic flows occurred with an average interval of 0.5-1 hours.

Visual observations from several post observatories during 16-22 January revealed ash eruptions, glowing lava flows and avalanches, and pyroclastic flows. Merapi ejected a dense, light-colored ash plume under medium to high pressure. Ash rose 850-1,300 m above the summit, with an estimated emission volume of 95 metric tons/day. Ashfall occurred on the surrounding areas of Babadan, Kaliurang, and Ngepos. Glowing lava avalanches, with more than 150 occurring per day, reached as far as 3.5 km from the summit into the Bebeng, Sat, and Senowo Rivers. Observers suggested more than one source vent for these flows. More than 20 pyroclastic flows occurred daily during the week, sending ash and gas a maximum of 3 km down the Bebeng River, 4.5 km down the Sat River, and an unreported distance down the Senowo River.

The Darwin VAAC issued an ash advisory on 19 January to advise pilots of ash emanating from Merapi. The advisory reported an ash plume up to an altitude of ~3,400 m. Prevailing winds were projected to carry ash to the E or SE; cloud cover prevented any further descriptions.

A new lava dome, termed "2001," grew on top of the 1998 dome that had collapsed around 16 January. Growth appeared continuous with the glowing dome visible at night. Researchers speculated that the failure of the 1998 dome and the instability of the new dome accounted for the high frequency and volume of pyroclastic flows.

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: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).