Gas at the Gendol solfatara field, on the S part of the summit, has been sampled since May 1992 (table 6). The gas analysis from 23 July shows a sharp increase in O₂ + Ar, N₂, CO₂, SO₂, and HCl, and a decrease in H₂O. Concentrations had returned to previous values by the 8 September sampling. VSI geologists noted that the increase in volcanic gases may have been related to pyroclastic flows generated by dome collapse. Blue sublimates were observed around the Gendol G.13 solfatara field during fieldwork at the summit on 8 September.

Shallow earthquakes (1.8 km depth) occurred beneath Merapi on 26 August at 1314 and 1325, with magnitudes of 1.1 and 1.5, respectively. Pyroclastic flows started at 1331, 1335, and 1341, flowing 2.5 km WNW down the upper Senowo River. Volcano observation stations at Selo (~5 km NNE) and Babadan (~4.5 km NW) reported 21 and 14 mm of rainfall, respectively, in the 2 hours before the pyroclastic flows. There were additional smaller pyroclastic flows on 28 August at 1715, 1909, and 1929. Geologists believe that the pyroclastic flows may correlate with rainfall, volcanic gas activity, and seismicity.

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: S. Bronto, MVO.