Activity during 13 August-12 September. "No more strong ash emissions occurred after 29 July, although prolonged and weak emissions have been reported. We assume that the record of weak emissions is incomplete due to weather conditions and frequent snowfall.

"After a strong phase until about 20 August, the amplitude of harmonic tremor decreased, but tremor remained continuous and maintained 5 Hz as a dominant frequency. Occurrence of seismic events reached a mimimum 17-23 August, with no events above the tremor level for >40 hours. Seismicity then increased again, reaching a maximum of ~120 events on 9 September. During this maximum, several long-period events were recorded, quite similar to but smaller than those observed before November 1985. About half of the remaining events were of high frequency (A-type) and most of the rest were B-types.

"COSPEC measurements resumed on 15 August. Rates of SO₂ emission ranged between 500 and 12,000 t/d, showing some correlation with tremor amplitude.

"The pattern of deformation changed in comparison to all previous months of 1986. By the end of August, most dry-tilt stations began to show fluctuating changes, with amplitudes 2-4 times as large as those observed since December 1985. Cumulative resulting vectors pointed approximately to the summit area."

Geological Summary. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: H. Meyer, INGEOMINAS, Manizales.