Servicio Geológico Colombiano’s (SGC) Observatorio Vulcanológico and Sismológico de Manizales reported that during 25-31 August seismicity at Nevado del Ruiz increased; the network detected a large number of long-period earthquakes, and several episodes of tremor associated with gas-and-ash emissions. Water-vapor-and-gas plumes rose 2 km above the crater and were sometimes tinged gray due to the presence of ash. Volcano-tectonic (VT) events occurred at depths between 0.77 and 6.77 km. The largest VT event was recorded at 0144 on 27 August, was a local M 1.1, and was 2.8 km SW of Arenas Crater at a depth of 4.69 km. Periods of very-high-energy tremor were detected on 31 August. According to a news article, La Nubia airport ceased operations on 31 August due to ash emissions.

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.

Sources: Servicio Geológico Colombiano (SGC), W Radio