On 9 June beginning at 2300 a swarm of volcano-tectonic earthquakes was recorded at Nevado del Ruiz. Next, hundreds of hybrid earthquakes were recorded, with more than 1,300 earthquakes occurring in 16 hours. High seismicity persisted for the following 3 days, with ~2,300 earthquakes recorded. This was the highest daily number of events recorded at Nevado del Ruiz since 1985. At the height of the activity the Alert Level was at Orange. In addition to heightened seismicity felt by residents near the volcano, jet-like sounds were reportedly heard that corresponded with some of the hybrid earthquakes, and the strong scent of SO2 was reported near the summit. No ash emissions were reported. By 13 June seismicity had decreased.

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: El Tiempo, Servicio Geológico Colombiano (SGC)