Seismicity under Colombia's Nevado del Ruiz continues at elevated levels, and the volcano is experiencing frequent ash emissions since May 17, 2016.
An eruption at about 10:40 UTC on May 22 sent ash 2.3 km (7 545 feet) above the crater (7.5 km / 24 606 feet a.s.l.) and temporarily closed La Nubia airport in Manizales, the capital of Caldas state.
Similar explosions were also observed on May 17, 18, 19 and 20 when ash fall was reported up to 30 km (18.6 miles) NW of the volcano.
The Volcanological and Seismological Observatory of Manizales, which is part of the Colombian Geological Survey (SGC), said in a statement the seismicity under the volcano remains elevated at fluctuating levels but has been showing a slight decrease overall during the past weeks. It continues to be dominated by internal fluid movements which reflect the interaction of water and magmatic gasses at a shallow depth that occasional results in explosions and more often in a strong degassing plume reaching up to 2 km (6 561 feet).
The observatory noted yesterday that continued ashfall on nearby communities cannot be ruled out and that recent visual assessment of ash emissions was favored by the atmospheric conditions. "Even though this type of seismic signals are presented several times during the day, ash emissions associated cannot always be verified visually."
Between May 10 and 16, seismicity at Nevado del Ruiz was characterized by long-period and very-long-period earthquakes, episodes of continuous tremor, and pulses of volcanic tremor associated with gas-and-ash emissions. Thermal anomalies near Arenas Crater were identified in satellite images on May 11 and 12. Significant amounts of water vapor and gas rose from the crater during the week.
A gas, steam, and ash plume rose 1.7 km (5 577 feet) above the crater and drifted NW and W on May 12. Based on information from SGC, the Washington VAAC reported that on May 17 an ash emission rose to an altitude of 7 km (23 000 feet) a.s.l.
The Alert Level remains at III (Yellow; the second lowest level on a four-color scale).
Nevado del Ruiz is under close observation since the 1985 eruption that killed more than 23 000 people and injured approximately 5 000.
Colombia's worst natural disaster
In November 1984, geologists observed an increased level of seismic activity under the volcano, increased fumarole activity, deposition of sulfur on the summit, and small phreatic eruptions. The activity decreased in October 1985, most likely due to new magma that rose into the volcanic edifice. In November 1985, volcanic activity increased once again as magma neared the surface and the volcano began releasing increasing quantities of gasses rich in sulfur dioxide and elementary sulfur. On November 13, the volcano began erupting, ejecting dacitic tephra more than 30 km (19 miles) into the atmosphere.
The eruption produced pyroclastic flows that melted summit glaciers and snow, generating four thick lahars that mixed with clay as they ran down to river valleys on the volcano's flanks at an average speed of 60 km/h (37.4 mph). After descending thousands of meters down the side of the volcano, the lahars were directed into all of the six river valleys leading from the volcano. While in the river valleys, the lahars grew to almost 4 times their original volume. In the Gualí River, a lahar reached a maximum width of 50 meters (200 feet).
One of the lahars virtually erased the small town of Armero in Tolima. Only one quarter of its 28 700 inhabitants survived. The second lahar, which descended through the valley of Chinchiná River, killed about 1 800 people and destroyed about 400 homes in the town of Chinchiná, in the department of Caldas.
Armero was located in the center of this photograph, taken late November 1985. Taken by Jeffrey Marso, USGS
The Armero tragedy, as the event came to be known, was the second-deadliest volcanic disaster in the 20th century, being surpassed only by the 1902 eruption of Mount Pelée, and is the fourth-deadliest volcanic eruption in recorded history. It is also the deadliest known lahar and Colombia's worst natural disaster.
Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers >200 sq km. 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. (GVP)
Featured image: Nevado del Ruiz erupting on May 20, 2016. Credit: Manizales Observatory (SGC)