1570, 1595, 1623, 1805, 1826, 1828 (perhaps), 1829, 1831, 1833 (perhaps), 1845, 1916, December 1984 – March 1985, September 1985 – July 1991, and possibly in April 1994. Many of these eruptions involved a central vent eruption, a flank vent eruption, and a phreatic (steam) explosion. Ruiz is the second-most active volcano in Colombia after Galeras. The 1595 eruption was the last major eruption of Nevado del Ruiz before 1985. The 1595 and 1985 eruptions were similar in many respects, including in the chemical composition of the erupted material. This mudflow flowed down the valley of the Lagunillas River for approximately , spilling out of the river channel and killing much of the local population.
In November 1985, volcanic activity once again increased
Eruption and lahars
thumb|upright=2|Hazard map for Nevado del Ruiz, with 1985 [[lahars shown in red]]
At 3:06 pm, on November 13, 1985, Nevado del Ruiz began to erupt, ejecting dacitic tephra more than into the atmosphere. The total mass of the erupted material (including magma) was 35 million tonnes The eruption reached a value of 3 on the Volcanic Explosivity Index. The mass of the ejected sulfur dioxide was about 700,000 tonnes, or about 2% of the mass of the erupted solid material,
The eruption produced pyroclastic flows that melted summit glaciers and snow, generating four thick lahars that raced down river valleys on the volcano's flanks. They ran down the volcano's sides at an average speed of 60 km per hour, eroding soil, dislodging rock, and destroying vegetation. 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 .
thumb|left|upright=1.35|[[Armero was located in the center of this photograph, taken late November 1985.]]
One of the lahars virtually erased the town of Armero in Tolima, which lay in the Lagunilla River valley. Only one quarter of its 28,700 inhabitants survived. In total, over 23,000 people were killed and approximately 5,000 were injured. and is the fourth-deadliest volcanic eruption in recorded history. It is also the deadliest known lahar, A young girl named Omayra Sánchez became a worldwide symbol of the disaster after images of her trapped under the debris of her former home, after the eruption made the news worldwide.
The loss of life during the 1985 eruption was due partly to the fact that scientists did not know precisely when the eruption would occur, and the authorities would not take costly preventive measures without clear warnings of imminent danger. Because the volcano's last substantial eruption occurred more than 140 years earlier, it was also hard for many to accept the danger the volcano presented; locals even called it the "Sleeping Lion". Another factor was the storm that hit that evening, causing electrical outages and hindering communications. Civil defense officials from four nearby towns tried to warn Armero the lahar was approaching in the hour or so before it reached Armero, but failed to make radio contact.
Scientists later looked back to the hours before the eruption and noticed that several long-period earthquakes, which start out strong and then slowly die out, had occurred in the final hours before the eruption. Volcanologist Bernard Chouet said that, "the volcano was screaming 'I'm about to explode'", but the scientists who were studying the volcano at the time of the eruption were not able to read this signal.
In 2006, heavy rains on Ruiz sent a mudslide down the Chinchiná River, killing nine children aged 12–19 on a scouting expedition near the volcano.
Current threats and preparedness
thumb|Nevado del Ruiz as seen from [[Manizales, 2006]]
thumb|upright=1.5|Photo of Nevado del Ruiz taken on approach to [[El Dorado International Airport, Bogotá. Nevado del Tolima is visible in the background on the left. Approximate altitude of photo: .]]
The volcano continues to pose a serious threat to nearby towns and villages. The most likely hazard is small-volume eruptions, which might destabilize glaciers and trigger lahars. Although small eruptions are more likely, the two-million-year eruptive history of the Ruiz–Tolima massif includes numerous large eruptions, indicating that the threat of a large eruption cannot be ignored.
As the Armero tragedy was exacerbated by the lack of early warnings, and evacuations due to volcanic hazards have been carried out. About 2,300 people living along five nearby rivers were evacuated when Nevado del Ruiz erupted again in 1989. When another Colombian volcano, the Nevado del Huila, erupted in April 2008, thousands of people were evacuated because volcanologists worried that the eruption could be another "Nevado del Ruiz". In October 2022, the International Union of Geological Sciences (IUGS) listed Nevado del Ruiz as one of 100 "geological heritage sites" for being among the "best-studied volcanoes in the world" and a "model for volcanic risk management".
Recent activity
During September and October 2010, the Colombian Geological Survey noted gradual increases in seismic activity near the Arenas crater. Sulfurous odors and phreatic eruptions reported at Nevado del Ruiz prompted an alert level of Yellow on October 1.