There had been no Substantial eruption of the volcano since 1845, which contributed to complacency; locals called the volcano the "Sleeping Lion".
1902
Colombia's worst natural disaster, the Armero tragedy (as it came to be known) was the second-deadliest volcanic disaster of the 20th century (surpassed only by the 1902 eruption of Mount Pelée).
It was the fourth-deadliest eruption recorded since 1500 AD. Its lahars were the deadliest in volcanic history.
Omayra Sánchez lived in the neighborhood of Santander with her parents Álvaro Enrique, a rice and sorghum collector, and María Aleida, along with her brother Álvaro Enrique and aunt María Adela Garzón.
Prior to the eruption, her mother had traveled to Bogotá on business.
The night of the disaster, Omayra and her family were awake, worrying about the ashfall from the eruption, when they heard the sound of an approaching lahar.
After it hit, Omayra became trapped under her home's concrete and other debris and could not free herself.
When rescue teams tried to help her, they realized that her legs were trapped under her house's roof with her dead aunt's arms tightly clutched around her.
Sources differ as to the degree to which Sánchez was trapped.
1972
Omayra Sánchez Garzón (August 28, 1972 – November 16, 1985) was a Colombian girl trapped and killed by a landslide when she was 13 years old.
1985
The landslide was caused by the 1985 eruption of the volcano Nevado del Ruiz in Armero, Tolima.
Volcanic debris mixed with ice to form massive lahars (volcanically induced mudflows, landslides, and debris flows), which rushed into the river valleys below the mountain, killing about 25,000 people and destroying Armero and 13 other villages.
After the lahar demolished her home, Sánchez was trapped beneath the debris of her house, where she remained in water for three days, as rescue workers did not have any way to render life-saving medical care if they amputated her hopelessly pinned legs.
Her plight was documented by journalists as she transformed from calmness into agony while relief workers tried to comfort her.
After 60 hours of struggling, she died, likely as a result of either gangrene or hypothermia.
Her death highlighted the failure of officials to respond correctly to the threat of the volcano.
A photograph of Sánchez taken by the photojournalist Frank Fournier shortly before she died was published in news outlets around the world.
On November 13, 1985, the volcano Nevado del Ruiz erupted.
At 9:09 pm of that evening, pyroclastic flows exploding from the crater melted the mountain's icecap, forming lahars which cascaded into river valleys below.
One lahar, consisting of three pulses, did most of the damage.
Traveling at 6 m per second (~13.5 miles per hour, ~22 km/h), the first pulse enveloped most of the town of Armero, killing as many as 20,000 people; the two later pulses weakened buildings.
Another lahar killed 1,800 people in nearby Chinchiná.
In total 23,000 people were killed and 13 villages in addition to Armero were destroyed.
Loss of life was exacerbated by the authorities' failure to take costly preventive measures in the absence of clear signs of imminent danger.
During September 1985, as earthquakes and phreatic eruptions rocked the area around the volcano, officials began planning for evacuation.
A hazard map was prepared in October; it highlighted the danger from falling ash and rock near Murillo, Santa Isabel, and Líbano, as well as the threat of lahars in Mariquita, Guayabal, Chinchiná, and Armero.
The map was poorly distributed to those at greatest risk: many survivors said they had not known of it, though several major newspapers had featured it.
Henry Villegas of the Colombian Institute of Mining and Geology stated that the maps clearly demonstrated Armero would be affected by the lahars, but had "met with strong opposition from economic interests".
He said that the short time between the map's preparation and the eruption hindered timely distribution.
The Colombian Congress criticised scientific and civil defense agencies for scaremongering, and the government and army were preoccupied with a guerrilla campaign in Bogotá, the national capital.
The death toll was increased by the lack of early warnings, unwise land use, as villages were built in the likely path of lahars, and the lack of preparedness in communities near the volcano.
1986
It was later designated the World Press Photo of the Year for 1986.
Sánchez has been remembered in music, literature, and commemorative articles.
2009
Zeiderman (2009) said she was "trapped up to her neck", while Barragán (1987) said that she was trapped up to her waist.
For the first few hours after the mudflow hit, she was covered by concrete but got her hand through a crack in the debris.
After a rescuer noticed her hand protruding from a pile of debris, he and others cleared tiles and wood during the course of a day.
Once the girl was freed from the waist up, her rescuers attempted to pull her out, but found the task impossible without breaking her legs in the process.
Each time a person pulled her, the water pooled around her, rising so that it seemed she would drown if they let her go, so rescue workers placed a tire around her body to keep her afloat.
Divers discovered that Sánchez's legs were caught under a door made of bricks, with her dead aunt's arms clutched tightly around her legs and feet.
Despite her predicament, Sánchez remained relatively positive: she sang to Germán Santa María Barragán, a journalist who was working as a volunteer, asked for sweet food, drank soda, and agreed to be interviewed.
