The eruption itself was small, but it melted part of the volcano's ice cap, generating lahars — fast-moving volcanic mudflows — that travelled down river valleys at speeds around 60 km/h.
Armero, 48 kilometres away, was buried under metres of mud roughly two and a half hours after the eruption began. Around 20,000 of its 29,000 residents died. Hazard maps published a month earlier had shown Armero in a high-risk lahar zone, and volcanologists had warned authorities, but no evacuation was ordered.
The tragedy is taught worldwide as a communication and governance failure rather than a scientific one, and led to the creation of Colombia's national volcanic monitoring service.
Why this event still matters
Historical incidents shape the rules that protect people today — building codes, evacuation policy, warning systems and inspection regimes almost always trace back to a specific disaster. Studying Nevado del Ruiz Eruption and Armero Lahar alongside current events is how preparedness improves.
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