Why the Ice Made a Small Eruption Lethal
Start with the thing that killed people, because it was not the eruption in the way most people picture it. Nevado del Ruiz is a 5,279-meter stratovolcano in Colombia's Cordillera Central, and its summit is capped with glaciers year-round. When it erupted on the evening of 13 November 1985, hot pyroclastic material and ashfall landed on that ice and began to melt it fast. The meltwater picked up ash, rock, and loose debris and became a lahar — a fast-moving slurry with the consistency of wet concrete.
Several lahars poured off the summit and funneled into the river valleys that drain the volcano. The largest raced down the Río Lagunillas, burst through a natural dam upstream, and hit the town of Armero — about 70 km away and well off the volcano itself — roughly two hours after the eruption began. Most of Armero was swept away or buried in a few short minutes. About three-quarters of its 27,000-plus residents died. The total toll across the region reached about 23,000.
The lesson that still stings: hazard maps for exactly this scenario existed before 1985, and warnings were issued the day of the eruption. Armero was not destroyed because nobody saw it coming — it was destroyed because the warnings did not reach and move people in time. Nevado del Ruiz is studied less as a geology problem than as a risk-communication failure.
1985 Was Typical, Not Freak — the Eruption Record Proves It
The instinct after a disaster is to treat it as a one-in-a-century freak. The data says the opposite. Our Smithsonian GVP record lists 24 recorded eruptions for Nevado del Ruiz, and the 19 that carry a size rating cluster tightly in the middle of the explosivity scale: four VEI 4, five VEI 3, eight VEI 2, and two VEI 1. In other words, Ruiz routinely erupts at exactly the modest scale that produced Armero. The 1985 eruption wasn't unusually big; it was unusually well-placed — a normal Ruiz eruption on top of a glacier, above a populated valley.
And it had happened before. A VEI-3 eruption in 1845 sent lahars down the same Lagunillas valley and killed around 1,000 people; the larger 1595 eruption did the same in the Gualí and Lagunillas drainages. Armero was built, over the following century, directly on top of the 1845 mudflow deposits. The valleys had told their story in rock and in the historical record. The 1985 lahars simply followed the path the earlier ones had carved.
| Eruption | VEI | What happened |
|---|---|---|
| 2023–2025 CE | 2 | The current unrest: ash and gas emissions, a shallow lava dome, and tens of thousands of daily earthquakes through 2023. The SGC raised the alert to Orange in 2023 before easing it back. Our record's most recent entry is 2025. |
| 1985 CE | 3 | The Armero catastrophe. A modest eruption that melted the summit glacier and sent lahars ~100 km down the valleys, killing about 23,000 people — the deadliest lahar in recorded history. |
| 1845 CE | 3 | A near-identical disaster 140 years earlier: lahars swept the Lagunillas valley — the same drainage that destroyed Armero — and killed roughly 1,000 people. The warning history was already written. |
| 1595 CE | 4 | The largest in our historical record for Ruiz — a VEI-4 explosive eruption that also produced deadly lahars in the Gualí and Lagunillas valleys. |
Source: VolcanoDB analysis of Smithsonian Global Volcanism Program records for Nevado del Ruiz (#351020). Pre-1985 dates are historical; the 1985 death toll follows USGS and published disaster studies. See the full Nevado del Ruiz data page and our deadliest eruptions ranking.
Nevado del Ruiz Today
Ruiz is not a museum piece. It has been restless since 2010, and in 2023 it entered an intense crisis — tens of thousands of earthquakes a day, a growing lava dome, and steady ash and gas emissions — that pushed Colombia's Servicio Geológico Colombiano (SGC) to raise the alert to Orange before easing it back to Yellow, where it has sat through 2026. Our record's most recent eruptive activity for Ruiz is dated 2025. Thousands of people in Manizales, Villamaría, and the Lagunillas and Gualí valleys live within reach of the same hazard that destroyed Armero.
The good news is that Colombia learned the 1985 lesson in blood and rebuilt its monitoring and evacuation planning around it — Ruiz is now one of the most closely watched volcanoes in South America. For the current alert level, always check the SGC bulletins directly; our live eruption dashboard and active volcanoes tracker pull from official feeds, but a glacier-capped volcano's status can change faster than any third-party page updates.
Where Armero Sits Among History's Deadliest Eruptions
What makes Nevado del Ruiz so important in the study of volcanic risk is that it breaks the intuitive link between size and death toll. Three of the four deadliest volcanic disasters on record came from genuinely enormous eruptions; Ruiz is the outlier that didn't:
| Eruption | Year | VEI | Deaths | Main killer |
|---|---|---|---|---|
| Tambora | 1815 | 7 | ~92,000 | Famine & ashfall |
| Mont Pelée | 1902 | 4 | ~30,000 | Pyroclastic flow |
| Krakatoa | 1883 | 6 | ~36,000 | Tsunami |
| Nevado del Ruiz | 1985 | 3 | ~23,000 | Lahar |
Ruiz is the smallest eruption on that list by a wide margin — a VEI 3 sitting beside a VEI 7, a VEI 6, and a VEI 4 — yet it ranks as the second-deadliest volcanic disaster of the entire 20th century, behind only the 1902 Mont Pelée pyroclastic flow. That is the single most important thing to take from it: a volcano's danger is set by its hazards and its neighbors, not by how big a number it scores on the explosivity index. If you live below a glaciated volcano, the question to put to a local hazard authority isn't “how big could it erupt” — it's “which valley does the mud come down, and how long do I have?”