The Hidden Killer: Lahars

The deadliest volcanic hazard is often not lava or ash but lahars — fast-moving rivers of mud, rock, and water that pour down volcano slopes. When hot volcanic material melts glaciers or mixes with heavy rain, the resulting mudflow can travel at 60 km/h and bury everything in its path under meters of concrete-like debris.

The Armero tragedy was caused by lahars, not lava. The mudflows traveled 74 kilometers from the summit and reached the town at night, while residents slept. Today, lahar-prone areas like the valleys around Mount Rainier in Washington have automated warning systems with sirens that give residents time to flee to higher ground.