The Anatomy of a Lahar
Nevado del Ruiz is a 5,389-meter-tall stratovolcano, a conical volcano built from layers of hardened lava and ash. Its summit is covered by a large glacial ice cap, a feature that makes it dangerous. The volcano's andesitic and dacitic lavas lead to explosive Plinian eruptions, which can launch pyroclastic flows—fast-moving currents of hot gas and volcanic matter—down its slopes. When these searing flows, with temperatures that can exceed 800°C, make contact with the ice, they trigger catastrophic melting.
This meltwater mixes with volcanic ash, soil, and rock debris, forming a lahar. The term, which is Indonesian in origin, describes a slurry with the consistency of wet concrete that can travel at speeds over 50 kilometers per hour. The unique combination of an ice cap and explosive eruption style at Nevado del Ruiz creates a permanent, high-risk environment for generating these destructive mudflows.
The Armero Tragedy
On November 13, 1985, Nevado del Ruiz produced a small eruption, registering only a 3 on the Volcanic Explosivity Index (VEI). The eruption itself was minor, ejecting about 35 million tonnes of material, just 3% of the volume from Mount St. Helens' 1980 blast. The eruption began at 3:06 PM, but the deadliest events happened hours later, at night and a storm that obscured the summit from view.
At approximately 9:00 PM, pyroclastic flows from the eruption melted a portion of the summit's snow and ice. This generated four distinct lahars that funneled into the river valleys draining the volcano. The largest, a torrent of mud and debris up to 40 meters thick, surged down the Lagunillas River valley at an average speed of 50 km/h. Two and a half hours after the eruption, it slammed into the town of Armero, 74 kilometers away, with no effective warning having reached the sleeping residents. The flow buried the town, killing approximately 23,000 of its 28,700 inhabitants.
This was not an unprecedented event. Deadly lahars had followed the same paths after eruptions in 1595 and 1845, killing 636 and over 1,000 people respectively. The town of Armero was built directly on top of the 1845 mudflow deposit. The 1985 tragedy prompted a global re-evaluation of volcanic hazards and led to the development of enhanced monitoring and warning systems, including acoustic flow monitors that can detect the ground vibrations from approaching lahars.