Few people living in the town of Armero, Colombia, realized the immediate danger they faced in the autumn of 1985. Nevado Del Ruiz volcano, 65 km from Armero, had reawakened after more than one hundred years of repose. During the previous year new magma had risen beneath this ice-capped Andean volcano (Figure 3.1) to within a few kilometers of the surface. Intermittent explosions showered the summit glacier with pyroclasts, fragments of rock propelled by the sudden expansion of volcanic gases within the ascending magma. After months of this intermittent explosive activity, a much larger explosive eruption sent pyroclastic flows, hot gaseous clouds loaded with pyroclastic rock fragments, across the summit glaciers. The ice melted rapidly and water mixed with pyroclasts swept into river channels that source high on the volcano.

When these flows, termed lahars, reached the rainforest on the flanks of the volcano, their energy was sufficient to completely strip the channel banks of vegetation (Figure 3.2). The lahars that descended the Rio Lagunilla canyon toward Armero had incredible momentum. The river takes a sharp bend 1 km upstream from the mouth of the canyon, where the flow debouched onto the alluvial plain and through the town of Armero. Here, the momentum of the flow, created by a loss of elevation of about 5 km along the 65 km flow path, carried the flow up and over the river's steep embankment.