THE ENORMOUS CHILLOS VALLEY LAHAR - AN ASH-FLOW-GENERATED DEBRIS FLOWFROM COTOPAXI VOLCANO, ECUADOR

The Chillos Valley Lahar (CVL), the largest Holocene debris flow in ar ea and volume as yet recognized in the northern Andes, formed on Cotop axi volcano's north and northeast slopes and descended river systems t hat took it 326 km north-northwest to the Pacific Ocean and 130+ km ea st into the Amazon basin. In the Chillos Valley, 40 km downstream from the volcano, depths of 80-160 m and valley cross sections up to 33700 0 m(2) are observed, implying peak flow discharges of 2.6-6.0 million m(3)/s. The overall volume of the CVL is estimated to be approximate t o 3.8 km(3). The CVL was generated approximately 4500 years BP by a rh yolitic ash flow that followed a small sector collapse on the north an d northeast sides of Cotopaxi, which melted part of the volcano's icec ap and transformed rapidly into the debris flow. The ash flow and resu lting CVL have identical components, except for foreign fragments pick ed up along the flow path. Juvenile materials, including vitric ash? c rystals, and pumice, comprise 80-90% of the lahar's deposit, whereas r hyolitic, dacitic, and andesitic lithics make up the remainder. The sa nd-size fraction and the 2- to 10-mm fraction together dominate the de posit, constituting approximate to 63 and approximate to 15 wt.% of th e matrix, respectively, whereas the silt-size fraction averages less t han approximate to 10 wt.% and the clay-size fraction less than 0.5 wt .%. Along the 326-km runout, these particle-size fractions vary little , as does the sorting coefficient (average = 2.6). There is no tendenc y toward grading or improved sorting. Limited bulking is recognized. T he CVL was an enormous non-cohesive debris flow, notable for its ash-f low origin and immense volume and peak discharge which gave it charact eristics and a behavior akin to large cohesive mudflows. Significantly , then, ash-flow-generated debris flows can also achieve large volumes and cover great areas; thus, they can conceivably affect large popula ted regions far from their source. Especially dangerous, therefore, ar e snow-clad volcanoes with recent silicic ash-flow histories such as t hose found in the Andes and Alaska.