Pleistocene cohesive debris flows at Nevado de Toluca Volcano, central Mexico

During the Pleistocene, intense hydrothermal alteration promoted a Bank failure of the southern portion of Nevado de Toluca volcano. This event produced a debris avalanche that transformed into a cohesive debris flow (Pilcaya deposit) owing to water saturation and weakness of the altered pre-avalanche rocks. The Pilcaya debris flow traveled along a narrow tectonic depression up to a distance of 40 km and then spread over a Bat plain reaching up to 55 km from the volcano summit. This transition zone corresponds with a sudden break in slope from 5 to 0.5 degrees that caused a rapid reduction in velocity and thickening of the flow that consequently reduced its competence to transport large particles. The resulting deposit thickens from 15 to 40 m, and contains boulders up to 15 m in diameter that form hummocky morphology close to the transitional zone. Sometime after thr emplacement of the Pilcaya debris flow, heavy rains and superficial drainage contributed to remobilize the upper portions of the deposit causing two secondary lahars. These debris flows called El Mogote, traveled up to 75 km from the volcano. The edifice collapse generated lahars with a total volume of 2.8 km(3) that devastated an approximate area of 250 km(2). The area versus volume plot for both deposits shows that the magnitude of the event is comparable to other cohesive debris flows such as the Teteltzingo lahar (Pico de Orizaba, Mexico) and the Osceola mudflow (Mount Rainier, Wa). The Pilcaya debris flow represents additional evidence of debris flow transformed from a Bank failure, a potentially devastating phenomenon that could threaten distant areas from the volcano previously considered without risk. (C) 2000 Elsevier Science B.V. All rights reserved.