A VOLUMINOUS AVALANCHE-INDUCED LAHAR FROM CITLALTEPETL VOLCANO, MEXICO - IMPLICATIONS FOR HAZARD ASSESSMENT

During the late Pleistocene the ancestral edifice of Citlaltepetl volc ano (also known as Pico de Orizaba) collapsed to form a clay-rich depo sit that extends 85 km from its source, has a volume of 1.8 km3, and c overs an area of 143 km2 east of the volcano. The deposit has clay con tent ranging from 10 to 16% and contains secondary alteration minerals such as smectite and kaolinite. The deposit's features suggest that i t had an origin as a sector collapse of hydrothermally altered rock th at transformed from a debris avalanche to a cohesive lahar very close to its source. The presence of glacier ice and a hydrothermal system d uring late Pleistocene times apparently provided a source of pore wate r which enhanced the hydrothermal alteration of the summit of Citlalte petl and was the origin of most of the water for the lahar. This depos it and several others suggest that glaciated volcanoes are sites where hydrothermal alteration and resulting cohesive lahars are most likely . Although cohesive lahars and debris avalanches both have origins as sector collapses, cohesive lahars are more mobile than similar-sized d ebris avalanches. Thus potential hazard of edifice collapse at glaciat ed volcanoes, especially those with large volumes of hydrothermally al tered rock, includes the possibility of large-volume cohesive lahars.