THE 1997 AND 2001 LAHARS OF POPOCATÉPETL VOLCANO (CENTRAL MEXICO): TEXTURAL AND SEDIMENTOLOGICAL EVIDENCE TO CONSTRAIN THEIR ORIGIN AND HAZARDS

Popocatépetl volcano is the most active volcano in central Mexico, and represents a high risk for more than 40 million people, including Mexico City. In 1994, volcanic activity at Popocatépetl renewed with the formation of ash-rich plumes up to 7-km high, and in April 1996, lava emissions filled the crater and were accompanied by a series of explosions that produced eruptive columns up to 8 km high. Associated with explosive events in 1997 and 2001, two major lahars formed, leaving debris flow deposits along Huiloac Gorge to the town of Santiago Xalitzinta. The 1997 debris flow deposit originated after a prolonged emission of ash which caused glacier melt and a rapid release of water (1 x 107 m3). The amount of melting water was sufficient to gradually erode the riverbed causing a flood that gradually transformed from a debris flow to a hyperconcentrated flow. In contrast, the 2001 debris flow that originated from a post-depositional remobilization of a pumice flow deposit, did not experiment any flow transformation and carried up to 25% water. The different behavior of these two lahars has important hazard implications. Both lahars reached Xalitzintla, but at that point, the 1997 lahar had already become a sediment-loaded streamflow. The 2001 lahar, however, maintained the characteristics of a debris flow, being more competent, and with greater destructive power. What happened with these lahars demonstrates how important it is to take into consideration secondary volcanic phenomena. Even though they were not large flows, they were capable of threatening populated areas, even during periods of volcanic quiescence or reduced magmatic activity. In fact, the eruptive history of Popocatépetl volcano reveals frequent laharic events, mostly associated with post-eruptive remobilization of voluminous ash and pumice fall deposits. Some of these events have been claimed responsible for the migration of ancient civilizations.