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... Preliminary signs of a possible reactivation of Cotopaxi in 2001 (Troncoso, 2005;Rivero, 2006) prompted a revaluation of the potential threat from the volcano and the publication of a new edition of the Cotopaxi hazard map (Hall et al., 2004 a, b). In addition, mapping and analysis of this area was recently initiated (Ettinger et al., 2005(Ettinger et al., , 2006, dedicated to the study of lahar deposits on the eastern flank of Cotopaxi. ...
Cotopaxi volcano (5897 m), located in the Eastern Cordillera of the Ecuadorian Andes, is one of the most active Ecuadorian volcanoes. During the last 2000 years, Cotopaxi has had at least one eruption of VEI 3 per century. Many of these past eruptions melted parts of the glaciers on Cotopaxi, generating large volume debris flows. These flows of great volume rapidly descended along the three main drainage systems of the volcano: the Pita-Esmeraldas rivers to the North, the Cutuchi-Pastaza rivers to the Southwest, and the Tambo-Tamboyacu-Napo river system to the East. We present a study of the 1877 lahar deposits in the eastern drainage of Cotopaxi. Sparse exposures left by the lahars suggest that flows evolved downstream. Grain-size distribution, textural trends in successive outcrops, and the spatial distribution of the lahar deposits demonstrate that past lahars on the eastern flank evolved from non-cohesive debris flows at and near the base of the volcano into a hyperconcentrated streamflow downstream due to the effect of topography changes on flow dynamics. Two major deposition zones have been identified: the first is situated at 3900 m a.s.l. at the base of the volcano; the second is an alluvial fan at the foot of the Eastern Cordillera, near the edge of the Amazon basin. The 1877 lahar deposits reveal a potential and dangerous hazard extending to the eastern basin located more than 90 km from the base of the volcano. Vulnerability to high volcano hazard is due to settlement within this zone over the past 60 years. A future lahar would impact people, villages along the riverbanks and the majority of local infrastructure.
