Ice-rain interaction determining the erosional-depositional behavior of adjacent drainages at Cotopaxi volcano (Ecuador)

Abstract

Cotopaxi is a glacier-clad volcano located in the Andes of Ecuador (South America). Since its last big eruption, occurred in June 1877, alluvial fans have developed in the lower flanks of the volcanic edifice due to water-related erosional-depositional processes. Two of those fans can be identified in the adjacent Pucarumi and Jatabamba drainages, whose source zones are closely located in the upper north-east flank of Cotopaxi volcano. Geological, ground-penetrating radar and sediment-componentry and granulometry data show that both fans have been deposited after the 1877 eruption and are similar in size, thickness and composition. These measurements thus suggest that the medium- to long-term development of the adjacent fans was controlled by similar processes affecting both drainages. However, when the current deposition in both drainages was surveyed during a one-year period using drone ortho-photogrammetry, the Jatabamba drainage proved to be much more active than the Pucarumi drainage. Although the observed depositional activity at Jatabamba can be broadly correlated to the year-round rain-pattern, the contrasting behavior observed at the adjacent Pucarumi drainage strongly suggest that rainfall alone is unable to entirely explain the erosional processes occurring at Cotopaxi volcano. Instead, recent satellite imagery shows that the source zone of the strongly active Jatabamba drainage currently displays a glacier tongue, which is absent in the case of the weakly active Pucarumi drainage. It is thus concluded that erosional processes occurring at Cotopaxi volcano are mostly driven by water originated by an interaction between rainfall and ice at the glacier border. The precise nature of such interaction remains to be determined.