Caracterización geológica de los lahares secundarios en el flanco oriental del volcán Cotopaxi

Abstract

Cotopaxi volcano is characterized by the presence of a 0.7 km3 glacier that covers the edifice from its summit down to the ~4900 m level. Since the last big explosive eruption of Cotopaxi in 1877, alluvium fans have formed all around the volcano by the erosion and deposition produced by secondary lahars. The quebradas Pucarumi and Jatabamba are two adjacent drainages in the NE flank of Cotopaxi, displaying very close source zones and clear post-1877 alluvium fans, and were the subject of the present study. The long-term development of the fans was assessed by measuring their total thicknesses with GPR surveys performed in segments of the drainages where the 1877 deposits are observed directly underlying the recent alluvium deposits and evaluating its granulometry to classify the deposits and determine their main components. The short-term development of the fans was observed with cm-resolution mapping of recent deposits through periodic drone surveys carried out between May 2018 and July 2019, and by comparing the mapped deposits with local rainfall records of the FONAG. GPR surveys and granulometry suggest that both drainages have developed similarly during the last 143 years, taking as reference that they were deposited from 1877; their deposits were classified as hyperconcentrated flows that have maximum thicknesses ranging between 6,5 to 8 m, and implying average long-term deposition rates of 4 – 5 cm/year. However, drone mapping shows a very contrasting current behavior between both drainages. First, Jatabamba drainage is much more active than Pucarumi, with big deposits mapped through the whole observed year. Second, there is no clear relationship between recorded rainfall and lahar size nor even occurrence. The only difference currently observed in the source zone is the presence of a glacier tongue in the Jatabamba drainage source zone. This suggests that rain-glacier interactions are significant processes in the erosional behavior of Cotopaxi volcano.