Comparative study of lahars generated by the 1961 and 1971 eruptions of Calbuco and Villarrica volcanoes, Southern Andes of Chile

The Villarrica and Calbuco volcanoes, of the Andean Southern Volcanic Zone, are two of the most active volcanoes in Chile and have erupted several times in the XX century. The 1961 eruption at Calbuco volcano generated lahars on the North, East and Southern flanks, while the 1971 eruption at Villarrica volcano generated lahars in almost all the drainages towards the north, west and south of the volcano. The deposits from these eruptions in the Voipir and Chaillupén River (Villarrica) and the Tepú River (Calbuco) are studied. The 1971 lahar deposits on Villarrica volcano show a great number of internal structures such as lamination, lenses, grading of larger clasts and a great abundance of large floating blocks on top of the deposits. The granulometry can be unimodal or bimodal with less than 5% by weight of silt + clay material. SEM images reveal a great variety of forms and compositions of clasts. The 1961 lahar deposits on Calbuco volcano have a scarce number of internal structures, steeper margins and features of hot emplacement such as semi-carbonized vegetal rests, segregation pipes and a more consolidated matrix. The granulometry usually is bimodal with great quantities of silt + clay material (> 10% by weight). SEM images show a uniformity of composition and forms of clasts.Differences on deposits reveal different dynamics on both lahars. The Villarrica lahar was generated by sudden melt of ice and snow during the paroxysmal phase of the 1971 eruption, when a high fountain of lava was formed. The melted water flowed down on the flanks of the volcano and incorporated sediments to become transition flows, highly energetic and were emplaced incrementally. Dilution of the flows occurs when the lahars reached unconfined and flatter areas. In cases where the lahar flow found large water streams, dilution is enhanced. The Calbuco lahars were generated by the dilution of block and ash pyroclastic flows by flowing over the ice or snow or by entering active rivers, transforming to debris flows. The differences on dynamics of both flows show the importance to understand initiation processes of lahars in order to make better hazard assessment due to laharics flows.