Calbuco volcano historic block-and-ash and pyroclastic flows: increasing threatening on surrounding communities, Southern Andes 41.5ºS

H. Moreno; J.A. Naranjo

¹Ovdas-Servicio Nacional de Geología y Minería, Cerro Ñielol, Casilla 641, Temuco, Chile

²Servicio Nacional de Geología y Minería, Av. Santa María 0104, Santiago, Chile

The Calbuco volcano (2003 m a.s.l.), located in the Southern Andes Volcanic Zone (SVZ) at 41.5ºS, is an andesitic compound truncated volcano that underwent a sector collapse on the early Postglacial leaving a 2 km wide horseshoe avalanche caldera opened to the northeast. Calbuco has a remarkably explosive Holocene eruptive record and a stratocone or dome grew inside the caldera overflowing its rim before it was destroyed. After the 1893 eruption a new dome-like volcano structure filled the avalanche theatre. Its latest eruptive activity has been developed on its summit that yields ca. 100 m below the western caldera rim. Consequently, most hazards tend to be driven through the northern, northeastern and southern drainages.

The most remarkable historic eruptions were those of 1893 ,1929 and 1961. The reports and chronicles review, in addition to geological evidences, together with the 1944 and 1961 aerial and field photographs suggest that the eruptive pattern and products were notably similar, but with different magnitudes. On 1961 one of the three 200-300 m diameter craters NW-SE aligned were the source of the 8 to 12 km high eruptive columns. The northern and southern craters extruded blocky lavas generating small collapsing block-and-ash and pyroclastic flows that triggered hot lahars toward the northeastern and southern feet of the volcano. The fast forest growth has efficiently obliterated the evidences of such products. In addition, the threaten population expansion is dramatically increasing the volcano risk especially to the northeastern and southern part of the volcano.