Mainly explosive Holocene eruptions of the Planchón volcano, southern Andes (35°15'S).

Abstract

The Holocene activity of Planchón volcano is reduced and mainly explosive occurred through five explosive craters as the main source of the post-glacial deposits distributed to the eastern side of the volcano. Three successive phases were involved during the first Holocene eruption. An explosive column produced a short-lived tephra-fall, subplinian deposit. Phreatomagmatic explosions and the collapse of the initial column, due to crater widening, including a thick ice cap, were the causes of the second phase, the Valenzuela Pyroclastic Surge (OPV), a highly fragmented and wet base surge. The presence of a thicker and more extensive glacial cap favoured an eruption dominated by a high water/magma ratio. This also generated highly voluminous laharic deposits to the west through the avalanche caldera opening. The final column collapse was caused by the diminishing water/magma ratio, increasing the density of the explosion columns, and leading to the third final phase: a pyroclastic flow heavily laden with solid scoriaceous bombs (FPLC). AMS dating indicates a Holocene age of approximately 7,000 BP for this eruption. The presence of dacitic magmas evolved from a system dominated by basaltic andesites at Planchón volcano is evidenced by the Los Baños pumice-fall deposit (PLB). This deposit was formed during a subplinian eruption which occurred around 1,000 years ago, this being a unique process during the entire Planchón evolution. An extended differentiation period of 6 ka within a fairly small magma chamber was the probable origin for the dacite explosive event. The minor eruptions of February 1991 and November 1998, were consequences of the interaction of phreatic water with overheated rocks above the magma chamber and/or with an uppermost differentiated dacitic small magma body. The small amount of dacitic magma was produced during a 150 years period, after the unique post-glacial lava (andesite) erupted at Planchón, in February 1837.