Accretionary lapilli in the 6.7 ka eruption of the Hudson volcano

SCASSO, R.A; CAREY, S.; NARANJO, J.A; KRATZMAN, D.; RUIZ, L.

San Carlos de Bariloche, Argentina

Congreso; IV Congreso Latinoamericano de sedimentología y XI Congreso Argentino de sedimentología; 2006

Asociación Argentina de Sedimentología

The 6.7 ka eruption is considered to be the largest for Hudson volcano and possibly for any volcano in the southern Andes during the Holocene (Stern, 1991; Naranjo and Stern, 1998).  It consisted on three major phases (P1 to P3) with an estimated bulk volume >18 km3.  The second phase (P2) produced a thick, distinctive accretionary lapilli-rich bed, whereas the initial phase (P1) produced a commonly reverse graded, coarse lapilli fall deposit and the final phase (P3) laid down an often normal graded, coarse lapilli fall unit. The melt phase was trachydacitic in composition and relatively uniform during the eruption. The accretionary lapilli layer (P2) is formed by silty-ash (modes between 20 and 40 µm) with rim-type accretionary lapilli diameters up to 2.3 cm at 35 kms, and 1.5 cm at 80 km from the volcano. It has been correlated with a widespread tephra in southern Patagonia, 900 km to the south of Hudson volcano (Stern, 1991). The occurrence of extensive, fine grained accretionary lapilli-bearing beds within this plinian eruption sequence may be related to magma/meltwater interactions triggered by eruption discharge through the summit glacier of Hudson volcano, probably related to the formation of its last superimposed or partially nested caldera (Orihashi et al., 2004). Accretionary lapilli is common in the geological record, particularly in facies of phreatomagmatic eruptions, associated with magma - meteoric water interaction and the generation of abundant fines. Aggregation processes are strongly particle-size dependant (Gilbert and Lane, 1994). Growth of accretionary lapilli is controlled by collision of liquid-coated particles due to differences in fall velocities across the eruption plume (Gilbert and Lane, 1994). Electrostatic charges can play a subordinated rol in the formation of accretionary lapilli by allowing contact between particles with different charges and the formation of the initial core. Then, the core will be surrounded by concentrically arranged layers bind together by capillary forces as it is shown in Hudson samples. Widespread thick beds of accretionary lapilli like the Hudson P2 will only form during big eruptions with high humidity, fragmentation and mass loading of fine particles in the ash cloud. Presence of accretionary lapilli in the sedimentary column can be considered indicative of location proximal to the vent and can be used as a paleoclimatic proxy of abundant meteoric water or ice in land-settled eruptions