Accretionary lapilli in the 6,7 ka eruption of the Hudson Volcano

SCASSO, R.A.

San Carlos de Bariloche

Congreso; 4° Congreso Latinoamericano de Sedimentología y 11 Reunión Argentina de Sedimentología; 2006

Asociación Argentina de Sedimentología

The 6.7 ka eruption is considered to be the largest for the Hudson volcano and, possibly, for any volcano in the southern Andes during the Holocene (Stern, 1991; Naranjo & Stern, 1998). It consisted of 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. The final phase (P3) laid down an often normally 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 km, 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 the 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 the Hudson volcano. The formation of Hudson’s last superimposed or partially nested caldera may be related to this eruption (Orihashi et al., 2004).Accretionary lapilli are 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 & Lane, 1994). The growth of accretionary lapilli is controlled by collision of liquid-coated particles due to differences in fall velocities across the eruption plume (Gilbert & Lane, 1994). Electrostatic charges can play a subordinate role 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 is shown by the 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. The presence of accretionary lapilli in the sedimentary column is generally regarded as indicative of a location proximal to the vent (Moore & Peck 1962) and can be used as a paleoclimatic proxy, suggesting either abundant meteoric water or ice in land-settled eruptions.