TOBA’S GIANT PUMICE DEPOSIT: CHARACTERIZATION, ORIGIN, AND TIMING OF A RAFTED PUMICE DEPOSIT AT TOBA CALDERA, SUMATRA, INDONESIA

The Toba Caldera is best known for its 74 ka super-eruption and classic resurgent dome. Consisting of two half-domes (Samosir Island and the Uluan Peninsula), resurgence of >1 km has taken place since the Youngest Toba Tuff eruption. A peculiar deposit of exceptionally large pumice blocks occurs within the sedimentary/volcanic sequences of these uplifted blocks. On southern Samosir, the principal exposure is 4 m thick and contains meter-sized pumice blocks with intervening pockets of fine white ash, lodged within clay-rich lake sediments. The pumice blocks have bread-crust surfaces, display radial cooling joints, are highly vesicular, and contain small phenocrysts (plag, amph, opx) set in a glassy matrix. They have distinct textures and mineralogy from all other volcanic rocks at Toba. Similar pumice clasts, that appear reworked, were found at 6 other secondary exposures on Samosir. Another primary exposure of large pumice blocks was identified on southern Uluan ~15 km from the principal Samosir exposure. Here they are lodged within clayey sediments and covered by a younger ignimbrite, but appear identical to pumices in the Samosir exposures. To determine whether these 8 separate exposures represent the same volcanic event, we conducted detailed petrography of the pumice and ash samples, acquired their bulk-rock and glass geochemistry, and dated driftwood found in the sediments. The collective results of this data imply that the 7 samples from Samosir were produced by the same eruption. The Uluan pumices are chemically distinct from those on Samosir, but the cause for their offset (alteration, magmatic variation, different eruption) has yet to be determined. ¹⁴C dates on wood from two secondary Samosir exposures suggest that the eruption occurred at ~22 ka. Granulometry, shard morphology, and glass geochemistry of the Samosir ash sample are consistent with phreatomagmatic fragmentation of vesiculating magma that also generated the giant pumice blocks. We suggest that the pumices spalled from a subaqueous lava dome before being rafted to their final positions along a former shoreline. The Pardepur lava domes that outcrop along the southern shores of Lake Toba are an unlikely source due to their distinct textures and geochemistry. Thus, the vent area for Toba’s giant pumice eruption may indeed be hidden beneath the lake.