HR: 0800h
AN: V31B-0484    [Abstracts]
TI: Explosive eruptions during the first 100-150 years of Kilauea's caldera
AU: * Swanson, D A
EM: donswan@usgs.gov
AF: USGS Hawaiian Volcano Observatory, PO Box 51, Hawaii National Park, HI 96718, United States
AB: The collapse of Kilauea's summit to form its modern caldera took place in 1480-1500 C.E. and was apparently almost nonexplosive. Only a layer of medium-coarse ash 1-4 cm thick at the base of the Keanakako`i Ash can reasonably be ascribed to the collapse itself. Soon thereafter, however, lava fountains probably much higher than 300 m played from multiple vents in the caldera, depositing a layer of nearly pure reticulite as thick as 65 cm on the rim. Multiple fountains, possibly from fractures bounding the collapsed blocks, best explain lateral changes in texture and componentry of the reticulite and its presence completely around the caldera. High fountains, related to high ascent rate, are required for reticulite production (Rust and Cashman, 2006). A paucity of denser material (pumice, Pele's tears) in the reticulite deposit indicates that only the top of the fountains cleared the caldera rim, with denser fallout trapped within the caldera. Thus the caldera was already several hundred meters deep when the reticulite erupted (about 1500 C.E., according to C-14 ages.) A lithic block fall and associated ash fall or surge, with subordinate vitric components, occurred soon (a few weeks to years?) after the reticulite eruption. This deposit occurs beyond the northern and northeastern rim of the caldera and is thickest and coarsest in the national park's housing area, where it contains clasts several tens of centimeters across. The block fall and ash are both pale pink, indicative of a dry, high temperature eruption. For the next 100-150 years, numerous small eruptions produced vitric ash containing several percent of lithic clasts in all grain sizes greater than 0.5 mm. The mixed deposits are dominated by poorly vesicular ash, have only small amounts of pumice, contain chunks of black glass with planar to gently concave surfaces, and commonly are somewhat palagonitized. Together, these features indicate that the explosions were phreatomagmatic, a conclusion also reached by most past workers. Involvement of external water suggests that the caldera was deep, likely near the water table (about 500 m below today's caldera floor), consistent with the interpretation of the reticulite eruption. Most of these deposits occur along the south rim of the caldera, where they have a total thickness up to 7 m. In contrast, the total thickness along the north rim of the caldera is at most 50 cm. This difference probably reflects the southwesterly direction of the predominant trade winds and perhaps the location of one or more vents in the southern part of the caldera. In addition, ash could have blown more easily over the south rim of the caldera than over the 120-m-higher north rim. Several thin surge deposits occur along the south rim, but none is known on the north rim. None of the many explosions was particularly strong or voluminous, to judge from how well the low- level trade winds controlled ash dispersal. Probably in the early 1600s, a powerful explosion, tentatively considered magmatic rather than phreatomagmatic, produced a widespread scoria fall (layer 6 of McPhie et al., 1990). Ash blew far southeast of the summit, reflecting dispersal by jet-stream westerlies of a column likely more than 10 km high (Swanson et al., 2006). After the layer 6 eruption, a few phreatomagmatic explosions produced mixed vitric-lithic deposits several tens of centimeters thick along the southern caldera rim. A period of erosion ensued, followed by repeated explosions of dominantly lithic debris that continued through 1790 and perhaps into the early 1800s. McPhie, J, Walker, GPL, Christiansen, RL, 1990, Bull. Vol., v.52, 334-354 Rust, AC, Cashman, KV, 2006, Eos, v. 87, V43A-1178 Swanson, DA, Rose, TR, Fiske, RS, 2006, Eos, v. 87, V33B-0646
DE: 8404 Volcanoclastic deposits
DE: 8428 Explosive volcanism
DE: 8440 Calderas
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting