Reticulite-Producing Fountains from Ring Fractures in Kīlauea Caldera ca. 1500 CE

A widely dispersed reticulite bed occurs close to the base of the Keanakīko'i tephra at Kīlauea Volcano. It can be divided into six subunits in the northern sector of the volcano; the reticulite also occurs in the southern sector, but outcrops are sparse owing to penecontemporaneous erosion and burial. Multilobate isopachs for each subunit and the total deposit suggests that multiple fountaining vents were distributed in the northern half of the caldera, possibly along ring fractures for the newly formed caldera. Isopach maps also show a sharp decline in thickness along the dispersal axis of each lobe, which could be explained by remobilization of tephra and/or inclined fountains. Despite such isopach characteristics, thinning rates calculated from the isopach data indicate that the fountains were among the most intense and powerful of all studied Kīlauea fountains. Density analysis of the pyroclasts suggests that fountaining was high (< 600 meters) yet complex, possibly due to lava ponding and re-entrainment. The calculated volume of the reticulite deposited around the caldera rim is approximately 0.2 km³, more voluminous than the deposits of the 1959 Kīlauea Iki eruption; this volume is a minimum, however, as the low density tephra is easily remobilized, and 600-m-high caldera walls probably trapped tephra within caldera, which is deeply buried today and not accounted for in the volume calculations. The duration of this eruption was most likely at least a few days to weeks, based on the calculated volume and estimated discharge rates as seen during the Kīlauea Iki 1959 eruption.