Eruption and fountaining dynamics of selected 1985–1986 high fountaining episodes at Kīlauea volcano, Hawai'i, from quantitative vesicle microtexture analysis

Tephra from the early Hawaiian fountaining episodes of the ongoing eruption of Pu'u 'Ō'ō in the East Rift Zone (ERZ) of Kīlauea provides an opportunity to study the vesicle microtextures of pyroclasts erupted from a single vent over a prolonged period of time. We report the results of microtextural analysis of pyroclasts from five of Pu'u 'Ō'ō's high (>200 m) Hawaiian fountaining episodes (episodes 32, 37, 40, 44 and 45) erupted during 1985–1986. This analysis was carried out to constrain the parameters that led to large variations in fountain height at Pu'u 'Ō'o, and the extent to which pyroclast residence times in the fountain modified microtextures. Our results confirm the finding of Stovall et al., 2011, Stovall et al., 2012 that pyroclasts from a single Hawaiian fountain can vary greatly in texture (from bubbly to foamy), and have vesicle volume densities (N^m_v) and vesicle to melt ratios (V_G/V_L) that vary by an order of magnitude. This range in vesicle texture and population is due to extensive growth and coalescence of vesicles within the fountain after fragmentation. Only one pyroclast from four of five episodes was found to have textures interpreted as indicative of the vesicle population near the moment of fragmentation: bubbly texture, high density (typically >500 kg m⁻³), high N^m_v (2.2 × 10⁶ to 4.4 × 10⁶), and low V_G/V_L of 2.06 to 4.65. We demonstrate a linear correlation between Δ(V_G/V_L) and peak fountain height across a range of Hawaiian fountains from Kilauea. This correlation could be used to infer peak heights of unobserved Hawaiian fountaining eruptions after further testing using well-recorded events.