What causes such eruptions? It is well established that their explosivity results from mixing of magma with water. New data from deposits of the most recent eruption, the Keanakakoi of A.D. 1790, indicate that magma was coming rapidly up the conduit when mixing took place. Evidence for this conclusion is that: (1) density measurements of 1800 clasts from the lower, vitric-rich subunit give a mean vesicularity of 73% with a standard deviation of 12%. Fewer than 2% of clasts have vesicularities >40%. Such uniformly high vesicularities are produced at Kilauea primarily during active lava-fountaining (M. Mangan, USGS-HVO, written commun.). (2) Bubble number densities of clasts (# of bubbles/cm^3 melt) range from 10^5 to 10^7. This is much higher than the 10^3-10^4 produced during slow effusive Kilauea eruptions (Mangan et al., 1993, Geology, 21:157-160), and higher even than the 10^4-10^5 of high Pu'u O'o lava-fountain eruptions of 1983 (M. Mangan, 1995, written commun.). The high number densities indicate rapid bubble nucleation, caused by rapid magma ascent.
These results indicate that magma could not have been sitting statically in the conduit, at a level below the water table, when water influx occurred. The level of magma must have extended at least to the ground surface or higher. Numerical modelling results (Mastin, Eos, 75(44):728) show that, under such conditions, water could enter the conduit only if the water table were, optimally, within 100 m of the surface, or, more realistically, at or above the ground surface. The present water-table depth is 500 m. A shallower water table in 1790 probaby reflected a lower caldera floor. In 1825, when first mapped, Kilauea's caldera floor lay some 300 m below its present elevation. If the rate of infilling of the caldera prior to 1823 were similar to that estimated from 1823-1841 (Wright and Fiske, in press), the caldera floor would have been well below the water table in 1790.
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