Weak-intensity, basaltic, explosive volcanism : dynamics of Hawaiian fountains

Parcheta, Carolyn Emily
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[Honolulu] : [University of Hawaii at Manoa], [May 2013]
Hawaiian fountains, typically occurring on basaltic volcanoes, are sustained, weakly-explosive jets of gas and juvenile ejecta. A broad range of Hawaiian fountaining styles occurred during twelve episodes of the Mauna Ulu eruption on Kīlauea between May and December 1969. The western episode 1 fissure system is currently well exposed, providing an exclusive opportunity to study processes of low-intensity fissure fountains. Episode 1 fountains occurred along a 5 km long fissure system that exploited the eastern-most kilometer of the Ko'ai fault system. A low, near-continuous, spatter rampart is present on the northern upwind and upslope side of the fissure. Most pyroclastic products, however, fell downwind to the south and little was preserved because of two processes: 1) incorporation of proximal spatter in rheomorphic lava flows 10--20 meters from the vents, and 2) downslope transport of cooler spatter falling on top of these flows >20 meters from vent. There is a clear 'lava-shed' delineation between lava that drained back into the fissure and lava that continued flowing into the flowfield. Vents range in surface geometry from linear--circular, with superimposed irregularity and sinuosity, and range from straight-sided--flaring cross-sectional geometries. Irregularity results from joints in the pre-existing wall rock. Sinuosity results from the local stress field. Geometry of non-flared vents could indicate the true geometry of the dike. Flared vents likely formed through mechanical erosion and thermo-mechanical abrasion. Vent positions along the fissure likely resulted from flow focusing. Uniquely, these vents drained and remain unobstructed (some >100 m depth), despite subsequent nearby eruptive activity. Three vents were imaged ≤16 m in depth at <4 cm resolution with tripod-mounted LiDAR. Textural analyses of pyroclasts from eruptive episodes 2--12 show three distinct degassing and outgassing paths: 1) rapid degassing and quenching with minimal outgassing, 2) prolonged degassing and outgassing, and 3) rapid degassing and quenching on pyroclasts' rims with prolonged internal bubble coalesce. Contrasting patterns of shallow degassing and outgassing were the dominant control driving Hawaiian fountain behavior and variability, and are probably the dominant control for many other Hawaiian-style eruptions.
Ph.D. University of Hawaii at Manoa 2013.
Includes bibliographical references.
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