WHAT IS A FISSURE ERUPTION AND HOW DOES IT WORK? (Invited Presentation)

This presentation will review my dissertation work and highlight Don Swanson's input and insights into fissure-fed basaltic lava fountains at Kilauea.

Hawaiian fountains are sustained, weakly-explosive jets of gas and juvenile ejecta. A broad range of Hawaiian fountaining styles occurred during twelve episodes of Kīlauea's Mauna Ulu eruption between May and December 1969. Part of the episode 1 fissure system is currently well exposed, providing an exclusive opportunity to study processes of low-intensity fissure fountains. Episode 1 was a 5 km long fissure system and exploited the eastern-most kilometer of the Koa`e 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. 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 downslope. Vents' surface geometry ranges from linear to circular, with superimposed irregularity and sinuosity, with straight-sided to flaring cross-sections. 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. 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 show a distinctly prolonged degassing and outgassing path.