Proximal lava drainage controls on basaltic fissure eruption dynamics

Hawaiian basaltic eruptions commonly initiate as a fissure, producing fountains, spattering, and clastogenic lava flows. Most fissures rapidly localize to form a small number of eruptive vents, the location of which may influence the subsequent distribution of lava flows and associated hazards. We present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of Mauna Ulu, KA << lauea, Hawai'i. Exceptional preservation of the deposits allows us to reconstruct vent-proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization. Through detailed field mapping, including measurements of the height and internal depth of lava tree moulds, we reconstruct high-resolution topographic maps of the pre-eruption ground surface, the lava high-stand surface and the post-eruption ground surface. We calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure. Aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface. By comparing with the inundation and drainage maps, we find that fissure segments that were inundated with lava to greater depths (typically 1-6 m) during the eruption later became foci of lava drainage back into the fissure (internal drain-back). We infer that, in these areas, lava ponding over the fissure suppressed discharge of magma, thereby favouring drain-back and stagnation. By contrast, segments with relatively shallow inundation (typically less than similar to 1 m), such as where the fissure intersects pre-eruptive topographic highs, or where flow away from the vent (outflow) was efficient, are often associated with sub-circular vent geometries in the post-eruption ground surface. We infer that these parts of the fissure became localization points for ongoing magma ascent and discharge. We conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity.