Intricate episodic growth of a Hawaiian tephra deposit: case study of the 1959 Kīlauea Iki eruption

Abstract

The 1959 Kīlauea Iki eruption on Hawai’i generated a succession of fountains, most reaching hundreds of meters high. The 16 episodes of fountaining persisted intermittently (with repose periods of between 7 h and 4 days) for 36 days. They produced tephra deposits that were dispersed several kilometers downwind of the vent, and a much larger volume of clastogenic lava which drained into the Kīlauea Iki crater to form a > 100 m deep lava lake. Field data from 211 tephra sample pits downwind of the vent reflect imperfectly the episodic nature of the fountaining behavior: only five composite stratigraphic subunits from the total of 16 fountaining episodes can be mapped in the field. However, isopach maps of these subunits were generated and, by the application of empirical deposit thinning relationships, volumes of each subunit were estimated. In combination with detailed observations made in 1959, our field data allow us to assign stratigraphic subunits to either single or aggregates of several fountaining episodes. The most voluminous subunits are linked with the highest fountaining, not with the longest in duration. In fact, significant downwind dispersal was possible only if the top part of the fountains reached above the crater rim, which was located about 100 m above the vent. The end of high fountaining episodes produced ash-rich partings, which are found on top of the lapilli-sized products of some episodes. However, the ash-rich intervals are strongly attenuated by wind advection and so are only present along the dispersal axes. The subunits follow slightly different dispersal axes. The maximum spread of 28 degrees between various dispersal axes is predominantly controlled by changing wind directions and spatter cone collapse into the vent, which modified its geometry. This study outlines the dependency of tephra dispersal on fountain height and emphasizes the capability of an episodic Hawaiian fountaining eruption to generate a seemingly monotonous downwind tephra deposit.