Hawaiian fissure fountains 1: decoding deposits-episode 1 of the 1969-1974 Mauna Ulu eruption

Deposits from episode 1 of the 1969-1974 Mauna Ulu eruption of KA << lauea provide an exceptional opportunity to study processes of low intensity Hawaiian fissure fountains. Episode 1 lava flows passed through dense forest that had little impact on flow dynamics; in contrast, the pattern of spatter preservation was strongly influenced by the forest (through the formation of tree molds) and the preexisting topography. A low, near-continuous spatter rampart is present upwind and upslope, on the north side of the fissure. Most of the pyroclastic products, however, fell downwind to the south of the fissure, but little was preserved due to two processes: (1) incorporation of proximal spatter in rheomorphic lava flows 10-20 m from the vents, and (2) the downslope transport of cooler spatter falling on top of these flows beyond 20 m from vent. The lava flow field itself shows a complex history. Initially, discharge from the fissure exceeded the transport capacity of the southern drainage pathways, and lava ponded dynamically to a maximum height of 4 m for 40-120 min, until fountains began to decline. During declining discharge, lava flowed both southward away from the fissure and increasingly back into the vents. There is a clear lava-shed or delineation between where lava drained northwards back into the fissure, and where it continued flowing to the south. The 1969 deposits suggest that care is needed when products of less well-documented eruptions are analyzed, as postdepositional transport of spatter may preclude the formation of classic paired (symmetrical) ramparts.