Effects of lava ponding on Hawaiian fountain heights: an experimental investigation

Abstract

Hawaiian eruptions are characterized by long-lived eruptions that produce lava fountains that last 300-10000 seconds and can reach 10s to 100s of meters high. During an eruption pyroclasts can fall back and accumulate proximal to the eruption site, forming ramparts or scoria cones that create topographic wells. Lava can accumulate within these topographic wells, creating lava ponds that may affect the behaviour of subsequent lava fountains. When a fountain ascends through a lava pond, it entrains previously erupted pyroclasts and accelerates them; this reduces the flow velocity of the ascending fountain, decreasing its overall height. Published studies have examined the relationship between ponding and variations in lava fountain heights from a theoretical perspective, though these studies have not yet received experimental verification. For this reason, an experimental kit is designed to conduct scaled analogue experiments to investigate the variation of fountain heights with ponding depth. Dimensional analysis is used to facilitate the comparison between laboratory and natural behaviours, while experiments are performed for varying parameters of; pressure head, ponding depth, conduit diameter and fluid viscosity. The collected dataset indicates that increasing volumetric flux corresponds to greater fountain heights, while increased ponding depth reduces fountain heights. A dimensionless model is then identified between dimensionless fountain height and dimensionless ponding depth, which allows the reduction in fountain height due to ponding to be evaluated.