When Heavy Rain Made the Lava Lake Spatter

On March 11, 2021, after bursts of heavy rain, monitoring cameras captured Kīlauea’s lava lake spattering briefly increasing, while a laser rangefinder noted coincident drops of up to 1 meter while tracking the height of the lake’s surface. In research presented at the 2021 American Geophysical Union Fall Meeting by geologist Matt Patrick, the USGS Hawaiian Volcano Observatory team noted that event might be one of the first documented “to show the influence of rainfall on a lava lake” even if temporary. Other “top-down” processes initiated at the surface have been previously documented at Kīlauea, such as the repeated rockfalls into the 2008 to 2018 Halemaʻumaʻu lava lake. Those rockfalls broke the lava crust and initiated increased outgassing, with the whole lake surface transforming from gray and crusted to red and bubbling within a minute, while dropping in level due to lost gas volume. Crustal overturning of stagnant lava lake surfaces is common, but while gas trapped beneath the crust must usually build enough pressure to break through (and will inevitably do so), the rockfalls induced an early gas release. In the 2021 case, the interaction between heavy rain and the lake’s surface produced a similar effect, triggering an unusual increase in spattering and outgassing according to the HVO research. Though the USGS did not speculate on the mechanism, one could imagine that the extra water might better cool the lava surface, allow it to harden, and prevent gas from easily escaping, at least temporarily through a now-stagnant lake surface. In the usual manner, the trapped gas eventually breaks through the cooled crust bringing up lava, at which point the excess drives lava spattering until it is spent. Regardless of the possible mechanism, somehow the cloudbursts of heavy rain correlated with increased spattering and drops in lava level. The delayed gas release would not slow the lava input into the lake, only its gas output into the atmosphere. In the February 2023 downpours, heavy rain may have had a similarly limited “top-down” effect on the lake surface, but it is not responsible for, nor have its effects been discernible within the larger change happening on the volcano. “Bottom-up” processes in the magma reservoir and its conduit to the surface most likely led to the recent shutoff of the eastern and central vents, despite overlapping with considerable precipitation on the surface. One might think that the immense volume of water must contribute to the forces acting on the volcano, and while they do, modeling suggests these forces are about a hundred times smaller than those exerted by the accumulation of magma. Kīlauea’s still-active western lake appears to now be stagnant with frequent overturning – perhaps there is where the “top-down” effect of the recent heavy rains might be most easily seen. Since its collapse in 2018, the story of magma movements within Kīlauea’s summit now includes three eruptions and three intrusions in less than five years, as well as a series of pauses in surface activity as recently as one year ago. The current decrease in Kīlauea’s 2023 activity fits neatly into that pattern, perhaps fortuitously since conditions for lava viewing have been sub-optimal. Still, for those wondering about the effect of so much rain on the eruption, we can now point to that well-documented instance when heavy rain made the lava lake spatter. Photo 1: Halemaʻumaʻu lava lake on March 11, 2021 just before the heavy rain, showing normal circulation, USGS-HVO KWcam. Photos 2 & 3: Halemaʻumaʻu lava lake on March 11, 2021 just after the heavy rain, showing increased spattering on the surface, USGS-HVO KWcam.