Ice-marginal lava delta in Iceland found on a nondescript shallow slope: An unexpected record of ice thickness late in deglaciation

Volcanism increases when glaciers melt because isostatic rebound during deglaciation decreases the pressure on the mantle, which enhances decompression melting. Anthropogenic climate change is now causing ice sheets and valley glaciers to melt around the world and this deglaciation could stimulate volcanic activity and associated hazards in Iceland, Antarctica, Alaska, and Patagonia. However, current model predictions for volcanic activity associated with anthropogenic deglaciation in Iceland are poorly constrained, in part due to uncertainties in past volcanic output over time compared to ice sheet arrangements. Further work specifically characterizing glaciovolcanic and ice-marginal volcanoes in Iceland is needed to reconstruct volcanic output during time periods with changing ice cover. Here, we describe a previously unrecognized ice-marginal volcanic lava delta on a broad, shallow slope southeast of Langjkull and the Jarlhettur volcanic chain in Icelands Western Volcanic Zone. Although previously mapped as interglacial lavas and sediments, canyons in this area revealed two ~20-30 meter-thick southwest-dipping sequences of pillow-bearing tuff-breccias between phoehoe lava flows above modern lake Sandvatn. Clasts within the tuff-breccias include a mixture of pillow lavas and phoehoe fragments, requiring that the subaqueous tuff-breccia facies were derived from subaerial flows. The upper subaqueous to subaerial transition in this sequence occurs around 400 m above sea level, much higher than any local topography that could dam water or the highest Icelandic marine transgression, necessitating ice damming. Quenched meter-scale cavities in coherent lava and cube-jointed facies show lava-ice contact, supporting evidence for an ice dam. We propose that an eruption melted through thin ice near Sklpanes during a deglacial period and lavas flowed downslope to the south, melting ice and forming an englacial lake. We constrain that the local ice thickness was tens of meters to a few hundred meters thick. This would represent a similar ice configuration as some interpretations of the ice extent at the time of formation of the Bui moraines around 11.2 ka, with higher ice flow down the valley of the Hvita river than off Langjkull, although it occurred during an earlier deglaciation. Importantly, this finding demonstrates that ice-marginal deposits that can provide paleo-environmental constraints may be hidden in terrains that do not conform to existing classifications of glaciovolcanic edifices.