Lightning-induced weathering of Cascadian volcanic peaks

Abstract

The process of meteorological lightning-induced modification of coherent volcanic rocks is examined by geochemical, textural, and experimental analysis of fulgurites from South Sister volcano, Oregon Cascades, USA. Lightning's effects on volcanic target rocks was simulated with an arc-welding device in order to reproduce the geochemical and textural features of natural fulgurites and to constrain temperatures of melting and devolatilization behavior during lightning strikes. Melting of volcanic target rocks produces melts of exceptional compositional diversity, ranging from those with pure mineral compositions (e.g., diopside and plagioclase), resulting from congruent melting reactions, to those that are highly mixed and compositionally identical to the target rock. Geochemical and textural observations on fulgurites suggest that melting proceeds rapidly, by an in situ modal batch process whereby individual crystals melt and contribute small aliquots to a larger batch that is then mixed by viscous flow. Hydrous species remaining in the glasses are trace (OH≪0.1 wt.%), and consistent with partial equilibration to high temperature (>2000 °C) supra-liquidus, 1-atm conditions. Lightning also induces physical weathering, in that the target rocks may be vapourized to form channels through the rock, representing injection of elemental components to the atmosphere. Finally, lightning acts as a multi-faceted producer of dry, chemically equivalent volcanic glass, which may be subject to long-term hydration by environmental waters at surface conditions. These re-hydration signatures contrast those of volcanic glasses, which have both magmatic and meteoric components, and are used herein to examine the post-lightning environmental history of these glasses, including age dating lightning events by diffusion chronometry. Our study of volcanic fulgurites illustrates the potential to use their hydration signatures to date paleoweather events at decadal to centuries scale. Because the lightning strike is in and of itself so effective at devolatilizing melts in an instant, the resultant fulgurites are a unique earth material that record individual weather events (i.e., a thunderstorm), and also longer-term paleoweather intervals. © 2020 Elsevier B.V.