From Magmatic Unrest to Eruption: Insights from Diffusion Chronometry in Iceland and Hawai‘i

Understanding when and how magmatic systems evolve toward eruption is key to forecasting and hazard mitigation. This study applies diffusion chronometry to two basaltic rift eruptions - the 2021 Fagradalsfjall eruption in SW Iceland [1] and the 1950 Mauna Loa SWRZ eruption in Hawai‘i [2] - to constrain the timing of magma mobilization, mush disaggregation, and eruption run-up.

At Fagradalsfjall - the first deep-sourced eruption on a mid-ocean ridge segment monitored with modern instrumentation - diffusion chronometry in olivine and plagioclase identifies three pre-eruptive phases: (1) a priming phase with long diffusion timescales indicating deep magmatic upheaval years before eruption, largely undetected geophysically; (2) a transition phase beginning ~1 year prior, coinciding with regional seismic and geodetic unrest; and (3) a run-up phase marked by steep gradients in both diffusion ages and seismicity, reflecting rapid magma ascent.

At Mauna Loa, diffusion chronometry of six olivine types reveals a shorter, yet similarly structured timeline: (1) a ~1.5-year priming phase, recorded by high-Fo olivine, intensifying in the final 8 months and preceding detectable seismicity by ~2 months; (2) a transition phase 6–4 months prior, marked by increased magma flux, mush disruption and seismic onset; and (3) a run-up phase starting ~2.5 months prior, involving more evolved olivines and culminating in a magnitude 6.4 earthquake days before eruption.
These case studies underscore the diagnostic value of diffusion chronometry in tracking the onset, escalation, and pace of pre-eruptive processes in basaltic systems.

[1] Kahl et al. (2023a) Geology 51, 184-188.

[2] Kahl et al. (2023b) Bulletin of Volcanology 85:75.