Unlocking the Off-Rift Record: Magma Storage and Mobilization in the Snæfellsnes Volcanic Zone

Constraining magma storage conditions and pre-eruptive mobilization timescales is essential for understanding how deep-seated magmatic unrest progresses toward eruption. While these processes are increasingly well constrained in Iceland’s on-rift volcanic zones, magma dynamics in off-rift flank regions remain poorly understood, despite their potential for hazardous explosive eruptions. Here, we investigate magma storage depths and pre-eruptive timescales of magma mobilization and crystal-mush erosion in the Snæfellsnes Volcanic Zone (SNVZ), a relict Tertiary volcanic belt hosting Iceland’s most extensive off-rift volcanism. We integrate olivine Fe-Mg diffusion chronometry with fluid inclusion and clinopyroxene-based barometry to reconstruct magma storage conditions preceding the Holocene Búðahraun and Berserkjahraun eruptions. Our results identify a dominant magma storage region at ~11-15 km depth [1]. The absence of fluid inclusions recording shallow storage indicates rapid olivine entrainment and swift magma ascent from mid-crustal depths to the surface. These storage depths broadly overlap with deep seismicity (15-20 km; median ~17 km) detected in the SNVZ since August 2024, consistent with possible reactivation of a mid-crustal magma domain by ongoing mantle-derived magma intrusion. Olivine diffusion chronometry indicates that mush erosion began ~4.9 and ~1.8 years prior to the Búðahraun and Berserkjahraun eruptions, respectively, with mobilization accelerating during the final ~1.5 months before eruption. These results suggest that once magmatic unrest in the SNVZ progresses toward eruption, magma mobilization may proceed rapidly, with eruptions potentially following within weeks to months. These timescales are comparable to those documented in Icelandic on-rift systems, suggesting broadly similar magma mobilization processes in off-rift and on-rift systems. In light of ongoing seismicity, our findings provide the first quantitative lead-time constraints relevant for monitoring and hazard assessment in this historically quiet, yet potentially active, off-rift volcanic zone [1].[1]: Kahl, M., Morgan, D.J., Wieser, P.E. et al. Crystal-mush remobilization timescales and magma storage depth in the Snæfellsnes Volcanic Zone (W-Iceland): insights from olivine Fe-Mg diffusion chronometry and fluid inclusion barometry. Bull Volcanol 87, 118 (2025). https://doi.org/10.1007/s00445-025-01892-3