Years of deep magmatic upheaval preceding the 2021 eruption at Fagradalsfjall, Iceland

Effective eruption forecasting and volcanic hazard management depend heavily on our ability to detect when a volcanic system switches from a state of unrest into a state of eruption. The 2021 eruption at Fagradalsfjall in SW Iceland, the first deep-sourced eruption on a mid-ocean ridge system monitored with modern instrumentation, presents an ideal opportunity to compare geophysical and petrological datasets to explore processes of deep magma mobilisation and eruption priming. Here we use diffusion chronometry to show that deep magmatic unrest in the roots of volcanic systems can precede apparent geophysical eruption precursors by a few years.  Early phases of magma accumulation and reorganisation in the near-Moho plumbing system, part of the priming for eruption, can occur in the absence of significant increases in shallow seismicity (<7 km depth) or rapid geodetic changes. In contrast, geophysical signals of unrest and crystal records of changing magmatic conditions both show significant increases in intensity in the months and days prior to eruption. This correlation may signal a rapid transition from a state of priming to full scale mobilisation in which magma begins to traverse the upper/ brittle crust. Our findings provide new insights into the dynamics of near-Moho magma storage and mobilisation.