Fluids play a fundamental role in driving seismic and volcanic activity. Unlike magma, which is molten rock, fluids are low-viscosity substances that can penetrate fractures in the Earth’s crust, modifying its mechanical properties. Changes in fluid pressure can reduce the effective strength of rocks, influencing fault slip, rock deformation, and magma transport. At divergent plate boundaries, where tectonic plates are spreading apart, the interactions between fluids, faults, and magma are key to understand rifting processes. However, most of these settings lie beneath the ocean along mid-ocean ridges, where direct observation is extremely challenging. Iceland offers a rare opportunity to study these processes directly, as the Mid-Atlantic Ridge is exposed above sea level. This makes it an exceptional natural laboratory for investigating volcano-tectonic dynamics typically hidden beneath the seafloor.
A notable opportunity for such study came with the 2021 eruption of the Fagradalsfjall volcano on the Reykjanes Peninsula. The eruption was preceded by an intense seismic swarm, whose detailed analysis showed that the earthquakes were driven not only by magma motion, but also by variations in fluid pressure within the crust. A study published in Communications Earth & Environment identified three distinct faulting regimes, controlled by changes in vertical stress and the presence of fluids. Detection of volumetric components in seismic sources indicated the opening and closing of cracks related to movement of fluids through permeable and impermeable rocks.
Fig. Reykjanes Peninsula as transtensional segment of the Mid-Atlantic rift zone with the volcanic-tectonic system of Fagradalsfjall.
Top right: Analysis of interpreted earthquakes reveals the orientation of the principal stress directions, compression axis (P) and tension axis (T). The T axes (blue) are predominantly oriented in NW–SE direction, consistent with the extensional regime of the Mid-Atlantic rift.
Read the full study:
„Stress variations and non-shear earthquakes before the 2021 Iceland eruption trace magmatic fluids“
Pavla Hrubcová, Václav Vavryčuk, Tomáš Fischer, Jana Doubravová