Feeders vs arrested dikes: A case study from the Younger Stampar eruption in Iceland

Understanding the mechanical conditions for dike arrest and associated surface deformation or, alternatively, dike propagation to the surface to supply magma to an eruption, is of fundamental importance for volcanology in general and for volcanic hazards in particular. Here we present the results of a study of an outcrop located in the Reykjanes Peninsula, SW Iceland, where one dike became arrested only 5 m below the surface of an active volcanic system, without inducing any brittle deformation at the surface. In the same outcrop, at a distance of 30 m, a feeder dike is exposed. Both dikes are associated with the Younger Stampar eruption (1210–1240 CE). We reconstructed a high-resolution 3D model, through drone surveys and Structure from Motion (SfM) techniques, on which we collected detailed structural data combined with field surveys. These data, integrated with petrographic and geochemical analyses, became inputs to Finite Element Method (FEM) numerical models, made using the COMSOL Multiphysics® software. Our results indicate that compression exerted by the intrusion of the feeder dike (inferred to have been emplaced first) can explain the arrest of the second dike and the absence of induced brittle deformation even if the dike tip is only 5 m below the surface. Furthermore, the contrasting mechanical properties of the layers that constitute the outcrop, with alternating stiff lavas and compliant tuffs, raise (concentrate) the compressive stresses in the lava flows ahead of the second dike, thereby encouraging its arrest. Both the dikes are basaltic, but the earlier emplaced feeder dike is crystal poor and slightly more evolved than the later emplaced arrested dike. The results throw a new light on the conditions for dike arrest and (the lack of) dike-induced brittle deformation at very shallow depths in active rift zones, with important implications for volcanic-hazard assessments.