Understanding the origin of magmatic necks: insights from Mt. Etna volcano (Italy) and analogue models

Abstract

Magmaticnecksarecommonlyfoundinvolcanicareas,andtheyoftenexhibitahomogeneousstructurewithacylindricalshape and a diameter of up to several hundreds of metres. Their massive and uniform structure poses a space problem for their emplacement in the brittle crust. Here, we use field data and analogue models to investigate how necks may emplace at shallow levels. Field analysis focuses on characterising the geometric, structural and magmatic features of two necks outcropping in the eroded portions of Mt. Etna, Italy. These are homogeneous and massive intrusive bodies, related to a single episode of emplacement at 400–600 m below the paleosurface. We further investigated their possible emplacement mechanism through analogue models, injecting vegetable oil within (a) a flat sand pack and (b) a sand cone. Dikes form with both configurations, erupting to the surface through vents. However, dikes injected within the cone are characterised by a larger thickening at shallow levels, in correspondencewiththe vent, where a neck-like structure forms.Thissuggeststhatthe gravitational loadimposed bya volcanic edificeprovidesthemostsuitableconditionsforthedevelopmentofmagmaticneck,asthedownslopeshearstressesenhancethe deformation of the cone slope during shallow dike emplacement promoting shallow dilation and thickening of the dike. Therefore, topography should be a further factor enhancing the development of necks, in addition to those mechanisms previously proposed. Our results are consistent with natural examples of feeder dikes thickening towards the surface and dikes transitioning to necks, supporting the reliability of the proposed conceptual model.