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

Abstract

Magmatic necks are commonly found in volcanic areas, and they often exhibit a homogeneous structure with a cylindrical shape 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 correspondence with the vent, where a neck-like structure forms. This suggests that the gravitational load imposed by a volcanic edifice provides the most suitable conditions for the development of magmatic neck, as the downslope shear stresses enhance the 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.