The evolution of lava flows from ephemeral vents at Mount Etna: Insights from vesicle distribution and morphological studies

Lava flow units from ephemeral vents at Mount Etna are characterized by a cross-flow subdivision into zones with different surface morphology, symmetrically distributed with respect to the centerline of the flow. These zones are: (1) a central zone (cp zone) with a relatively smooth surface near the vent; (2) a lateral zone (lp zone) covered by a carpet of clinkers overlying the massive lava body; and (3) a lateral levée zone (ll zone). In many cases the cp and lp zones are separated by an inward-dipping groove, and the cp zone is from some decimeters up to more than 1 m thicker than the adjacent lp zone. In sections perpendicular to the flow direction, the vesicles corresponding with the cp zone are systematically distributed in a ring-like region and are characterized by a sub-elliptical shape with their minor axes along a direction radial to the center of such region. Vesicles corresponding with the lp zone are still elliptical and their major axes tend to be parallel to the nearest cooling surface. Vesicularity is the lowest at the center of the ring-like region where vesicles approach a spherical shape. In the upper part of the ring-like region, vesicle accumulation and coalescence below the crust produces one or more gas-rich layers which act as levels of preferential detachment for the formation of lava tubes. The observed morphological characteristics and reconstructed vesicle distribution patterns are consistent with a radial distribution of velocity within the lava flow unit and suggest the existence of an inner plug flow region where the velocity gradient is zero. A simple evolution scheme is proposed in which a lava flow unit from ephemeral vent invariably evolves to a lava tube through the downward migration of the plug flow region. The progressive reduction of the cross-flow section due to cooling produces a pressure increase within the flowing lava body and the thickening of the active central portion of the lava flow unit in order to satisfy mass conservation.