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 b y a cross-flow subdivision into zones with different surface morpholog y, symmetrically distributed with respect to the centerline of the flo w. These zones are: (1) a central zone (cp zone) with a relatively smo oth surface near the vent; (2) a lateral zone (Ip zone) covered by a c arpet of clinkers overlying the massive lava body; and (3) a lateral l evee zone (II zone). In many cases the cp and Ip zones are separated b y an inward-dipping groove, and the cp zone is from some decimeters up to more than 1 m thicker than the adjacent Ip zone. In sections perpe ndicular to the flow direction, the vesicles corresponding with the cp zone are systematically distributed in a ring-like region and are cha racterized by a sub-elliptical shape with their minor axes along a dir ection radial to the center of such region. Vesicles corresponding wit h the Ip zone are still elliptical and their major axes tend to be par allel to the nearest cooling surface. Vesicularity is the lowest at th e center of the ring-like region where vesicles approach a spherical s hape. In the upper part of the ring-like region, vesicle accumulation and coalescence below the crust produces one or more gas-rich layers w hich act as levels of preferential detachment for the formation of lav a tubes. The observed morphological characteristics and reconstructed vesicle distribution patterns are consistent with a radial distributio n of velocity within the lava flow unit and suggest the existence of a n 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 lav a flow unit in order to satisfy mass conservation.