Vesicle Analysis of the 1973 Eldfell (Iceland) Tephra: Comparison of the Opening Fissure and Late-Stage Cone

Long manifested as a fissure swarm, Iceland’s Vestmannaeyjar volcanic system is slowly evolving into a central volcano. This study quantified tephra density, vesicularity, and vesicle volume distribution (via 2D image analysis) from the most recent Vestmannaeyjar eruption (Eldfell, 1973) to assess both pre- and syn-eruptive magma dynamics. Tephra from the late-stage cinder cone is dominated by bubble coalescence textures, consistent with the observed shift to a Strombolian eruptive style later in the Eldfell eruption. Tephra from the opening fissure preserves evidence of bubble ripening, which requires long term magma residence at shallow depths, a characteristic of central volcanoes.