75-26 Change is Bubbling Up: How Vesicles in the 1973 Eldfell (Iceland) Tephra Record Syn-Eruptive Style Changes and Long-Term System Evolution

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Abstract:

The 1973 Eldfell eruption (Iceland) is the most recent eruption of the Vestmannaeyjar volcanic system and forced the evacuation of the entire island of Heimaey. Within the first ​​week of the eruption, 2.5 million m³ of tephra was expelled from the vent, covering the island and its town. The eruption began as a Hawaiian style eruption along a fissure at least 1.5 km long, later transitioning to a single source vent and a Strombolian eruption style. The tephra from these different stages preserves a record of the associated gas phase, in the form of vesicles. To assess how bubble processes relate to the different eruption styles, we quantified tephra density and vesicularity (180 clasts in the 16–32 mm size fraction), and vesicle volume distribution (detailed 2D image analysis of two clasts). Tephra from the opening fissure is less dense and has a more restricted vesicularity range (63% to 88%). Tephra from the late-stage cone is denser on average but spans a wider range of vesicularity (20% to 83%). Tephra from the late-stage cinder cone is dominated by bubble coalescence textures, as suggested by its bimodal vesicle volume distribution, peaking at bubble sizes of 2 mm and 0.08 mm. Such a distribution is consistent with the Strombolian eruptive style observed later in the Eldfell eruption. Tephra from the opening fissure preserves evidence of bubble ripening, based on a vesicle volume distribution peaking sharply at large vesicle sizes (2 mm). The interpretation of bubble ripening is consistent with previous work showing crystal ripening textures in the early Eldfell magmas, which requires long term magma residence at shallow depths. The Vestmannaeyjar volcanic system, located on the tip of Iceland’s southwestward propagating East Volcanic Zone, is currently classified as a fissure swarm, characterized by relatively mafic, monogenetic eruptions fed by deep sources. In contrast, central volcanoes are fed by shallower magma chambers and have the potential to result in larger, more explosive eruptions. The new evidence presented here supports the emerging hypothesis that the Vestmannaeyjar volcanic system is slowly evolving from a fissure swarm into a central volcano.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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