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Abstract:
Shallow ocean/emergent basaltic phreatomagmatic volcanism observed on November 14, 1963 on the south-eastern end of the Vestmannaeyjar archipelago and the volcanic island of Surtsey appeared on the next day. Surtsey island is the only remaining subaerial part of the constructs from the 1963-1967 eruptions, has two tuff cones in its northern sector while the southern sector is a lava field mostly built on a lava delta. The SUSTAIN drilling in 2017 revealed the existence of an underlying diatreme, giving the island some of the characteristics of a maar volcano. Gravity data obtained on Surtsey in 2014 with a gravity station spacing of 100 m. Density measurements were obtained for surface samples of seafloor sedimentary rock bombs, and lava, tuff and tephra. With the best measured density value for gravity data correction, we use forward modeling to study the vent and surface lava characteristics by considering the four geological units defined for the island: tuffs above sea level, tuffs below sea level, lavas above sea level and a lava delta below sea level, composed of breccias over which lava advanced during the effusive eruption. The boundaries between the bodies defined from the eruption history and mapping done during the eruption. The data reveal low density of the vent areas within the lavas, best explained by cavities within the very shallow magma conduits. However, the small size of the island precludes accurate determination of the dimensions of the deeper diatremes, formed by excavation of the pre-eruption sea floor, through gravity data models.
