Characterization of tephra deposits with limited exposure: the example of the two largest explosive eruptions at Nisyros volcano (Greece) |
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Authors: | Celine Longchamp C Bonadonna O Bachmann A Skopelitis |
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Institution: | 1.IGAR, Université de Lausanne,Lausanne,Switzerland;2.Section des sciences de la Terre et de l’environnement,Université de Genève,Geneva,Switzerland;3.Department of Earth and Space Sciences,University of Washington,Seattle,USA |
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Abstract: | Explosive eruptions associated with tephra deposits that are only exposed in proximal areas are difficult to characterize.
In fact, the determination of physical parameters such as column height, mass eruption rate, erupted volume, and eruption
duration is mainly based on empirical models and is therefore very sensitive to the quality of the field data collected. We
have applied and compared different modeling approaches for the characterization of the two main tephra deposits, the Lower
Pumice (LP) and Upper Pumice (UP) of Nisyros volcano, Greece, which are exposed only within 5 km of the probable vent. Isopach
and isopleth maps were compiled for two possible vent locations (on the north and on the south rim of the caldera), and different
models were applied to calculate the column height, the erupted volume, and the mass eruption rate. We found a column height
of about 15 km above sea level and a mass eruption rate of about 2 × 107 kg/s for both eruptions regardless of the vent location considered. In contrast, the associated wind velocity for both UP
and LP varied between 0 and 20 m/s for the north and south vent, respectively. The derived erupted volume for the south vent
(considered as the best vent location) ranges between 2 and 27 × 108 m3 for the LP and between 1 and 5 × 108 m3 for the UP based on the application of four different methods (integration of exponential fit based on one isopach line,
integration of exponential and power-law fit based on two isopach lines, and an inversion technique combined with an advection–diffusion
model). The eruption that produced the UP could be classified as subplinian. Discrepancies associated with different vent
locations are smaller than the discrepancies associated with the use of different models for the determination of erupted
mass, plume height, and mass eruption rate. Proximal outcrops are predominantly coarse grained with ≥90 wt% of the clasts
ranging between −6ϕ and 0ϕ. The associated total grainsize distribution is considered to result from a combination of turbulent
fallout from both the plume margins and the umbrella region, and as a result, it is fines-depleted. Given that primary deposit
thickness observed on Nisyros for both LP and UP is between 1 and 8 m, if an event of similar scale were to happen again,
it would have a significant impact on the entire island with major damage to infrastructure, agriculture, and tourism. Neighboring
islands and the continent could also be significantly affected. |
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