Characterization of tephra deposits with limited exposure: The example of the two largest explosive eruptions at Nisyros Volcano (Greece)


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Characterization of tephra deposits with limited exposure: The example of the two largest explosive eruptions at Nisyros Volcano (Greece)
Bulletin of Vulcanology
Longchamp C., Bonadonna C., Bachmann O., Skopelitis A.
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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 ?6? and 0?. 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.
Tephra deposits, Explosive volcanism, Eruptive parameters, Inversion, , Volume
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25/11/2013 20:43
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14/02/2022 8:55
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