Geoscience Reference
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explosion momentum occurs during the first 6-7 s and corresponds to the main discharge of
large ballistic blocks and dense lapilli- and ash-laden plume, with an initial maximum
followed by a rapid decrease. The following phase, with lower velocities and echo power,
results from the final emptying of the gas slug tail causing the relatively milder release of
gas and finely fragmented ash at lower concentrations.
Fig. 10. (
left
): Time series of power (
top
) and maximum radial velocities (
bottom
) of a
Strombolian outburst recorded by VOLDORAD aiming downward into Yasur's southern
crater on 27/09/2008. Data are smoothed with 3 incoherent integrations (0.21 s).
Fig. 11. (
right
): Strombolian explosion at southern crater of Yasur volcano, Vanuatu.
Incandescent pluridecimetric lava blocks resulting from the magma surface disruption by
the pressurized gas slug are visible ahead of the ash plume (about a hundred meters high)
generated by the magma fragmentation during gas pressure release. Photo courtesy: A.
Finizola (2008).
5.2.4 Tephra mass loading
Gouhier & Donnadieu (2008) presented a method to estimate the particle loading
parameters (mass, number, volume) of eruptive jets from the inversion of the echo power.
The inversion algorithm uses the complete Mie (1908) formulation of electromagnetic
scattering by spherical particles to generate synthetic backscattered power values. Assuming
a log-normal shape for the particle size distribution, they estimated the total mass of tephra
emitted during Strombolian explosions at Etna at around 58 and 206 tons for low and high
concentration lava jets respectively. Derived parameters such as mass flux, particle kinetic
and thermal energy, and particle concentration can also be estimated. As for particle
concentrations, they must be regarded as minima in the case of small scale phenomena
(relative to the range gate dimensions) like lava jets, because they are spatially highly
heterogenous and might not completely fill the sounded volume. More reliable
concentration values could be obtained when several range gates are filled completely, such
as one expects from large ash plumes. For instantaneous events like Strombolian explosions,
a total mass can be calculated from the echo power maximum amplitude assuming that all
particles are present in the beam at the instant of the peak power. The instantaneous mass
flux that can be derived in this way differs, however, from the initial mass flux as the power
