Geoscience Reference
In-Depth Information
Ash is the major tephra component and is the main source of the ensuing hazards to
humans, infrastructures and aviation, as shown by the 2010 eruption of Eyjafjallajökull in
Iceland. This is because fine ash can remain in the atmosphere for hours to days, forming an
ash cloud in the distal part. Although termed an ash plume, the proximal part does not
comprise only ash, especially in the gas thrust and convective regions sounded by
VOLDORAD. Below, examples are described which give insight into the ash plume
dynamics close to the source.
6.1 Discriminating ballistics and ash
Tephra emissions are commonly explosive, having initial excess momentum compared with
purely buoyant plumes. Therefore the explosive emission driven by the expansion of
overpressured gas propels ash, lapilli and blocks in the air. Ash-sized particles closely
follow the turbulent gas regime whereas inertial blocks mainly follow ballistic trajectories.
So both are strongly decoupled, although a continuum of dynamic behaviors occurs in
between for intermediary particle sizes. Because the spatiotemporal distribution of their
velocity field and mass loading are contrasted, the dynamics of ballistics and ash can be
discriminated when radar targeting the gas thrust region of the volcanic jet. Figure 14
illustrates the distinctive Doppler signatures for a jet plume at Arenal volcano similar to
that shown in figure 13b and recorded with the beam aiming upward (27°) toward the
summit. Although not obvious from the analysis of the time sequence of Doppler spectra the
discrimination becomes particularly conspicuous on velocigrams. The velocigrams represent
the power spectral density (dB color scale) as a function of radial velocities (y-axis) and time
(x-axis) in 5 contiguous 120 m-wide range bins from 2367 to 2847 m. The 2607 m range bin,
located above the vent, first records the jet plume onset. A 3-D representation of the
velocigram at 2607 m is shown in the inset (cf. also topic cover image). The ballistics are
characterized by a short-lived signal (10-15 s) rapidly transiting through the gates. Range
gates above the vent show positive radial velocities shifting to negative in a matter of
seconds, as a result of the progressive bending of the ballistic trajectories through the
radar beam.
Contrastingly, blocks only enter range gates located down-beam with negative radial
velocities. So the time evolution of the spectral shape of this signal holds information about
the ejection geometry (height, angles, orientation) and mass load spatial distribution, in
addition to source parameters retrieved in section 5. Single streaks from individual blocks
are sometimes visible on the velocigrams, and power-derived sizes are often decimetric.
Considering lapilli to block sizes ranging between 0.04-1 m, Valade & Donnadieu (2011)
found a mass of ballistics in the range 0.5-7 tons, i.e. a dense rock equivalent volume of 0.2-
2.8 m
3
, for a similar event at Arenal. The second signal characterizes the ash plume, with
lower backscattered power (by 10-20 dB), longer duration (>1 mn), slower transit through
the gates, and with only negative velocities because the wind pushes the ash toward the
radar. Interestingly, these characteristic maximum radial velocities may be used to constrain
the effect of the wind and the buoyant ascent velocity. Although clearly smaller than for
ballistics, the particle size distribution in the ash plume is poorly constrained, and so is the
ash mass. Also, the longer duration and wider spatial coverage of the ash cloud requires
spatial and temporal integration to obtain the total mass, which is nevertheless presumably
greater than the mass of ballistics.
