Geoscience Reference
In-Depth Information
6.2 Ash plume dynamics
The onset impulsivity of the same jet plume is nicely displayed on the time series of figure
15a, showing a sharp onset in maximum radial velocities, rapidly peaking over 80 m/s and
then decreasing exponentially over about 10 seconds. The power maximum amplitude is
also at the onset, indicating a mass flux maximum at the beginning, and P+ decreases
rapidly after a few seconds and then more gently after about 10 s from the onset. The
contribution of falling ballistics keeps P- high for a longer time. The 10 second phase of
strong velocities and echo power corresponds to the initial mushroom-like ash plume head
heavily charged with large blocks which dominate the signal. After this phase, radial
velocities remain steady and low (10-20 m/s) and the power decreases more gently. The
ballistics might therefore come from the disruption of the solidified lava plug by the gas
overpressure accumulated underneath. This would clear the vent or fracture and open a
way to a milder sustained gas release remobilizing variable quantities of ash and possibly
fragmenting the lava to form juvenile ash.
Not all tephra emissions are impulsive at Arenal, and there are a wide variety of eruptive
behaviors (Mora et al., 2009; Valade et al., 2012). Some emissions comprise mainly ash and
are sustained typically for a minute or so (Fig. 15b). In contrast to the impulsive signal of jet
plumes (Fig. 15a), the peak in echo power comes about 10 s after the onset. The second
striking feature is the large oscillations correlated between P+ and P-, and having a
remarkable periodicity of about 3 s. This indicates pulsations in the amount of material
emitted, suggesting a staccato pressure release (Donnadieu et al., 2008). This observation
supports the clarinet model of Lesage et al. (2006) for the volcanic tremor at Arenal, in which
intermittent gas flow through fractures produces repetitive pressure pulses. The repeat
period of the pulses is stabilized by a feedback mechanism associated with standing or
traveling seismic waves in the magmatic conduit. Moreover, these rhythmic variations
might well be a common feature of persistently active volcanoes with intermediate lava
composition. In eruptions of Santiaguito volcano, Guatemala, Scharff et al. (2012) also
observed multiple explosive degassing pulses occurring at intervals of 3-5 s, with common
velocities of 20-25 m/s.
Figure 15c illustrates the signature of a larger ash plume of Popocatépetl, in Mexico,
reaching a few kilometers above the volcano. Because its summit culminates at nearly 5450
m a.s.l., even small ash plumes generate hazards to the aviation, to the surrounding
infrastructures and airports and the 30 million inhabitants leaving within 100 km of the
volcano in important cities like México and Puebla. Its crater is 600 by 800 m wide with a
growing lava dome inside. The relatively low velocities (<35 m/s) measured at 5085 m,
along with the velocity peak not reached immediately after the onset, suggest low excess
momentum and a mainly buoyant uprise, like most ash plumes in 2007 at Popocatépetl.
Note that, in the case of ash plumes, radial velocities might reflect plume velocities more
closely because convection and turbulence create eddies entraining particles that would
tend to generate echoes with radial velocities toward and away from the radar, with
comparable amounts of echo power in the absence of wind. From the time lag of P- relative
to P+, however, it can be inferred that wind was blowing with some component toward the
radar, i.e. to the north. In addition the power backscattered in 3 range gates simultaneously
reveals that the horizontal dimension of the plume at the beam level was 2-3 times the radial
resolution, i.e. >300 m. The comparable level of echo power at 5085 m and 5235 m, along
