Environmental Engineering Reference
In-Depth Information
related to the energetics of the eruption (Heiken
et al
.,
1985
; Zimanowski
et al
.,
2003
). Sub-micron sized particles are present in every eruption and are often found
adhering to larger particles. The style of magma fragmentation (e.g. magmatic vs.
phreatomagmatic) influences the shape of the resulting ash particles (Sigurdsson
et al
.,
1999
). Ash particles are very rarely spherical, and shapes can range from
equant blocky fragments to angular cusps formed from broken bubble walls.
The composition of volcanic ash can be highly heterogeneous, and typically
contains a mixture of glass and crystal fragments, and lithic (rock) components
that were entrained in the magma, originating from, for example, the surrounding
country rock or walls of the volcanic conduit (De Rosa,
1999
). The glass particles
may themselves contain phenocrysts (large crystals), microlites (micro-crystals)
and bubbles formed by the exsolution of magmatic volatile phases.
A backscattered electron (BSE) image of a volcanic ash sample from the
May 2010 Eyjafjallajökull eruption (
Figure 7.2
) shows that the fragments vary
in composition, have irregular shapes and occupy a variety of sizes from a few
microns up to 0.5 mm.
All of these properties in
uence how ash is transported and how long
it remains in the atmosphere. They also in
uence how electromagnetic radia-
tion is transmitted, absorbed and scattered when it interacts with a cloud of
ash particles. Having an understanding of this relationship allows us to retrieve
information on these properties using the remote-sensing techniques previously
described.
7.3.2 Microphysical properties
The physical attributes relevant to remote sensing of fine ash, defined here as
particles with diameters less than
c
.60
μ
m, include the size distribution and
composition, with less importance on shape. The spectral region between 8 and
13
fine-ash properties, but limits the range
of particle radii that can be retrieved to between
c
.1
μ
m has mainly been exploited to infer
m. There are
few measurements of the size distribution of ash while resident in the atmosphere
and so this often has to be determined from deposits on the ground, which is not a
satisfactory situation. Some airborne data exist from the Mt St Helens eruptions of
May 1980 (Hobbs
et al
.,
1981
) and from the recent eruptions of Eyjafjallajökull
(Schumann
et al
.,
2011
) and Grímsvötn (Vogel
et al
.,
2012
) in Iceland. These data
suggest a bimodal size distribution with peaks near to 0.5
μ
mto
c
.16
μ
m radius
and have been modelled mathematically using gamma and log-normal distribu-
tions. There is very little experimental evidence of signi
μ
m and 3
-
6
μ
cant amounts of ash
with particle radii
ne
ash, e.g. Gislason
et al
.(
2011
), Lieke
et al
.(
2013
); for the purpose of remote
>
50
μ
m. There are measurements of the composition of
