of safety, only plumes and relatively small volcanic clouds have
been directly sampled in this way. The particles consist of two
represent fragments of the magma. These are glassy pyroclasts
and minerals, which represent the crystalline fraction of the
magma. Their shape is angular, and basaltic and andesitic
eruptions give rise to particles that have moderate aspect ratios
(Riley et al, 1999), while rhyolitic eruptions can generate an
abundance of glassy pyroclasts with a platy geometry and extreme
aspect ratios (Rose and Chesner, 1987). The diameters of silicate
pyroclasts generated during explosive eruptions range from meters
to microns. Those in volcanic clouds are smaller, generally less
than about 50 microns. The mass proportions of silicate particles
with diameters less than about 1 micron are very small (Rose et al
Non-silicate particles are
related to reactions among the constituents of the volcanic gases.
These particles are generally smaller than the silicates, usually
less than 1 micron in diameter. The most common composition for
these is sulfate, especially H2SO4, which
forms as submicron spherical droplets which also contain H20
(typically about 25% by volume--Zhao et al, 1995). A total of at
least 28 different phases have been observed also (see Table 3 in
Rose et al, 1982 for a partial list) including native sulfur,
sulfates, haloids, metallic oxides, and such exotic species as
silver sulfide and even native gold (Meeker et al, 1985). Overall
the analogy between the observed phases and fumarolic
incrustations and sublimates at gas vents (Stoiber and Rose, 1974;
Bernard, 1985; Symonds et al, 1987) suggests that these phases
originate from reactions among the volcanic gases, sometimes
involving the atmosphere and volcanic silicates.
Besides these two broad
types, a wide variety of other, unexplained materials have been
observed in volcanic clouds. They consist largely of phases that
are amorphous and have uncertain compositions (Chuan et al, 1987).
Many or most of these particles are likely to be non-volcanic in
origin, and represent accidental material of surficial or
Direct sampling and analysis
in volcanic clouds has been done only rarely (e.g. Cadle et
al, 1979) although analysis of CO2 has been done much
more extensively during CO2 flux determination surveys
(Harris et al, 1981; Gerlach et al 1999). Other information about
gases has been collected from extensive airborne remote sensing of
volcanic plumes using the correlation spectrometer (COSPEC)
instrument. These results show that the volcanic gases in volcanic
clouds are mixed and highly diluted by the ambient atmosphere, and
the concentrations of volcanic SO2 and CO2
are less than a few ppmv (McGee, 1992).
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