Jökull - 01.12.1985, Page 5

TABLE1
Volcanic Volatile Mass from Historic Eruptions
TAFLA 1. Massi rokgjarna gosefna í sex eldgosum
Measured
Ice-core estimate volcanic
Total erupted Petrologic estimate (g) a) (g)(H2S04+HX)b) aerosol(g)
Eruption Eldgos rhass (g) Heildarmassi gosefna Mat byggt á bergfræðirannsóknum H2S04 HCl HF(+) Total acids Mat byggt á ís- kjarnarann- sóknum Mæld rokgjörn gosefni
Laki 1783 3.4X1016 9.03X1013 1.6 xlO12 n.d.(++) 9.19X1013 lOxlO13 —
Tambora 1815 2.4X1017 5.24X1013 2.16X1014 1.26X1014 3.94X1014 2xl014 -
Mount St. Helens 1980 6.5xl014 7.9 xlO10 3.52X1010 5.3 xlO10 1.67X1011 - 3x 10uc,d)
Soufriere 1979 4 xlO13 1.53X1010 3.5 xlO10 n.d. 5.03X1010 - 2.3xl09e)
Agung 1963 2.4xl015 2.84X1012 1.53X1012 o T—1 X 00 5.17X1012 2xl013 -
Krakatau 1883 2.7xl016 2.94X1012 3.75X1012 n.d. 6.69X1012 5.5xl013 -
(+) Data for fluorine from Devine and Sigurdsson (1984) except for new analyses for Agung (720 ppm F in glass
inclusions and 400 ppm in matrix glass) and for Mount St. Helens (360 ppm F in glass inclusions and 280 ppm in
matrix glass).
(++) n.d.=not detected c) Kent 1982
a) Devine et al. 1984 d) McCormick 1982
b) Hammer et al. 1980 e) McCormick et al. 1981
Laki is the latitudinal dependence of acidity. The data
from four cores, ranging from 65°N to 82°N (Hammer
1977, 1984) show a regular northward decrease,
approximating 0.7 p equiv. H+/degree latitude. This
trend should form a useful basis for refinement of
quantitative estimates of global volcanic acid aerosol
fallout from ice core data. We note on the other hand,
that the 90Sr fallout pattern, used in modelling dispersal
of volcanic aerosol mass (Hammer 1977, 1980) shows
the opposite trend, with increase in radionuclide activity
from 71°N to 77°N. The applicability of 90Sr fallout as a
model for dispersal of volcanic aerosols from a point-
source may be complicated by the fact that the USSR
nuclear tests from September 1961 to late 1962, which
are the sources of most of the fallout, were carried out
over a range of latitudes, from sites at Novaya Zemlya
(75°N), Siberia (52°N) and Semipalatinsk (50°N) (Koide
etal. 1979).
MOUNT ST. HELENS 1980
Although relatively small, the May 1980 explosion of
Mount St. Helens volcano was well monitored from
every angle. The total aerosol mass was determined as
3xlOu g (Kent 1982, McCormick 1983). By compari-
son, the petrologic estimate (Table 1) of volcanic vola-
tile yield is 1.67xlOn g HF, 3.5xl010 g HCl and 7.9x
1010 g H2S04. This estimate was based on a total
erupted tephra volume of 0.25 km3 (dense-rock equiva-
lent), but the recent discovery of large-volume pyroclas-
tic flow deposits indicate that this may be low by factor
of two (C. W. Criswell, pers. comm. 1984). Thus the two
estimates of the Mount St. Helens volcanic volatile mass
are very close.
The glass inclusion data imply a higher proportion of
chlorine and fluorine from Mount St. Helens than gen-
erally assumed in volcanic plumes and imply a HCl/SCL
ratio of 0.7. Chlorine compounds were observed in the
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