USGS/Cascades Volcano Observatory, Vancouver, Washington
Sulfur Dioxide Scrubbing During the 1992 Eruption of Crater Peak,
Mount Spurr Volcano, Alaska
by:
Michael P. Doukas 1
and Terrence M. Gerlach 2
1 U.S. Geological Survey, Alaska Volcano Observatory,
4200 University Dr., Anchorage AK 99508-4667
2 U.S. Geological Survey, Cascades Volcano Observatory,
5400 MacArthur Blvd, Vancouver, WA 98661-7095
IN:
Keith, T.E.C., ed., 1995, The 1992 eruptions of Crater Peak vent, Mount Spurr
volcano, Alaska: U. S. Geological Survey Bulletin B-2139, 220p, p.47-57.
Address reprint requests to T. M. Gerlach (tgerlach@usgs.gov)
Permission granted to cite and distribute this document
ABSTRACT
Sulfur dioxide scrubbing by liquid water masked SO2 emissions
from shallow magma during the 1992 eruptions of Crater Peak
and effectively prevented observation of SO2 emissions from
shallow magma both before and after explosive eruptions and
seismic crises. Airborne ultraviolet correlation spectrometer
(COSPEC) measurements from July 22, 1991, to September 24,
1992, indicate only background to minor (<100 t/d) noneruptive
SO2 emissions from Crater Peak, even though this period
included the onset of precursory seismicity beneath Crater
Peak (August 1991), the peaking in frequency of volcano-
tectonic earthquakes and the initiation of volcanic tremor
bursts (June 5, 1992), and three explosive eruptions that
produced SO2 emissions of 200 to 400 kilotons (kt).
The low, nonexplosive SO2 emissions are explained by
hydrolysis of SO2 to aqueous H2S
and sulfate from interactions with liquid water:
4H2O(l) + 4SO2(aq) ===>
H2S(aq) + 3H+(aq) + 3HSO4-(aq).
Sulfur dioxide hydrolysis also explains the increase in
the sulfate content of Crater Peak lake water prior to the
first eruption, the strong H2S odor during periods of
background to low SO2 emission, the TOMS evidence for
significant H2S emissions during the explosive eruptions, and
the observed decline of SO2 during periods of volcanic tremor.
Abundant, local sources of melt water and a high permeability
for the Mount Spurr volcanic edifice are probably the chief
factors responsible for masking SO2 emissions by scrubbing,
and possibly for quenching shallow intrusions that were
ascending. Large SO2 emissions unencumbered by scrubbing were
only possible during the three explosive eruptions when magma
penetrated through liquid water zones under Crater Peak and
reached the surface. Nonexplosive SO2 emissions of as much as
750 t/d were possible, however, for a brief period when dry
pathways to the surface existed from September 25 until about
October 10, 1992. Airborne infrared spectrometer (MIRAN)
measurements of CO2 emissions indicate that in addition to the
degassing of magma through dry pathways, degassing through
boiling water with the loss of SO2 by scrubbing was also
important during that time. The CO2 emission data indicate
that magma degassing was taking place, and CO2/SO2 values
calculated from MIRAN and COSPEC data are in the range 10 to
100, which supports the hypothesis of SO2 loss by scrubbing.
Because of its strong preference for the vapor phase during
boiling, CO2 emissions from degassing magma are less likely to
be masked by the presence of water, whereas SO2 emissions may
be lost totally from interactions with water; thus misleading
COSPEC results are obtained. We recommended prompt and early
monitoring of CO2 when Cook Inlet volcanoes become restless.
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