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Eos, Transactions, American Geophysical Union, 74:19, pp 217 and 221-222.NOTE: This version is lacking the final edits by the Eos editors (mostly punctuation).
On 27 June, 1992, the Crater Peak vent on the south side of Mt. Spurr
awoke from 39 years of dormancy and burst into sub-plinian eruption
after 10 months of elevated seismicity. Two more eruptions followed
in August and September. The volcano lies 125 km west of Anchorage,
Alaska's largest city and an important international hub for air travel.
The Alaska Volcano Observatory (AVO) was able to warn communities and
the aviation industry well in advance of these eruptions. Background Chronology
The Spurr massif is a large andesitic stratovolcano which collapsed in
the late Pleistocene to produce an ashflow-covered debris avalanche and
a 5 by 6 km caldera. Post-collapse effusions formed a central
silicic-andesite summit dome, and Crater Peak, a basaltic-andesite cone
in the caldera breach (Figure 1
; Riehle, 1985;
Nye and Turner, 1990). Crater Peak produced some 40 major mid- to
late-Holocene tephra eruptions and the only other historic eruption in
1953 (Riehle, 1985; Juhle and Coulter, 1955). The volcano has been
monitored seismically for 10 years by a system which now incorporates
technical advances developed in response to the eruptions at Redoubt
Volcano in 1989-90 and Mt. Pinatubo in 1991 (March and Power, 1990;
Murray et al., 1991). Eight seismometers were in place, including one
on the crater rim, just before the June eruption. Normally, about 10
earthquakes were located per month, but swarms of scores of events shook
the north caldera rim in 1982 and 1989.
AVO monitors potentially hazardous Alaskan volcanoes, particularly those
in Cook Inlet. When an eruption is believed to be imminent or is confirmed,
AVO conducts an emergency call-down to key federal, state, and municipal
agencies. AVO also communicates hazard information through periodic
public updates, as well as through scientific publications.
The first sign of reawakening was a swarm of small volcano-tectonic (VT)
earthquakes beneath Crater Peak in August 1991. Following two months of
relative calm, seismicity beneath the volcano increased to about ten times
the pre-August level (Figure 2).
Pulses of seismicity occurred in February, May, and June 1992. Most events were concentrated
beneath Crater Peak and Spurr's summit, but some occurred under the north
caldera rim. On 3 June AVO issued an advisory to government agencies and
the airline industry to plan for an eruption. The daily number of located
events reached 28 on 5 June, after which VT earthquakes declined to about
3-5 events/day and bursts of shallow tremor began. An overflight on 8 June
revealed upwelling in the crater lake, which had turned from green to gray.
One to 24 shallow tremor bursts per day, each 1-10 minutes long, were recorded
at stations within 10 km of Crater Peak between 6 and 26 June. On 11 June, a
field party found the lake temperature to be 50 C, pH 2.5, and SO4/Cl 2.7;
temperature and pH were similar to the 1970s, but SO4/Cl had increased one
hundred fold. Small new geysers erupted in the talus pile at the base of the
crater wall.
On behalf of AVO, the Alaska State Seismologist briefed the Alaska Division
of Emergency Services on 17 June. Tremor duration increased abruptly at
1534 ADT (all times in this article are Alaska Daylight Time, UT-8 hours,
unless otherwise noted) on 24 June with the onset of a tremor episode
which lasted 154 minutes. Twelve hours later a similar episode lasted 142
minutes. Eight additional tremor bursts occurred within the following 18 hours.
On the morning of 26 June a field party reported that the crater lake had
mostly disappeared and that lithic blocks, presumably forcefully ejected, had
impacted the residual mudflat. At 1204 continuous tremor began. Meanwhile,
at 1630, AVO issued formal warning of concern and went on 24-hour staffed
duty. The final stage of the prelude began at 0300 on 27 June with a swarm
of VT at 0-2 km depth, which built to rates of 1 event every 2 minutes, and a
few long-period (LP) earthquakes. This swarm probably reflected the forceful
injection of magma into the upper conduit. Abrupt doubling of tremor amplitude
at 0705 heralded onset of eruption, which soon destroyed the closest seismic
station, 400 m from the vent. Tremor amplitude gradually increased throughout
the eruption, peaking between 0935 and 1025, and was recorded on stations more
than 100 km away. Midmorning pilot reports estimated the plume to be as high
as 9 km, and the National Weather Service (NWS) estimated a plume height of
14.5 km based on C-band radar measurements (pers. comm.). Small volumes of
hot pyroclastic debris mixed with snow late in the eruption, forming flows
which swept down the south side of the cone to the Chakachatna River, 6 km
from the crater. The eruption lasted 4 hours. Seismicity decreased and by
8 July was below pre-August 1991 levels. In intensity and brevity, the
volcano had repeated its 1953 performance (Juhle and Coulter, 1955).
The 27 June airfall-tephra formed a narrow black stripe that passed northward
into sparsely populated areas, broadening in the lee of the Foraker-McKinley
massif in the Alaska Range (Figure 3).
Ash thickness was about 2mm in Denali National Park, 260 km downwind, and 1mm in Manley Hot
Springs, 420 km downwind. Tephra volume was about 50x106 m3 [20x106 m3
dense-rock equivalent (DRE)], and was mostly juvenile andesite. The plume
passed north to the Beaufort Sea, turned southeast into Canada and the
contiguous United States, and drifted eastward.
Seismicity remained low through July and the first half of August.
Seismic monitoring of the volcano was somewhat compromised by the destruction
of the crater rim station. Despite repeated attempts to reinstall the crater-rim
station, the closest seismometer was now 4.8 km away. Only one shallow and two
deep events were recorded between 12 August and 17 August. Perhaps the 27
June eruption "opened" the conduit, and allowed magma to rise undetected.
At 1538 on 18 August a 16-min episode of weak tremor including several LP events
began. At 1548 a pilot reported an ash-rich plume. The main eruption began
at 1642 when strong tremor was recorded on all Spurr stations. By 1658 a
subplinian column thrust through low clouds to reach 11 km altitude. Large
bombs were thrown 750 m above the vent Ultimately, the radar-determined plume
top reached about 14 km -- pilot reports were higher. Small pyroclastic flows
descended the east and southeast flanks of Crater Peak. Some flows were dry
and hot, and left coarse, clast-supported deposits with lobate, steep-fronted
margins. Other flows mixed with snow and ice high on the cone and were more
mobile and cooler. A late shower of mostly lithic blocks as large as 1 m
were hurled as far as 3.8 km southeast of Crater Peak. The southeastward
distribution of these deposits was controlled by the position of the vent
against the northwest crater wall. More than 170 lightning strikes were
detected by the AVO lightning detection system during the second half of
the eruption. Eruption ended after 3 hours and 28 minutes at 2011, but
intermediate and deep crustal seismicity increased afterward to levels
comparable to those of mid-June.
The volume of August tephra is about 110x106 m3 (40x106 m3 DRE). Upper-level
winds took the tephra plume east-southeast directly over Anchorage
(Figure 3,
Figure 4),
where sand-sized ash fell as thick as 3 mm. Beyond Anchorage, the axis of
the plume crossed the Chugach Mountains and followed the coast toward
Yakutat Bay. At Yakutat, 550 km downwind, ashfall was significant; at
Juneau, 1000 km downwind, the plume was opaque enough to disrupt air traffic.
Ashfall forced the closing of Anchorage International Airport for 20 hours.
Air-quality alerts were issued during the ashfall and on the following day, as
vehicular traffic resuspended the ash.
16-17 September Eruption:
Seismicity remained elevated after the August outburst, but signs of an
impending eruption remained so weak, even on the newly reinstalled station
400 m from the vent, that AVO had a crew in the crater on the afternoon of 16
September. At about 1930 on 16 September, seismic activity - both discrete
events and weak tremor - began to increase at the crater rim station.
Tremor amplitude increased again at 2225. An eruption, accompanied by brief
incandescence, began at 2236 but lasted only 11 min. Weak tremor followed for
the next hour. At 0004 on 17 September the main phase of the eruption began
impulsively, accompanied by intermittent bright incandescence which could be
seen both from Anchorage and with AVO's telemetered slow-scan camera system,
130 km to the south-southeast. This eruption lasted 3 h 36 min. Pyroclastic
flows swept down the south, east, and east-northeast flanks of Crater Peak,
entraining snow to become lahars. Other flows moved down the south flank.
Although these flows look like pyroclastic flows, they were cool and
water-saturated by the afternoon of 17 September. A narrow ballistic field
extends at least 10 km east from the vent. This time a strong swarm of
about 50 VT shocks occurred between depths of 5-10 km during the last part of,
and for a few hours following the eruption, perhaps reflecting readjustment of
the system after magma withdrawal.
The ash cloud rose to a radar-determined height of nearly 14 km (NWS, pers.
comm.) and curved eastward across Alaska, only slightly dusting Anchorage but
depositing enough ash to prompt air quality alerts and air traffic disruptions
in Palmer, Wasilla, and surrounding communities (Figure 3).
There was substantial ashfall in Glenallen, 350 km east, and detectable
ashfall at Burwash Landing, 700 km east. Tephra volume is about 50x106 m3
(20x106 m3 DRE).
Post-eruption:
Rates of seismicity remained high through 1992, with two notable seismic
crises. Nearly 24 hours of continuous tremor on 1-2 October followed by 72
hours of intermittent, quasi-periodic, tremor drove AVO to its highest
concern-code, although no eruption occurred. AVO again went to its highest
concern-code at 2207 AST on 9 November, a half hour into a 3.5 hr swarm of
more than 170 earthquakes, but once again no eruption followed. These
earthquakes had mixed frequency contents, occurred at three locations
a few hundred meters apart and 1.2 km below Crater Peak, and had magnitudes
up to 1.7. They probably were caused by the shallow intrusion of magma.
The duration of this swarm was similar to those of the three eruptions.
Seismicity is gradually returning to background level at this time.
Chemistry and Petrology
Most proximal bombs are cauliflower-like porphyritic hornblende-bearing
andesite with brown, microlite-rich andesitic groundmass glass. Subordinant
light grey-green andesite scoria has clear rhyolitic glass. Rare light-colored
pumice composed of nearly microlite-free rhyolitic glass with a few percent of
plagioclase phenocrysts and rare quartz grains also occurs. Much of the distal
tephra appears to be the same as the proximal bomb material. Despite large
variations in groundmass glass composition, the andesite is uniform in major and
trace element composition throughout the eruptions. The 1992 andesite differs
from 1953 andesite in having similar or lower concentrations of highly
incompatible elements at higher SiO2 (Figure 5).
1992 andesite is therefore not a simple fractionate of magma that fed the
previous eruption, but represents a new batch of magma. Prehistoric Crater
Peak lava flows record the rise of small, chemically unrelated, batches of
magma which fed only a few eruptions.
August and September pyroclastic flows contain several percent of gneissic
xenoliths that are partly melted and highly inflated. Smaller proportions of
similar material were also found in proximal June 27 deposits. These xenoliths
consist of clear, vesicular, rhyolitic glass with crystals of plagioclase,
orthopyroxene, cordierite, sillimanite, garnet, and spinel. Subordinate
xenolith lithologies include white plagioclase-quartz-glass rock, which is
siliceous (76 wt.% SiO2), yet, compared to the andesite, strongly depleted in
all incompatible elements except U, and fewer xenoliths of
garnet-plagioclase-wollastonite skarn. All xenoliths are metamorphic, which
is remarkable because most of the country-rock exposed near Spurr is granitic.
Cordierite-bearing samples fall off the trend of Spurr lavas towards high
concentrations of both compatible transition metals and K, Cs, Rb, Ba, REE,
Nb, Ta Y, U, Th, and Pb. Nye and Turner (1990) suggested that silicic "sweat"
from the upper crust forms a significant component of all Spurr magmas.
These xenoliths in bulk cannot be this component, because of their high
concentrations of compatible transition metals, but their interstitial melt
(as yet unanalyzed) may be. The xenoliths provide direct evidence of
extensive partial melting of country rock beneath Spurr.
Gas Monitoring
AVO routinely monitors SO2 emission from Cook Inlet volcanoes by airborne UV
correlation spectroscopy (COSPEC). In addition, airborne IR spectroscopy of
CO2 was started on 25 September, 1992. There were slight increases in SO2
during premonitory seismic activity in August 1991 and May-June 1992, but
COSPEC-determined SO2 concentrations were slight on 8 June. Surprisingly, SO2
concentrations were down to the detection limit two days after the June
eruption, despite the fact that Total Ozone Mapping Spectrometer (TOMS) data
indicate that the eruption cloud contained 185 kilotonnes of SO2 (Global
Volcanism Network, 1992a). SO2 values were low after the August eruption,
and before and after the September eruption, although again TOMS data
indicate that the plumes contained 300 and 190 kilotonnes of SO2, respectively
(Global Volcanism Network 1992b). The lack of SO2 between eruptions, despite
the S-rich nature of the magma, may reflect absorption of SO2 by an active
hydrothermal system during non-eruptive degassing. Alternatively, magma may
have remained deep, below the depth of gas-saturation, until just before each
eruption.
Discussion
Spurr has departed from its 1953 behavior by erupting three times instead of
once. Yet all four eruptions share a commonality in volume and duration, as
if controlled by some unchanging parameters involving initial overpressure,
conduit volume, and magma-production dynamics in the deep source. What is not
shared is the associated pattern of seismicity -- a vexing problem for would-be
forecasters. We have observed most possible permutations: seismicity
preceding but not following eruption, seismicity following but not preceding
eruption, and seismicity both preceding and following eruption. We also
observed intense seismicity without eruption in October and November.
This fascinating story, and the character of magmatic behavior it implies,
continues to unfold.
Acknowledgments
This work was supported by the Alaska Volcano Observatory. AVO is a cooperative
program of the US Geological Survey (USGS, 4200 University Drive, Anchorage,
AK 99508 and 345 Middlefield Road, Menlo Park, CA 94025 ), University of
Alaska Fairbanks Geophysical Institute (UAFGI, University of Alaska Fairbanks,
Fairbanks, AK 99775), and Alaska Division of Geological and Geophysical Surveys
(ADGGS, 794 University Avenue #200, Fairbanks, AK 99709) under the Volcano
Hazards Program of the USGS. The State of Alaska provided additional funding
during the Spurr crisis through its Division of Emergency Services.
Principal Investigators
Principal contributors to the work summarized here are, from the USGS,
T Casadevall, B Chouet, J Dorava, M Doukas, I Ellersieck, C Gardner, R Hoblitt,
A Jolly, T Keith, J Lahr, T Mattox, B May, G McGimsey, D Meyer, T Miller,
C Neal, R Page, J Paskievitch, J Power, C Stephens, D Trabant, and R Waitt.
From the UAF/GI, J Beget, J Davies, K Dean, J Eichelberger, M Harbin, J Kienle,
S McNutt, S Swanson, and G Tytgat. From ADGGS, R Motyka, C Nye, and G March.
Requests for reprints may be sent to Chris Nye at UAFGI.
References
Global Volcanism Network, Bulletin of the Global Volcanism Network,
Smithsonian Institution, 17, 6-8 , 1992a.
Global Volcanism Network, Bulletin of the Global Volcanism Network, Smithsonian
Institution, 17,-3, 1992b.
Juhle, W. and H. Coulter, The Mt. Spurr eruption, July 9, 1953; Transactions,
Amer. Geophys. Union, 36, 199-202, 1955.
March, G.D. and J.A. Power, A networked computer configuration for seismic
monitoring of volcanic eruptions, U.S. Geol. Survey Open-file Report 90-442, 19 pp, 1990.
Murray, T.L., J.A. Power, G.D. March, A.B. Lockhart, J.N. Marso, and A. Miklius,
Application of a real-time data acquisition and analysis system in response to
activity at Mount Pinatubo, Phillippines, 1991 (abstract), Eos, 72, 47, 1991.
Nye, C.J., and D.L. Turner, Petrology, geochemistry, and age of the Spurr volcanic
complex, eastern Aleutian arc, Bull. Volcanol., 52,05-226, 1990.
Riehle, J.R., A reconnaissance of the major Holocene tephra deposits in the
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