Studies by the U.S. Geological Survey in Alaska, 2005

U.S. Geological Survey Professional Paper 1732–B

Geothermal Disruption of Summit Glaciers at Mount Spurr 

Volcano, 2004–6: An Unusual Manifestation of Volcanic 

Unrest

By Michelle L. Coombs, Christina A. Neal, Rick L. Wessels, and Robert G. McGimsey

Abstract

Mount Spurr, a 3,374-m-high stratovolcano in the Cook 

Inlet region of Alaska, showed signs of volcanic unrest beginning in 2004 and lasting through 2006. These signs included 

increases in heat flow, seismicity, and gas flux, which we 

interpret as the results of a magmatic intrusion in mid-2004. In 

response, debris-laden meltwater beneath the glacier in Mount 

Spurr’s geothermally active summit basin accumulated as the 

overlying snow and ice melted. As heat output increased, the 

icecap subsided into a growing cavity over a meltwater lake, 

similar to that observed during subglacial volcanic activity in Iceland. An ice plug collapsed into the lake sometime 

between June 20 and July 8, 2004, forming an ice cauldron 

that continued to grow in diameter during 2004 and 2005. A 

freefall of ice and snow into the lake likely caused a mixture of water and debris to be displaced rapidly upward and 

outward along preexisting englacial and, possibly, subglacial 

pathways leading away and downslope from the summit basin. 

Where these pathways intersected crevasses or other weak 

points in the sloping icefield, the mixture debouched onto the 

surface, producing dark, fluid debris flows. In summer 2004, 

the occurrence of two sets of debris flows separated in time by 

as long as a week suggests two pulses of summit ice collapse, 

each producing a surge of water and debris from the lake. A 

single debris flow was also emplaced on May 2, 2005. This 

event, which was captured by a Web camera, occurred simultaneously with a lake-level drop of ~15 m. To the east of the 

ice cauldron, a spillway that fed the debris flows has apparently maintained a relatively constant lake level for months 

at a time. Aerial photographs show that the spillway is in the 

direction of a breach in the summit crater. Melting of snow 

and ice at the summit has continued through 2006, with a total 

meltwater volume of ~5.4 million m3 as of March 2006.

Introduction

Mount Spurr is a 3,374-m-high ice- and snow-covered 

stratovolcano situated in rugged wilderness 125 km west of 

Anchorage (fig. 1). Explosive eruptions from Crater Peak, a 

satellite vent 3.5 km south of the summit (fig. 2), occurred in 

1953 (Juehle and Coulter, 1955) and 1992 (see Keith, 1995, 

and references therein), producing ash falls on populated areas 

of south-central Alaska and small-volume pyroclastic flows 

and lahars at the volcano. The summit of Mount Spurr, which 

is largely ice covered, has been interpreted as a lava-dome 

complex, on the basis of limited sampling (Nye and Turner, 

1990). This feature lies within a 5.5-km-wide, largely ice 

filled collapse caldera that formed in late Pleistocene or early 

Holocene time (fig. 2; Nye and Turner, 1990). On the basis of 

studies of tephra deposits, the latest eruption at the summit is 

believed to have occurred ~5.2 ka (Riehle, 1985).

In summer 2004, increases in seismicity, heat flux, and 

gas flux and resulting drastic morphologic changes at the summit of long-dormant Mount Spurr heightened concern about a 

possible eruption. In response, the Alaska Volcano Observatory (AVO) enhanced its geophysical monitoring, mounted 

a series of observation flights and airborne gas and thermal 

measurements, increased acquisition of remotely sensed data, 

and examined historical imagery of the volcano in an effort 

to understand the significance of the observed changes. As of 

early 2006, seismicity remains above background level, but 

no eruption has occurred. Regardless, the ongoing volcanic 

unrest has been interpreted as an injection of magma beneath 

the summit, producing a prolonged increase in magmatic-gas 

output and heat flux (Power, 2004; Power and others, 2004; 

Cervelli and others, 2005; De Angelis and McNutt, 2005). 

This chapter describes and interprets the morphologic changes 

in glaciers at the summit of Mount Spurr in response to this 

magmatic intrusion.

Geology of the Summit

The Spurr volcanic complex, characterized by Nye and 

Turner (1990), is a long-lived stratovolcano that is the easternmost active volcanic center in the Aleutian Arc. The complex 

consists of ancestral Mount Spurr, which is as old as middle 

Pleistocene, on the basis of K-Ar dating of stratigraphically 

low lavas (>250 k.y.; Nye and Turner, 1990) and two Holo-
   Studies by the U.S. Geological Survey in Alaska, 2005

cene vents. A much older volcanic center may have existed in 

the same place, on the basis of isolated exposures of Pliocene 

lavas (Nye and Turner, 1990; M.L. Coombs and A.T. Calvert, 

unpub. data, 2005). Ancestral Mount Spurr was eviscerated by 

a catastrophic landslide that formed a large debris-avalanche 

deposit to the south and southeast and an avalanche caldera, 

~5 km in diameter. The date of this event is unknown but is 

probably early Holocene because the debris-avalanche deposit 

was not affected by late Pleistocene glaciation (Nye and 

Turner, 1990; Waythomas and Nye, 2002).

Since caldera formation, eruptive activity at Mount Spurr 

has focused at two vents: the summit and Crater Peak (fig. 2). 

Over the past several thousand years, most eruptions of the 

Spurr volcanic complex have been from Crater Peak, a satellite 

vent at the southern caldera margin. These eruptions produced 

basaltic andesite tephra falls and local flow deposits (Riehle, 

1985; Nye and Turner, 1990). All historical eruptions of the 

complex have been from the Crater Peak vent, including the 

most recent in 1992 (see Keith, 1995, and references therein).

Less is known about the eruptive history of the summit. 

The mostly snow and ice covered summit dome complex rises 

460 m above the icefield that fills the caldera. Several samples 

collected from the summit in the 1980s are dacitic (Nye and 

Turner, 1990). On the basis of chemical correlation, a single 

tephra fall dated at 5.2 k.y. is attributed to the summit vent 

(Riehle, 1985). Other relatively silicic, Holocene deposits may 

also have their origins at the summit, including a pyroclasticflow deposit along the Chakachatna River (C.J. Nye, written 

commun., 2004).

Despite the relative scarcity of evidence for summit eruptions, the morphology of the summit dome complex suggests 

that this feature has not been extensively eroded by glaciers. 

Since the 1970s, the amount of snow and ice at the summit has 

fluctuated, at times allowing views of the crater that sits at its 

top. More discussion of historical observations follows in a 

later section.

The Spurr volcanic complex has been seismically monitored by AVO with a network of at least six stations since 1989 

(Jolly and others, 1994). During the 1992 eruptions of Crater 

Peak, seismicity was at times focused beneath the summit and 

not at Crater Peak (Power and others, 1995); and since 1993, 

most earthquakes have occurred beneath the summit. Volcanicgas measurements at the complex have focused on Crater 

Peak, the historically active vent. Airborne gas measurements 

Figure 1. Cook Inlet region, southern Alaska, showing locations of Mount Spurr and other 

volcanoes (triangles). Dots, towns and villages. Map courtesy of Janet Schaefer, Alaska Division 

of Geological and Geophysical Surveys.
at Crater Peak indicate a background, noneruptive rate of SO2

degassing of no more than a few tens of tonnes per day; Crater 

Peak also has consistently degassed a small amount of CO2

(typically <200 t/d) since 1994 (Doukas, 1995; M.P. Doukas, 

written commun., 2004). Before the 2004–6 volcanic unrest, 

no airborne gas measurements had been made at the summit of 

Mount Spurr.

Onset of Unrest, Generation of Summit 

Ice Cauldron, and Debris Flows

In early July 2004, AVO seismologists noted an increase 

in the rate of volcanotectonic and long-period earthquakes 

occurring beneath the summit of Mount Spurr (Power, 2004; 

Power and others, 2004; De Angelis and McNutt, 2005). AVO 

was also contacted by a long-time Alaskan pilot who flew near 

the volcano on July 11 and reported a small steam plume from 

the ~1,675-m level of the east side of Crater Peak, as well 

as an odor of SO2. Both observations were slightly unusual 

for the Crater Peak area. On the basis of this report and the 

increase in seismicity, AVO launched a fixed-wing observation flight to the volcano on July 15 (table 1). Although 

clouds obscured the summit, the east flank was visible and 

streaked with nearly a dozen dark debris-flow deposits (fig. 

3; McGimsey and others, 2004)—to our knowledge, the first 

observation of debris flows from the summit of Mount Spurr. 

On August 2, an AVO overflight also confirmed the presence 

of a steep-walled collapse pit in the summit basin. We describe 

these features in sequence below and discuss their relation and 

significance to the volcanic unrest at Mount Spurr.

Summer 2004 Debris Flows

The summer 2004 debris flows produced sheets and 

elongate lobes of dark material that cascaded down the steep 

glacier surface, draping and flowing into and across crevasses 

(fig. 4). The flows emerged primarily from crevasses in the 

glacier on the precipitous (20–45°) east flank of the summit. 

Several flows appeared to have cascaded down the steep, 

Figure 2. Spurr volcanic complex (see fig. 1 for location). Summit of Mount Spurr sits in an ice-filled collapse 

caldera, breached to the south; Crater Peak, a historically active satellite vent, sits in breach. Debrisavalanche deposit in foreground, which was formed during caldera formation, is dated at early Holocene 

(Waythomas and Nye, 2002). View northward; photograph 666–6 by Austin Post, taken September 4, 1964.

Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 
   Studies by the U.S. Geological Survey in Alaska, 2005

Table 1. Chronology of events and observations relating to volcanic unrest at Mount Spurr, Alaska, 2004–6

[ASTER, Advanced Spaceborne Thermal Emission and Reflection Radiometer; AVO, Alaska Volcano Observatory; FLIR, Forward-Looking Infrared; NASA, 

National Aeronautics and Space Administration; USGS, U.S. Geological Survey]
Table 1. Chronology of events and observations relating to volcanic unrest at Mount Spurr, Alaska, 2004–6—Continued

[ASTER, Advanced Spaceborne Thermal Emission and Reflection Radiometer; AVO, Alaska Volcano Observatory; FLIR, Forward-Looking Infrared; NASA, 

National Aeronautics and Space Administration; USGS, U.S. Geological Survey]

Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6   
   Studies by the U.S. Geological Survey in Alaska, 2005

Figure 3. East face of summit cone of Mount Spurr (fig. 1), showing debris-flow deposits, lettered in order of 

deposition (see figs. 4, 5): a–c, June 20 to July 8, 2004; d–h, July 10–15, 2004. True summit is obscured by clouds. 

Area of dark bare ground and debris on south face (upper left) is a long-lived zone of frequent avalanches of mixed 

rock, ice, and snow. View northwestward; photograph by C.A. Neal, taken July 15, 2004.

exposed bedrock cliff on the northeast summit flank (hereinafter referred to as the northeast face, the site of scattered 

boiling-point fumaroles; Turner and Wescott, 1986).

The debris-flow deposits ranged from about 50 to 1,000 

m in length and from 20 to 100 m in width. Dark brown to 

black, they appeared to be uniform in texture from the air. No 

large clasts were visible in magnified aerial photographs, suggesting that the debris flows were devoid of boulder-size material. The deposits also appeared to be fairly thin, as indicated 

by their diffuse appearance along flow margins where the 

underlying snow and ice were varyingly visible. Lateral levees 

were visible on some deposits. Several flows were marked 

by central rills, meters deep, indicating downcutting during 

or after emplacement. Some of the rills were washed clean, 

whereas others were mantled with sediment. Rills extended 

beyond the debris-flow deposits in a few places, indicating 

that water or watery debris flows traveled beyond the lobate 

deposit termini and caused simultaneous erosion of the ice 

surface. Flowing material may have been warm enough to melt 

surface snow and ice after coming to rest because several termini were incised below the level of the surrounding glacier. 

Alternatively, solar heating of the dark debris during the days 

after emplacement may have been sufficient to cause differential melting and relative subsidence of the deposit areas.

Comparison of satellite images indicates that the debris 

flows occurred in at least two pulses separated by several days 

to several weeks. Advanced Spaceborne Thermal Emission 

and Reflection Radiometer (ASTER) data from July 8 and 

Landsat 7 Enhanced Thematic Mapper Plus (ETM+) data on 

July 10 show some, but not all, of the deposits observed on 

July 15. Thus, some debris flows occurred between June 20 

and July 8, and the rest between July 10 and 15.

Using the July 15 aerial photographs in combination with 

ASTER and QuickBird satellite imagery from August 10, 

we created a map of the distribution of debris flows (fig. 5). 

The debris flows were confined to the northeast and east. The 

June 20–July 8 debris flows emerged from the glacier in two 

areas: adjacent to a bedrock ridge that trends southeast from 

the summit (deposit a, figs. 3–5), and above the northeast face 

(deposits b, c). The July 10–15 debris flows were smaller and 

emerged primarily from lower-elevation points directly east of 

the summit (deposits d–h). Flows in the first set were larger in 

volume and traveled a greater distance. July 15 Forward Looking Infrared (FLIR) images show that the earlier flows were 

slightly warmer than the later flows, suggesting that the earlier 

flows were thicker and retained heat more efficiently.

The total area of the deposits observed in summer 2004 is 

132,000 m2; their thickness was not measured. A lahar deposit 
that formed from a crater-lake outbreak at Chiginagak Volcano 

on the Alaska Peninsula in 2005 averaged 15 cm in thickness 

(Schaefer and others, 2005); using this thickness as a proxy 

suggests a combined volume of ~20,000 m3, excluding any 

water that must have been present.

Another debris flow that formed in May 2005 is described 

in detail below.

early August through early December 2004, the ice cauldron 

enlarged gradually as blocks of ice ringing the depression 

toppled into the lake. The collapse pit, which was mapped in 

a geographic information system (GIS), using a combination 

of vertical and oblique imagery (fig. 8; table 1), increased 

systematically in area over time until August 2005 (fig. 9). 

At first, in midsummer 2004 through March 2005, its area 

increased fairly linearly. The growth rate increased somewhat 

from March to midsummer 2005, to an area of 68,900 m2 by 

August 1. This increase in area was accomplished by expansion southward as large seracs became unstable, pulled away 

from the south wall of the summit crater, and toppled into the 

lake. From August 2005 to spring 2006, the area of the ice 

cauldron remained relatively constant, likely owing to a combination of decreasing heat flux, as evidenced by lower SO2

emissions (M.P. Doukas and K.A. McGee, unpub. data, 2006) 

and lower seismicity rates (fig. 9B), and walling in of the collapse pit by rock on its north, west, and south sides.

The height of the ice cauldron walls likely remained relatively constant. Before a sudden drop in lake level in May 2005 

(discussed below), the water level also appeared to remain constant, as tracked by features in the pit walls. A high-resolution 

digital elevation model (DEM) of the summit area, generated 

from vertical aerial photographs taken on September 23, 2004, 

indicates that the elevation of the lake was 10,605 ft. Along the 

southwest rim, closest to the summit, the snow elevation was 

~10,900 ft, and along the northeast rim it ranged from 10,810 

to 10,830 ft. Thus, the average depth of the ice cauldron from 

snow surface to lake level was ~255 ft (78 m). The depth of the 

lake is unknown. Assuming an average elevation difference of 

78 m throughout the period of volcanic unrest and assuming 

vertical walls, the total volume of melted snow and ice (as of 

Mar. 22, 2006) was 5.4 million m3.

Lake, Fumaroles, and Rockwalls

Throughout much of the observation period, the color of 

the ice-cauldron lake remained dark gray as circular, ice-free 

zones, tens of meters across, appeared near the northeast 

bedrock shoreline and at several points farther from the lakeshore (fig. 10). In late February 2005, AVO noted roiling of 

the water surface within these circular zones for the first time, 

although previous aerial photographs show circular ice-free 

patches within the lake in early August 2004. These patches 

most likely represent upwelling of warm, gas-rich water 

from deeper within the lake, similar to the upwelling zones 

observed in the Crater Peak lake before the 1992 eruption 

(Keith and others, 1995). During observation flights through 

fall 2004 into early 2005, the area of open water persisted near 

the northeast shoreline.

Five FLIR surveys documented gradually increasing temperatures of the crater lake, fumaroles, and exposed 

bedrock outcrops between July 15, 2004, and April 16, 2006. 

The thermal surveys indicated that the lake-water temperature 

gradually increased from about 1°C (when the lake was mostly 

Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 

Summit Ice Cauldron and Lake

AVO overflights on August 2 and 4, 2004, revealed a 

circular collapse pit, ~50 m in diameter, in the eastward-sloping summit icecap (fig. 6D). The pit was at approximately the 

north edge of a broad depression in the summit icefield, encircled by nested arcuate crevasses that indicated a much larger 

area of active subsidence. The crevasses mimicked the overall 

open-to-the-east shape of the summit basin. The pit walls were 

vertical to overhanging and exposed cavernous tunnels in the 

ice, as well as one conspicuous area of steeply sloping, gray 

bedrock that formed the lower northeast wall (fig. 6E). The 

bottom of the pit was an ice-choked lake with small areas of 

blue-gray open water.

The collapse pit and surrounding depression became 

known as the ice cauldron, following usage of the term at 

ice-covered Icelandic volcanoes that have had subglacial eruptions or transient subglacial geothermal events (for example, 

Björnsson, 1988; Gudmundsson and others, 2004). Unsafe 

landing conditions throughout the period of volcanic unrest 

prevented any ground-based observations of the summit ice 

cauldron, and so the record of its formation is reconstructed 

here on the basis of helicopter and fixed-wing overflights, 

aerial photographs and video, and satellite-image analysis. A 

review of pre-August 2, 2004, aerial photographs and mountaineering accounts provides some constraints on the timing of 

formation of the ice cauldron (table 1). Climbers in mid-March 

2004 who traversed the slope that would later collapse noted 

no disturbance or evidence of incipient subsidence (fig. 6A). 

The first clear indication of summit deformation is in aerial 

photographs taken on June 20 (fig. 6C). Here, an east-facing, 

arcuate scarp across the summit basin displays subsidence of 

several meters. The position of the escarpment coincides with 

the well-developed outer set of nested crevasses visible on 

August 2 and 4 when AVO first observed the fully formed ice 

cauldron. An ASTER satellite image taken on July 8 shows a 

nearly circular depression with sharply defined walls enclosing a partially darkened area, suggesting ice-choked water (fig. 

7B). Thus, collapse of 4,200 m2 of the summit icefield likely 

occurred sometime between June 20 and July 8, 2004.

Growth of the Ice Cauldron

Overflights throughout late 2004 and 2005 documented 

the growth of the ice cauldron, continuing deformation and 

collapse of the surrounding icewalls, and variation in the 

area of open water on the surface of the crater lake. From 
 Studies by the U.S. Geological Survey in Alaska, 2005

Figure 4. Closeups of debris-flow deposits on summit cone of Mount Spurr. Same labels as in figures 3 and 5. 

Photographs by C.A. Neal, taken July 15, 2004. A, Main debris-flow lobes drape east sector of summit cone. Dark areas 

in upper left are outcrops and not flowage features. True summit is obscured by clouds. View north-northwestward. B, 

Reddish-brown outcrop of volcanic bedrock in upper right is northeast face, a longstanding feature that hosts diffuse 

boiling-point fumaroles and remains snow free much of the year. Several debris-flow deposits emerge from cracks and 

holes in glacier and cascade over crevasses; additional flows course over northeast face and extend outward onto glacial 

ice. Note central rill visible in deposit and narrow incised channel that extends from toe of debris flow at right (d). 
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 

Deposits at middle right (b) appear to have melted surface snow, resulting in negative topography. C, East-southeast face, 

showing nearly all of longest debris-flow deposit (a) that emerged from a point just obscured by clouds at top of photograph. 

Note diffuse debris cover along margins and clean ice along central part of flow, suggesting waterflow late in emplacement. Ice 

cliff at center, partly covered by a short debris-flow lobe to right of flow a, marks source of May 2005 debris flow. D, Main debrisflow field on east flank. Terminus of longest flow (a), visible in lower right, is where material flowed over and around crevassed 

ice. Note large patches apparently washed clean of debris. View northward.
Figure 5. Mount Spurr area, showing locations of 2004–5 debris-flow deposits and other near-summit features. Same labels as in figures 3 awith dashed outlines are approximate zones or point sources of documented thermal activity, including areas of warm rock, anomalous-tempeby forward-looking-infrared imagery, and aerial photographs of steaming ground or crevasses. See figure 14 for cross section A–A’. Base from visible- and near-infrared-band 4, 0.62-m-resolution QuickBird satellite image, acquired August 10, 2004 (catalog ID 1010010003295802, copyrigContour interval, 100 ft; contours from U.S. Geological Survey topographic map.
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 11

Figure 6. Stages in formation of ice cauldron 

at summit of Mount Spurr. A, Summit icefield on 

March 19, 2004, from atop a place that would later 

become ice cauldron. View south-southwestward; 

photograph by Cody Arnold. B, Summit on May 5, 

2004. At this time, no deformation or subsidence 

is visible on summit plateau. View southward; 

photograph by Stuart Parks. C, Summit on June 

20, 2004. By this time, ice cauldron had begun to 

subside, as shown by arcuate scarp (ends denoted 

by black arrows) marking edge of eventual pit. 

Note linear escarpment 

(south side down, denoted 

by red arrows) to east 

of protocauldron; this 

structure may mark 

boundary of bedrock ridge 

with a subsiding ice sheet 

to south. View westward; 

photograph by Bruce 

Hopper. D, Summit on 

August 2, 2004. First clear 

view of ice cauldron. View 

southward; photograph 

by Kristi Wallace. E, 

Ice-filled lake at summit, 

exposed rockwalls on 

north margin, dark debris 

(tephra?) sloughing out 

of ice, and crescentic ice 

tunnels providing potential 

spillways eroded into 

ice. View northeastward; 

photograph by Donna 

Eberhart-Phillips.
12 Studies by the U.S. Geological Survey in Alaska, 2005

Figure 7. Satellite images of summit of Mount Spurr, cropped and scaled to same footprint. A, View from 

Ikonos, taken 21:34:42 u.t.c. June 15, 2002 (image ID 20020615213442300000100079111, copyright © Space 

Imaging), showing no obvious pit or subsidence at summit. B, View from Advanced Spaceborne Thermal 

Emission and Reflection Radiometer (ASTER), taken 21:49:42 u.t.c. July 8, 2004 (granule ID AST_L1A_

00307082004214924_073020044131349), showing beginning of ice cauldron and first set of debris-flow deposits. 

C, View from Landsat 7 Enhanced Thermal Mapper Plus (ETM+), taken 21:09 u.t.c. July 10, 2004 (scene ID 

7070017000419251), showing same debris-flow deposits as in figure 7C but not all flows observed on July 15, 

2004, overflight. D, View from QuickBird, taken August 10, 2004 (catalog ID 1010010003295802, copyright © 

Digital Globe). Same base as in figure 5.
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 13

filled with ice) to over 11°C in April 2006. The rise in lake 

temperature likely is partly controlled by a decrease in the 

volume of water over time, coupled with a decrease in the volume of snow and ice falling into the lake. The temperatures of 

the circular upwelling areas were not significantly higher than 

the average lake temperatures during a June 21, 2005, survey; 

however, an April 2006 survey indicated that the temperature 

of the upwelling area in a slightly shallower lake was at least 

25°C, while the average lake temperature was 11°C.

The area of exposed bedrock near the bottom of the ice 

cauldron also increased and occasionally produced a visible 

vapor plume. By August 2005, bedrock was visible around 

the lakeshore in all quadrants except the south-southeast (fig. 

10). Yellow-tinted snow, ice, and rock in the vicinity of the 

lake indicated sulfur deposition near the lake margin. Ongoing release of volcanic gases, including SO2 and CO2, was 

confirmed by periodic airborne gas measurements (Neal and 

others, 2005; K.A. McGee and M.P. Doukas, unpub. data, 

2005). The FLIR-measured temperature of fumaroles in the 

bedrock outcrop on the northwest shore of the lake increased 

from 60°C in June 2005 to over 150°C in April 2006.

May 2005 Debris Flow

From August 2004 through late 2004, the lake level 

remained fairly constant. In early May 2005, however, another 

debris-flow event occurred in conjunction with a rapid drop 

in lake level. After a pilot reported of a dark flow on the flank 

of Mount Spurr, an AVO overflight on May 3 revealed that 

the lake level was ~15 m lower since a flight on April 25—the 

first decrease of this magnitude noted since the lake had 

become exposed (fig. 11). Newly exposed rock was steaming 

Figure 8. Summit of Mount Spurr, showing outlines of growing ice cauldron between July 8, 2004, and March 23, 

2006. Digital elevation model from vertical aerial orthophotograph, taken September 23, 2004.
14 Studies by the U.S. Geological Survey in Alaska, 2005

0

50

100

150

200

Mar-04 Jul-04 Nov-04 Mar-05 Jul-05 Nov-05 Mar-06

DATE

EARTHQUAKES PER WEEK

Mar-04 Jul-04 Nov-04 Mar-05 Jul-05 Nov-05 Mar-06

AREA OF ICE CAULDRON,

IN SQUARE METERS

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

A

B

vigorously. About 600 m southeast of the lake at ~2,865-m 

elevation, a new debris-flow deposit (i, fig. 5) was observed in 

the same area as the 2004 flows. This 190-m-long, 30-m-wide 

deposit was also dark, composed of fine material, and apparently emplaced with abundant water, as evidenced by erosion 

of snow and ice. Using the area of the ice cauldron mapped 

from April 25, 2005, images and a decrease in depth of 15 m, 

we calculate that ~600,000 m3 of water drained from the lake 

during this event.

Emplacement of the May 2 debris-flow deposit was 

recorded by a Web camera installed for monitoring purposes 

by AVO in September 2004. Images show that the flow 

reached its full length in 12 minutes or less (fig. 12). Web 

camera images from the morning of May 3 show a steam 

plume rising several hundred meters above the summit. 

Because such a strong steam plume had not been observed 

previously, we conclude that the onset of steaming reflected a 

drop in lake level. Although the debris flow was emplaced at 

~11:00 a.m. A.d.t. May 2, clear Web camera views from that 

afternoon show no steam; this delay may have been caused by 

changing meteorologic conditions.

To our knowledge, the May 2005 debris-flow event 

was the only such incident after July 2004. In November 

2005, however, AVO received photographs from a pilot that 

showed rilling of the snow and ice cover along the same swath 

affected by the May 2005 debris flows (fig. 13). The timing of 

this event is uncertain. Although any dark, solid component 

involved in outflow may have been covered by new snow, the 

Figure 9. Correlation between growth of ice cauldron at summit (A) and weekly frequency of earthquakes in Mount Spurr area from 

March 2004 through March 2006 (B). Earthquake data from Dixon and others (2005) and Alaska Volcano Observatory (unpub. data, 2006).
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 15

Figure 10. Summit of Mount Spurr, showing fully developed ice cauldron, approximately 200 m across north-south. 

A, Area of consistent upwelling and roiling in lake is visible along north shore (to right). View westward. B, Closeup of 

upwelling area and shoreline fumaroles. Photographs by Dave Schneider, taken June 21, 2005.
16 Studies by the U.S. Geological Survey in Alaska, 2005

event may also have involved simply water and remobilized 

snow and ice with no suspended sediment.

Discussion

Source of Sediment and Water in Debris Flows

We were unable to directly sample the 2004 debris-flow 

deposits, which rapidly became covered with snow in steep 

terrain. A likely source of the sediment fraction is lapilli- and 

ash-size tephra-fall deposits from the 1992 eruptions of Crater 

Peak (Neal and others, 1995; McGimsey and others, 2001). 

These deposits are visible as conspicuous dark-gray bands in 

seracs and in the walls of the ice cauldron (fig. 6E). The June 

1992 eruption cloud traveled northward from the vent, depositing as much as 50 cm of tephra on the summit of Mount Spurr 

(McGimsey and others, 2001). The August 1992 eruption of 

Crater Peak may have just dusted the summit, and the September 1992 eruption deposited <1 cm of material in the vicinity of 

the summit. Two fine ash falls from the 1989–90 eruptions of 

Mount Redoubt (fig. 1; Scott and McGimsey, 1994) would also 

have left measurable, fine-ash tephra layers on the summit of 

Mount Spurr. Subsequently, these lapilli and ash deposits were 

progressively buried in the glacier. Gradual melting and collapse 

of the glacier to form a meltwater lake would have incorporated 

this material; additional sediment for entrainment in the debris 

flows may have come from the crater floor and wallrock.

The generation of debris flows requires a heat source to 

create and maintain liquid water at the 3,374-m-high summit 

of Mount Spurr. Long-term observations suggest that transient geothermal heating has occurred in the summit area for 

decades, including reports from climbers of bare and warm 

rock along the summit ridge and diffuse, boiling-point fumaroles on the northeast face of the summit and in selected areas 

around the summit ridge, as described by Turner and Westcott 

(1986) and Nye (1987). A more recent increase in geothermal 

output is indicated by exposures of snow-free bedrock along 

the lakeshore, adjacent areas of (sometimes roiling) open 

water, and hand-held and helicopter-mounted FLIR measurements of the inner collapse pit that showed areas as hot as 

39oC (D.J. Schneider and R. Wessels, unpub. data, 2005). De 

Angelis and McNutt (2005) suggested that long-period earthquakes in the recent swarm are caused by the flux of hot gases 

through fluid-filled cracks, a likely mechanism for the current 

heat transfer to the summit.

Debris-Flow Formation and Summit Hydrology

Draining of geothermally heated subglacial or ice-cauldron lakes is a common occurrence in Iceland, where much of 

the active neovolcanic zone is buried by glaciers. The volumes 

of documented glacial-outburst floods, or jökulhlaups (0.05–5 

km3; Björnsson, 1992), are typically much greater than those 

of the debris flows observed at Mount Spurr, and occur at the 

toe of the glacier, although the discharge and flow mechanisms 

for such jökulhlaups may be similar. Like many of the icecauldron lakes in Iceland, the summit lake at Mount Spurr is in 

a preexisting crater of volcanic origin and formed in response 

to geothermal heating. This crater became exposed as more 

and more snow and ice has melted, revealing that much of the 

lake is walled by rock, not ice.

What triggers a sudden outflow from such subglacial 

reservoirs? A subglacial or ice-cauldron lake with a perfect 

ice-rock seal would drain only when the hydrostatic pressure, 

which is a function of water depth, exceeds the ice-overburden 

pressure (Nye, 1976). Most of the ice-cauldron lakes that have 

been studied in Iceland, however, appear to drain at water levels below this critical depth (Björnsson, 1992). In such lakes, 

cracks in the glacier, or basal channels or permeable layers 

in the ice or in underlying sediment or bedrock, facilitate 

the release of water at hydrostatic pressures less than those 

required to “float” the surrounding ice (Nye, 1976; Fisher, 

1973). This is likely true at Mount Spurr as well, although the 

water depth of the ice-cauldron lake there is unknown.

Because water release typically occurs below this critical 

depth, other mechanisms must be responsible, such as (1) sudden 

changes in the discharge or drainage system beneath the ice dam, 

leading to enlargement of subglacial conduits (Röthlisberger 

and Lang, 1987); (2) an increase in lake temperature, leading to 

accelerated growth of conduits (Björnsson, 1992); or (3) a rockslide displacing water within the lake (Kjartansson, 1967).

The coincidence in time and space of debris flows and formation of an ice cauldron at the summit of Mount Spurr suggests a 

causal relation. Melting-induced collapse of an ice plug into the 

lake occurred sometime between June 20 and July 8, 2004, and the 

first debris flows were emplaced in two pulses sometime between 

June 20 and July 15. The May 2005 lake-level drop and debris 

flow have an even more constrained temporal relation, occurring 

on the same day, probably simultaneously.

One possible mechanism for sudden water release (and 

resulting debris flows) may have been the collapse of ice and snow 

into the lake. Such collapse could have caused the water-debris 

mixture to be displaced rapidly upward and outward along preexisting englacial and, possibly, subglacial pathways leading away 

and downslope from the summit basin. Where these pathways 

intersected crevasses or other weak points in the sloping icefield, 

the mixture debouched onto the surface, producing debris flows. 

In 2004, the occurrence of two sets of debris flows separated in 

time by as long as a week suggests two pulses of collapse, each 

producing a surge of debris and water from the lake (fig. 14). In 

May 2005, observations indicate that lake-level drop and debrisflow formation were not accompanied by significant ice collapse. 

This drainage could have instead occurred in response to sudden 

enlargement of an englacial or subglacial conduit by erosion, heating, or glacier flow.

The ice-cauldron lake at Mount Spurr maintained a 

fairly constant level—as determined by comparing features 

on consecutive photographs—during the first 9 months of its 
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 17

Figure 11. Summit of Mount Spurr, showing ice cauldron before (A) and after (B) lake-level drop that occurred in early May 

2005. A–D, features common to both views. A, April 25, 2005. Photograph by Dave Schneider. B, May 3, 2005, showing lake-level 

drop of approximately 15 m, estimated from 90-m overall height of predrainage west wall calculated in September 23, 2004, 

digital elevation model. Photograph by M.L. Coombs.
18  Studies by the U.S. Geological Survey in Alaska, 2005

Figure 12. Series of Web camera photographs of Mount Spurr 

taken May 2, 2005, showing emplacement of new debris-flow 

deposit (yellow circle) on east flank. Debris flow occurred 

sometime after 10:57 A.d.t. (A) and came to rest by 11:09 A.d.t. (C).

most rapid growth. The first major drop in lake level occurred 

in early May 2005 (discussed above), and a second, smaller 

drop between January and March 2006. For the period of 

constant lake level, water must have been exiting at a rate 

approximately equal to the input by melting collapsed blocks 

of snow and ice and precipitation. Either water was percolating through a porous crater floor into the edifice, or water was 

draining over a low point(s) in the enclosing rim and flowing 

beneath or within the glacier. Constancy of lake level even 

during times when the influx of snow and ice was reduced, 

including late 2006 when the cauldron reached its apparent 

maximum size, argues against steady loss of water through 

the crater floor. Furthermore, high-temperature alteration of 

volcanic rock produces clay minerals that are likely to create a 

relatively impermeable lake floor.

Our preferred interpretation is that lake water exits the 

crater through a spillway (or multiple spillways) at approximately the same rate as it is being generated. No such spillway 

is clearly visible despite many good views and aerial photographs of all sections of the lake margin. As the ice cauldron 

grew to its present and apparently stable size (figs. 8, 9), sheer 

rockwalls became exposed around much of the lake. As of 

this writing (April 2006), ice remains in contact with the lake 

surface in the east quadrant. This quadrant, which coincides 

with the distribution of debris flows in 2004–5 (fig. 5), is the 

best candidate for a spillway; it is also the “breached” part of 

the crater structure that exists at the summit of Mount Spurr 

(see next section; Turner and Westcott, 1986).

Although AVO aerial observations and photographs have 

not unequivocally shown that such a spillway exists in the 

east quadrant, stranded cavernous openings on the east and 

southeast sides of the growing lake, visible early in ice-cauldron formation, may have reflected drainage pathways during 

debris-flow emplacement (fig. 6E). Photographs taken by 

climbers on the summit of Mount Spurr on March 22, 2006, 

show a surface sag in the snow and ice wall over the southeastern part of the lake (fig. 15). A similar sag in a thin tephra 

layer tens of meters below the lake surface demonstrates that 

the sagging is not a surface feature but involves a thick layer 

of the glacier. The sagging may reflect the presence of an outflow channel or englacial tunnel carrying water from the lake 

out of the summit crater and down the east-southeast flank. 

Outside the ice cauldron in this quadrant are several long-lived 

steaming holes in the ice (fig. 5); it is unknown whether these 

holes reflect buried fumarolic sources or, possibly, trace the 

path of warm water exiting the lake. The glacier that covers 

the east side of the summit cone plunges steeply (20–45°) and 

is heavily crevassed.

An ice-mantled bedrock ridge extends about 750 m northeastward from the summit and terminates in several exposures 

of permanently warm and largely snow free bedrock, the largest of which, the northeast face, has long been described as an 

area of diffuse boiling-point fumaroles (Turner and Wescott, 

1986). During the initial formation of the ice cauldron, the ice 

mantling this ridge became bounded on its southern margin 

by a linear crevasse that formed as ice draining the southeast 

sector of the summit crater subsided (figs. 5, 6C). At least temporary englacial tunnels or pathways must have existed atop 

this ridge in 2004, on the basis of the appearance of debris 

flows lower on the northeast face. A line of pits and openings 

in the ice may trace the path of such a drainage pathway to 

the upslope edge of the northeast face (fig. 5). A second broad 

bedrock ridge with scattered thermal features extends about 

700 m east-southeastward from the summit of the volcano; 

vapor plumes, never more than a few tens of meters high, 

emanate from areas of bare ground and crevasses. A steep 

rockface on the south-southeast flank of the summit area was 

determined by FLIR images on April 14, 2006, to be warmer 

than expected from solar heating alone, suggesting some 

geothermal component. Still other areas of steaming ground 

and warm rock occur along an escarpment that separates the 

broad bedrock ridge from glacial ice draining the east flank of 

the summit cone.
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 19

Figure 13. Southeast face of summit cone on Mount Spurr in 

November 2005. A, Rilling and avalanching of snow from ice cliff 

at approximately 9,300-ft elevation that was outlet point for May 

2005 debris flow. B, Closeup of area of rilling and avalanching. 

Photographs by Diana Maroney.
20 Studies by the U.S. Geological Survey in Alaska, 2005

Other Examples of Geothermal Heating, Glacial 

Response, and Release of Crater-Lake Water

Other examples of sudden release of a large volume of 

crater-lake water are not limited to Iceland. Sudden breaches 

of volcanic lakes, not associated with eruptions, have led to 

the formation of lahars at Mount Ruapehu, New Zealand, in 

1953 and at Agua Volcano, Guatemala, in 1541 (Delmelle and 

Bernard, 2000).

Recurring debris avalanches and manifestations of increasing geothermal activity at Mount Baker in Washington provide 

a particularly good analogy for several aspects of the 2004–6 

volcanic unrest at Mount Spurr (Frank and others, 1975; Frank 

and others, 1977). Ice-filled Sherman Crater near the summit 

of Mount Baker is similar in size to the summit crater of Mount 

Spurr and has also been the site of long-term fumarolic activity. In March 1975, an increase in thermal output produced a 

series of new and enlarged depressions and other disruption of 

the ice, including a 50- by 70-m collapse pit enclosing a small 

lake (Frank and others, 1977, fig. 10). Fumaroles were active on 

the lakeshore, and as many as five circular zones of upwelling, 

similar to those at Mount Spurr, were observed.

No debris-flow outbursts were recorded during the 

~1-year-long episode of increased heating and melting of 

the summit ice at Mount Baker; however, unlike at Mount 

Spurr, a well-formed spillway of meltwater beneath Boulder 

Glacier was able to accommodate the increased melting and 

discharge freely. Interestingly, a distinctive water-chemistry 

anomaly was detectable in Boulder Creek several kilometers 

downstream from Sherman Crater (Frank and others, 1977), 

suggesting that if the summit lake at Mount Spurr is, indeed, 

Spurr

summit dome

Previous ice level

Debris flow 

exits the glacier at 

9,500-ft elevation

11,500

9,000

9,500

10,000

10,500

11,000

0 500 1,000 1,500

DISTANCE, IN METERS

ELEVATION, IN FEET

West

A’

East

Ice

cauldron

A

Predrop

lake level

losing water to a specific drainage, it might be documentable 

through systematic water sampling.

A prolonged period of increasing volcanic heat flux 

resulted in ice-cauldron formation, the loss of >40 million m3

of ice, and ephemeral bodies of standing water in the summit 

caldera of Mount Wrangell, Alaska, in the 1960s and 1970s 

(Benson and Motyka, 1978; Motyka, 1983). Also interpreted 

as the result of degassing of a shallow magma body, the Mount 

Wrangell geothermal event was not associated with any known 

outbursts of water or debris.

An additional analog appeared in Alaska less than a year 

after the onset of volcanic unrest at Mount Spurr. In early summer 2005, a much larger (400-m diam) ice cauldron formed 

at Chiginagak Volcano on the Alaska Peninsula. Ice-cauldron 

formation was accompanied by rapid release of an acidic sediment-water mixture that exited beneath and within the glacier, 

partially draining the summit crater, although the relative 

timing of events is poorly constrained (fig. 16; Schaefer and 

others, 2005). The formation of a temporary, steaming circular 

depression in the icefield on Snowy Volcano, Alaska (Neal and 

others, 2004), without subsequent collapse and exposure of 

standing water may represent a less well developed, geothermally induced subsidence feature.

Farfield Effects of Lake Drainage

The effect of the 2004–5 debris flows on the icefield of 

Mount Spurr remains uncertain but likely was minor. Some 

sediment and much of the water must have eventually drained 

into openings in the glacial ice and the englacial plumbing 

Figure 14. Cross section A–A’ (fig. 5) of summit of Mount Spurr. Surface elevation of ice (snow) and lake level are calculated from 

digital elevation model of September 23, 2004; bedrock surfaces are inferred from estimated thickness of ice.
22 Studies by the U.S. Geological Survey in Alaska, 2005

system. Glacial-drainage maps by March and others (1997) 

indicate that Kidazgeni Glacier (fig. 2) is the most likely part 

of the Mount Spurr icefield to have absorbed runoff from the 

2004–5 debris flows. Aerial observations and photographs 

of the glacier in summer 2004, however, revealed no obvious 

surface effects, such as sagging over tunnels or point sources 

of clearly anomalous water discharge. The limited extent of 

individual debris flows suggests that the volume of liquid 

water was quite small and may have been accommodated 

without significant modification of the icemass. March and 

others (1997) reported a 1981 ice thickness of >150 m near the 

toe of Kidazgeni Glacier; small, deeply buried tunnels may not 

be sufficiently voluminous to perturb the surface.

In addition, we have no direct evidence of abnormal 

outflow at the snout of Kidazgeni Glacier (fig. 2). Aerial photographs taken in early March 2005 and again in early April 

2006, however, showed water exiting the front of the glacier 

at a much higher rate than at adjacent glaciers draining the 

Tordrillo Range (fig. 17). The outflow stream was open bank 

to bank, unlike other glacial outlets nearby, with the single 

exception of the Crater Creek drainage, which is partly fed by 

warm springs on the slope of Crater Peak. Because we have 

no data on glacial stream discharge or composition at Mount 

Spurr, we cannot state with certainty whether these observations are significant. In April 2006, a single water sample 

collected from the outflow stream of Kidazgeni Glacier just 

above the confluence with the Chakachatna River (fig. 2) 

showed high conductivity and anion concentrations suggestive 

of geothermal input (W.C. Evans, written commun., 2006). 

Frank and others (1977) documented a clear change in downstream water chemistry after the increase in subglacial fumarolic activity at Mount Baker in 1975 (Bortleson and others, 

1976). By analogy, further sampling of water discharging from 

glaciers draining the summit of Mount Spurr is warranted.

New Insights into the Geology of the Summit 

Revealed by Ice-Cauldron Formation

The 2004–5 events at Mount Spurr more clearly exposed 

the structure of the summit. Though briefly noted by previous 

workers (Turner and Wescott, 1986; March and others, 1997) 

and retrospectively quite clear in mid-20th-century aerial photographs, the size and significance of this feature was not well 

known before the present volcanic unrest.

Aerial photographs of gradually increasing exposure of 

the crater walls show multicolored, altered rockfaces with 

steep, gray talus ramps descending to the lake edge on the 

south, west, and north. Conspicuous, steeply dipping, red 

oxidized horizons are complexly interbedded with pale-yellow-gray, hydrothermally altered zones (fig. 10). Cavernous, 

steaming openings in gray, clastic-appearing material on 

the northeast lakeshore suggest sites of long-term fumarolic 

discharge. Without the benefit of samples or closer examination, we cannot say with certainty much about the details of 

exposed outcrops; however, their overall appearance suggests 

that the walls are largely composed of volcanic breccia and 

lava-flow or dome rock that has been pervasively altered. The 

steep, fairly regular north, west, and south crater walls are 

reminiscent of Crater Peak, and so we suggest that the summit 

of Mount Spurr most likely represents a young, explosively 

reamed vent structure, little modified by ice.

Over the course of the current volcanic unrest, new thermal 

features—defined as zones of visible vapor production, anomalous bare ground, and, in some places, visible sulfur precipitate—have appeared in the summit area of Mount Spurr. The 

approximate distribution of these newly recognized thermal 

areas, added to those known from previous decades of observation but never mapped, defines several structures (fig. 5) of 

potential significance to our understanding of the summit area.

Exposed fumaroles and steaming bedrock at the summit 

partly outline the 200- by 300-m-diameter crater now well 

exposed by the ice cauldron. This crater is at least 160 m deep 

as measured from the true summit to the surface of the lake. 

As described by earlier workers, the crater rim appears to be 

much lower on the east and may be poorly defined or absent in 

the east-southeast sector. A glacier filling the crater flows eastward into the icefield filling the caldera of the Spurr volcanic 

complex, feeding the east branch of Kidazgeni Glacier (fig. 2).

Earlier Geothermal-Heating Events and 

Fumarolic Activity at Mount Spurr

How unusual is the formation of an ice cauldron at the 

summit of Mount Spurr? To our knowledge, this is the first ice 

cauldron containing open water at the summit, at least since 

written records of volcanic activity in Alaska (mid-1700s) and 

certainly since regular overflights began in the mid-1900s, as 

well as the first known occurrence of debris flows from the 

summit. Historical reports and photographs, however, document significant variation in the snow and ice cover at the 

summit that at times has been low enough to expose a basin 

or breached-crater structure (Turner and Westcott, 1986; Nye, 

1987) in the same area as the current ice cauldron. When snow 

and ice volume is low, such as in aerial photographs taken in 

1954, 1957, and 1964, a distinct arcuate wall enclosing an 

ice-filled basin becomes visible (fig. 18), which was described 

by March and others (1997) as an ~200- to 300-m-diameter 

feature open to the east-northeast. Aerial photographs show a 

hole in the icecap near the base of the north crater wall. The 

crater wall itself is devoid of snow and ice, suggesting warmth. 

Although no open water is visible, the 1957 aerial photograph 

shows several dark patches that could represent warm or 

exposed bedrock. Photographs from the USGS High Altitude 

False Infrared Series, taken throughout the 1980s and 1990s 

and as recently as spring 2004 (figs. 6A, 6B), show a summit 

crater that is more subdued and difficult to discern, but no hole 

or perforation in the ice, possibly owing to greater net ice and 

snow accumulation during this period.

Summit ice thickness can be influenced by several factors, including variations in heat flux, ice mobility, snowfall, 
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 21

Figure 15. Formation of ice cauldron at summit of Mount Spurr between 2005 and 2006. Arrows denote bare, warm 

rockface; circles identify area of active upwelling in lake. A, April 25, 2005. Note tunnel that may lead to drainage 

along rock-ice interface, as well as near-horizontal tephra layer within ice. View southeastward; photograph by Dave 

Schneider. B, March 22, 2006. Note faint, deformed tephra horizon in icewall across lake; sag may reflect subsidence 

of surface ice over a drainage pathway beneath ice. Lower lake level is shown by position of upwelling zone relative 

to lakeshore. View southeastward; photograph by Nathan Bluett.
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 23

Figure 16. Summit of Chiginagak Volcano on the Alaska Peninsula. Sometime between two photographs, 

increased geothermal heating led to formation of a crater lake that drained and sent acidic lahars and floods 

downstream (Schaefer and others, 2005). A, August 24, 2004. B, August 20, 2005. Photographs by Janet Schaefer, 

Alaska Division of Geological and Geophysical Surveys.
24 Studies by the U.S. Geological Survey in Alaska, 2005

Figure 17. Termini of Kidazgeni Glacier, which descends in two arms from ice field of the Spurr volcanic 

complex. East arm (E) drains east flank of summit of Mount Spurr (arrow; March and others, 1997). Discharge from 

glacial outlet (lower left) at time of photograph (March 2005) appeared to be more vigorous and open than that 

from other glacial-outlet streams then and again in April 2006. Photograph by C. A. Neal, taken March 2, 2005.
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 25

snowdrift, and ice accumulation as a function of climatic conditions. Few long-term measurements of glacial mass balance in 

south-central Alaska are available to evaluate climatic control 

on ice volume. Data from Wolverine Glacier, 200 km southeast of Mount Spurr on the Kenai Peninsula (Mayo and others, 

2004), suggest a decline in glacial mass in 1965–76 (a period 

of distinct summit-crater morphology at Mount Spurr) that may 

be a reasonable analog to the Spurr glacial system. This decline 

was followed by a net ice accumulation through 1988 (see URL 

http://ak.water.usgs.gov/glaciology/all_bmg/3glacier_

balance.htm). After 1988, data for Wolverine Glacier suggest a net mass loss that continues to the present, consistent 

with glacial wastage throughout Alaska (Arendt and others, 

2002). These trends are largely consistent with climatic shifts 

documented for the North Pacific Ocean in 1977 and 1989 

(Hare and others, 2000), suggesting that these trends may be 

widespread across southern Alaska. Such shifts may have 

influenced the “disappearance and reappearance” of the summit crater at Mount Spurr during the late 20th century and into 

the current cycle of ice-cauldron formation. Further analysis 

of meteorologic and hydrologic records is required to fully 

evaluate climatic effects on the Mount Spurr icefield. Nonetheless, the 2004–5 events demonstrate that heat (and gas) flux can 

play a role in snow and ice thickness at the summit of Mount 

Spurr and lead to the formation of transient melt and collapse 

features.

Geothermal activity at Mount Spurr has been intermittently robust enough to produce visible steam plumes since 

at least the early 20th century. Capps (1935) noted “* * * the 

highest peak of the mountain, from vents near the top of which 

Figure 18. Pre-2004 summit of Mount Spurr. Yellow dashed lines mark approximate position of summitcrater rim; star marks topographic summit. North is at top in all photographs. A, August 29, 1954. U.S. 

Geological Survey EROS Data Center’s Mission M4H55, frame 104, roll 1. B, August 7, 1957. U.S. Geological 

Survey EROS Data Center’s Mission M–237, frame 10115, roll 76. C, August 25, 1964. University of Alaska, 

Fairbanks, Geophysical Institute, Geodata Center’s North American Glacier Photography, Ice, and Climate 

Project, roll V644, exposure 199, altitude 13,000 ft. Aerial photograph by Austin Post.
26 Studies by the U.S. Geological Survey in Alaska, 2005

steam sometimes still issues.” Juehle and Coulter (1955) 

stated, “During the past three decades, a vapor plume has 

issued from the summit of Mt. Spurr at irregular intervals.”

Pilots reported an increasing discharge of steam from the 

summit of Mount Spurr during late spring 1953, just weeks 

before the July 1953 eruption of Crater Peak. The original 

report, which appeared in the Fairbanks News-Miner (May 21, 

1953, p. 9), further describes steam emerging from a “fissure 

300 ft beneath the peak on the northeast, or Anchorage, side.” 

The pilot described the steaming source as “a big chimney” 

with “smoke [sic] coming out under pressure,” and mentioned 

that he had been smelling sulfur for the past week. In 1960, 

climbers on the first successful ascent of Mount Spurr reported 

“interesting crystal formations” that they surmised to have 

resulted from sulfurous (“lemon-yellow”) steam escaping from 

numerous vents (Wilson and others, 1999).

Two newspaper articles (Anchorage Daily Times, Feb. 2, 

1965, p. 1, 9; Fairbanks Daily News-Miner, 1965, v. 43, no. 

28, p. 9) describe steam discharge from Mount Spurr; however, 

it is unclear whether the source was Mount Spurr or Crater 

Peak. The second ascent party of Mount Spurr in June 1969 

again described fumaroles on the summit cone, but subsequent 

accounts by climbers through 1978 reported no fumarolic 

activity. Climbers in 1983 reported several crevasses near the 

summit venting sulfurous gases (Wilson and others, 1999).

A photograph of the summit of Mount Spurr by Tom 

Miller of AVO in fall 1972 shows a small vapor plume above 

the summit basin. Boiling-point fumaroles that occasionally 

produce vapor plumes were reported on exposed bedrock 

(the northeast face) at the 2,925- to 3,050-m level on the east 

face of the summit peak (Turner and Westcott, 1986). Nye 

(1987) observed, but did not visit, fumaroles on the south and 

southwest flanks of the summit. AVO field crews conducting a 

gas-measurement flight in 1993 spotted a possible vapor source 

on a rib of exposed rock on the south flank of the summit. 

Climbers in 1997 reported no fumarolic activity or sulfur smell 

near the summit (Dale Letourneay, written commun., 2004). 

We note that the local Dena’ina Indian name for Mount Spurr, 

K’idazq’eni, means “one that is burning inside,” suggesting that 

plumes related to thermal output at the summit had been visible 

for some time before the mid-1700s and the onset of written 

documentation in Alaska (Wilson and others, 1999, p. 28).

Systematic gas-measurement flights and aerial observations 

of the summit were not conducted before 2004, largely owing to 

its assumed level of inactivity and absence of geologic evidence 

for late Holocene eruptions. March and others (1997) mentioned the summit crater and possible outburst floods from water 

impounded in this crater and their potential impact on Kidazgeni 

Glacier and, ultimately, the Chakachatna River (fig. 2).

Conclusions

The 2004–present ice cauldron at the summit of Mount 

Spurr reflects a prolonged period of increased magmatic-gas 

and heat flux from an intracaldera vent that is believed not to 

have erupted during the past ~5 k.y. Melting and collapse of 

snow and ice has exposed a breached crater open to the east. 

Exposed bedrock appears to be a combination of hydrothermally altered rock and bedded pyroclastic deposits, suggesting 

that the summit of the volcano is an explosive vent feature 

atop a glaciated dome complex. Should elevated geothermal 

activity continue in the summit area, increasing exposure of 

the summit bedrock will allow a more complete interpretation 

of the characteristics and origin of this structure.

Formation of an ice cauldron in response to increasing volcanic heat flux at Mount Spurr, though not unique, 

is the first such feature of which we are aware in Alaska to 

be associated with debris flows. The formation of a similar 

feature at Chiginagak Volcano in summer 2005 (Schaefer and 

others, 2005), with an outburst of acidic waters, underscores 

this process as a potentially hazardous consequence of any 

volcano capable of sustaining a lake over an active geothermal 

area. Numerous other iceclad volcanoes in Alaska retain the 

potential for geothermal melting of significant volumes of ice, 

formation of meltwater lakes, and periodic debris flows.

Acknowledgments

We thank the staff of AVO, who made many essential 

observations during the period of volcanic unrest at Mount 

Spurr. In particular, Peter Cervelli, Dave Schneider, Chris 

Waythomas, and Mike West contributed ideas on debris flows 

and melt-pit growth. Our colleagues at the Cascade Volcano 

Observatory, Carolyn Driedger, Willie Scott, Cynthia Gardner, 

Mike Doukas, Ken McGee, and Terry Gerlach, also shared 

helpful ideas and perspective, and David Frank kindly offered 

his thoughts in comparing this activity to that at Mount Baker. 

Excellent reviews by Willie Scott, Larry Mastin, and David 

Frank improved the manuscript. We also thank colleagues and 

citizens who freely shared their photographs and observations 

of Mount Spurr. Many of our observations were collected 

during observational flights over rugged terrain, and we thank 

the capable pilots who made it possible. ASTER data were 

provided by NASA/GSFC/METI/ERSDAC/JAROS and the 

U.S./Japan ASTER Science Team.

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Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6 29

Appendix: Volcano Monitoring

Increasing seismicity, in combination with observations of debris flows from the summit, prompted concern about accelerating volcanic unrest at Mount Spurr. On July 26, 2004, AVO elevated the Level of Concern color code to yellow (the first 

degree of elevated concern above green, or background), and maintained it through the end of 2005. Nearly all weekly summaries and daily status reports emphasized that despite the departure from background activity at Mount Spurr, there were no signs 

of imminent eruptive activity. On October 8, 2004, AVO announced the public availability of Web camera images of Mount 

Spurr on the AVO Web site (URL http://www.avo.alaska.edu/). The Web camera rapidly became a popular public page and an 

important monitoring tool for AVO.

AVO issued three Information Releases on Mount Spurr activity in 2004 and 2005, in addition to summarizing the situation 

there in standard weekly updates on all Alaskan volcanoes. In March 2005, as the Alaska Range climbing season began, AVO 

distributed a special information release discussing specific hazards in the immediate vicinity of the summit, a popular climbing target for mountaineers. Several articles appeared in the Anchorage Daily News (July 28, 2004, p. A1; Aug. 8, 2004, p. B3). 

In response to the yellow Level of Concern color-code declaration, the National Weather Service issued a one-time Volcanic 

Ash Advisory (VAA), and the Federal Aviation Administration (FAA) issued a Notice to Airmen (NOTAM) on July 26, 2004, 

that was canceled on November 9, 2004 (NOTAM 4/2284; B. Brown, FAA, written commun., 2005). After more than a year of 

slowly declining seismicity and gas emissions, AVO downgraded the Level of Concern color code for Mount Spurr to green on 

February 21, 2006.

AVO Information Releases

Monday, July 26, 2004 12:05 PM ADT (2005 UTC)

MOUNT SPURR VOLCANO (CAVW#1103–04)

61°18′ N 152°15′ W, Summit Elevation 11,070 ft (3,374 m)

Previous Level of Concern Color Code: Green

Current Level of Concern Color Code: YELLOW 

AVO has identified an increase in earthquake activity beneath the summit of Mount Spurr volcano located about 130 km (80 Spurr volcano located about 130 km (80 

mi) west of Anchorage. Some of these earthquakes can be interpreted to reflect the beginning stages of volcanic unrest. Because 

this is a notable departure from the normal background seismicity of the volcano, AVO is raising the level of concern color code 

to YELLOW. However, there are no indications that an eruption is imminent. Often this type of seismicity will decline without 

producing an eruption.

Retrospective analysis suggests that the current increase began slowly, perhaps as early as February 2004. At present, AVO is 

locating 15–20 earthquakes below the volcano each day. This is a rate greater than any observed since the last eruptive period 

in 1992. All earthquakes are less than magnitude 1.5 and range in depth between 1 and 6 km (0.6 and 4 miles) below sea level. 

To date, relatively few earthquakes have been located beneath the Crater Peak vent, the site of the 1953 and 1992 eruptions. The 

current earthquake activity differs markedly from that seen prior to the1992 eruption, which started with a small cluster of earthquakes directly beneath Crater Peak nearly a year before the eruption began.

AVO geologists visited Spurr by fixed-wing aircraft on July 15 and observed no indications of recent volcanic activity. On July 

11, a pilot flying by the volcano reported a sulfur smell and new area of steaming, but neither was noted during the AVO flight, 

although a significant portion of the Crater Peak area was obscured by low clouds. AVO geologists did document several freshlooking, dark debris flow/avalanche deposits on the southeast and south face of the summit dome complex suggesting water flow 

at the surface, but this may simply reflect recent unseasonably warm and sunny conditions.

AVO will continue to monitor activity at Mount Spurr closely using seismic data, satellite images, and overflights, and will issue 

further information releases as appropriate.

Spurr volcano is an ice- and snow-covered stratovolcano located on the west side of Cook Inlet. The only historical eruptions 

in 1953 and 1992 occurred at the Crater Peak flank vent located 3.5 km (2 mi) south of the Spurr summit. These eruptions were 

explosive, brief in duration, and produced towering columns of ash that rose up to 20 km (65,000 ft) above sea level and deposited several mm of ash on populated areas of south-central Alaska, including approximately 6 mm of ash in Anchorage in 1953. 

The summit dome complex of Mount Spurr is largely covered in ice; its last known eruption was approximately 5,000 years 

ago. Primary hazards from future eruptions at Mount Spurr and Crater Peak include far-traveled ash clouds, ash fall, pyroclastic 

flows, and lahars or mudflows that could impact drainages primarily on the south and east sides of the volcano.
30 Studies by the U.S. Geological Survey in Alaska, 2005

Tuesday, August 3, 2004 2:00 PM ADT (2200 UTC)

MOUNT SPURR VOLCANO (CAVW#1103–04)

61°18′ N 152°15′ W, Summit Elevation 11,070 ft (3,374 m)

Current Level of Concern Color Code: YELLOW

An overflight of Mount Spurr by AVO scientists yesterday afternoon revealed a circular depression in the icecap just northeast Spurr by AVO scientists yesterday afternoon revealed a circular depression in the icecap just northeast 

of the summit. The depression is approximately 50 meters (165 feet) in diameter and about 25 meters (82 feet) deep. The floor 

of the depression contains an icy pond, with small areas of open water. No steam or volcanic emissions were observed. Depressions of this sort may have existed on Spurr before, but AVO is not aware of any in recent decades. The depression may have 

formed by heat from the volcano melting the ice, or, the release of stored water which might account for the minor mudflows 

observed several weeks ago on the upper flanks. If due to heat, it is not known if the flow of heat to the surface has increased and 

is related to the recent increase in earthquake activity.

There is no evidence or suggestion of any explosive activity; no fresh ash was observed anywhere on the mountain. Crater Peak, 

the vent responsible for the eruptions in 1953 and in the early 1990s, showed no sign of any unusual activity.

Earthquake activity continues beneath Mt. Spurr with no significant change over the past week.

Based on these observations and our continuing seismic monitoring of the volcano, there are no signs that an eruption is imminent or even certain. Ephemeral summit craters have been described before at Mt. Spurr and temporary increases in earthquake 

activity below a volcano often decline without producing an eruption. AVO will continue to monitor activity at Mount Spurr 

closely using seismic data, satellite images, and overflights, and will issue further information releases as appropriate.

Spurr volcano is an ice- and snow-covered stratovolcano located on the west side of Cook Inlet. The only historical eruptions 

in 1953 and 1992 occurred at the Crater Peak flank vent located 3.5 km (2 mi) south of the Spurr summit. These eruptions were 

explosive, brief in duration, and produced towering columns of ash that rose up to 20 km (65,000 ft) above sea level and deposited several mm of ash on populated areas of south-central Alaska, including approximately 6 mm of ash in Anchorage in 1953. 

The summit dome complex of Mount Spurr is largely covered in ice; its last known eruption was approximately 5,000 years 

ago. Primary hazards from future eruptions at Mount Spurr and Crater Peak include far-traveled ash clouds, ash fall, pyroclastic 

flows, and lahars or mudflows that could impact drainages primarily on the south and east sides of the volcano.

Thursday, August 12, 2004 11:40 AM ADT (1940 UTC)

MOUNT SPURR VOLCANO (CAVW#1103–04)

61°18′ N 152°15′ W, Summit Elevation 11,070 ft (3,374 m)

Current Level of Concern Color Code: YELLOW

AVO received a pilot report at approximately 8:15 AM ADT, today, August 12, 2004, of possible emission of ash from Mt. Spurrr

volcano. At this time we have no evidence that an eruption has occurred or is in progress. Crater Peak, the vent responsible for 

the eruptions in 1953 and in the early 1990s, also shows no sign of any unusual activity.

Earthquake activity continues beneath Mt. Spurr with no significant change over the past several weeks.

Based on these observations and our continuing seismic monitoring of the volcano, there are no signs that an eruption is imminent or even certain. Ephemeral summit craters have been described before at Mt. Spurr and temporary increases in earthquake 

activity below a volcano often decline without producing an eruption. AVO will continue to monitor activity at Mount Spurr 

closely using seismic data, satellite images, and overflights, and will issue further information releases as appropriate.

Tuesday, February 21, 2006 3:15 PM AKST (015 UTC)

SPURR VOLCANO (CAVW#1103–04)

61°17′58′′ N 152°15′4′′ W, Summit Elevation 11070 ft (3374 m)

Current Level of Concern Color Code: GREEN

Previous Level of Concern Color Code: YELLOW

The purpose of this Information Release is to summarize the recent period of volcanic unrest at Mt. �Spurr �olcano and present 

AVO’s rationale for downgrading the Level of Concern Color Code from Yellow to GREEN.
Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6  31

Summary

AVO identified an increase in earthquake activity beneath the summit of Mount Spurr Volcano beginning in February 2004. The 

increase marked a notable departure from the normal background seismicity of the volcano, and AVO raised the level of concern 

color code to YELLOW on July 26, 2004. By this time, the seismicity had increased to a rate greater than that observed since 

the last eruptive period in 1992. Seismicity remained elevated until about April 5, 2005, declined gradually since about June 

25, 2005, and has remained relatively stable since then. The majority of located earthquakes have been smaller than magnitude 

1.5 and range in depth between 1 km above and 6 km (0.6 and 4 miles) below sea level. Relatively few earthquakes have been 

located beneath the Crater Peak vent, the site of the 1953 and 1992 eruptions. In response to the elevated seismicity, AVO geologists first observed Mt. Spurr Volcano by fixed-wing aircraft on July 15, 2004, and observed no indications of obvious volcanic 

activity, but did observe several fresh-looking, dark debris-flow deposits on the southeast and south face of the summit edifice 

suggesting water flow at the surface. On August 3, 2004, AVO scientists observed a circular melt depression; about 50 meters 

(165 feet) in diameter and about 25 meters (82 feet) deep in the snow and ice cover just northeast of the summit of Mt. Spurr. A 

small ice-rich pond on the floor of the depression also was observed. Since then, the melt depression and pond have increased in 

size although the volume of water in the depression has fluctuated somewhat. Occasionally a small steam plume has been visible 

above the depression from Anchorage and by passing pilots. Intermittent observations made from August 2004 through September 2005 chronicled the enlargement of the melt depression, documented areas of warm rock on the floor of the depression and 

in other areas on the edifice with an infrared camera, and noted at least two additional debris flows. Localized but vigorous roiling within the medium-grey colored lake was observed on numerous occasions and is consistent with emission of hot volcanic 

gases, whereas the persistent grey color usually indicates high acidity. Intermittent airborne measurements of volcanic gases 

(CO2, H2S, and SO2) from August 2004 to September 2005 indicated small but distinctly measurable quantities, although by 

September 2005 gas emissions had declined to low levels. As of the last AVO over flight of Mt. Spurr in January, 2006, the melt 

depression persists, the lake remains open, but had a distinctly greenish cast. 

The declining seismicity, the reduced gas emissions, and the changing summit lake color, all suggest a reduced level of activity 

at the volcano. Although the overall level of seismicity has remained higher than it was prior to the recent period of unrest, it 

now appears that this level denotes a new background condition for Mt. Spurr Volcano. Thus, based on available information and 

an increased level of monitoring of Mt. Spurr, AVO concludes that the current episode of unrest at Mt. Spurr has ended and the 

likelihood of eruption in the near term has diminished significantly. Thus the level of concern color code for Mt. Spurr is returning to GREEN.

Present Hazards

Although the overall level of unrest at Mt. Spurr Volcano has declined to a new background level, conditions on the mountain 

could remain hazardous to skiers, snowboarders, mountaineers, pilots (especially those landing near the summit area) and other 

visitors for some time. It is possible for unstable snow and ice conditions to persist in and around the summit area, especially 

the steep and unstable walls of the melt pit, and for low-level gas emissions to continue. Thus, visitors to the area may encounter 

foul-smelling and possibly dangerous levels of volcanic gas that might be impossible to detect from a distance. Occasional felt 

earthquakes may occur and these could be large enough to disturb steep or unstable areas of ice and snow. Water still impounded 

in the summit melt depression could be released quickly and without warning and this could lead to hazardous fast-moving 

floods of debris or water on the upper flanks of the summit. 

Additional information about Mt. Spurr hazards can be found on the AVO Web site at http://www.avo.alaska.edu/activity/Spurr.php.

Special Information Release for Climbers and Other Backcountry Users

March 5, 2005

MOUNT SPURR VOLCANO (CAVW#1103–04)

61°18′ N 152°15′ W, Summit Elevation 11,070 ft (3,374 m)

Current Level of Concern Color Code: YELLOW

Continued volcanic unrest at Mount Spurr has led to several conditions on the mountain that could be hazardous to skiers, Spurr has led to several conditions on the mountain that could be hazardous to skiers, 

snowboarders, mountaineers, pilots (especially those landing near the summit area) and other visitors. This information release 
32 Studies by the U.S. Geological Survey in Alaska, 2005

is intended to provide the public with an inventory of the main volcanic hazards that may exist on Mount Spurr. It should not be 

taken as an exhaustive list, since some volcanic hazards are inherently unpredictable.

(1) Unstable snow and ice conditions near the summit of Mount Spurr. Higher than normal heat flow at Mt. Spurr’s summit has 

created a large (>100 meters across) melt pit, just below the topographic summit of the mountain. The rim of this melt pit is 

extremely unstable—we have direct evidence that large pieces of the rim fail and slide into the pit regularly, perhaps several times 

per day. The high heat flow has also created several additional melt holes and may have increased steam and gas output from preexisting fumaroles near Spurr’s summit. Other melt holes, some likely covered by a thin crust of snow or ice, likely exist.

(2) High concentrations of volcanic gases near the summit of Mt. Spurr and Crater Peak. Both Spurr and the Crater Peak cone, 

3 km south of Spurr, are emitting volcanic gases, including carbon dioxide (CO2), sulfur dioxide (SO2), and hydrogen sulfide 

(H2S). In high concentrations, all of these gases can be lethal. Carbon dioxide is of particular concern since it is heavier than air 

and ponds in low-lying areas or depressions. Carbon dioxide can also accumulate in the snow pack; snow caves built in CO2-

saturated snow pack can be extremely dangerous, with the gas causing dizziness, headaches, muscular weakness, respiratory distress, and, in high enough concentrations, death (see http://seis.bris.ac.uk/~glcjh/ivhhn/guidelines/gas/co2.htm http://seis.bris.ac.uk/~glcjh/ivhhn/guidelines/gas/co2.html and http://pubs. l and http://pubs.

usgs.gov/fs/fs172-96/).

(3) Potentially unstable snow and ice conditions down slope from Mount Spurr. Because hotter than normal water may be flowing from the summit melt pit, unusual or unseasonable snow and ice conditions may exist within the drainages flowing from the 

summit. A sudden release of water impounded in the summit melt pit is also possible.

(4) Acidic water. Because of dissolved volcanic gasses, the lake in the summit melt pit is likely acidic, possibly with sufficient 

strength to burn skin. It is also possible that this acidic water is emerging from as of yet unknown points on the volcano.

(5) Earthquake-caused rock or ice fall. Mt. Spurr is currently experiencing seismicity rates that are much higher than normal. So 

far, these earthquakes have been too small to cause rock or ice fall, but the potential for larger earthquakes cannot be ruled out. 

Such earthquakes could add to the background avalanche hazard.

(6) Higher than normal probability of volcanic eruption. Although there are no signs that an eruption is imminent, the current 

unrest at Mt. Spurr may result in an eruption either from the summit itself or from Crater Peak. Earthquake swarms will almost 

certainly precede an eruption, but the earthquakes may be too small for people to feel. Even a relatively minor eruption could be 

extremely dangerous to anyone within several kilometers of the vent. For information about hazards and a description of recent

activity, refer to http://www.avo.alaska.edu/avo4/atlas/volc/spurr/spurr2004/index.html or call the Alaska Volcano Observatory. http://www.avo.alaska.edu/avo4/atlas/volc/spurr/spurr2004/index.html or call the Alaska Volcano Observatory.

NOTAM and VAA Texts

Volcanic Ash Advisory issued by the Anchorage VAAC and NOTAM issued by the FAA when AVO declared Level of 

Concern color code yellow. These formal warning notices to aviation are required as part of the Interagency Operating Plan for 

Volcanic Ash Episodes in Alaska (National Oceanic and Atmospheric Administration, Federal Aviation Administration, and 

U.S. Geological Survey, 2004).

VAA Issued by the Anchorage VAAC

Subject: FVAK20 PANC 262116

FVAK20 PANC 262116

VAAAK0

VOLCANIC ASH ADVISORY-WATCH

ISSUED 2004JUL26/2115Z VAAC: ANCHORAGE

VOLCANO: SPURR 1103–04

LOCATION: N6118W15215

AREA: ALASKA

SUMMIT ELEVATION: 11,070 FT (3,374 M)

ADVISORY NUMBER: 2004/01

INFORMATION SOURCE: ALASKA VOLCANO OBSERVATORY

AVIATION COLOR CODE: UPGRADED FROM GREEN TO YELLOW
REMARKS:

AVO HAS IDENTIFIED AN INCREASE IN EARTHQUAKE ACTIVITY BENEATH THE SUMMIT OF MOUNT SPURR 

VOLCANO LOCATED ABOUT 130 KM (80 MILES) WEST OF ANCHORAGE. THERE ARE NO INDICATIONS THAT 

AN ERUPTION IS IMMINENT. OFTEN THIS TYPE OF SEISMICITY WILL DECLINE WITHOUT PRODUCING AN 

ERUPTION. HOWEVER, AN ERUPTION IS STILL POSSIBLE IN THE NEXT FEW WEEKS AND MAY OCCUR WITH 

LITTLE OR NO WARNING.

NEXT ADVISORY:

NO ADDITIONAL WATCHES WILL BE ISSUED AT THIS TIME.

ERW JUL 04

NOTAM issued by the FAA

FDC 4/7470

A) ZAN

B) WIE

C) UFN

E) AK. SEISMIC ACTIVITY ADVISORY FOR MOUNT SPURR VOLCANO AK. /6118N15215W/. ALASKA VOLCANO 

OBSERVATORY HAS REPORTED INCREASED SEISMIC ACTIVITY IN THE VICINITY OF MOUNT SPURR VOLCANO, WHICH INDICATES THE POSSIBILITY OF A VOLCANIC ERUPTION. AIRCRAFT SHOULD REMAIN ALERT 

FOR POSSIBLE ERUPTIONS, STEAM OR ASH CLOUDS AND REPORT ANY SIGHTINGS TO ATC IMMEDIATELY. 

CONTACT ANCHORAGE ARTCC 907–269–1103 FOR ADDITIONAL INFORMATION.

Geothermal Disruption of Summit Glaciers at Mount Spurr Volcano, 2004–6  33