|Start:||July 2004 ||Observed|
|Stop:||February 2006 ± 2 Months||Observed|
|Fumarolic or hydrothermal activity: ||
|Eruption Type:||Not an eruption.|
For a complete report of the unrest at Spurr between 2004 and 2006, please see Coombs and others (2006), available online at http://pubs.usgs.gov/pp/pp1732/pp1732b/index.html
From Neal and others (2005): "In early July 2004, AVO seismologists noted an increase in volcano-tectonic and long-period earthquake activity beneath the summit of Mount Spurr Volcano (Power, 2004; Power and others, 2004). About the same time, AVO was contacted by a long-time Alaskan pilot who flew near the volcano on July 11 and saw a small steam plume from the approximate 5,500 foot level of the east side of Crater Peak. She also reported an unusual sulfur dioxide odor. Based on this pilot report and the increase in seismicity, AVO launched a fixed-wing observation flight on July 15. Clouds prevented a view of the summit of Spurr, but Crater Peak and the lower south and east flanks of the Spurr summit dome were clear. Crater Peak appeared unchanged from previous views following the 1992 eruption and nothing unusual was noted along any of the glacier margins or termini around Crater Peak. The east flank of the Spurr summit dome, however, was marked by as many as a dozen dark debris flow lobes that emanated primarily from point-sources within the glacial cover [see fig. 5 in original text] (McGimsey and others, 2004).
"Increased daily counts of shallow (5-10 km or 3-6 mi below sea level) earthquake activity combined with observations of debris flows from the summit prompted concern about the possibility of volcanic unrest at Spurr. On Monday July 26, AVO elevated the Level of Concern Color Code to YELLOW. A second AVO overflight on August 2 revealed a circular depression in the Spurr summit ice cap, approximately 50-60 m (165-200 ft) in diameter and 25 m (82 ft) deep [see figs. 6A and B in original text]. The pit contained an ice-encrusted pond with small areas of open water that were distinctly blue-gray in color [see fig. 6C in original text]. This feature became known as the 'ice-cauldron' following usage of the term at ice-covered Icelandic volcanoes.
"From early August though early December, the summit ice-cauldron gradually enlarged as blocks of ice ringing the depression sagged and then collapsed into the pit [see figs. 6A, B in original text]. Careful measurements from images taken on August 10 and October 30 indicate that the pit enlarged from about 65 x 95 m (210 x 310 ft) across to 130 x 130 m (430 ft x 430 ft) across in two months' time (M. Coombs, written commun., 2004). Overflights throughout the late summer and fall documented the changing size of the feature, continuing deformation and collapse of surrounding ice walls, and the variability of open water on the surface of the lake. The lake remained a dark battleship gray color, and circular ice-free zones perhaps 5-10 m (16 x 33 ft) across occurred near bedrock lake shoreline and at several points further from the shore [see fig. 6C in original text]. By early December, the areas of exposed bedrock near the bottom of the cauldron had grown and were occasionally observed steaming. Yellow-tinted snow, ice, and rock outcrops in the vicinity of the lake reflected sulfur deposition near the lake margin.
"AVO staff conducted several airborne Forward Looking Infrared Radiometer (FLIR) measurements using both a hand held and helicopter-mounted camera and video system. FLIR data confirmed the presence of at least two prominent areas of warm bedrock -- with temperatures as high as ~39o C or 102o F -- on the margins of the lake [see fig. 7A in original text] and on the outer flanks of the summit dome. Lake surface temperatures as measured by FLIR ranged from -10o to 0o C (14o to 32o F) for areas of floating ice and snow debris as well as open water [see fig. 7B in original text].
"AVO also gathered an extensive library of satellite imagery of the Spurr edifice and increased satellite analysis frequency using the standard AVO monitoring imagery (Geostationary Operational Environmental Satellites [GOES], Advanced Very High Resolution Radiometer [AVHRR]) and higher resolution imagery (Advanced Spaceborne Thermal Emission and Reflection Radiometer [ASTER], Moderate Resolution Imaging Spectroradiometer [MODIS]). ASTER imagery showed the first signs of a summit thermal anomaly in nighttime thermal infrared data on August 17, 2004; as the summit lake grew in size, the intensity of the ASTER thermal infrared anomaly increased (R. Wessels, oral commun., 2005).
"Five fixed-wing gas measurement flights of the Mount Spurr plume were conducted between early August and the end of October. Emission-rate measurements of SO2, H2S, and CO2 gas were made during each of these flights following protocols developed by the USGS (Gerlach and others, 1997; Gerlach and others, 1999; McGee and others, 2001 -- see original paper for complete references). Preliminary results show that CO2 degassing from the summit of Mount Spurr increased from 600 tonnes/day (t/d) in August to 1,300 t/d in September and finally to 1,400 t/d in October. At Crater Peak, CO2 emission rates were 160 t/d, 1,000 t/d and 120 t/d for the same measurement periods. Very small amounts of H2S (=3 t/d) were consistently measured on all of the flights at both degassing locations while no SO2 was detected at all.
"Crater Peak has consistently degassed a small amount of CO2 since 1994 that, except for an anomalously higher value in 1997, is typically <200 t/d (Doukas, 1995; M. Doukas, pers. commun., 2004). Carbon dioxide degassing from the summit of Mount Spurr had previously not been detected, although airborne measurements directed specifically at the summit have been rare. The absence of SO2 throughout this period is likely caused by the extremely wet environment at this glacier-clad volcano, where abundant groundwater dissolves SO2 (Doukas and Gerlach, 1995). This scrubbing process would also be greatly enhanced by the presence of the lake at the summit, and the distinctive battleship gray color of this lake might be partly due to dissolved sulfur compounds. The low but positive values for H2S can reflect the release of H2S from a boiling hydrothermal system (Symonds and others, 2001 -- see original text for full citation). This is consistent with historical reports of pressurized fumaroles described by climbers in the summit region and the presence of diffuse boiling-point fumaroles on outcrops of bedrock on the east side of the Mount Spurr summit dome (Turner and Wescott, 1986; C.J. Nye written commun., 2004).
"Seismicity at Mount Spurr remained consistently above the pre-July 2004 background level for the remainder of the year, although daily rates of seismicity varied considerably from several to several tens of volcano-tectonic (VT) events per day. The largest tally of identifiable earthquakes in one day was 80 on October 26. Particularly energetic swarms of VT earthquakes located within 20 km (12 mi) of Mount Spurr occurred on October 26 (6.6 earthquakes per hour), November 4 (5.8 earthquakes per hour), August 14 (2.6 earthquakes per hour), and August 21 (1.8 earthquakes per hour). Throughout the unrest in 2004, VT seismicity was concentrated within 5 km (3 mi) of the Mount Spurr summit, in stark contrast to the pre-1992 seismicity (Power, 2004; Power and others, 2004). Located long-period (LP) events occurred at an average depth of approximately 7 km (4 mi) and at variable rates, peaking in November. Deep earthquakes (> 20 km or 12 mi) were located beneath and south of Crater Peak in the same area as the deep seismicity associated with the end of the 1992 eruption of Crater Peak.
"Although no eruptive activity ensued in 2004, AVO did experience an eruption response drill. A pilot report of possible ash from Mount Spurr on August 12, followed by a public ash fall advisory issued by the NWS, prompted a daylong flurry of calls, inquiries, and media attention. AVO issued a special Information Release stating that no eruption had occurred. This event— certainly not the first or last of its kind in AVO history—underscored the level of public concern regarding the situation at Mount Spurr and likely reflected a fresh memory of ash fall in 1992. The drill also facilitated review and improvement of communication protocols between AVO and its partner in ash warnings, the National Weather Service.
"How unusual is this drastic change in the summit morphology at Mount Spurr? To our knowledge, this is the first documented episode of significant geothermal heating and generation of a substantial lake at the summit, as well as the first known occurrence of watery debris flows from the summit. Historical reports and aerial photographs from the 1950s, 60's and 70's, however, document significant variability in the snow and ice cover at the Mount Spurr summit. During periods of lower-snow levels, a crater-like structure becomes visible. This feature was described in March and others (1997) as a ~200 to 300 m (650-1,000 ft) diameter feature open to the east-northeast. In this same 1957 aerial photograph, a steep-walled, snow and ice pit, 20-30 m (65-100 ft) wide, is located in the ice cap near the base of the north summit crater wall. No open water can be seen in the bottom of the pit, however, several dark patches occur and could possibly represent warm bedrock.
"AVO interprets this 2004 period of elevated seismicity and heat flux, summit melting, debris flow generation, and magmatic gas emission from both Spurr and Crater Peak to be the result of new injection of magma to a shallow level beneath the Spurr edifice (Power, 2004; Power and others, 2004). Magmatic gas flux from both Crater Peak and Mount Spurr suggests an open connection to the surface from the zone of intrusion or magma storage along two conduits. An alternative interpretation invokes release of volatiles from the still-cooling intrusions from the 1992 eruption series (Power and others, 1998; 2002).
"Mount Spurr remained at Level of Concern Color Code YELLOW through the end of the year. Nearly all information release statements, weekly summaries, and daily status reports emphasized that despite the departure from background activity at Spurr, there were no signs of imminent eruptive activity. As part of this response, AVO mounted a number of observation flights, gas measurement and FLIR imaging flights, increased the frequency of satellite analysis, and installed six new seismometers and 3 permanent, continuous GPS receivers to improve seismic monitoring and track deformation of the volcanic edifice. On October 8, AVO announced the public availability of Internet web camera images of Mount Spurr on the AVO web site (http://www.avo.alaska.edu).
"AVO issued three Information Releases on Mount Spurr activity in 2004 in addition to summarizing the Spurr situation in standard weekly updates on all Alaskan volcanoes. A number of articles appeared in the Anchorage Daily News (Anchorage Daily News 2004). In response to the YELLOW Level of Concern Color Code declaration, NWS issued a one-time Volcanic Ash Advisory (VAAS) and the FAA issued a Notice to Airmen (NOTAM) on July 26. The NOTAM was cancelled on November 9 (NOTAM 4/2284; B. Brown, FAA, pers. commun., 2005)."
From McGimsey and others (2007): "During 2005, elevated seismicity continued beneath Mt. Spurr, and the summit ice-collapse pit enlarged - becoming a large cauldron - as heat was supplied to the summit area. The lake changed in size, and the amount of ice debris on the lake varied. Lake level declined in May, seemingly associated with the generation of a small debris flow on the upper southeast flank. With the decline in water level, subaqueous fumaroles emerged and the area of hot, steaming wall rock increased. Temperatures of the warm zones measured with Forward-Looking Infrared Radiometer (FLIR) increased somewhat over the year. Emissions of CO2 and SO2 decreased. The Level of Concern Color Code for Mount Spurr remained at Yellow for all of 2005 (see table 6 in original text)."
Neal and others (2009) report that the"Level of Concern Color Code for Mount Spurr at the beginning of 2006 was YELLOW due to continued above-background seismicity, magmatic degassing, and the presence of an open, warm lake within a 300-m-diameter (980 ft) rock- and ice-walled cauldron atop the summit cone. Following months of no significant change in activity, AVO downgraded the Level of Concern from YELLOW to GREEN on February 21 . The information release cited a steady decrease in shallow seismicity between April and June 2005, after which earthquake activity remained slightly elevated above background levels through the remainder of 2005 and into 2006. By May 2006, seismicity at Mount Spurr ahd returned to background levels and remained there with few exceptions through the end of 2006. Intermittent observations permitted sporadic documentation of ongoing changes in the summit area as the geothermal activity continued to disrupt the ice field around the summit cone [see table 5 in original text]."