Mount Spurr lies some 80 nm (150 km) due west from Anchorage. It is a stratovolcano that is visible on most days across Cook Inlet and lies on the Alaska Range. The mountain is 9,800’ (3,000 m) high and is topped with a 3 by 4 nm (5 by 6 km) caldera. The active vent is on the south end of the caldera and called crater peak. The mountain suffered a crater collapse around 10,000 years ago that created Chakachamna Lake. There is an active ice field in the caldera and multiple glaciers. Crater Peak last erupted in 1992, putting around a half inch (1 cm) of ash on Anchorage. http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Spurr
Perhaps the most interesting things about Mount Spurr are the location and the flank collapse debris flows of the south side of the volcano. The volcano sits at the south edge of a break in the Alaska Range. From the volcano northward are relatively high mountains, glaciers and active ice caps. There is also another volcano that was only recognized as a volcano in the mid 1970s, Hayes volcano. And for skiers, there are nearly 10,000’ (3,000 m) of vertical snow for the extreme skiers to play on.
The Alaska Volcano Observatory has several seismic monitors on the mountain and a webcam located on an oil and natural gas production platform in Cook Inlet looking north to the volcano.
There is a river valley and lake that sits in the notch between the southern flank of Spurr and the southern mountains of the Alaska Range. It is into this valley that the southern portion of the flank collapses and refills on occasion. I have not found any evidence that historic flank collapses have opened up the vent for a lateral blast like Mount St.Helens.
Additionally, the valley periodically gets pyroclastic flows, lahars, and other volcanic discharges that periodically dam the Chakachatna River flowing out of the lake. There is even a glacier that occasionally creates an ice dam that lake outflow gets to deal with. Lake water is very cold and supports a run of sockeye (red) salmon. There is geologic evidence of multiple floods in the flood plains below the lake as flowing water wins the argument with dams made of material that be easily eroded.
The caldera was thought to be formed some 4,000 – 10,000 ago. The volcano is primarily constructed out of andesitic lava and pyroclastic deposits. The current active vent, Crater Peak sits on the southern edge of the caldera. There is a dome in the main crater. It is also covered with an ice cap and anchors several valley glaciers down the mountain in all directions.
At the time of the caldera formation, the river valley to the south of the volcano and the entire Cook Inlet to the east were full of ice – as much as half a mile (one kilometer) deep. The ice started retreating in this part of the world about 5,000 years ago.
The Crater Peak vent formed some 6,000 years ago and has for the most part been the center of eruptive activity since then. The USFS Hazard assessment of Mount Spurr suggests that there have been more than 30 ash producing euptions out of Crater Peak since it was formed. http://geopubs.wr.usgs.gov/open-file/of01-482/of01-482.pdf
The two most recent eruptions were 1953 and 1992. Both produced significant ash fall, sub-Plinian plumes, lahars and small pyroclastic flows. The 1953 eruption consisted of a single blast which dusted Anchorage and Cordova. The 1992 eruption was marked by three eruptions over the course of three months, two of which deposited ash on Anchorage. The eruption plume topped out around 8 nm (14 km) and was tracked all the way to the Atlantic coast of the US. Both eruptions were marked with steam emissions before and after the blasts. They were visible from Anchorage. http://geopubs.wr.usgs.gov/open-file/of01-370/
In 1994, the Robotics Institute of Carnegie – Mellon University out of Pittsburgh brought a semi-autonomous robot to Alaska to climb down the Crater Peak crater. The project was a joint project with NASA. It operated for a week and was controlled from Anchorage. The robot was named Dante II and made it to the crater floor. It was tethered at the top of the crater and the tether allowed it to traverse steep slopes of the crater. The tether also supplied power and (I think) transmitted telemetry. The robot fell over on its side on the way out of the crater which ended the test. Helicopter retrieval was unsuccessful and only dropped it deeper into the crater where it was abandoned. This test was briefly characterized in the (awful) 1997 Dante’s Peak. http://www.ri.cmu.edu/pub_files/pub2/bares_john_1999_1/bares_john_1999_1.pdf
In 2004, the monitoring stations picked up an increase in earthquakes indicating that magma was moving in the mountain. Warnings were made and the mountain was closely watched. No change in Crater Peak was noted.
However, the summit of Mount Spurr itself developed a hole in the ice cap – not unlike that seen on Mount St. Helens as it woke up. The hole in the ice turned into an ice filled crater lake that grew from August through October 2004.
The seismic activity continued through the next year and the lake now renamed as the “cauldron” continued to grow on the peak of Mount Spurr. There were over a dozen debris flows from the summit in the summer of 2004. Most of the flows traveled for some distance under the ice cap before surfacing. There was a smaller flow in 2005.
Seismic activity decreased in 2005 and into 2006 and 2007. No eruption was observed. Eventually the cauldron iced over as the heat flow beneath the ice decreased. http://pubs.usgs.gov/pp/pp1732/pp1732b/index.html
Local power companies have looked into a pair of power generation proposals associated with Mount Spurr. The first one proposes to tap Lake Chakachamna and route water through a 10 – 12 nm (18 – 20 km) tunnel through the mountains to the south to a power plant and would empty water into the river south of the lake. This project would not be subject to the flank collapse, debris flow, pyroclastic flow, glacier problems of the Chakachatna River valley and the south flank of Spurr. Cost would be in the neighborhood of a couple billion dollars and produce about a quarter of electricity for SouthCentral Alaska. http://www.petroleumnews.com/pntruncate/839208091.shtml
The second one is a proposed geothermal power plant tapping into the magma chamber of Spurr. A geothermal company named Ormat Technologies with worldwide operations has been drilling test wells on the southern flanks of Spurr since 2011. Most of the operation was funded by the State of Alaska which was pursuing renewable energy at the time. They were looking at a geothermal power plant in the 50 MegaWatt (MW) range. To date, they have not found enough heat or the right rock to allow them to proceed. The drilling has found a lot of gravel and little solid rock, making it difficult and much more expensive install a pressurized system needed for their particular flavor of geothermal generation. http://www.alaskajournal.com/Alaska-Journal-of-Commerce/AJOC-November-6-2011/Ormat-says-it-isnt-giving-up-on-Mount-Spurr-geothermal/
Being as close to over half the population of the State of Alaska (Anchorage and the MatSu Valley) and its major port and international airport, eruptions of Mount Spurr are an important part of our daily life. We watch the mountain closely and pay close attention when it is active. It has demonstrated the ability to drop ash across Cook Inlet and central Alaska. Fortunately, it has not recently been significantly violent, though with any active volcano that is operating out of the remains of a caldera, this may be more wishful thinking than anything else.