One of the things I find interesting are recent discoveries of long-time large dormant stratovolcanoes. Two examples come to mind.
The first and most obvious is Pinatubo in the Philippines. It was so eroded that it was not recognized as a volcano until the late 1970s. By the time 1991 rolled around, it reminded everyone that it was indeed there, active and a threat to neighbors with one of the largest eruptions in the 20th Century a VEI 6, ejecting some 11 km3 of tephra, ash and pyroclastic flows.
The second and not nearly as obvious is Hayes Volcano in upper Cook Inlet, Alaska. Like Pinatubo, it was not identified as a volcano until 1975. Like Pinatubo, it also erupts infrequently, with the last eruption being some 300 years ago. The largest known historic eruption was 1550 BC, a VEI 5.4, ejecting just over 8 km3 of tephra, ash and pyroclastic flows.
As with all large stratovolcanoes in this part of the state, it is mantled with a permanent cap of ice, snow and glaciers. Some estimates have it carrying some 60 km3 of frozen water in various forms.
Hayes Volcano is located some 140 km NW Anchorage in a remote part of the Alaska Range called the Tordrillo Mountains. There are no nearby roads so it is accessible via air, dogsled and snowmachine during the winter. Nearest villages are Skwentna some 76 km NE and Beluga some 82 km SE. The Smithsonian GVP lists only 151 people within 100 km of the volcano. The Iditarod trail goes through Skwentna every February, which puts it north of Hayes Volcano. There is an active extreme skiing operation with access via helicopter operating in these mountains.
The volcano itself tops out at around 2,800 m. As previously mentioned, it is mantled in a permanent cover of ice, snow and glaciers. It is named after the nearby Hayes Glacier. Hayes is located some 39 km NNW of Mount Spurr, its closest neighbor to the south. Hayes is the northern most of all Cook Inlet volcanoes. http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Hayes
The volcano does not show an obvious cone, mostly due to the action of ice, snow and glaciers on the soft material constructing it. The GVP lists the major component as dacite, which is an evolved andesitic magma. Wouldn’t surprise me if the eruptions were due to destabilized magma chamber.
Volcano Discovery describes Hayes as being mostly destroyed by a series of explosive eruptions some 3,500 – 3,800 years ago. That cone destruction was followed by caldera formation. Yet I cannot find a hint of caldera in maps and satellite photos of the volcano. http://www.volcanodiscovery.com/hayes.html
Threats from future eruptions include but are not limited to tephra, pyroclastic flows, lahars, flank collapses and lateral blasts.
The thing that makes Hayes interesting to is its series of eruptions some 3,600 to 4,400 years ago. At the time, it was the most prolific of all known volcanoes in Cook Inlet. Little is known about its eruptive history before that time. After that series of eruptions, Hayes has been pretty quiet.
USGS Scientific Investigations Report 2014-5133 analyzes a series of tephra and pyroclastic deposits along the Hayes River some 31 km NE of Hayes Volcano and details extensive activity of Hayes over the last 4,000 years. The report includes new data about a large eruption some 4,450 years ago. It ups the 1550 BC eruption to some 10 km3 worth of tephra and pyroclastic flows. Several other unrecognized eruptions of Hayes were discovered at the site. http://pubs.er.usgs.gov/publication/sir20145133
There are six regional tephra layers from that period, indicating Plinian to sub-Plinian eruptions, with the largest being the VEI 5.4 described earlier. Average debris from these eruptions average some 2.4 km3 apiece, with the 1550 BC eruption being the largest.
Final eruption was dated around 900 AD based on a relatively small tephra layer in the ice, snow and glaciers on the volcano. http://volcano.si.edu/volcano.cfm?vn=313050
There are no active fumaroles on Hayes and it is not one of the monitored Alaskan volcanoes. Even photographs of it are difficult to find.
As it sits on the far northern end of Cook Inlet, Hayes Volcano is pretty far away from the part of the Inlet that is dominated by subduction. While there is a subducting plate underneath, the dominant tectonic process is the plastering of terranes onto the southern edge of Alaska by the Pacific Plate. The most recent piece is the Yakutat Terrane which sits some 300 km east of Cook Inlet.
Hayes Volcano sits between two major fault lines. These are the Lake Clark fault line some 70 km to its south and the Denali Fault to its north. Movement along the Denali Fault, the major geologic structure in this portion of North America, is strike-slip. There have been some 38 km of displacement on either side of the fault over the last 38 million years. http://gsabulletin.gsapubs.org/content/85/12/1883
Movement along the Lake Clark fault is right-later strike-slip. There has been some 26 km offset along the Lake Clark fault since the late Eocene. http://pebblescience.org/pdfs/USGS_Lake_Clark_Fault.pdf
A 2012 paper by Hartman, Armstrong and Arkle makes the case that the area under Hayes has two subducting pieces underlying it. The deepest would be the Pacific Plate. The shallower piece is the subducting piece of the Yakutat Terrain, which they call the Yakutat slab. It underlies the entire region of the MatSu stretching all the way to Denali and the Denali fault at the farthest NW point.
According to the paper, not only is the Yakutat Terrane welding itself to the Chugach Terrane which was the last most recent addition to southern Alaska, but it is also going underneath part of the Chugach, forcing it up in a process the paper refers to as underplating. http://www.searchanddiscovery.com/pdfz/documents/2012/30250hartman/ndx_hartman.pdf.html
To top the entire mess off, the region above the subducting Yakutat Terrane is referred to as the Southern Alaska Block and it is moving roughly WSW over geologic time. It is also rotating counterclockwise as it meshes with the Bering Block to its west.
As Hayes is located just to the west of what is believed to be the subducted / underplated Yakutat Terrane, and on the western portion of the Southern Alaska Block, it should have plenty of magma making its way to the surface via various weak points in this tectonically complex region.
Hayes Volcano dacitic eruptive products indicate that eruptible magma is not always present. But when the magma chamber is remobilized likely with an injection of basalt from near the boundaries of the block and Yakutat Terrane, it can produce multiple vigorous eruptions. Given the complex geology underlying and near Hayes, future eruptions are not out of the question. For now, it is a benign, well behaved mountain. How long that lasts is anyone’s guess.