Nearly a month ago, one of the strongest earthquakes in 25 years rattled Cook Inlet. It was originally rated as a 6.3 Richter quake that was upgraded to a 6.46 Richter. Location was some 14 km south of Mount Iliamna with a depth of 117 km. Due to the depth, local geologists determined it was tectonic in nature, with the Pacific Plate being pulled into the mantle under its own weight. http://www.aeic.alaska.edu/quakes/iliamna_20150729.html
It was strongly felt here in Anchorage as a two-parter. The initial jolt was felt followed by what could be best described as a buzz as the short period vibrations passed. The surface waves arrived a few seconds later and really rolled the house. Total time of the quake was around a minute. Local time of the quake was around 1830, suppertime.
Reporting on the blogs mostly focused on possible activity at Redoubt, the most recent Cook Inlet volcano to erupt. It occurred to me that few out there know about Mount Iliamna. Perhaps it is time to introduce yet another Alaskan stratovolcano.
Mount Iliamna is located on the west side of Cook Inlet roughly midway between Redoubt to the north and Augustine to the south. It is a composite stratovolcano that is perpetually covered with ice and snow. There are several active glaciers on the cone. The cone itself is layered with andesitic lava and pyroclastic deposits. There are flank collapse / avalanche deposits down the cone through valleys. Over the lifetime of the cone the action of glaciers and avalanches has whittled it away to the point where it no longer has an obvious crater or dome complex at the top of it.
Iliamna is some 3,050 m tall and is located 218 km SW Anchorage. It does not have a noticeable crater any more.
If you were to stand at the Kenai City Dock and look across the Inlet to the west, you would look directly at Redoubt as the closest volcano some 80 km west. To your right some 100 km NW would be Mount Spurr. To your left some 116 km SW is Mount Iliamna. It is an impressive view.
Iliamna has vigorous active fumaroles two thirds of the way up the mountain. Activity from these fumaroles has been mistaken for steam eruptions from time to time. There are no known ash eruptions from Iliamna since European settlement of Cook Inlet. Webcams and webicorders can be found at the AVO’s Mount Iliamna page on their web site. http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=Iliamna
Due to the heavy snow cover and internal heat, snow, ice and rock avalanches down the flanks are relatively common. The last set of landslides was February 2013. These are normally picked up by the multiple webicorders on the mountain. They also throw snow and ice into the air and are mistaken for steam emissions from the fumaroles.
There were two recent occurrences of what the Alaska Volcano Observatory (AVO) refers to as volcanic – tectonic events. These were a swarm of small earthquakes determined to be volcanic in nature. The most recent swarm was 2011. The one before that was 1999. None of the earthquake swarms led to an eruption. An older 1996 earthquake swarm was tied to a dike emplacement under the mountain.
I was not able to find any references to any flank collapse or caldera forming event at Iliamna, though they are most certainly observed on other Cook Inlet volcanoes (Augustine, Katmai, Redoubt, Spurr).
Also could not find any reasonable dating of the volcanic edifice but perhaps have enough to make a (poorly) educated guess. Iliamna’s large neighbor to the north is Redoubt. It is around 800,000 years old. It’s new neighbor to the south, Augustine has rocks dated as old as 40,000 years. I would guess that Iliamna was in the neighborhood of 500,000 – 1 million years old. It could be much, much younger than that.
As discussed previously, fumarole activity has been mistaken for steam eruptions from time to time. These emissions have been as high as 1 – 3- km above Iliamna.
The most recent ash eruptions are pyroclastic flows from 300 years ago. Volcano Discovery lists a pyroclastic flow some 140 years ago, but that listing is not verified by AVO.
The most recent explosive event, once again a steam eruption listed by AVO was in 1953. It put steam a couple kilometers above the peak. It is listed as a VEI 2.
A pair of eruptions in 1876 and 1867 are listed as VEI 3 and 2 respectively. They are likely steam explosions.
The largest eruptions in the Smithsonian GVP database are 2050 and 5050 years ago. Both are listed at VEI 4 events. There is a discrepancy between the GVP database and the VOGRIPA database on these eruptons. VOGRIPA lists them as roughly 2600 and 5900 BC. Both eruptions expelled some 0.10 km3 of ash and pyroclastic flows. http://volcano.si.edu/volcano.cfm?vn=313020
At this time, the most serious threat posed by Iliamna is that of avalanches down the flanks of the mountain. Should the magma chamber underneath it be recharged or destabilized you can add ashy eruptions, possible flank collapse, lateral blasts and significant lahars to the list. With this structurally weak edifice, expect lahars early and often.
The line of active volcanoes in Cook Inlet stretch from Hayes in the north past the Katmai – Novarupta Volcanic Field in the south. They are variously referred to as Cook Inlet volcanoes and the Eastern Aleutian Volcanic Arc.
All are forearc volcanoes powered and supplied by subducted and melted Pacific Plate as it dips under the North American plate. To make Cook Inlet more interesting, it is also being formed by the collision and indentation of the Yakutat terrane some 300 km to the east. The Aleutian Trench sits to the east of the Alaska Peninsula and south of Cook Inlet.
As such, the basin is full of folds, discontinuous structures, fault lines, oblique-slip faults. The further you travel south in the basin, the more geology trends toward pure subduction with the subducting plate traveling roughly from east to west. The farther north you get in the basin, the subduction trails off and the geology is driven by forces associated with welding of terranes on the southern part of Alaska itself.
Faults in the basin are capable of driving 6 – 7+ Richter earthquakes, the last of which was felt in July 2015.
Most of the eruptive products are andesitic and basaltic in nature.
Based on the vigorous fumarole activity on Mount Iliamna, I would posit that it is not yet finished. One of the things you tend to see with large subduction-fed volcanoes is that once the magma feed is established, it tends to exploit that weak spot for an extended period of time.
Perhaps there is a reason the Mount Iliamna is one of the better instrumented volcanoes in Alaska.