Granyia ran across a photo of Kanga after its new webcam went live last month and wondered if it was a candidate for a flank collapse. The photo showed a fissure running across the top of the mountain near the crater topping the stratovolcano. This prompted me to look into Mount Kanaga / Kanaga Volcano for a post.
I did e-mail AVO and asked if this fissure demonstrated that Kanaga was a candidate for another flank collapse. They responded Friday that it is related to the shallow hydrothermal system and less likely to create a flank collapse.
Mount Kanaga is a 1307 m tall stratovolcano on the northern end of Kanaga Island, Aleutian Islands, Alaska. It is some 1941 km SW from Anchorage and 37 km WNW from Adak, formerly Adak Naval Station which currently has a population of 326.
Adak services a commercial fishing fleet working the Bering Sea, with a fish plant that processes groundfish and crab that is offloaded at the port. As a former military base (Army and Navy), Adak has a large and modern airport. Alaska Air Lines flies out twice a week. The Aleut Native Corporation owns and operates most facilities and infrastructure in Adak.
Adak is also in an active volcanic neighborhood, close to Great Sitkin discussed in July, Andrew Bay volcano and Moffett volcano both around 10 km to its north and Kanaga to its west. There are other Aleutian volcanoes strung along the line of the Aleutian Islands. All of these are andesitic – basalt stratovolcanoes. Most have demonstrated some sort of flank collapse, mostly to the north into the Aleutian Basin of the Bering Sea.
Weather in the vicinity is generally overcast, moderate temperatures and frequent cyclonic storms as the remains of tropical storms / typhoons rattle up the Aleutian Islands after doing their damage along the Pacific Rim and Japan. Wind gusts during the storms in excess of 100 km/hr are not uncommon. While there is annual snowfall, it does not tend to persist and is generally melted by rain.
Kanaga is monitored by the Alaska Volcano Observatory. You can find its web page including webicorder here: http://www.avo.alaska.edu/volcanoes/volcinfo.php?volcname=kanaga
Webcam can be found here: https://www.avo.alaska.edu/webcam/Kanaga.php
The volcano itself is a steep, symmetrical stratovolcano built with layers of andesitic lavas, basaltic lavas, mudflow deposits, and numerous tephra layers. There are also mudflow layers. Unlike many of the neighboring volcanoes, there is no evidence of glaciers on the volcano.
Kanaga is flanked on the south, east and west by the scarp remaining of a flank collapse. The ancestral volcano, named Mount Kanaton was a low, shield volcano built largely by effusive activity. Sometime around 12,000 years ago, it collapsed to the north. Due to the lack of massive pyroclastic deposits on the island itself, it is not believed that the collapse was eruptive in nature. Blocky deposits have been mapped covering some 364 km2 north of the Island. Total volume of the debris avalanche is estimated at some 25 km3. Total drop was around 1.3 km above to 3.3 km below sea level. The toe of the debris field is some 38 km from the origin of the ancestral Mount Kanaton. The original vent appears to be close to the location of the modern cone.
Following the collapse, the primary vent continued to be active and the volcano rebuilt itself, perhaps several times only to be destroyed by as many as three (disputed) caldera-forming events. Timing of these large eruptions is in dispute, with supportive evidence being tephra deposits as far away as Adak (which is also disputed). One paper suggests that Mount Moffett on the NW corner of Adak Island is the more likely source of Adak tephras.
However, there are no locations in the caldera or on the north part of the island with the typical thick pyroclastic flow deposits typical of large caldera forming eruptions. These would be deposits tens of meters thick on the flanks and slopes stretching tens of kilometers away from the vent.
The current cone is covered with blocky basaltic and andesitic lava flows that reach the sea on the north and western sides of the volcano.
While not glacier covered, the volcano is usually covered with a snow pack making lahars an active threat. There are active fumaroles near the summit and hot springs exist near the base of the volcano.
There are several andesitic lave domes near the base of the scarp that defines the remaining ridge from Mount Kanaton. These have not been dated but are thought to predate the new Kanaga edifice. Neighboring tephras have been dated to 11,000 years BP. The presence of these domes may have participated in the destabilization of Kanaton, introducing gas-laden magmas, which in turn will introduce substantial water pressure into that portion of the edifice which will in turn fracture, lubricate and expand portions of the ancestral cone upward. Eventually the cone structurally failed and two thirds of it slid north into the Bering Sea.
Kanaga has erupted numerous times over the last 11,000 years with the most recent eruption being a phreatic explosion in 2012. Prior to that, the most recent eruption was 1993 – 1995. A fissure in the cone near the crater on top was first observed following the 2012 eruption.
Long-term activity is generally episodic, with long periods of inactivity punctuated by periods of rapid deposition of volcanic materials during eruptions.
Deposits of pyroclastic materials indicate the volcano erupted explosively at least four times in the last 11,000 years with the most recent explosive event being some 500 years ago. These eruptions included significant falls of tephra and pyroclastic flows.
Historic activity was observed in 1763, 1768, 1786, 1827 and 1933. A series of lava flows were observed in 1906, perhaps starting as early as 1904. The 1993 eruptions began as steam and ash plumes as high as 7.5 km. They were accompanied with explosions and ash fell as far away as Adak. During the August 1994 part of the eruptions, lava flows originated in the crater and made their way to the sea.
The February 2012 explosion was accompanied by volcanic tremor, ash fall and a steaming new fissure across its summit. Cloudy conditions prevented direct observations of the eruption for much of February. The new fissure was about 600m in length. In places it was 15 m in width. This is what Granyia picked up upon with her flank collapse question.
Like other volcanoes in the western Aleutians, Kanaga is a subduction driven and supplied system. Interestingly, there appears to be a change in the chemistry of magmas over time. One paper describes the Aleutian arc as never rifted, relatively thick crust compared with other Pacific volcanic arcs, the islands are relatively large and host extensive exposures of Eocene to Miocene plutons as well as Eocene to Holocene lavas. When you figure that the Eocene ended some 33 MY ago, these islands are very long lived.
Initial lavas were calc-alkaline parental magmas with higher silicates and high viscosity. More recent magmas appear to be contaminated by interaction of melt with the lower crust. Kanaga magmas are best described chemically as partial melting of a subducted mantle wedge that has been variably modified by fluid. Fluid sources are subducted sediments and a crustal component of serpentine and periodite.
The Aleutian crust is 20 – 30 km thick and no back arc spreading zone has developed behind the line of volcanic islands. There are canyond perpendicular to the line of volcanic islands indicating the segmentation of the line into blocks. These blocks have shown some rotation based on the motion of the incoming subducting Pacific Plate. The Pacific Plate also has three fracture zones which introduce additional sediment into the subduction trench.
Kanaga is yet another highly evolved, long lived volcanic center in the central Aleutians. It has demonstrated at least one structural failure and flank collapse of its ancestral cone, a large shield volcano. The caldera left by that collapse was filled with one to several steep andesitic stratovolcanoes. We have observed a fissure opening near the top of the active cone following the 2012 eruption. There is no reason to believe this volcanic system is going to calm down any time soon.