Fourpeaked Volcano is a little-known volcano in the NE part of Katmai National Park. It consists of isolated andesitic rock outcrops above Fourpeaked Glacier. With one possible exception, prior to the first historic eruption September 17, 2006, it was thought to be dormant. As such, it was not directly monitored, though the neighboring Katmai volcanic system 100 km to the SW and Augustine Volcano 70 km to the N are.
Alaska Volcano Observatory (AVO) received several citizen calls mainly from the Homer region of a dark plume fed by what appeared to be two sources near Cape Douglas on the other side (west) of Cook Inlet from Homer. The plume reached over 6 km on satellite photos. Due to its remoteness and bad weather, it took three days to confirm Fourpeaked was the source of the eruption, and another three days to reveal the source of the phreatic blast.
Following the 2006 eruption, AVO installed four seismometers around Fourpeaked and neighboring Mount Douglas. Over the years, three of them failed and today neither volcano is considered actively monitored, though the AVO home page map for Fourpeaked / Mount Douglas does list station CDD with a currently active signal.
Fourpeaked Volcano is located on the west side of the Shelikof Strait, at the entrance of Cook Inlet, just to the west of Kodiak Island, placing it nearly 70 km S of Augustine Volcano, 155 km SW of Homer, Alaska, 13 km S of neighboring Mount Douglas volcano, 27 km N of neighboring Kaguyak volcano and caldera. It is in the northern portion of Katmai National Park, 103 km NE of Katmai – Novarupta.
This portion of the Alaska Peninsula is heavily glaciated, and likely has been ice-covered since the beginning of the last ice age over 100,000 years ago. As such, anything identifiable as a volcano must be recently active as the erosive action of glaciers will quickly remove the soft materials building the cones of these predominantly andesitic volcanoes.
Neighboring Mount Douglas to the north is a heavily eroded, glaciated stratovolcano that has an active crater lake and uneroded lava flows on its NW flank. There are also active fumaroles but no eruptions in historic times.
Kaguyak volcano to the south has a 2.5 km caldera and crater lake. The caldera-forming eruption is dated by the Smithsonian GVP around 5,800 years ago. There is a post-eruption lava dome. Voluminous dacitic pyroclastic flow deposits surround the caldera.
The region is uninhabited, with no permanent residents. Less than 200 live within 100 km from Fourpeaked itself.
Fourpeaked Volcano was originally viewed by local Alaskans as multiple rock outcroppings through the Fourpeaked glacier rather than a proper stratovolcano. AVO and USGS volcanologists knew otherwise. Prior to 2006, it was often referred to as Fourpeaked Mountain.
It is an andesitic stratovolcano surrounded by lava flows and extensive hydrothermal alteration of rocks near what is now thought to be the summit. It tops out at 2,105 m. The outcrops and ridges are andesitic with some dacites present. Its recent dormant period is estimated to have lasted the last 10,000 years.
The volcano is not well known and there has been limited hands-on reconnaissance of it. Scientists have sampled nearby lavas and ashfall as long ago as 1984.
The range of sizes and styles of past eruptions is not well constrained. Past eruptions of andesite and dacites suggest that eruptions can be explosive. The interaction of magma and an ice cap covering the summit will create volcanic hazards of explosive eruptions, possible debris flows, lahars and jokulhlaup.
In early 2014, all monitoring systems at Fourpeaked failed, preventing seismic monitoring of the volcano. It was removed from the list of monitored Alaskan volcanoes that February.
The 2018 USGS Volcanic Threat Assessment carries Fourpeaked as a high threat system, likely due to its proximity to Anchorage and the air routes in and out of Alaska. It is listed as #53 of 161 ranked volcanic systems in the US.
Fourpeaked Volcano only has three known short periods of historic activity. The first was a report of steam and gas plumes near the summit of Fourpeaked in July 1965. The second was a phreatic eruption Sept. 17, 2006. The final event was a minor increase in seismicity April – May 2013.
The 1965 activity was reported by a setnetter in October 1965. He reported seeing a steam plume at or near the top of Fourpeaked in July or August while passing through Shelikof Strait. He did not report any ash on the snow or discoloration in the plume.
The short 2006 eruption took place in the afternoon of Sunday, Sept. 17. It was reported by multiple observers across the Shelikof Strait and lower Cook Inlet. The unusually clear (for this time of year) and calm conditions allowed observers over a hundred kilometers away to see and photograph the plume. They reported a plume issuing from two sources near the top of the mountain in the saddle between Fourpeaked and neighboring Mount Douglas.
The plume persisted throughout the night, drifting only 20 km downwind. Radar and satellite observations of the plume showed it topped out just over 6,000 m. Weather deteriorated the following day, though a pilot reported a strong sulfur smell 300 km NW of the volcano. Pilots in the Lake Iliamna area also reported strong sulfur smell the next morning. There was a report of light ash and sulfur smell 110 km WNW of the volcano. Satellites reported a thermal anomaly in the vicinity of Fourpeaked.
An AVO overflight on Sept. 20 confirmed Fourpeaked as the source. There were several vapor clouds rising through the cloud deck above Fourpeaked, discolored snow and ice N and W of the summit. An AVO helo photographed a dark debris flow tongue from the glacial ice below the summit, and patches of grey ash on the glacial ice around the summit.
Weather cleared enough for a more thorough observation on Sept. 23, when they found a linear series of pits extending over 1,200 m across the glacial ice cover on the N side of the summit. There were multiple sources of vapor from these vents, the likely source of the double plume observed on Sept. 17. There were snow-mantled rills extending downslope from one of the pits, indicating several episodes of surface debris or water flow. Finally, there was dusting of dark material around some of the pits.
Subsequent visits found as many as nine craters or pits in the ice, half of which were actively steaming at any one time. Most craters were surrounded by dark debris collars and had elevated temperatures in IR imaging. There was a glacial outburst associated with the eruption, though it never breached the surface of the glacier. Visitors reported loud rumbling noises likely associated with water outbursts above 1,000 m up the north flank of the volcano. Water from these outbursts remained beneath the ice. SO2 emissions continued to be elevated through mid-Nov.
AVO did not actively monitor the volcano until the 2006 eruption and subsequent unrest. There was an existing seismic station to the NW of Mount Douglas. It was supplemented with three additional short-lived seismic stations across Fourpeaked, two pressure sensors, and a web cam. These monitored the volcano through 2007 until batteries ran down.
The new seismic stations reported low level seismicity and persistent steam emissions through the end of 2006. Airborne gas surveys in Nov. 2006 showed elevated SO2 emissions, about half of that measured immediately after the Sept. 17 eruption. There was an earthquake swarm in Jan. 2007. By Feb. 8, AVO started reporting small explosive events on seismic and acoustic sensors. There was a possible large steam plume. A swarm of 13 earthquakes took place on Feb. 18. One of these M 1.8 was the largest seismic event of the period.
Occasional small eruptions continued through March. Seismicity gradually decreased, as did decreased steam emissions from the summit vents. This continued through May. AVO lowered the Aviation Color Code and Volcanic Activity Alert Levels to green and normal respectively.
Analysis of the events in 2011 suggest that a new magma intrusion took place in the upper 10 km of crust. This activated fractures, releasing volcanic gasses which in turn powered the surface activity. Activity ceased when the new magma cooled to the point that it was no longer releasing volcanic gasses.
There was a minor increase in seismicity at Fourpeaked volcano April – May 2013. This included a single M 4.5 three days into the swarm. It took place 15 km W of Fourpeaked and 25 WSW of Mount Douglas at 15 km deep. There was a single report of steaming near Mount Douglas and / or Fourpeaked. AVO follow-up found that the crater lake on Mount Douglas previously frozen over did not freeze in 2013, making it the likely source of the emissions.
Tectonics of this region are driven by the ongoing subduction of the Pacific Plate beneath the North American Plate. All volcanic systems are at some level, subduction volcanoes. The Katmai segment lies at the north end of the eastern Aleutians Arc. This part of the arc is segmented, with larger, caldera forming volcanic centers near segment boundaries and smaller stratovolcanoes located in intersegment portions.
The eastern Aleutian arc is divided into two segments, the Cook to the north and the Katmai segment to the south. Alignment of volcanoes on these two segments differs by 35 degrees which is interpreted as a warping of the subducting plate. The plate dips at a 45-degree angle beneath the arc with a maximum depth of 200 km. Cook segment volcanoes line up close to the 100 km depth of the subducting plate. Katmai segment volcanoes vary 75 – 100 km depth.
Crustal tectonics appear to play a role in localizing volcanic activity. Linear groups of volcanoes appear to be associated with deep crust fault under the volcanic front. Transverse elements divide the arc into subsegments and localize larger magma reservoirs.
Intrasegment volcanoes in both Cook and Katmai segments erupt andesites and minor dacites. Chemical composition of all these products are similar. Segment boundary volcanoes erupt a wider variety of magmas including basalts to rhyolites. Greater crustal thickness in the eastern Aleutian arc allows magma ponding at shallow crustal levels, allowing differentiation / evolution of the magmas into dacites and rhyolites.
The first historic eruption of Fourpeaked volcano was a surprise to local observers and a reminder to all of us that these systems are never truly extinct, especially if they are located in close proximity to multiple other vents (Augustine, Mount Douglas, Kaguyak and the greater Katmai system) that have been historically active.