Like Akutan, Makushin Volcano in and Aleutians showed up in the 2018 USGS update of National Volcanic Threat Assessment. It is 2,036 m high and located nearly equidistant between Bogoslof, Okmok and Akutan on Unalaska Island. The volcano is located some 28 km west of Dutch Harbor and Unalaska, the largest population centers in the Aleutians. It is located nearly 1,300 km SW Anchorage.
Unalaska Island is the second largest of the Fox Islands. It is 2,720 km2 with a permanent population just over 4,500. This population can approach 10,000 during fishing season.
The first western discoverer was Vitus Bering in 1741. The island had long been settled by Aleuts at the time the first Russian settlement was created in 1759. They were mostly fur traders. It took only four years for relations between the two groups to deteriorate to the point of active hostilities. The Aleuts killed 175 settlers and burned four ships. Russian reprisals afterwards took the lives of 5,000 Aleuts. The Spanish made it this far north in 1788 when a Spanish expedition claimed Unalaska for Spain.
The US purchased Alaska from Russia in 1867. They started fortifying Dutch Harbor in 1940. Japanese attacked it in 1942.
The port of Unalaska / Dutch Harbor is the main port and support base for the Bering Sea king crab fishery. It is the largest fishery port in the US in terms of volume of seafood caught and led in total dollar value until 2000 when New Bedford, MA passed Dutch Harbor. There is a reality TV show called Deadliest Catch that features the king crab fishery shot on location at Dutch Harbor and features port facilities and local bars. https://www.discovery.com/tv-shows/deadliest-catch/
There was a pilot project to produce bio-diesel out of fish oil. The Alaska Energy Authority explored the possibility for a geothermal electrical generation operation on the island. Currently, diesel and heavy fuel oil are barged in from out of state to support the fishing fleet and processing operation.
Like Akutan, climate is subpolar oceanic bordering on subarctic. Summers are cool and wet with plenty of rain and fog. Winters are wet with intense storms as typhoon remnants from the Pacific Rim work their way up the Aleutians and to the east. It rains 225 days a year with December being the wettest month. Heaviest snowfall months are December – March.
Makushin is monitored by the Alaska Volcano Observatory (AVO) with a pair of webicorders and a webcam.
The volcano is a broad stratovolcano topped with a 3 – 4 km caldera. The volcano is 16 km in diameter and occupies most of the NW portion of Unalaska Island. There is a small cinder cone, remains of other cones, and fumaroles in the crater. The volcano is capped by an icefield measuring 40 km2. Glaciers from this icefield flow into flanking valleys as low as 305 m above sea level. It erupts primarily andesites, basaltic andesites, basalts, with some trachyites and dacites. The 2000 Preliminary Volcanic hazard Assessment dates volcanic activity back at least 1 Ma. Most deposits of ancient eruptions have been buried by more recent eruptive deposits. This is further complicated by the action of ice during at least ten ice age cycles during the life of the volcano.
Makushin was constructed in two primary periods separated by a period of quiet with significant glacial erosion of the volcanic cone. The first period started before 0.93 Ma and includes extensive lava flows and pyroclastic flows, both basalts and andesites. These enlarged the island by several kilometers along the north coast.
There were two massive eruptions, 8,800 and 8,000 years ago. The second one appears to have formed the caldera. Pyroclastic flow deposits as thick as 100 m were produced during the caldera forming eruptions. They fill valleys E, N and W of the caldera. There are surge deposits in the valley bottoms, ridges and slopes surrounding the volcano. There are also young lava flows, debris avalanche, lahar and flood deposits in valley bottoms surrounding the volcano.
More recent activity built satellite monogenetic satellite vents, andesitic and basaltic lava flows, and scoria cones in the caldera. Pakushin cone is a satellite cone 8 km SW on the flank of the main volcano. Tabletop Mountain is an eroded pyroclastic cone surrounded by lava flows 20 km NE. Wide Bay cone is a small symmetrical cone on the NW edge of Unalaska Bay. Sugarloaf cone is built of bedded pyroclastics 14 km to the SW. Point Kadin vents are 10 small cones and explosion craters (maars?) aligned along a rift zone extending NW from the caldera. These are located just south of a valley-filling ash flow. There are at least 15 recent tephra layers on northern Unalaska. The exact number of these from Makushin is uncertain.
There are small maars and cinder cones along a fault on the north side of the volcano. There is a large lava flow named the Lava Ramp covering over 50 km2 on the NE flank. There are active hot springs and fumaroles in the summit caldera and in valleys on the E and SE sides of the volcano.
A 1986 report by Nye et al studied lava samples from Makushin on Unalaska Island. It concluded that there was not a single shallow magma chamber feeding the central volcano and all satellite vents over the course of their collective lifetimes. The magma plumbing system feeding Makushin over its lifetime is relatively small volume and frequently resupplied with magma from deeper. Several km3 sized magma bodies rose and erupted on the flanks of the volcano to form satellite eruption centers. One of these was particularly large (5 km3 erupted) and formed the Lava Ramp. It did not erupt all available magma and is described as longer lived than is normal for Makushin. It is thought to be the heat source for the existing geothermal field.
Makushin had two large, caldera forming eruptions and at least 17 historical eruptions VEI 3 or greater. The large eruptions date 8,800 and 8,050 years ago. These eruptions sent pyroclastic flows downslope and across open water as far away as the current location of Dutch Harbor.
The oldest deposits since the end of the last ice age came from the series of eruptions that formed the caldera. They began 8,800 – 8,400 years ago. The first large deposit was a debris avalanche on the north flank of the volcano that traveled the 10 km to the coast and may extend an additional 3 – 5 km offshore into the Bering Sea.
The VOGRIPA database lists the caldera forming eruption as the one around 8,000 years ago, a VEI 5.9, ejecting 7.5 km3 of andesites. Numerous pyroclastic flow deposits fill valleys E, NE, N and W of the volcano. These date 8,100 – 8,000 years ago before the caldera forming eruption no younger than 7,950 years ago. The pyroclastic flow deposit at the head of Makushin Valley is over 100 m thick and traveled 20 km to the shore of Unalaska Bay, where it is 3 m thick.
The Driftwood Pumice was deposited from a smaller eruption 7,700 years ago. It put ash and pumice as far away as the town of Unalaska. This was the most recent large explosive eruption from Makushin. This was deposited generally NE of the volcano and is in three main layers topped by an ash layer. Thickness is generally in the 1.5 m range. There is a repeated occurrence of light tephras overlain by dark tephras. This suggests a common and repeated mechanism for explosive Makushin eruptions. Total volume erupted of the Driftwood Pumice may have been larger than 1 km3, with the VEI in the 4 – 5 range. Column was 10 – 25 km high and the duration was greater than 12 hours. This eruption was similar in size to that of Mt. St. Helens.
It appears that the lowest pumice layer came from the upper, most fractionated part of the magma chamber. The eruption was triggered by the injection of mafic magma into a more evolved crystal-poor mush. The new magma did not mix all that well with the magma already in the chamber and erupted the upper part of the chamber through a stable conduit or a dome (end product of previous eruption). The main stage of the eruption was a powerful eruption through a stable vent. Final portion of the pumice eruption transitioned into a Strombolian, more mafic eruption. Repeated light – dark ash layers indicate that the Driftwood eruption was the last of a series of similar, smaller, less violent eruptions triggered by a similar magma mixing process. This process is a typical cause for much of Makushin’s recent activity.
There is a 40-m thick ignimbrite exposure at Point Tebenkof on the western side of Driftwood Bay. This may be associated with the Driftwood Pumice. This ignimbrite is not welded. Indicating a lower temperature or water cooling of the flow. Analysis of the erupted material gives a maximum depth of crystallization of around 7 km with a possible stall of the magma on the way up at a kilometer below the surface. There is a possible second eruption that deposited a scoria layer on top of the ignimbrite. This ignimbrite formed from a phreatomagmatic eruption of a low temperature, relatively shallow magma chamber.
Another large eruption in the VOGRIPA database dates at around 5,650 years ago. It was a VEI 4 and ejected 0.10 km3 of bulk volume.
There have been frequent smaller eruptions since the caldera forming eruption that have deposited at least 26 ash layers, some up to 5 cm thick in the valleys over the last 4,000 years. There are several smaller pyroclastic flows that traveled as far as 8 km from the vent. Lahar and flood deposits can be traced 8 – 10 km down the valley from the vent. A large post-caldera event was over 2,500 years ago and produced a debris avalanche over 15 m thick over 10 km from the volcano.
Eruptions over the last 300 years have been relatively small. Half of the six 20th Century eruptions were minor phreatic explosions due to interaction of summit ice, the volcano hydrothermal system, and the hot conduit system. The most significant activity in 1907 was limited to the central caldera. The largest previous recorded eruption was in 1826. Eruptions were not recorded before the Russian fur traders shows up on the island. Smaller eruptions during this period are likely, though not recorded.
Explosions in 1983 were heard by local geologists but never correlated with specific deposits.
The most recent eruption in 1995 was small and ashy. Uplift was observed by InSAR the two years before the eruption around 5 km E of the caldera. Modeling suggests there was 0.022 km3 of magma injected 7 km below sea level. This inflation reversed itself following the Jan 1995 eruption. Ash deposits were found in 1996 in the summit and on the volcanoes flanks. These were probably produced by undocumented eruptions later in 1995.
Chemistry of erupted magmas suggest small batches of chemically distinct magmas entering a shallow magma system. Flank vent lavas are more mafic (more andesitic / more basaltic) than the lavas from the main volcano. This means the main volcano has a more evolved chamber with chemical zoning, multiple injections and many eruptions.
Volcanic activity in this portion of the Aleutians is driven by the nearly head-on convergence of the Pacific Plate under the North American Plate at the Aleutian Trench. Convergence rate is 66 mm / yr. Crust under this portion of the Aleutians is 35 – 37 km. There is a broken section of the Kula Plate under this section of the Aleutians. There have been three periods of heightened volcanic activity in the Aleutians 38 – 29 Ma, 16 – 11 Ma, and 6 Ma to present.
A 2018 paper by Prejean and Hill studied what is referred to as dynamic earthquake triggering, when seismic waves from a distant earthquake trigger seismicity at distant sites. This has been recognized for 25 years. The authors took a look significant increases in seismicity rates at 19 Alaskan volcanoes 2006 – 2013 within 3 days following earthquakes greater than M 7.0. Makushin was one of these. They concluded that while some volcanoes and geothermal areas are prime for dynamic earthquake triggering, Alaskan volcanoes respond weakly, meaning that eruption triggering by distant earthquakes are unlikely in Alaska over the time scale of days. There were no consistently strong triggered responses of Alaskan volcanoes studied. Weak responses by half of the studies were observed. Only 3 of the 19 volcano complexes studied showed more than one episode of dynamic triggering. These were Katmai, Pavlof and Makushin.
An analysis of seismicity under Makushin done using data collected 1996 – 2009 found the majority of the seismicity located beneath and SE of the caldera. There was also a cluster NE of the caldera south of Wide Bay Cone. The majority of seismicity under Makushin extends 15 km E of the caldera 5 – 10 km deep. SAR data modeled an inflating source 7 km deep 5 km east of the caldera that inflated before and deflated after the most recent eruption Jan. 1995. The source is modeled as a magma body overlain by brittle rocks. There was an associated swarm 3 – 7 km, 5 km E of the caldera. There is a second shallow seismically active area 15 km NE of the caldera at 2 – 8 km deep. Earthquakes here do tend to occur in small groups over the span of an hour to several days. There is deeper seismicity 10 – 37 km deep, 5 km SW of the caldera which might indicate deeper migration of magma.
There are three areas of seismicity at a distance from the central volcano. The first is below Unalaska Bay, 3 – 11 km deep. These may related to deflation of the volcano. Second is south of Unalaska Harbor beneath the 13 Ma Captain’s bay pluton generally 2 – 12 km deep. These occur in 2 km bands offset by a couple km. Final area is beneath the far E end of the island peninsula N of Beaver Inlet. The majority of this was in late Dec. 2008 at a depth 5 – 10 km. This is thought to be due to crustal stresses not related to volcanic activity.
Makushin is a large, dangerous, active volcano. Like its close neighbor Akutan, it is connected to a well-established, prolific magma source. It does not surprise me that USGS listed it among the most dangerous volcanoes in Alaska. The combination of a prolific basaltic to andesitic magma supply with an active ice cap almost guarantees violent, explosive eruptions at least in the early stages.