With the Fisher Caldera as a neighbor, you are living in a rough neighborhood. Shishaldin Volcano is located just east of the center of Unimak Island in the Aleutian Islands. Its neighbor to the west is the Fisher Caldera system.
Shishaldin has been in the volcano news lately: After a week of seismic unrest, with elevated surface temperatures from the summit crater and weak tremor, the Alaska Volcano Observatory (AVO) had made a helicopter flight over Shishaldin on July 23, 2019. They found that a spattering lava lake had formed in the main crater. In response, AVO raised the Aviation Color Code/Alert Level to Orange/Watch.
Shishaldin is the western portion of a four volcano edifice stretching to the east. The next volcanoes to the east in order are Isanotski, Unimak 5270 and Roundtop volcanoes. False Pass is the closest village and lies 13 km to the east of the edifice, 45 km to the east of Shishaldin itself.
Shishaldin is one of the more active Aleutian volcanoes. AVO lists 63 reported active events since westerners discovered it in the late 18th Century. The most recent was a period of unrest in 2017. The most recent large eruption was 1999.
Shishaldin sits under the air routes between North America and Asia. Explosive eruptions are capable of injecting ash clouds into those air routes. The 1999 eruption put ash 17 km into the atmosphere prompting diversion of scheduled air traffic at the time. Ash forecasts can be found here: https://www.ready.noaa.gov/READY_traj_volcanoes.php
AVO monitors Shishaldin with four webicorders. https://avo.alaska.edu/volcanoes/volcinfo.php?volcname=Shishaldin
It is also visible via webcam located on the south flank of Isanotski. https://www.avo.alaska.edu/webcam/Shishaldin.php
This portion of Unimak Island it not occupied by humans, though it is visited by mountain climbers, extreme skiers, hikers, fishermen, hunters, geologists, biologists and archeologists. The volcano itself is part of the Eastern Aleutians Wildlife Refuge which precluded development near the volcano. The closest village is False Pass with a permanent population of around 50. It supports commercial fishing as a primary industry. Former Aleut villages around the island have been abandoned. Climate is subpolar oceanic bordering on subarctic. Summers are cool and wet with plenty of rain and fog. Winters are wet with intense storms. There is significant snow during the winter.
Isanotski is the immediate eastern neighbor to Shishaldin. It is a snow and ice covered stratovolcano measuring some 10 km in diameter. It is heavily eroded. It has four possible active events in 1795, 1825, 1830, and 1845. It is possible that historic fumarole activity has been mistaken for this reported activity or that some or all of the eruptions took place from neighboring Shishaldin. Historic eruptions are considered unlikely due to the extreme current eroded state of the volcano. The volcano has not been mapped or geologically analyzed. https://avo.alaska.edu/volcanoes/volcinfo.php?volcname=Isanotski
Unimak 5270 is an unnamed volcanic center between Roundtop and Isanotski. 2008 aerial observations during Shishaldin fieldwork found lava flows dipping radially away from the summit. This led to the volcano being named as a separate vent / volcano. There is no reported historic activity from Unimak 5270. https://avo.alaska.edu/volcanoes/volcinfo.php?volcname=Unimak%205270
Roundtop is an eroded and glaciated stratovolcano located 13 km SW from False Pass. Hot springs were discovered on its slopes in the 1930. There are pyroclastic flows, a group of domes south of Roundtop, and a thick silicic pre-Holocene flow. There is no reported historic activity from Roundtop, though there is evidence of a major VEI 5 eruption 9,100 – 10,000 years ago that erupted an estimated 7 km3 of rhyolite and created a 3 km diameter currently ice-filled caldera. The eruption produced pyroclastic flows and a widespread rhyolitic ash layer. A group of domes was constructed south of the volcano after the major eruption. The analyzed layer was deposited between deglaciation of Cold Bay and overlain by a layer of Fisher ash. https://avo.alaska.edu/volcanoes/volcinfo.php?volcname=Roundtop
Shishaldin is a symmetrical cone near the center of Unimak Island around 16 km in diameter. It rises over 2,850 m. It has a small summit crater that emits a steady stream of steam. The upper 2,000 m is covered by snow and an ice cap. It has at least 54 monogenetic parasitic cones, mostly on the NW flank. A few of these were built since the 18th Century, though most appear to be older. Lava flows from the monogenetic cones stretch more than 5 km downhill. Other lava flows extend from the flanks of the volcano itself. A vent 1,300 m on the NE side sourced a 20 km2 lava flow. It may have been erupted in 1825.
There are active fans of lahar and flood debris surrounding the volcano. These are thought to be a result of frequent eruptions from the ice and glacier covered volcano.
The cone itself is quite young, less than 10,000 years old. It occupies a glacially eroded ancestral somma and built on a shield.
The ancestral Shishaldin was destroyed by a flank collapse during a large eruption shortly after the end of the last great ice age. The cone was quickly rebuilt, which requires high eruption rates.
It is one of the most active volcanoes in the Aleutians, with at least 63 reported eruptions since 1775. Major eruptions took place in 1830 and 1932. There are eight historical eruptions that produced lava flows.
Shishaldin’s activity before the end of the last ice age has mostly been buried by the output from newer activity, so we will start our discussion there. Since the end of the last ice age, there have been four large events. Two of these are dated around 1,000 years ago. They are a sequence of tephra layers on the north side of Shishaldin above a young lava flow dated 1,000 years ago. An older large eruption deposited a 2 m tephra layer near the volcano. The deposit was 3 cm thick 95 km from the volcano at Cold Bay.
The largest known eruption out of Shishaldin took place 9,500 and produced the Cape Lapin debris avalanche. This landslide traveled over 20 km to the Bering Sea, burying the NW flank of Shishaldin. The description of this avalanche describes the modern cone of Shishaldin as completely regrowing after the collapse event. Debris from this eruption traveled at least 20 km from the volcano and is exposed at the cliffs on the north shore of the island. There is a rim of lava flows around the W and S sides of the volcano tracing a horseshoe 1,500 – 1,800 m up the side of the volcano.
Debris from this debris avalanche was covered a few hundred years later by tephra deposits and pyroclastic flows from the catastrophic eruption that created Fisher Caldera.
Layered tephras exposed near a large maar on the SE flank document eruption frequency since the debris avalanche and Fisher Caldera eruption. This location has over 100 tephra layers, some of them 5 cm thick. It is capped by the pyroclastic apron of the maar formation. They date activity over the last 9,000 years, indicating an eruption large enough to deposit a layer takes place at least once a century.
While there was a period of increased background seismicity and infrasound in Dec. 2017, the most recent eruption was Jan. 2014 – Jan. 2016. Persistent steam plumes increased and started being accompanied by increased surface temperatures in the summit crater and intermittent tremor. By Feb 7, there was an ice-rich steam plume 7.6 km high. There were also explosion signals recorded in infrasound by Feb. Lava started showing up in the crater in March. Passing mariners photographed a darkened area around the crater rim, likely from volcanic ash. Activity was consistent through October, though it fell off for three weeks in November. Activity increased sharply by the end of November for a couple days and then tailed off a bit. There was active lava in the crater Jan. 2015. Elevated surface temperatures in the crater, seismicity above background levels, weak ash emissions, and infrasound recordings of explosions continued through Oct 2016. The volcano returned back to normal levels of activity by Feb 2017.
The most recent large eruption took place summer 1998 – May 28, 1999. There was an increasing period of seismic restlessness. Weather observers reported larger than normal steam plume Jan. 9, rising to nearly 5 km. By Feb 2, there were a high level of seismic events beneath the volcano. Thermal anomaly showed up Feb 12 in a satellite image. During the next three weeks seismic tremor increased as did the thermal anomaly and steam emissions. Activity waned a bit, but increased heating at the summit melted snow around the crater rim.
Seismic tremor increased steadily Apr 17 and strombolian fountaining began. Activity escalated to the point where Shishaldin put an ash cloud nearly 14 km above sea level on April 19. The significant eruption began 1145 Apr 19. Ash went as high as 17 km and dispersed south at higher altitudes and north at lower altitudes (wind shear). The eruption lasted about 7 hours, though strombolian activity continued. The next large explosive event was Apr 23 followed by persistent strombolian activity. Ash rich plumes persisted through most of May. AVO returned the volcano back to normal levels of activity Jun 25.
The eruption produced ashfall, minor mudflows down the flanks, significant ashfall on the southern flank, and a significant lahar on the north flank. Eruptive product was an evolved basalt. This eruption produced two hours-long periods of explosive eruptions. It was classed as a VEI 3.
There was a flood due to this eruption that originated on the north side of the volcano, swept down its flanks and eroded a deep channel into glacier ice. It was more than 30 m deep in a valley on the north flank of the volcano and deposited sediment as far as 20 km to the north, almost to the Bering Sea.
Magma source for Unimak Island volcanic activity is driven by subduction of the Pacific Plate under the North American Plate. Subduction line offshore is the Aleutian Trench. Subduction rates under Unimak is 6.5 cm/year. Large subduction related earthquakes are common in this part of the Aleutians.
Rasmussen et al analyzed the extended run-up to the 1999 eruption and identified three magma injection events. This was unusual due to the long run-up time and the lack of deformation observed. The first mixing event took place around 11 months before the eruption. The run-up began following a deep, long period earthquake swarm. The second mixing event was the major one and took place around 50 days before the eruption. It coincided with an M 5.2 shallow earthquake. The final mixing event took place a week before the eruption.
Analysis of earthquake depths during the recharge suggest that the recharge magma originated at least 20 km below the volcano. It mixed with a shallow magma chamber less than 3 km below the volcano. It was stored 0 – 2.5 km below the volcano just before the eruption, a shallow, vertical reservoir which could also explain the absence of observable deformation. The immediate run-up to the eruption began with the 50-day injection event. Signs of unrest became continuous at this point. The long run-up times may be related to the time needed to fill the shallow reservoir with new magmas.
InSAR analysis of Unimak Island was unable to identify significant deformation for the 1995, 1999 and 2004 eruptions. This may indicate a deep magma source below 10 km, fast refilling system, and / or a shallow magma source. There are regions 15 km east of Shishaldin and north of neighboring Fisher Caldera (30 km south) under the Tugamak Range that may contain some deformation. There may be other deformation sources under Unimak Island that are not yet identified. What they do know is that there is not enough information yet.
A 2014 paper by Cusano, et al analyzed long period (LP) earthquakes in the year leading up to a minor eruption in 2004. They found that the LP earthquakes could be used a proxy for magma travel between the deeper magma chamber (>10 km) and a shallow presence of magma (3 – 5 km) which is also co-located with the volcano’s hydrothermal system. The shallow system has mostly degassed magma. The deeper chamber hosts mostly basaltic magma like was erupted in 1999. Magma slowly migrates upwards and gas is released. When the internal pressure reaches a critical point, the eruption begins. The paper suggests observation of pressurization of the magma system via analysis of LP earthquakes may be useful for predicting potential eruptions.
Unimak Island, with Westdahl, Pogromni, Fisher, Shishaldin, Isanotski and Roundtop, is home to at least three recently active, and vigorously active volcanoes. Shishaldin is a beautiful stratovolcano that has already had one significant flank collapse / debris avalanche. In less than 10,000 years, it has rebuilt the cone almost completely. That cone is generally ice and snow covered, virtually guaranteeing subsequent eruptions of basalts will be explosive and produce lahars and floods down its flanks. The volcano has an active magma supply that may have been more prolific at the end of the last ice age, but shows no sign of slowing down.
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