NASA Landsat image of Little Sitkin island. Image courtesy SJ Smith via AVO, 2001
Little Sitkin island is a volcanic island in the Rat Islands archipelago of the western Aleutian Islands.
The Rat Islands are a group of six larger and multiple smaller islands located to the west of the Andreanof Islands in the western Aleutians. They are quite volcanically active hosting Kiska, Segula, Davidof, Little Sitkin and Semisopchnoi volcanoes. There are at least two other subsurface vents (seamounts) associated with the islands.
Location of Rat Islands and Little Sitkin. Image courtesy Rat Islands Research Project, University of Buffalo, 2016
The islands are quite remote and uninhabited, with Little Sitkin 2,161 km to the SW of Anchorage. The closest habitation is Adak, 332 km to the E. The island group includes Amchitka Island, where the US did three underground nuclear tests in 1965, 1969 and 1971. The Rat Islands are currently part of the Alaska Maritime National Wildlife Refuge and a UNESCO Biosphere Reserve.
They were named by visiting Russians in 1827 based on their encounter with a brown rat infestation on Rat Island introduced around 1780 via a Japanese shipwreck. As of 2009, an eradication effort was thought to successfully remove all rats from the island. Unfortunately, there was significant collateral damage to bird populations due to overuse of poison by the contractor. Rat Island was renamed to its Aleut name, Hawadax Island, in 2012.
Rat Islands to the west of the 180-degree longitude line. Andreanof Islands to the east. Aerial volcanoes are all labeled. Amchatka Island is the long island to the south of Little Sitkin and Semisopochnoi center left of the image. Rat Island is the smallest visible island immediately to the NW of the NW end of Amchatka. Image courtesy Plate Tectonic.ru
Little Sitkin island was named Ostrov Malyy Sitkhin (Little Sitkin island) on an 1848 Russian Hydrographic Dept. chart. Little in this context means “western.” The name Sitkin may be related to the Aleut word “sitxan” meaning “from beneath or from under” or more likely “excretion.” The Aleuts found Little Sitkin island inhospitable and did not leave the remains of any settlements. The 1959 USGS Bulletin 1028-H suggests that they may have tried to inhabit the island when it was active, leading to the name “sitxan.” This may also explain a similar name used for Great Sitkin 300 km to the east.
Sea-facing cliffs on Rat Island. There are no active volcanoes on the island as it erodes back into the Bering Sea and Pacific Oceans. Image courtesy Pre-Tend Magazine, 2015
We discussed several volcanic islands in this part of the Aleutians over the last several years. Semisopchonoi of the Rat Islands in 2015, Great Sitkin of the Andrenof Islands in 2016, and Kanaga of the Andrenof Islands in 2016.
Climate is cold maritime with high humidity, low temperatures, and abundant wind, rain and fog. Low leafy plants, moss and grasses grow on Little Sitkin below 500 m. Animal life on Little Sitkin includes blue foxes, a few sea birds and insects. Sea otters and seals are sometimes seen offshore, but there are no known rookeries on the island itself. There have been multiple expeditions in recent years along the volcanoes of the western Aleutian volcanic arc. The following videos are from Flight to Wonder, 2018.
Volcano
Little Sitkin Island is a volcanic island about 5 km east of Davidof Island in the Rat Islands. It is a mountainous island some 10 km in diameter built by a single volcanic vent currently topping out at 1,174 m. There have been three cone building episodes since the island was first formed. The base of the current cone is 4.5 km in diameter at its base. It has a summit crater. It erupts andesites, dacites and basaltic andesites.
The geologic history of Little Sitkin began with mid-Tertiary uplift of the Aleutian Islands arc. Initial activity erupted a base of hyaoclastic material on a platform at least 100 m below the surface of the ocean. Early magmas were andesites rich in volatiles, as the initial above surface eruptions were rich in ash and pyroclastic materials. The first cone is estimated at 2,100 – 2,400 m. There was a long period of erosion following the construction of the first cone. This likely included glacial ice caps. Most of the deposits of this round of erosion are beyond current island margins.
InSAR spaceborne radar image of Little Sitkin island. Current active cone clearly visible right of center. Most recent dacite flow south from the crater clearly labeled. Smallest curved line is the boundary of Caldera Two. Largest curved line is boundary of Caldera One. Calderas are located close to the eastern side of the island. Image courtesy InSAR Imaging of Aleutian Volcanoes: Monitoring a Volcanic Arc from Space
The first caldera forming eruption created the 5 km diameter Caldera 1 which for a time held a crater lake. Post-caldera eruptions produced tuffs deposited offshore and in the caldera lake. This eruption may also have included the first flank collapse / debris avalanche to the SE. It took place before the end of the last ice age and is not listed in the VOGRIPA database.
A new vent formed on the NE side of the island following this eruption and produced a thick series of basalts and andesites. New dacitic magmas started erupting. There was some mixing as some of the dacite had andesitic inclusions. These dacites were relatively poor in volatiles and did not produce significant pyroclastics. Instead, most eruptions were thick massive flows.
urrently active cone of Little Sitkin. Photo taken from Williwaw Cove on the NW side of the island by Steve Ebbert, USF&WS, 2000 via Smithsonian GVP
Over time these flows built a large central cone in the confines of Caldera 1. This cone was then eroded over a period of relative quiet preceding the Caldera 2 eruption.
The newer caldera, Caldera Two was built almost entirely of lava flows within Caldera One and topped out around 900 m. A second caldera-forming eruption formed this elliptical 2.7 x 4 km caldera. Its eastern and southern margins share those of Caldera One. The Caldera Two eruption tilted a large block of the remaining original stratovolcano southward during the eruption with the Caldera Two northern margin acting as a hinge.
There is a deposit of partly welded tuff up to 100 m thick associated with the Caldera Two eruption which produced voluminous welded pyroclastic flows and welded dacite ash on the NW side of the island. The caldera forming eruption was followed by a vulcanian phase which relatively cold ash and a minor amount of angular debris was erupted. Some lava flows in the lower part of Little Sitkin dacite erupted during this time. Creation of Caldera Two took place after the end of the last ice age.
Proposed growth and destruction of Little Sitkin Island over time. 1 – growth of andesitic composite cone on initial island platform. 2- formation of Caldera One after a long period of erosion. 3 – Post caldera activity, crater lake, and East Point lava flows. 4 – Erection of second stratovolcano dacite cone. 5 – Caldera Two formation with pyroclastic flow deposits. A large block dropped a little bit into the new caldera. 6 – extrusion of low-silica dacite lava continuing until the present. Screen capture from USGS Geology of Little Sitkin Island Alaska GL Snyder Geological Survey Bulletin 1028-H, 1959
Magma remained close to the surface following this eruption, powering fumarole and an active hydrothermal system. This magma was likewise poor in volatiles and included mainly low-silica dacites and new andesitic lava flows. Two aa flows were erupted either in the 1828 eruptions or around the turn of the 20th Century. They have well-developed levees.
The volcano has a currently active hydrothermal system with steam jets and hot springs in a valley on the NW side of the island. Fumaroles are near the western coast, along the NW margin of Caldera One, and within a kilometer of the summit crater down the southern flank.
Sonar view of both suspected flank collapses from Little Sitkin. Note how the largest one to the SE (up is north in the image) extends all the way into the caldera structure. Image courtesy Montanro and Beget, 2011
There is a hummock field on the SE side of the island that covers an area of 190 km2 on the ocean floor. It is a possible landslide that extends 17 km offshore. It originates at a horseshoe-shaped scar off the Caldera One rim. This was initially attributed to the Caldera Two event but was more likely formed by a flank collapse.
The NE part of the island is truncated by a 500 m cliff. The submarine area below this cliff has another field of large blocks via side scan imagery. The blocks extend more than 8 km offshore and cover an area of 80 km2. This cliff is now interpreted as a scarp left by another relatively recent debris avalanche.
West Cove lava flow and vent area. Visible are Davidof caldera center and Segula right of center with its top in the low clouds. Image courtesy CA Neal, AVO, 2005
Eruptions
Following the second caldera-forming eruption, the three most recent eruptions were 1776, 1828 and 1900. The 1776 eruption was explosive. The other two were for the most part effusive. There was an earthquake swarm in 2012.
The 1776 report has Mt. Sitignak belching fire was documented by Grewingk in 1850. This was interpreted in 1950 as a likely eruption from Little Sitkin.
This report comes from Russian reports that Little Sitkin along with seven other Aleutian volcanoes was smoking in 1828. The same captain reported Shishaldin still smoking two years later, leading to the conclusion that whatever was taking place in 1828 on Little Sitkin was not taking place two years later.
The suspected 1900 activity comes from mapping of two dacitic flows on the south and west flanks of the current crater. These flows are similar in elevation to lava flows from Kanaga in 1906 and are no more vegetated, implying that they are not older than the Kanaga flows. This leads to a conclusion that Little Sitkin erupted lavas at least once around the turn of the century. This activity is classified as a questionable event. Actual dating of these lava flows ties them to the 1828 eruption.
Sampling a hot spring within the Little Sitkin hydrothermal region. Image courtesy Taryn Lopez, AVO, 2015
The 2012 unrest started toward the end of August with a swarm of high-frequency earthquakes at Little Sitkin. AVO raised the Aviation Color Code and Volcano Alert levels to yellow and Advisory respectively. The rate of earthquakes varied for the next couple of days. By the first week in September, seismic activity had decreased significantly, but was still considered elecated. There were two additional earthquake swarms Sept. 11 and 13. Seismic activity remained elevated through October and decreased through the end of the year. AVO reported Jan. 9 2013 that activity was back to background levels and reset the Aviation Code and Alert levels to Green and Normal. There was no increase in hot spring activity on Little Sitkin observable via satellite, though there was a couple centimeters of inflation beneath the current Little Sitkin cone. This event is interpteted as an intrusion of new magma.
Schematic of Aleutian volcanic arc and significant earthquakes. Image courtesy Plate Tectonics.ru
Tectonics
The Rat Islands is on the western Aleutians. This region is tectonically governed by the subduction of the Pacific Plate underneath the North American Plate. It is one of the largest tectonic margins in the world, stretching nearly 4,000 km from the Gulf of Alaska to Kamchatka. Subduction rates vary from 5.4 – 7.8 cm/year in the far west. There is significant zonal variation with the angle of the suducting plate varying from nearly horizontal in the east to nearly vertical in the west, where the relative plate motions become oblique strike-slip.
The subduction zone has produced some of the largest earthquakes ever recorded. There were seven with magnitudes greater than M 7.6 from 1938 – 1979. The M 8.6 Rat Islands earthquake ruptured 600 km of megathrust fault, initiating near the Andreanof Islands and propagating NW. The June 23, 2014 M 7.9 event is at the end of this rupture.
The Rat Islands is one of the most seismically active regions in the world, with nearly 12,000 events since 1898. Seismicity takes place down to 300 km and includes strike – slip and normal events in the overriding North American plate.
Schematic of 23 June 2014 Rat Island earthquake. Showing possible hypocenters. Image courtesy Ye, et al, 2014
The 23 June 2014 M 7.9 Rat Island earthquake struck in the vicinity of Little Sitkin island. It was the largest intermediate depth earthquake measured with modern instruments. Intermediate depth earthquakes (70 – 200 km deep) are rare, with only 14 ever recorded. These typically rupture downgoing slabs in tension. They tend to occur closer to onshore populations and create more intense shaking than shallower events of similar magnitude. The physical processes that cause them are poorly understood. Intermediate depths usually have temperatures and pressures too high to allow brittle failure to occur. One proposed mechanism is dehydration embrittlement. Another might be thermally induced shear instabilities where slip leads to heating and weakening of the slab.
The 2014 earthquake was 102 km deep. It was felt on Adak and Shemya 200 km away. Both islands were far enough from the epicenter to not suffer damage or casualties. There were over 1,800 aftershocks greater than M 2.5 within the first month, the largest of which was M 6.6.
Two interesting observations came out of this earthquake. First is that the location of most aftershocks is not in the fault plane of the main quake. The aftershock sequence was long duration and compact. These are thought to argue for a fluid release due to dehydration as the mechanism that caused the quake. Second is that it appears that the event must have ruptured well into the oceanic mantle.
Little Sitkin island from the NW. Image courtesy Roger Clifford, AVO 2012
Conclusions
Like the other volcanoes we have reviewed in this part of the Aleutians, Little Sitkin has a complex, violent history including a pair of caldera forming events and at least two known flank collapses / debris avalanches into the Bering Sea that surrounds it. The volcano continues to be active, most recently producing a pair of lava flows. While remote, it is well monitored, and activity reported. It continues to have a healthy, active hydrothermal system and fumaroles. These modify previous eruptive products into clay. The island itself is sufficiently inhospitable that sea mammals and birds have not used it for rookeries.
North coast Little Sitkin. Image courtesy CA Neal, AVO, 2005
Additional information
https://avo.alaska.edu/volcanoes/volcinfo.php?volcname=Little%20Sitkin
https://en.wikipedia.org/wiki/Little_Sitkin_Island
https://volcano.si.edu/volcano.cfm?vn=311050
http://volcano.oregonstate.edu/little-sitkin
https://pubs.usgs.gov/bul/1028h/report.pdf
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL061153
https://pdfs.semanticscholar.org/aa1f/13e4b432e0628fe148eb99356e9c7132d081.pdf
https://watchers.news/2012/09/01/quake-swarm-little-sitkin/
https://flighttowonder.com/2018/07/02/alaskas-sitkin-volcanoes/
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL065023
https://www.seoe.sc.edu/yogodzinski/Genes_Web_Site/SeafloorVolcanism.html
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