Ushishur (also spelled Ushishir) volcano is located in the central Kuril Islands. It is a small breached caldera with at least four domes since the caldera-forming eruption. The Ushishur Islands have Ushishur (also known as Yankich / Yankicha Island) and Reponkich Island to the north. There are a number of sea rocks above and below the surface surrounding both islands.
The island is quite small at 5 km2. The 1 km diameter crater lake is breached to the south, forming a sea water crater bay. Average depth of the bay is estimated at 30 m with a vigorous hydrothermal system driving frequent bubbles, numerous fumaroles, hot springs, altered water chemistry, and bacterial mats. The volcano is considered an active one with the most recent eruption in 1884.
There are two small lava domes in the central lagoon. These are thought to be the most recent domes. A pair of older domes nearer to the southern rim of the crater are connected to the crater rim by a sand bar. The highest point on the island is 401 m.
The neighboring island to the north, Reponkich (Ryponkicha) is the larger of the pair and is part of the volcano’s original flank. Erosion and flank collapse landslides have surrounded it with sheer cliffs.
Ushishur had no permanent habitation prior to European contact. It was visited by Ainu people who regarded the island as a sacred land, home of their thunder god. Remains of Ainu huts can be found on the northern slopes of the crater bay. There are multiple locations on both main islands where visiting Ainu left archeological evidence of their visits.
Like the rest of the Kurils, ownership of the islands transferred multiple times between Japan, the Soviet Union and Russia over the last several centuries. It is now part of the Russian Federation, the Sakhalin Oblast. The central Kurils are sparsely populated, with less than 300 within 100 of Ushishur. Sea approaches to the island are difficult due to frequent fogs, unpredictable and strong ocean currents and numerous small offshore rocks from past flank collapses. Detailed geologic study of the island began in the 1950s.
Today, Yankicha Island has become a tourist destination, with visitors entering the caldera on Zodiac boats. They partake of the hot springs as hot baths, climb up and down the walls of the caldera, and look at the wildlife (otters, seals, foxes and birds). Eugene Kaspersky posted photos and a description of a Aug 2014 visit in his blog. The photos are stunning. There was also a 2019 article for Silversea by Andy Shuman that described a similar visit five years later. Both articles described a sea level cave that penetrates the side of the caldera wall.
The climate of the Kuril Islands is generally severe with long, cold stormy winters and short, notoriously foggy summers. Annual precipitation is generally 100 – 130 cm, much of it falling as snow. As a result, much of the vegetation is arctic tundra with grasses and low bushes.
Volcano and Eruptions
The Kuril Island Arc has 125 seamounts, and multiple islands with partially or completely submerged calderas or volcanic craters.
Yankich Island has the active Ushishur Volcano and is roughly 2.5 km in diameter. The breached crater is 1.6 km in diameter, open to the S, and has at least four domes. The older domes are connected to the southern crater rim by a sand spit. The tallest of these is 125 m. The central part of Kraternaya Bay (Bight) has two flattened, well preserved domes, 200 x 300 m and 100 x 200 m. They are respectively 32 and 12 m above the water. All domes are extrusive andesitic domes.
The two most recent domes were formed sometime after it was first visited by westerners in 1769. They are flattened and well preserved. The two older domes in the southern part of the bay have peaked sides and have been extensively eroded. One researcher believes the four domes mark the location and size of 500 m diameter volcanic conduit.
Ushishur is basically an andesitic system, erupting the occasional basaltic andesite and dacite. The newest caldera was created in a VEI 4 eruption some 10.6 ka that ejected 0.10 km3 of bulk material. There is a suspected ancient caldera centered on Yankich Island up to 5 km in diameter. Neighboring Ryponkich Island is 1 x 3 km, 130 m at its highest point. It is thought to be the last remaining part of the flank of the original volcanic island.
Ushishur has a vigorous hydrothermal system that powers thermal springs inside the crater, both above and below the water level. There is a large plume of discolored seawater from the outer submarine slope of the volcano, indicating a significantly larger release of hydrothermal fluids outside the crater than in.
Internal thermal springs were investigated starting in the 1960s. Thermal waters were investigated more recently for their use as therapeutic hot springs. These are generally located on the SE side of the crater. There has been no significant changes in levels of activity in recent years. Gases and fluid output do not vary significantly between sites in the northern and southern part of the field. The southern part covers around 1 km2. Hydrothermals have high mineralization and low pH, with temps 86-100 C.
There is a cluster of strong fumaroles and hot springs along the SE caldera shoreline. Vigorous hydrothermal activity continues to modify the chemistry of the caldera bay. The only historic eruptions from Ushishur have been phreatic, with a possible phreatic explosion in the area of solfatara fields in 1884 being the last known eruption.
These waters are recommended for treatment and prevention of diseases of the circulatory system, muscles, skeletal system, and skin diseases. They are used by tourists and fishermen who swim in diluted waters near the shore. At least one paper concludes that they can be used in treatment and prevention of a wide range of diseases.
As with most marine ecosystems in the near Arctic, any time you can find a protected location that injects heat (but not too much), metals and minerals into the water, the wildlife responds very well. The vigorous hydrothermal system beneath the crater lake has been closely studied. The water in the crater lake is stratified, with the top 5 m being warmer and saturated with volcanic CO2 and oversaturated with O2. It also has a high concentration of chlorophyll. Hydrogen sulfide is found to 15 m.
The plankton community has high rates of production and destruction of organic matter in the top 5 m layer. Vertical migration of this plankton causes local red tides. There are bacterial, algae-bacterial and diatom mats on the bottom of the crater lake in the zone where hydrothermal waters are circulating. These species are not found outside the crater lake breach in the surrounding ocean. Some microscopic species found inside are markedly different in size from those found outside.
All this microbial and algal life attracts things that feed on them. In the case of Ushishur, Kraternaya Bight (the crater lake lagoon) has a vigorous community of many species of bivalve mollusks, which end up being the dominant animals in the lagoon. They occur at depths from 3 – 55 m depending on species. Different species occupy different depth ranges, likely due to how they feed (filter feeders, deposit feeders) and different levels of tolerance for water temperatures, chemicals and dissolved metals released into the Bight by hydrothermal activity. Their numbers also vary in relation to changes in hydrothermal activity. As a result of the volcanic activity, Kraternaya Bight has a high content of heavy metals in water and bottom sediments. The bivalve mollusks concentrate heavy metals from the environment in large amounts and can be used as an indicator of heavy metal concentrations in their habitat areas.
The underwater slopes of the crater are boulders and gravel, covered with sandy and silty sand to a depth of 25 m. Below 30 m, the slopes pass to a flat bottom with silty sand. Silts are mostly found in the deepest part of the bight. Recent volcanic activity takes place though breaks in the bottom sediments of the Bight. There are several areas of surface and underwater hydrothermal activity in the crater. The most intensive activity is a surface solfatara field with a depression, several billing hot water pits and a small hot lake.
Water in the bight has limited water exchange with the ocean and is stratified by temperature, salinity, pH values, gas content, nutrients, dissolved organic matter, suspended matter and dissolved metals.
We have covered several volcanic systems in the Kurils over the years, Simushir Island, Raikoke, Sarychev and Ebeko. Tectonics of Ushishur is identical to neighboring Simushir Island and reproduced in whole below.
Like the rest of the Kurils, tectonics of subduction of the Pacific Plate beneath the Okhotsk Plate drives volcanic and earthquake activity in the central Kurils. Crust thickness is less than 15 km beneath the island.
The central Kuril Islands are also subject to frequent great earthquakes. There are 20 – 22 tsunami deposits recognized on Matua Island (Sarychev volcano) over the last 3,300 years. Average reoccurrence is around 150 years. Simushir Island has recorded 34 – 35 tsunami deposits over the last 2,350 years, or about 65 years between events. Tsunamis at least 11 m higher than modern tsunamis in 2006 and 2007 occurred every 300 years or so. Most recent runup from the 2006 – 2007 earthquakes is in the 5 – 15 m range. Timing of volcanic activity does not seem to be connected to these frequent earthquakes, though there is no shortage of Russian-authored papers that attempt to make that connection.
Ushishur volcano is an active subduction volcano. It has a vigorous hydrothermal system, erupts infrequently, and has at least two calderas, one above the surface of the ocean, one below. Erosion has removed most of the original cone. It is a remote, visually spectacular volcanic system that has become a haven for wildlife above and below the surface of the ocean. It has an active magma source that powers a vigorous hydrothermal system inside and outside of the flooded caldera. This system may surprise us in the future.
Distribution of bivalve mollusks Macoma golikovi Scarlato and Kafanov, 1988 and macoma calcarean (Gmelin, 1791) in the shallow water hydrothermal ecosystemof Kraternaya Bight (Yankich Island, Kuril Islands): Connection with feeding type and hydrothermal activity of Ushishir volcano, Kamenev, et al, Jan 2004