Part 2 will focus on the rest of the volcanoes on Iturup, tectonics, conclusions and additional information.

Sashiusu Yama (Sashiusudake / Baransky)

Baransky is a volcanic complex along the Pacific coast in central Iturup Island, 60 km NE from Stokap.  It is an eroded volcano capped by a 1,126 m high recent stratovolcano.  The crater is 650 m in diameter.  There is a summit lava dome cut by a NW – SE chain of small explosion craters.  There are flank cones farther NW with similar alignment.  Lava flows from the summit partly surround the flank cones.  The system erupts andesites / dacites.  Lava flows reach the Pacific Ocean along a broad front. 

The SW flank crater is located 700 m.  It is open downslope with an amphitheater shape.  There are spring outflows and mud pots below the crater along a creek.  Magma chamber is estimated at 4-5 km deep. 

The only historic eruptive activity took place 1951 with 45 days of weak explosions from the summit crater (VEI 1).  There are confirmed historic eruptions in 1570 and 1460.  Wells showed a strong temperature increase in 1989, 4-5 months before an eruption from Grozny.  Temperatures decreased after the eruption.  There is a small, cold, crater lake occupying a 40 m wide explosion crater created in the summit lava dome Oct 1987.    

A small hydrothermal explosion took place during 1991 at an exploratory well in the geothermal field.  An exploration well was drilled into a zone of hot water 4.5 km SW of the summit in the Kipyaschiy creek valley summer 1991.  Drilling was temporarily halted with no casing installed.  Two weeks later, a hydrothermal explosion took place 50 m upslope forming a crater over 10 m wide.  A mixture of vapor, water, and a large quantity of rock fragments erupted for several days, destroying vegetation within 50 m of the crater.  The explosion is thought to be due to hot water moving up the borehole after drilling halted.   

The complex has a vigorous hydrothermal system, with strong solfataras at the summit crater and several flank craters.  Starozavodskoe solfataric field is one of two large clusters of solfatars and thermal mud on the W flank of Baransky.  It was more accessible of the two and was chosen for hydrothermal development, which went poorly.  The hydrothermal system may interact with that of neighboring Grozny, 13 km SW. 

Odamoi Yama (Odamoisan / Tebenkov)

Odamoisan is located at the center of Iturup Island, 50 km NE from Stokap.  It is a 1,208 m forested andesitic stratovolcano immediately N of Ivan Grozny dome complex.  The volcano is topped with a 3 – 3.5 km caldera.  Its Machekh crater, immediately S has strong fumarole activity.  It shows up clearly in satellite photos as an area without vegetation.  Grozny is 4 km SW.  There are no known historical eruptions from this volcano. 

Etorofu Yakeyama (Ivan Grozny)

The Grozny Group is a volcanic complex located in the center of Iturup Island, 46 km NE from Stokap.  The complex tops out at 1,158 m and erupts andesites / basaltic andesites.  The system is topped with a 3.5 km diameter caldera open to the S.  A large andesitic dome was emplaced near the opening.  Several other domes were extruded over last 10 ka to the NE in the caldera.  These domes rearranged a post-caldera crater lake.  The Grozny Group spans 45 km along the Pacific coast of the island. 

The basement of the Grozny group are the remains of pre-glacial volcanoes.  The cone of Tebenkov is located in the SE part of a 2 km caldera like depression open to the E.  It has erupted andesite lavas.  Machekha stratovolcano is located S of Tebenkov.  It also erupted andesites and basalts.  The Grozny group is located SW from this pair.  It has a 3 – 3.5 km diameter caldera, with 200 – 240 m high walls.  The caldera was probably formed sometime over the last couple ice ages, though it is currently undated.  The N part of the caldera was located under what was at the time of the eruption, a large lake.  The Grozny dome is located on the S edge of the caldera.  The Drakon dome is located NE from Grozny dome.  Yermak dome is located W of Grozny.   

The oldest historic eruption took place in 1968 from the central dome.  It was shortly followed by a series of VEI 1 eruptions.  Six months of these took place in 1970.  Two weeks of activity took place in Jan 1973.  Activity resumed in May 1973 with the first VEI 2 over a single day. 

The VEI 2 series of eruptions in 1989 started in early May with an abrupt increase in fumarole emissions and weak explosions producing black ash.  Seismometers 30 km SW recorded multiple earthquakes at various depths and short period of tremor over a period for 5 days.  A strong explosion ended this part of the eruption with a plume almost 2 km above the summit.  The eruption also caused a mudflow 1 km down the E flank. 

Fumarole activity remained vigorous through the rest of May into June, with an explosion Jun 19.  There was another explosion early August that formed new fumarole vents on the N flank of the dome.  Geologists on the dome in Sept found new fumaroles from small craters, two new mudflows 1.5 – 2 km down the N side of the dome.  Another small new mudflow was found on the S side of the dome.  Explosive activity mostly calmed down after Aug.

After 23 years of quiet, explosions started from the dome Aug 2012 and continued for 11 days.  Plume from these reached 5 km.  Once again, fumarole emissions increased significantly before the eruptions began.   Activity continues through April 2013, with ash over 2 km above the dome. 

Hitokappu Volcanic Group (Stokap / Bogatyr Ridge)

The highest point (Nishihitokapuyama / Stokap) rises 1,629 m.  It is considered to be a composite stratovolcano and pyroclastic cone. 

The Hitokappu Volcanic Group is a ridge of small cones built over the last 2 Ma along a NE – SW trend.  The newest cone, Nishihitokappuyama (Stokap) cone is the newest eruptive center at the SW end of the string.  It is capped with a complex of 8 – 10 cones and explosion craters, one of which holds a small lake. 

The ridge is aligned with the spine of the 200 km long island, generally NE – SW trending line.  The S end of the chain is Nishihitokappuyama (Stokap) basaltic to andesitic stratovolcano.  One of the craters has a lake.  The chain was constructed over a high glaciated base covered by lava flows that stretch to the Pacific Ocean to the E and Sea of Okhotsk to the W.  Lava flows from Stokap stretch to the sea.  There is weak hydrothermal activity on its E slope. 

Etorofu Atosa Dake (Atosonupuri / Fuji Etorofu)

Etoforu – Atosanupuri is a 1,206 m stratovolcano forming another peninsula on the SW side of Iturup Island.  It is located some 17 km W of Stokap.  The isthmus is only 30 m above the ocean.  Eruptive products are andesites and basalts.  There is a 1 – 2 km diameter caldera at the summit.  It is considered one of the most beautiful active volcanoes on the island. 

The ancestral volcano was destroyed in a somma eruption, leaving the caldera rim exposed 900 m on the SE side.  It formed an island some 1,500 m high.  Material eroded from the cone later connected it to the main island.  There is a fault that offsets the NW side of the somma.  The 2 km diameter caldera was filled by a new central cone forming the current summit.  Recent Strombolian eruptions erupted andesites, andesitic basalts and basalts.  There are few lava flows exposed.  The only two known historical eruptions were a VEI 1 in 1812 and a VEI 2 in 1932.  The 1932 eruption put lava into the ocean.   

The E half of the ancestral cone forms the somma.  W part of the rim is overlapped by the new cone.  The Somma walls are 20 – 30 m high.  The slopes of both cones are not glacially eroded.  Lavas from the ancestral volcano cover a submarine terrace 140 m deep, meaning that the somma was formed toward the end of the last glaciation when water levels were near their lowest, making it an extremely young system.

Urumonbetsu (Urbich)

Urbich is a composite stratovolcano and caldera.  Eruptive products are andesites and dacites.  Highest point is 908 m somma wall.  It is located 26 km SW from Stokap.  In Google Maps, the caldera measures 6 km in diameter.  There is a 3 km diameter crater lake in the center of the caldera.  The walls are 500 m above the floor.  Like neighboring Lvinaya Past, there are no traces of glacial erosion on the inner walls of the caldera, meaning caldera formation was post-glacial, within the last 10 ka.  The caldera forming eruption produced dacites.  The pre-caldera volcanic complex had several small stratovolcanoes along a NW – SW trending lineament, similar to many other volcanic complexes on Iturup. 

Krasivoe Lake is located in the Urbich caldera.  The highest point of the caldera is the Klinok mount, 745 m.  The lake surface is 82 m above sea level.  Its area is 38 km3.  There is a channel connecting the lake through the SE part of the caldera with the Pacific Ocean.  The Urumpet River is 3.5 km long.  Maximum depth of the lake is 50 m, with an average depth of 26 m.  There is no hydrothermal activity known in the lake as of 2021.  The lake supports the largest population of sockeye (red) salmon in the S Kuril Islands.  Salmon populations in caldera lakes is not uncommon in the northland.  Aniakchak on the Alaska Peninsula supports a small population of sockeye salmon.   Sediment on the bottom of the lake is estimated to be 80 – 90 m thick.

Unnamed

There was an unnamed submarine eruption located some 33 km NW from Stokap in the Sea of Othosk.  Google Maps satellite view shows the location as the top of a known, mapped seamount.  The submarine eruption in 1967 took place 17.5 km NW of the shore of Itirup Island.  Surveys at the time reported an increase in water temperature and acids at a depth of 1,350 m.  There is a seamount reaching within 930 m of the surface.  Google Earth 2015 shows the seamount within 195 m of the surface. 

Moikeshi (Moekeshiwan / Lvinaya Past / L’vinaya Past)

Lvinaya Past is a 7 x 9 km diameter caldera on S Iturup Island, located 38 km SW from Stokap.  It is breached to the NW with a floor 550 m below sea level.  Highest soma wall is 512m.  Eruptive products are andesites, basalts and dacites.  The Lvinaya Past caldera is located on a tectonic line connecting Atsonupuri and Beraturabe volcanoes.  The ancestral stratovolcano was 12 – 13 km in diameter.  The caldera is slightly elongated N-S.  Caldera walls are about 1,000 m above the caldera floor. 

Lvinaya Past is Russian for Lion’s Mouth.  The caldera rim defining the bay is said to resemble a lion’s mouth.  The two ends of the breached caldera rim are called Fang and Jaw peninsulas.  To complete the grouping, there is an island rising from the submerged rim called Kamen Lev (Stone Lion).  The submerged rim is 5 km long and 50 m deep.  Depth in the middle of the caldera varies 400 – 450 m with a maximum of 535 m.  Numerous lavas and pyroclastic flows are pierced by dikes that form outcrops on the caldera inner walls.

The caldera formed 10.6 ka, in a VEI 6.9 eruption that ejected some 75 km3 of material.  The thick dacite deposits form the 50 – 60 m high Yuzhny Isthmus, connecting Lvinaya Past and Berutarube to the rest of the island.  There may have been more than one caldera-forming eruption, though this is in dispute.  This speculation is based on the shape of the ring fault system.  If it exists, there was an eruption a few thousand years earlier than the known 10 ka eruption.

There is no evidence of glacial activity on the slopes of the ancestral cone, confirming that the caldera forming eruption must have taken place the last 10 ka.  Material at the base of the pyroclastic deposits date around 9.4 ka.  About 20 km3 of dacite pumice from the eruption surrounds the caldera and fills low parts of the island between it and neighboring Berutarube (SW) and Urbich (NE).  Pyroclastics are up to 100 m thick.  There are multiple pyroclastic units identified, with no trace of reworking, erosion or paleosols between the layers.  This means they were deposited in a very short period of time. 

Bertuarubesan (Berutarube / Perutarube)

Berutarube is a 1,221 m stratovolcano forming the SW tip of Iturup Island, 53 km SW from Stokap.  Like other volcanoes on Iturup, it erupts andesites, basaltic andesites and dacites.  The flanks are deeply eroded by wide glacial valleys, river valleys and multiple waterfalls.  There is a low saddle on the NE side connecting it to the Lvinaya Past caldera. 

The only known activity in the last 10 ka built a small pyroclastic cone in the complex, eroded summit.  The summit cone has been hydrothermally altered.  It is the source of two small lava flows.  There is an active hydrothermal system with fumaroles on the walls of the summit crater.  The most recent listed eruption is an unconfirmed 1812 eruption listed in Smithsonian GVP. 

Berutarube is one of the most inaccessible and poorly studied active volcanoes in the Southern Kurils.  The cone is relatively flat with a basement diameter of 11 km.  The summit is highly eroded with a chain of merged amphitheaters from the W and E sides with a small ridge separating them.  The S part is the volcano is its highest point.  The top is complex due to multiple amphitheaters.  There are no recent explosion craters.

The volcano has a vigorous hydrothermal system with multiple hot springs, fumaroles and solfataras, many of them near the summit ridge.  Plumes are visible up to 15 km from the source.  The three hydrothermal fields are Verkhny (upper), Tsentral’ny (central), and Nizhny (lower).  Verkhny is the highest at 940 m.  It contains the largest fumarole on the volcano.  Tsentral’ny is located a bit farther down the slope at 860 – 900 m.  The Nizhny field is located 800 – 810 m.  Maximum measured temperatures from steam gas plumes is 96.6° C, though crystalized molten sulfur flows are present.  These require temperatures over 115° C to emplace.  Waters from the hot springs are highly acidified (1.5 pH) with dissolved chloride sulfates.  Cold springs also exist.  There have been no significant changes in the nature of hydrothermal activity in recent years. 

Tectonics

The Kuril Island arc is a subduction system where the Pacific Plate subducts under the Okhotsk Sea Plate along the Kuril – Kamchatka Trench.  There is a constant convergence velocity of nearly 9 cm/yr.  Age of subducting oceanic crust varies 90 – 118 Ma, older to the S.  The old age of the plate correlates with 650 km depth of deep earthquakes.  The trench is filled with a mixture of continental material, biogenic material and volcanic ash.  Percentage of continental material in the trench increases to the S.

The arc started to develop no later than 23 Ma.  It stretches 1,150 km and is 100 – 200 km wide.  The Kuril Basin was formed 32 – 15 Ma.  It is triangular shaped extension structure W of the volcanic arc. 

The surface of the subducted plate is around 93 km below the volcanic arc.  Crustal thickness varies based on location along the arc, 28 – 33 km in the S, 25 – 30 km in the central, and 32 – 36 km in the N.  Volcanic rocks of the islands vary from basalts to rhyolites, with basalts to andesites being most common.  By 2.5 Ma, there were four relatively small volcanic islands located at the site of the modern Iturup Island.  The entire island was above sea level by 120 ka. 

Tectonics of the Kuril arc divides it into two arcs, the NE (Central and Northern Kuril Islands) and SW (Southern Kuril Islands) arcs separated by the Bussol Strait.  The arcs differ tectonically, with the dip angle in the NE 48° – 55° and 38° – 46° in the SW.  The Kurils are divided into two sets of islands, the Greater Kuril and Lesser Kuril island chains.  The Lesser Kurils are a short extension from the Nemuro Peninsula of E Hokkaido.  These include Hanomai and Shikotan Islands and the Vitayz Ridge.  The Greater Kuril chain extends 1,150 km from Hokkaido to Kamchatka with more than 30 volcanoes active over the last 10 ka.  The largest eruptions in the S Kurils over the last 100 ka took place on Kunashir Island with Golovnin and Mendeleev 40 – 38 ka.  The largest eruption in the last 10 ka took place on Iturup with Lvinaya Past 9.4 ka, creating a caldera now flooded by the sea.  Eruptive activity in the S Kurils was vigorous through 10 ka and weakened since 8 ka. 

Conclusions

The volcanoes of Iturup Island provide a good example of a vigorous group of subduction driven volcanic systems. There is an ample supply of eruptible magma.  The island has multiple active volcanoes, calderas and associated hydrothermal systems.  All calderas have post-caldera activity.  Eruptive styles vary from effusive to highly explosive with at least one 100 km3 eruption taking place in the not so distant future.  The best thing about this island is that it is sparsely populated.  The worst thing is that it underlies great circle air routes and not particularly well monitored. 

Additional information

Volcanoes of Japan, JMA

Active volcanoes of the Kurile islands:  A reference guide for aviation users, USGS Open-File Report 2008- 1162

Petrology of Bogdan Khmelnitskiy volcano (Iturup Island, the Kurils):  a model of fractionation and mixing in the magma chamber, Larin, et al, 1997

National catalogue of the active volcanoes in Japan (the fourth edition, English version), JMA

Geology of the calderas of Kamchatka and Kurile Islands with comparison to caldera of Japan and the Aleutians, Alaska, E Erlich, Open File Report 86-291, USGS, 1986

Geochemisty of magmatic gasses from Kudravavy volcano, Iturup, Kuril Islands, Turan,. Et al, Jan 1997

Volatile trace-element transport in high-temperature gases from Kudriavy volcano (Iturup Islands, Russia), Wahrenberger, et al, 2007

High temperature volcanic gas geochemistry (major and minor elements) at Kudryavy volcano, Africano, et al, Jan 2003

Decade-long study of degassing at Kudriavy volcano, Iturup, Kurile Islands (1990 – 1999):  gas temperature and composition variations, and occurrence of 1999 phreatic eruption, Korzhinsky, et al, Sept 2001

Hydrocarbons in magmatic fluid in phenocrysts of eruption products of Men’shii Brat volcano (Itirup Island): data from pyrolysis-free gas chromatograpy – mass spectrometry of a melt a …, Nizametdinov, et al, Mar 2022

The 2017 activity of the Kurile Islands volcanoes, Rybin, et al, 2018

Gas emissions from volcanoes of the Kuril Island arc (NW Pacific):  geochemistry and fluxes, Taran, et al, 2018

Vetrovoy isthmus of Iturup Island – Holocene strait, Afans’ev, et al, 2019

Water in parental basaltic magmas of the Menshiy Brat volcano (Itutup Island, Kurile Islands), Nizametdinov, et al, Jul 2019

Holocene climatic changes and environmental history of Iturup Island, Kurile Islands, northwest Pacific, Razjigaeva, et al, Jul 2000

Basaltic volcanism of Medvezhia caldera on the Iturup Island of Kurile Isles:  impact of regional tectonics on subduction magmatism, Martynov, et al, Feb 2022

Age, source and distribution of Holocene tephra in the southern Kurile Islands:  evaluation of Holocene eruptive activities in the southern Kurile arc, Razjigaeva & Matsumoto, Sept 2015

The structure of the volcanic lake in the Urbich caldera (Iturup Island, the Kuril Islands), DN Kozlov, 2021

Parental melts and magma storage of a large-volume dacite eruption at Vetrovoy isthmus (Iturup Island, southern Kuril Islands):  insights into the genesis of subduction-zone dacites, Smirnov, et al, Aug 2019

Felsic magmas of the caldera-forming eruptions on the Iturup Island:  the first results of studies of melt inclusions in phenocrysts from pumices of the Lvinaya Past and Vetrovoy Isthmus calderas, Smirnov, et al, Jan 2017

Specific structure of the vegetation cover of the Starozavodoskoe solfataric field (Baransky volcano, Iturup Island) in terms of the development of hydrothermal resources, Romanyuk & Kordyukov, 2020

PGE in the modern hydrotherms of Kudryavy volcano (Kuril Islands), Distler, et al

Modern postvolcanic activity of Beratarube volcano, Iturup Isl., the Kuril Islands, Degterev, et al, 2020

Ultra acid sulfate chloride waters of Baransky volcano on Iturup Island, Kurils.  The composition and output of magmatic and rock-forming components, Kalacheva, et al, Oct 2022

Hydrogeochemistry of thermal waters of Baransky volcano, Iturup Island (Southern Kurils), Bragin, et al, Jul 2015