Bayonnaise Rocks. Aerial photo courtesy Wikimedia Commons, 2015
The Bayonnaise Rocks are the barely exposed rim of a largely submarine caldera 8 – 10 km in diameter located 408 km south of Tokyo. It is part of the Izu archipelago and has at least nine neighboring calderas along this volcanic arc. The rocks were discovered (and named in the western world) by the French corvette Bayonnaise in 1850.
There are at least three volcanic systems within the local region. They include the Bayonnaise Knoll, the Myojin Knoll, and the Myojin-sho caldera. All three are located within 25 km of one another. Recent volcanic activity in this area has come from the Myojin-sho caldera. The Japanese reported a single historic volcanic event in the vicinity of Bayonaise Rocks, though it may very well have been from Myojin-sho. As far as I can find, neither the Myojin Knoll or the Bayonnaise Knoll have been recently active nor have they been observed erupting.
Topographic representation of the three caldera systems in the vicinity of Myojin—sho. The blue and red lines represent subsurface surveys of Bayonnaise and Nyojin Knolls. Image courtesy Yamashita, et al 2015
The active system in this part of the Izu archipelago is the Myojin-sho caldera. Its internal measurement is 6.6 x 8.3 km. The Bayonnaise Rocks are on the western rim of the caldera, the emergent tip of a stratocone (Kita-Bayonnaise or Kitabeyonesu) that grew on the caldera’s rim. The eastern side of the caldera is the active Myojin-sho (Myojinsho) system, which generally tops out around 50 m below the waves. It is this system that has been responsible for submarine eruptions, emergent ephemeral islands, and at least one eruption that sunk a research vessel in 1952.
Vegetation is sparse at best as the three main Bayonnaise rocks top out at 9 – 11 m above the waves. Numerous smaller rocks poke the surface of the ocean. The rocks are a resting place for migratory birds. The surrounding ocean is quite rich, with abundant sea life and popular with sports fishermen.
Map of Izu Islands. Image courtesy Wikipedia
Region
The string of islands along the Izu archipelago (Izu-Bonin Volcanic arc or the Izu-Ogasawara arc) stretching S and E of Tokyo and the Izu Peninsula are occasionally called the Seven Islands of Izu, even though there more than a dozen islands. Nine of them are currently inhabited. All are volcanic, some recently active, and there are at least 9 calderas along the string. Total population of all the islands is just under 25,000.
Aogashima is the southernmost inhabited island of the Izu archipelago. It is 358 km S of Tokyo and is an active, complex stratovolcanic island. The island itself is constructed on the remains of at least 4 collocated submarine calderas. There is a neighboring 5.4 x 9.9 km Higashi – Agogashima caldera 12 km E of Agoshima. Kita-Bayonnaise / Myojin-sho caldera is some 40 km south of Agogashima. The 6.6 x 8.3 km Sumisu caldera (Smith Rock) is 110 km S of Aogashima. The smaller 2 x 3 km Minami-Sumisu is another 20 km farther south of Sumisu. Finally, the 4 km Torishima caldera is located 5 km NNE of Torishima Island, 242 km S of Aogashima Island. There are rocks associated with Kita-Bayonnaise, Higashi-Agogashima and Sumisu.
Shaded relief map of mining survey around North Myojin Rift. Image courtesy Tanahashi et al, 2008
This area is thick with calderas, active volcanoes, and silicic hydrothermal systems.
Recent oceanographic surveys of the region have been oriented toward seafloor mining targeting what are referred to as kuroko deposits, massive sulfide deposits that contain concentrated ores (as much as 20% by weight) of combined copper, lead and zinc. The ores also contain gold and silver. These ores are concentrated by hydrothermal systems associated with subsea volcanic activity. They are found in significant quantity in hydrothermally produced sulfide deposits in all three Bayonnaise – Myojin caldera systems. There are also significant sulfide deposits associate with volcanic structures around the North Myojin Rift system to the west of Bayonnaise – Myojin.
Kaiyo Maru No 5. Image courtesy shipsnostalgia.com. Image taken 1952
Tragedy
The 1952 – 1953 eruption sequence from Myojin-sho (Myojin Reef) began with an undersea eruption on Sept 15 forming a new island. It was discovered by a fishing boat. The Japanese sent an oceanographic ship to investigate. The new island disappeared completely on Sept 23. The Sept 23 eruption was observed and did create a tsunami. There were multiple explosions detected Sept 17 – 23.
The next day on Sept 24, 1952, the oceanographic survey vessel Kaiyo-Maru No 5 was destroyed by a submarine eruption. The ship, its crew of 31 including 9 scientists were lost. This was the biggest disaster in the history of Japan’s Hydrographic Department. Major activity including explosions and tsunamis continued after the sinking through Sept 26.
An array of USN SOFAR (Sound Fixing and Ranging) stations in California detected the double explosion associated with the sinking. This was the first volcanic eruption positively identified by SOFAR.
Volcanic explosion from Myojin-sho on Sept 23. Image courtesy Asahi Shimbun via Smithsonian GVP
Tsunamis were observed on an island 130 km north of the Myojin-Sho. Looking at the data gathered for the previous week, it became apparent that the system was picking up multiple explosions from Myojin-sho starting Sept 17.
Wreckage from the Kaiyo-Maru No 5 drifted on the ocean current after the accident. Pieces of fragmented lavas and pumice were found stuck to pieces of the boat. These chemically matched samples from the volcano.
A 1953 paper by Dietz and Sheehy analyzed SOFAR data before and after the eruption and surmises that the volcano was quiet, in a temporarily dormant state when overflown at noon on Sept 22. The assumption is that the Kaiyo Maru No 5 must have considered the reef safe to approach, only to be caught by an unexpected and intense explosion.
Photo of ephemeral island raised by the 1952 -1953 eruption sequence. 1952 image courtesy Wikipedia
This is not the first time that a volcanic eruption was suspected of sinking a ship. Kick ‘em Jenny is suspected of sinking the Island Queen in the Caribbean in 1944, killing all 67 onboard. There is now an exclusion zone around the volcano as a result of this. In 2006, the yacht Maiken encountered a pumice raft near the Vava’u Islands. After they crossed the raft, a subsurface eruption broke the surface of the ocean behind them.
As with any relatively unknown natural phenomena, subsurface volcanic eruptions in this part of the world provided input for western conspiracy theorists, who tagged this part of the ocean as the Devil’s Sea (or the Dragon’s Triangle) with multiple unexplained instances of missing ships over the last 70 years, the functional equivalent of the Bermuda Triangle off the east coast of the US. Unlike the Bermuda Triangle, this part of the sea has multiple submarine volcanoes that erupt from time to time in addition to bad weather rolling through. Interestingly, all this speculation has been from the western world as the description is relatively unknown in Japan itself.
Images of three explosions from Myojin-sho on Sept 23, 1952. Image C is a surge traveling across the ocean. Image courtesy Fiske, et al 1998
This disaster was significantly impressive to the Japanese that similar scenes started showing up in popular Japanese monster moves by the Toho Studios in the following decade. They typical scene has something bubbling up underneath an unsuspecting ship, sinking it and killing the crew. The first of these that I can remember is 1954’s Gojira.
Humans do learn when responding to disasters. One of the positive fallouts of this sinking was the development of unmanned radio-operated survey craft. The department has used them to investigate hazardous sea areas since the accident. The other positive was the use of SOFAR for observation and correlation of subsea volcanic eruptions.
Bathymetry of Myojin-sho caldera and edifice. Bayonnaise Rocks, caldera and active Myojinsho volcano all clearly visible. Image courtesy Fiske, et al 1998
Myojin-sho Caldera
The greater caldera is moderately complex, with pumice-mantled walls, rhyolitic lava flows, rhyolitic pumice, and what appears to be at least one active somma. There is a post-caldera lava dome that rises some 250 m above the floor of the caldera. Smithsonian GVP lists the caldera as 7 – 9 km in diameter. Base diameter of the caldera is 22 x 18.5 km. The circular caldera is surrounded by a somma at a depth of 600 – 700 m. The 260 m central cone has a basal diameter of 2 km. The caldera is generally 500 – 900 m above the sea floor. Its age is unknown but may be as young as a few thousand years. There is an active hydrothermal field in the eastern portion of the caldera with chimneys up to 30 m high.
The foot of the double volcano is 1,400 – 1,500 m at depth. The somma is circular at 7.9 km and tops out at 1,000 – 1,400 m. The diameter of the caldera floor is 5.6 km and around 1,100 m at depth. The central cone was formerly known as Takane-Sho, 770 m high, topping out at 328 m below the surface of the water.
Post eruption domes from Myojin-sho. Top photo is Sept 22. Bottom photo is April 14. Image courtesy Fiske, et al 1998
Myojin-Sho is a post-caldera cone in the NE part of the somma. It is a single conical cone 550 m high, within 50 m of the surface. It is considered to be currently active, though that activity is low today.
The Bayonnaise Rocks are the exposed top of a volcanic cone, Kita-Bayonnaise on the western ridge of the somma. They are 9 – 11 m above the water. There are three larger rocks with multiple smaller rocks.
The base of the complex is basalt. Most recent eruptive materials are dacites.
The active hydrothermal system on the caldera floor seems to be mostly confined to the eastern part of the floor. The floor is covered with ash to cobble-sized pumice. The walls are pumice and rhyolitic lava flows. The floor sediments are enriched with varying amounts of sulfides and other mineralized metals. Samples were taken in 1992. This deposit is referred to as the Sunrise Deposit. Neptune Minerals has mining applications for the caldera region.
Bathymetric survey map of Myojin Knoll. Image courtesy Honsho, et al, 2016
Myojin Knoll
A 1995 paper by Fiske et al describes the caldera-forming eruption of Myojin Knoll. The eruption is undated and thought to have been about twice the size of the 1883 Krakatau eruption, producing an estimated 36 km3 of rhyolite pumice. It is tentatively dated sometime in the last several thousand years and created a collapse caldera 5 – 7 km diameter. The eruption took place at water depths of 200 – 400 m. Much of the material likely flowed downslope in cold submarine pyroclastic flows. It left a 130 – 200 m thick pyroclastic deposit on the top of the caldera walls. Caldera formation likely created tsunamis. The age and recurrence intervals of large submarine events like this and similar nearby volcanoes are unknown, meaning future risks cannot be evaluated. Virtually nothing is known about large pyroclastic eruptions on the sea floor.
Submarine topography of Myojin Knoll. Not post-caldera dome in the center of the caldera. Image courtesy Murakami, Geological Survey of Japan via Fiske, Smithsonian GVP
Base diameter of this caldera is 15 x 22 km. It tops out at 336 m below the ocean surface with walls of 1,000 m.
Total caldera volume is estimated at 18.1 km3. The rhyolite pumice deposit on the caldera walls was imaged during Shinkai 2000 dives. A significant volume of the pumice produced by the eruption likely floated at least for a time, making estimates of total volume produced a lower end estimate at best. There was also a significant volume of material carried in suspension and deposited over 20 km from the eruption.
There was a large polymetallic sulfide deposit on the caldera floor in discovered in 1999. Additional surveys were conducted 2001 and 2008.
Structural survey of Bayonnaise Knoll via mapping in 2010. Upper schematic is suspected magma location and shows it powering the hydrothermal system of the caldera. Image courtesy Yamashita, et al 2015
Bayonnaise Knoll
The Bayonnaise Knoll caldera is a conical silicic caldera to the west of the Myojin-sho caldera. Like neighboring Myojin Knoll, there are numerous faults, and caldera formation was controlled by back-arc rifting. The caldera has evidence of an active hydrothermal system like the other calderas in the region, mostly on the eastern side of the floor and walls. It is located 20 km W of the Myojin-sho caldera. The caldera measures 2.5 x 3.0 km in diameter. The summit of this knoll is 600 m below the ocean surface. It has a wall height of 400 m and a base diameter of 10 km.
The caldera is a well-formed conical shaped caldera with a central dome. Large faults were visible in seismic sampling with a tuned air gun and hydrophone array. Velocity returns under the western caldera wall suggest fractured rhyolitic lavas. There may also be relatively shallow magmas under the western part of the caldera. The caldera walls are pumiceous sand and gravels. There were also thick volcanic breccias. Returned signals from the knoll are similar to those of the Sumisu system to the south that have been verified by oceanic drilling.
Comparison between Bayonnaise and Myojin Knolls. Screen capture from Yamashita, et al 2015
Metallic minerals abundant in active hydrothermal sulfide deposits on the current volcanic front are also present in inactive hydrothermal sulfide deposits on the Bayonnaise Knoll caldera. The hydrothermal system appears to be powered by a magmatic intrusion, with water intruding along a fault line on the eastern outer edge of the caldera. It is heated and forms a hydrothermal field in the caldera. This is similar to other volcanic activity distributed along the east side of the Izu-Ogasawara volcanic arc.
Discolored water and steam emission from Myojin-sho volcano six days after beginning of 1952 – 1953 eruption sequence. Photo taken Sept 22. Later that day, the eruption became very explosive and the dome was destroyed. Three cycles of dome growth and destruction took place thru Oct 1953. Image by Helen Foster, 1952, USGS via Smithsonian GVP
Eruptions
Most historic eruptions have taken place from Myojin-Sho. The first observed submarine volcanic eruption was 1869. There have been at least 10 eruptions since that time. Unless specifically noted, all these eruptions were at Myojin-Sho, the most active portion of the complex.
These eruptions built and destroyed an ephemeral volcanic island multiple times since 1869.
Phreatomagmatic eruptions sook place in 1870. It created a small island. There was a submarine eruption in 1871. The 1896 eruption also created a small island and a violent surge.
The 1906 submarine eruption took place over a week in April. It created a volcanic plume and a large amount of pumice was found floating in the sea following the eruption. The 1906 eruption was described in some detail in 1908’s Publications of the Earthquake Investigation Committee in Foreign Languages No. 22. The description described eruption of a submarine volcano near Bayonnaise Rocks. It put an impressive white plume around 100 m wide by 300 m high. The beginning of the eruption was not observed, though the end was. It went on for over 24 hours and put a significant amount of pumice on the ocean, some of which was collected. The pumice was pure white with black inclusions. Analysis suggests some of the crystal inclusions were already present in the magma before erupting and concluded that the erupted magma was an andesite.
There were at least two eruptions in 1915. The first one was a submarine eruption. The second created rock, steam and seawater plumes.
Eruptions in 1934 and 1945 were both submarine eruptions. The 1946 eruption created multiple ephemeral islands, the tallest being 100 m high. They had all returned below the sea by Dec.
Myojin-sho eruption plume. Image taken Sept 27, 1952. Image courtesy Ashai Shinbun, Sept 1955, via Wikimedia Commons
The 1952 – 1953 eruption sequence was one of the largest recorded from this system. There were multiple appearances and disappearances of the island during the sequence. The island grew to several tens of meters high before being destroyed in a large eruption in 1953. The eruptions produced air-fall pyroclastics, pyroclastic surge, pyroclastic density current, floating pumice on the sea surface.
The new island appeared as a result of a large explosion on Sept 17. There was also an undersea eruption. It sank on Sept 23. The No 5 Kaiyo-Maru was sunk on the following day. The island reappeared around Oct 11 and disappeared Mar 11, 1953. It reappeared once again April 5 – Sept 3.
Two eruptions took place in 1954 – 1955.
The only eruption thought to be near the Bayonnaise Rocks took place in 1957. Bodies of dead deep-sea fish floated to the surface. They were suspected to be killed by submarine volcanic activity.
A phreatomagmatic eruption took place in 1960. This put a 3 km plume above Myojin-Sho and produced floating pumice.
There were at least five episodes of explosions, discolored water, bubbles and floating pumice in 1970.
Water discoloration was observed in 1979.
Water was discolored in 1980, 1982, 1983, 1986, 1987 and 1988.
The most recent submarine eruption was Mar – Nov 2017 which discolored and bubbled seawater.
Personal Note:
For such an active system, it has been surprisingly difficult to find photos of active, ongoing eruptions. It appears Getty Images acquired rights to all news photos of the 1952 – 1953 eruption sequence. There are 29 great photos available. This means that images that would now be in the public domain are now licensed for use at $499 a pop.
Tectonic setting of northern Izu arc. Image courtesy Kimura, et al 2010
Tectonics
The entire region is called the Izu – Bonin – Mariana (IBM) arc system. The three segments are named in order of their distance from Tokyo with Izu being closest and the Marianas being the farthest. The system is a convergent tectonic plate boundary with the Pacific Plate subducting under the Philippine Sea Plate. All three arcs have associated back arc volcanism, some of it quite old (over 40 Ma in the Bonin arc). There is at least one extinct spreading center, the Shikoku Basin to the north and the Parece Vela Basin to the south. Guam bounds the southern end of the Marianas with the Challenger Deep, the deepest place on the planet at the far southern end of the IBM arc ridges. Overall, the entire system can be described as an intra-oceanic convergent margin. The volcanic arc associated with this system is over 1,200 km long, generally oriented N – S.
Schematic of history of Izu – Bonin – Mariana arc / back-arc system. Image courtesy Calvert, et al, 2011
The system has always been an arc system under strong extension, which has spread out its components over 1,000 from W – E. Subduction has produced at least three volcanic arc systems over the last 50 Ma and one extinct spreading center between the oldest and newer volcanic arcs. Initial volcanic arc stabilized around 30 Ma as the Parece Vela and Shikoku Basin spreading centers began forming. These widened until around 15 Ma. Arc volcanism waned during much of this period and restarted 20 – 17 Ma.
The northernmost portion of the system started colliding with Honhsu around 15 Ma. Rifting started to form the Mariana Trough some 10 Ma, meaning the current Mariana volcanoes cannot be older than 3 – 4 Ma. By contrast, the Izu – Bonin volcanoes could be as old as 25 Ma. Andesitic – basaltic volcanism in the back arc region began around 17 Ma and continued until 3 Ma. Older front arc volcanism is found generally on the western portion of the arc and has migrated eastward over time.
Izu rifting began around 2 Ma. This zone is primarily back-arc rifting divided into 4 subareas. From N to S Hachijo Rift, Agoshima Rift, Myojin Rift, and Sumisu Rift. Boundary faults for the rifts are generally E – W. The general depression runs N – S and has at least 5 pumice layers dating 1,000 – 131,000 years ago. Fault movement has continued during deposition of the pumice layers. Volcanoes on the volcanic front generally produce calderas while those in the back-arc region are small volcanoes of conical or elongated shape.
Schematic of subsea topography of Myojima-sho caldera. Image courtesy Vulkaner.no blog
Conclusions
Submarine volcanoes, especially explosive ones, are incredibly dangerous locations. It is accidents like the Kaiyo-Maro No 5 that drive maritime authorities to stand up exclusion zones around known submarine volcanoes. There is no reason to believe that the Myojin-sho system has calmed down, though it is currently quiet. The accident has opened the door to better eruption / explosion detection from a distance, development of radio-operated subsurface survey craft, and even an extended look into seafloor mining of metal deposits created by volcanic hydrothermal systems. The loss of life is terribly unfortunate, but it is difficult to find a system that has triggered such an improvement in detection, science and economic activity.
Bayonnaise Rocks. Underwater eruption in the vicinity of Bayonnaise Rocks Nov 2017. Image courtesy Strange Sounds blog, 2017
Additional information
https://volcano.si.edu/volcano.cfm?vn=284061
http://www.vulkaner.no/v/volcan/japan/bayonnaise.html
http://volcanolive.com/bayonnaise.html
https://www.jstage.jst.go.jp/article/shigenchishitsu1992/45/249/45_249_1/_pdf
https://gbank.gsj.jp/volcano/Act_Vol/aogashima/text/eng/exp07-5e.html
https://somwritinglab.utdallas.edu/~rjstern/pdfs/IshizukaEPSL06.pdf
https://earth-planets-space.springeropen.com/articles/10.1186/s40623-015-0214-2
http://www.godac.jamstec.go.jp/catalog/data/doc_catalog/media/shinkai11_29.pdf
https://onlinelibrary.wiley.com/doi/full/10.1111/j.1751-3928.2008.00063.x
https://www.terrapub.co.jp/journals/JO/JOSJ/pdf/0902/09020053.pdf
https://studylib.net/doc/8439680/66.-beyonesu–bayonnaise–rocks–including-myojinsho-
agimarc
Spica
Granyia
volcanohotspot.wordpress.com 
Looks like the expected eruption in western Iceland, 20 km from Reykjavik, started overnight. So far, it is small. Fagradalsfjall. Cheers –
https://www.nbcnews.com/news/world/iceland-volcano-dormant-6-000-years-erupts-not-far-reykjav-n1261637
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