is a

submarine volcano Submarine volcanoes are underwater vents or fissures in the Earth's surface from which magma can erupt. Many submarine volcanoes are located near areas of tectonic plate formation, known as mid-ocean ridges. The volcanoes at mid-ocean ri ...

located about 450 kilometers south of

Tokyo Tokyo (; ja, 東京, , ), officially the Tokyo Metropolis ( ja, 東京都, label=none, ), is the capital and largest city of Japan. Formerly known as Edo, its metropolitan area () is the most populous in the world, with an estimated 37.46 ...

on the Izu-Ogasawara Ridge in the

Izu Islands The are a group of volcanic islands stretching south and east from the Izu Peninsula of Honshū, Japan. Administratively, they form two towns and six villages; all part of Tokyo Prefecture. The largest is Izu Ōshima, usually called simply � ...

. Volcanic activity has been detected there since 1869. Since then it has undergone more

eruptions Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...

, the most powerful of which resulted in the appearance and disappearance of a small

island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An isla ...

. The name Myōjin-shō derives from a fishing boat, ''No.11 Myōjin-Maru'' of Yaizu City,

Shizuoka Prefecture is a prefecture of Japan located in the Chūbu region of Honshu. Shizuoka Prefecture has a population of 3,637,998 and has a geographic area of . Shizuoka Prefecture borders Kanagawa Prefecture to the east, Yamanashi Prefecture to the northea ...

, the crew of which first witnessed the major volcanic eruption of 1952.

Eruption of 1952-1953

The volcanic eruption from 1952 to 1953 was one of its biggest eruptions on record, with the repetitious appearance and disappearance of an island, which at one point reached over ten metres above sea level, before sinking after a major volcanic eruption in September 1953. On September 24, 1952, a survey vessel, ''Kaiyo Maru No. 5'' of the Hydrographic Department of the Maritime Safety Agency, was destroyed by the volcano, with the loss of its crew of 31 (including the nine scientists studying the eruption). Consequently, the Department developed ''Manbou'' (Sunfish), an unmanned radio operating survey boat, and has used it for the research of dangerous sea areas such as submarine volcanoes. This was the first time that volcanic activity had been detected using the SOFAR channel. "Transpacific detection of Myojin volcanic explosions by underwater sound"

Survey of 1998-1999

In 1998 and 1999, the Hydrography Department conducted comprehensive sea bottom surveys around Myōjin-shō, using the state-of-the-art survey vessel ''Shoyo'' and ''Manbou II'', the second generation ''Manbou''. As a result of these surveys, a detailed picture of the

seabed topography The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

around Myōjin-shō was made for the first time. ''Manbou II'' conducted the survey of the sea area within a radius of 3

nautical mile A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude. Tod ...

s (about 5.4 kilometers) of Myōjin-shō. ''Shoyo'' conducted the survey of the sea area within a radius of about 10 nautical miles (about 18.5 kilometers) but farther than the area of the radius of . ''Manbou II'' works by the order of preprogrammed instructions and measures depth and water temperature. Bathymetric survey of ''Manbou II'' was carried out by using the "PRD-601" echo sounder at intervals of 0.2 nautical miles (about 370 meters). ''Shoyo'' conducted a comprehensive survey including the geological and geophysical surveys of sea bottom. Bathymetric survey of ''Shoyo'' was carried out by using a "Seabeam 2112" echo sounder at intervals of 0.5 nautical miles (about 930 meters).

Structure

Previously, Myōjin-shō was considered to be the central cone of a double volcano with the

Bayonnaise Rocks is a group of volcanic rocks in the Philippine Sea about south of Tokyo and south-southeast of Aogashima, in the south portion of the Izu archipelago, Japan. The rocks were discovered by the French corvette ''Bayonnaise'' in 1850, while surv ...

(rocks of 9.9 meters in height above the sea level) as a portion of the

somma A somma volcano (also known as a sommian) is a volcanic caldera that has been partially filled by a new central cone. The name comes from Mount Somma, a stratovolcano in southern Italy with a summit caldera in which the upper cone of Mount Ves ...

(Mita, 1949). As a result of the survey, however, the authors found that both Myōjin-shō and the Bayonnaise Rocks are cones on the somma of a double volcano. The foot of this double volcano lies 1,400 to 1,500 meters in depth and the size is about 30 by 25 kilometers east-west, north-south. The somma is almost a circle in the diameter of 7 by 9 kilometers and the height is 1,000 – 1,400meters. The diameter of the caldera floor is 5.6 kilometers and about 1,100 meters in depth. The central cone is a high formerly known as Takane-shō, 328 metres below sea level. Myōjin-shō is a post caldera cone formed in the northeastern part of the somma of the double volcano. It is a single conical cone and its height is 550 meters with the shallowest depth 50 meters. A record that suggests a gushing of bubbles near the summit was obtained and micro-earthquakes were observed near Myōjin-shō, showing that the volcano is still active, although at a low level

Gallery

1952 eruption

File:Myojin-sho_19520917.jpg, Myōjin-shō, 1952 File:Myojinsho.JPG, Myōjin-shō, 1952

References

External links

Survey of Myojin-sho
{{DEFAULTSORT:Myojin-sho Izu Islands Seamounts of the Pacific Ocean Ephemeral islands Natural disasters in Japan 1952 natural disasters Postwar Japan Former islands from the last glacial maximum