The Bayonnaise Rocks submarine-volcano is one of the Japanese Izu Islands
It raises only about 11 m asl (36 feet), and its relative position is 31.88°N 139.92°E
Bayonnaise Rocks represent part of the barely exposed rim of a largely submarine 8-9 km
wide caldera. Formation of the caldera was followed by construction of a large (2.6 cu km)
lava dome and/or lava flow complex on the caldera floor, originally located at a depth
of 1000-1100 m.
Most historical eruptions, recorded since the late-19th century, have occurred from
Myojin-Sho, a large post-caldera lava dome constructed on the NE rim of the caldera.
Deposits from submarine pyroclastic flows associated with growth of the dacitic lava dome
mantle the conical dome and extend into the NE part of the caldera and down its outer slopes.
It was 1869 when a volcanic eruption was first observed. Withinthe 130 years that followed,
it has repeated more than ten volcanic activities of eruption and the appearance and
disappearance of a new island.
An explosive submarine eruption from Myojin-Sho in 1952 destroyed a Japanese research
vessel, killing all 31 on board. Submarine eruptions have also been observed from other
points on the caldera rim and outside of the caldera.
March 1st, 2004
According to ongoing survey by SWVRC, this volcano is forecast to erupt in 2004
with an accuracy of 95% probability. So now we just have to wait and see.....
The volcanic eruption from 1952 to 1953 was one of its biggest activities on record with
the repetitious appearance and disappearance of an island. The island grew to more than
several ten meters high at a certain point, but it disappeared with severe volcanic
explosion in September 1953. The name Myojin-Sho derives from a fishing boat,
"No.11 MYOJIN-MARU " of Yaizu City, Shizuoka Prefecture, the crew of which first
witnessed the major volcanic eruption of 1952.
On September 24, 1953, a survey vessel,"No. 5 KAIYO-MARU" of the Hydrographic
Department of the Maritime SafetyAgency, met with disaster, costing thirty-one persons
including those of nine scientists.This is the biggest disaster in the history of Japanese
oceanographic research and an unforgettable tragedy for the Hydrographic Department.
Determined to learn a lesson from this tragedy, the Hydrographic Department developed
"MANBOU" (meaning Sunfish) an unmanned radio operating survey boat, and has used it
for the research of dangerous sea areas such as submarine volcanoes, etc.
The U.S. Navy SOFAR (Sound Fixing and Ranging) Stations off California detected the
severe explosion, and Tsunami waves that accompanied theexplosion were observed in
Hachijojima Island, about 130km north of the Myojin-Sho.
Previously, Myojin-Sho was considered to be the central cone of a double volcano
with the Bayonnaise Rocks (rocks of 9.9 meters in height above the sea level) as a portion
of the somma (MITA, 1949). As a result of the survey, however, the authors found that
both Myojin-Sho 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 sizeis about
30 25 kilometers east-west, north-south. The somma is almost a circle in thediameter
of 7 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-Sho, height is 770 meters with theshallowest depth 328 meters.
Myojin-Sho is a post caldera cone formed in the northeastern part of the somma of the
double volcano. It is the single conical cone and the height is 550 meters with the shallowest
depth 50 meters. A record that suggests a gushing of bubbles near thesummit was obtained
and the micro earthquake were observed near the Myojin-Sho.
Therefore, the authors consider that the Myojin-Sho continues to be active now although
the level of the activity seems low.
02/70 Explosions and discolored water on 29 January; larger eruption on 3 February
03/70 Explosions in January and February detected by hydrophones
04/70 Eruption on 23 April sends water and smoke 200 m high
05/70 Detailed observations of activity from fishing boats and observation flights
07/70 Explosion bubbles and floating pumice on 18 June
07/79 Water discoloration observed on 13 July 1979
11/80 Discolored water
12/80 Discolored water seen on 23 December
10/87 Discolored water within 100 km
03/88 Discolored water
Info on this page mainly from the Smithsonian 'Global Volcanism Program'
and from THE SURVEY OF MYOJIN-SHO,
by Kunio YASHIMA, Azuza NISHIZAWA and Yasyio OOTANI, Japan
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