Birth of a Submarine Volcano "Teisi Knoll".

Abstract

Frequent sea bottom surveying in an area of earthquake swarms in Sagami Bay about 2 to 3km from the east coast of Izu Peninsula clearly disclosed the birth of a new submarine volcano and processes of its formation. Bathymetric soundings, taken on July 9, 1989, showed that the seafloor, where the new volcano was to be born, was characterized by undeformed sedimented bottom in depths of about 100m. Large-amplitude tremors, beginning on the evening of July 11, were associated with the formation of a new volcanic knoll (cryptodome). This was confirmed by the survey vessel "Takuyo" on July 13 at 18 h 28m when it was found that the knoll had risen 25m above the surrounding seafloor. Five minutes later, after the existence of this newly formed bulge was established, a submarine eruption from the knoll began. While escaping from the hazardous area, Takuyo took photographs and made video tape recordings of the effects of the eruption on the sea surface and in the atmosphere. In addition, audio records of a series of frightening acoustic shocks of the volcanic activity were made. These shocks suggest frequent discharges from the seafloor of high pressure gasses associated with sub-bottom phreatomagmatic eruptions. On July 15, an unmanned automated survey craft, the Manbou, recorded a column of gas bubbles rising from the 200m diameter crater in the center of the knoll. The eruption of July 13 had lowered the height of the dome from 25 to 10m. Three months later a detailed bathymetric survey, using a highly accurate multibeam sounding system, Hydrochart, confirmed the maar-like topography of the volcano and also recorded the weakened emission of gasses.

Earthquake Prediction Research in Japan.

“…In this figure, the northwestward migration of swarm activity and the decreasing focal depths are notable. A submarine volcanic eruption occurred on July 13 nearly at the center of the focal region of the third of the three above-mentioned swarms, and formed a new crater (Oshima et al, 1991). When the earthquake swarm began in July, just before the eruption, the Coordinating Committee for Earthquake Prediction remarked that the focal depths of earthquakes in the swarm were extremely shallow.…”

Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits

“…This depth is generally recognized to be about 2001 000 m and less, depending on magma composition and volatile content (e.g. Kokelaar, 1986;Bonatti and Harrison, 1988;Gill et al, 1990;Oshima et al, 1991;Heikinian et al, 1991;Binard et al, 1992;Wright, 1996Wright, , 1999White, 1996;Kano, 1998;Fiske et al, 1998Fiske et al, , 2001Hunns and McPhie, 1999) and is referred to as the volatile fragmentation depth (Fisher and Schmincke, 1984).…”

Volcanic Tremor Accompanying the 1989 Submarine Eruption off the East Coast of Izu Peninsula, Japan.

“…There are some clues suggesting that the source of a pair of waves also locates near the erupted position. The shock wave accompanying the submarine eruption was recorded by audio cassette tape on a ship about 500m apart from the erupted position (Oshima et al, 1991). The appearance times of high frequency in the isolated one.…”

“…The echo sounding survey showed a knoll 25m high just before the eruption of July 13 and a knoll 10m high on July 15 in the erupted area, while no topographic high knoll was found there on July 9, 1989 (Oshima et al, 1991). Since the earthquake swarm from June 30 and the volcanic tremor from July 11 should relate to the creation of the new volcanic vent, the investigations on them should give us knowledge on dynamics of magma in new volcanic vent and the mechanism of volcanic eruption.…”