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 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.