伊豆・小笠原弧北部域の明神海丘における海底カルデラの形成機構 The Forming Machanism of the Submarine Caldera on Myojin Knoll in the Northern Part of the Izu-Ogasawara (Bonin) Arc
Myojin Knoll Caldera is located on the broad volcanic edifice of Myojin Knoll and is one of the twelve calderas along the front of the Izu-Ogasawara (Bonin) Arc. The caldera floor is about 3×4 km in diameter and lies at a depth of about 1, 400 m.<BR>Myojin Knoll is composed of three acoustically defined stratigraphic units, 1A, 1B, and 1C. These are correlated respectively with three lithologic units : stratified volcanic breccia, massive volcanic breccia and rhyolitic lava. These same lithologies were observed during submersible dives using Shinkai 2000. The volume of these units making up Myojin Knoll is Unit1A, 17.5 km<SUP>3</SUP>; Unit 1B, 64 km<SUP>3</SUP>; and Unit 1C, 48.3 km<SUP>3</SUP>.<BR>The mean density of Myojin Knoll is about 1.84 g/cm<SUP>3</SUP>, based on the calculated volumes and assumed densities of Unit 1A, 1B, and 1C. Using this density for Bouguer corrections, there is neither a high nor a low Bouguer anomaly associated with the caldera. This pattern is more typical of anomalies associated with volcanic craters rather than with Krakatau-type calderas. Three dipole type magnetic anomalies have been recognized, one on the north caldera rim, another on the west rim, and a third on the central cone. The anomaliy associated with the central cone is small, reflecting its relatively small volume.<BR>Three mechanisms are suggested for the formation of the Myojin Knoll Caldera : (1) the caldera formed as a pumice cone, (2) it formed by collapse of the pre-caldera stratovolcano that formed Units 1B and 1C, and (3) it formed by the destruction of the pre-caldera stratovolcano as a result of the explosive eruption of the huge volume of pumice associated with Unit 1A. The gravity and magnetic data presented here suggest that the models of the pumice cone and the explosive destruction of the pre-caldera stratovolcano are preferable to the model of a collapsing stratovolcano.