Detailed morphology and bubble plumes of Daiichi-Amami Knoll in the central Ryukyu Arc

High-resolution geophysical mapping operations were conducted from a survey vessel and autonomous underwater vehicle (AUV) in the Daiichi-Amami Knoll, Ryukyu island arc. For the first time, both the detailed morphology and the extent of the bubble plume in this region are revealed. The knoll is a conical edifice rising 500-700 m from the surrounding seafloor with a peak depth at 247 m. The summit region of the knoll has a complex structure and is divided into three parts: the Northeastern Rim, Central Caldera, and Southern Cones. Among the three parts, the Southern Cones has characteristic morphological features such as cones and a depression. Two cones, cone A and cone B, rise ~ 30 m above the surrounding basement, and have fissure-like linear features. The depression, depression D1, faces the steep slope of cone A and is bounded by cone A and B. The horizontal dimensions are ~ 500 m × 200 m and it is 10-40 m lower than the surrounding basement. Water column anomalies detected on side-scan sonar images, which are thought to be bubble plumes, were intensively distributed in and around depression D1 and cone B with horizontal dimensions of ~ 300 m × ~ 800 m. The volcanic morphology and bubble plumes, indicators of hydrothermal activity, suggest that the Daiichi-Amami Knoll is an active volcanic knoll. This result proposes a new insight that dual volcanic chains exist in the north to central Ryukyu Arc. The Daiichi-Amami Knoll is located behind the volcanic front, not on the front. The knoll, surrounding volcanoes, and volcanic edifice behind the front are aligned with one straight line ~ 20 km behind and parallel to the front, making a rear-arc volcanic chain. The presence of dual volcanic chains in the north to central Ryukyu Arc, revealed by our detailed geophysical survey, provides valuable information to consider toward the subduction processes and magma generation in the Ryukyu Arc.