Detailed volcanic morphology of Daisan-Miyako Knoll in the southern Ryukyu Arc

High-resolution bathymetric mapping using an autonomous underwater vehicle and a survey vessel was conducted at Daisan-Miyako Knoll in the southern Ryukyu Arc. This paper presents for the first time the detailed volcanic morphology of a submarine edifice that previously was poorly explored, and discusses its formation process and relation to arc volcanism and tectonics. Bathymetry shows morphological evidence that Daisan-Miyako Knoll is a stratovolcano with three caldron-like depressions (>1 km in diameter), a central cone, and multiple lava flows at the summit. The main edifice is conical with a basal diameter of 8-10 km and relief of 1000-1100 m, with a summit depth of 779 m. The central cone that stands inside the depressions has a circular crater. Numerous blocks up to 10 m wide are distributed around the crater, indicating a previous explosive eruption. Lava flows erupted from the central cone have a maximum thickness of 46 m at the flow margins and a maximum flow length of 1.9 km. The total flow area is 1.5 km2 and the estimated volume is 0.03 km3. The relatively thick lava flows (up to 46 m) with steep-sided margins (30-40°) and curtain-folded surface textures suggest that silicic magmas erupted, and their fresh appearance indicates that lava-flow eruptions (the most recent volcanic activity) occurred during the Quaternary. The volcanic features at Daisan-Miyako Knoll are aligned mainly in the NW-SE direction, suggesting that volcanism here may be controlled primarily by arc-parallel extension. No volcanic edifices have been confirmed on the trench side of Daisan-Miyako Knoll, therefore this edifice defines the location of the arc volcanic front in this region. From the larger rhyolitic edifices (>10 km wide) around Daisan-Miyako Knoll along the front, it is inferred that silicic arc volcanism is extending from the central to southern submarine Ryukyu Arc.