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Submarine volcanism accounts for c. 75% of the Earth's volcanic activity. However, difficulties with imaging their exteriors and interiors mean deep-water volcanoes remain poorly understood in terms of their extrusion dynamics and erupted volume. Here, we use high-resolution 3-D seismic reflection data to examine the geometry, distribution, and extrusion dynamics of two Late Miocene-Quaternary, well-imaged, deep-water (>2.0 km emplacement depth) volcanoes in the South China Sea buried beneath ~55-330 m of sedimentary strata. The volcanoes, which have crater-like basal contacts that truncate underlying strata, erupted lava flows feeding lobate lava fans. The lava flows are >9 km and contain lava tubes that have rugged basal contacts defined by ~90±23 m high erosional ramps. We suggest the lava flows eroded down into and were emplaced at shallow depths within wet, unconsolidated, near-seafloor sediments. Extrusion dynamics were likely controlled by low magma viscosities, high hydrostatic pressures, and soft, near-seabed sediments, which collectively are characteristic of deep-water environments. Because the lava flows and volcanic edifices are imaged in 3D, we calculate the lava flows account for 50–97% of the total erupted volume. Our results indicate deep-water volcanic edifices may form a minor component of the extrusive system, and that accurate estimates of erupted volume requires knowledge of the basal surface of genetically related lava flows. We conclude that 3D seismic reflection data is a powerful tool for constraining the geometry and extrusion dynamics of buried, deep-water volcanic features; such data should be used to image and quantify extrusion dynamics of modern deep-water volcanoes.
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