Extrusion dynamics of deep-water volcanoes

Submarine volcanism accounts for c. 75 % of the Earth's volcanic activity. Yet difficulties with imaging their exteriors and interiors mean the extrusion dynamics and erupted volumes of deep-water volcanoes remain poorly understood. Here, we use high-resolution 3-D seismic reflection data to examine the external and internal geometry, and extrusion dynamics of two Late Miocene-Quaternary, deep-water (> 2 km emplacement depth) volcanoes buried beneath 55–330 m of sedimentary strata in the South China Sea. The volcanoes have crater-like basal contacts, which truncate underlying strata, and erupted lava flows that feed lobate lava fans. The lava flows are > 9 km long 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 sub-surface 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 thus form a minor component (~ 3–50 %) 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.