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. Submarine volcanism accounts for ca. 75 % of the Earth's volcanic activity.Yet difficulties with imaging their exteriors and interiors mean that theextrusion dynamics and erupted volumes of deepwater volcanoes remain poorlyunderstood. Here, we use high-resolution 3-D seismic reflection data toexamine the external and internal geometry and extrusion dynamics of twolate Miocene–Quaternary deepwater (&gt; 2 km emplacement depth)volcanoes buried beneath 55–330 m of sedimentary strata in the South ChinaSea. The volcanoes have crater-like bases, which truncate underlying strataand suggest extrusion was initially explosive, and erupted lava flows thatfeed lobate lava fans. The lava flows are &gt; 9 km long and containlava tubes that have rugged basal contacts defined by ∼90±23 m high erosional ramps. We suggest the lava flows eroded downinto and were emplaced within wet, unconsolidated, near-seafloor sediments.Extrusion dynamics were likely controlled by low magma viscosities as aresult of increased dissolved H2O due to high hydrostatic pressure andsoft, near-seabed sediments, which are collectively characteristic ofdeepwater environments. We calculate that long-runout lava flows accountfor 50 %–97 % of the total erupted volume, with a surprisingly minorcomponent (∼3 %–50 %) being preserved in the main volcanicedifice. Accurate estimates of erupted volumes therefore require knowledgeof volcano and lava basal surface morphology. We conclude that 3-D seismicreflection data are a powerful tool for constraining the geometry, volumes,and extrusion dynamics of ancient or active deepwater volcanoes and lavaflows.

Extrusion dynamics of deepwater volcanoes revealed by 3-D seismic data