How do deep-water volcanoes grow?

Deep-water volcanoes are emplaced in water depths >1.0 km and are widespread along continental margins and in ocean basins. Whilst the external morphology of deep-water volcanoes can be mapped using bathymetric surveys, their internal structure and true volume remain enigmatic. It is thus difficult to determine how deep-water volcanoes grow. We investigate 13 Late Miocene-to-Quaternary, deep-water volcanoes that are imaged in 3D by seismic reflection data from the northern South China Sea, which allow us to quantify their external morphology and examine their internal structure. These deep-water volcanoes were emplaced in water depths >1.5 km, are relatively small (<3.0 km diameter, <0.56 km tall, and <0.92 km³ in volume), and have steep slopes (up to 42°). Most of the volcanoes have erosional, ‘crater-like’ bases, infilled with sub-horizontal seismic reflections. These crater-like bases are overlain by downward-converging, conical seismic reflections delineating the classical volcano morphology. We suggest the crater-like bases formed by excavation of cold, wet, and poorly consolidated near-seabed sediment during expulsion of hydrothermal fluid, and not by explosive magmatic eruptions or gravitational subsidence. Erupted igneous material infilled the precursor craters with the observed sub-horizontal layers, likely comprising hyaloclastites. After this initial phase of volcanism, the buildup of volcanic material produced layers that are now represented by the flank-parallel or downward-converging, conical seismic reflections. We suggest high hydrostatic pressures of >15 MPa, which are typical of water depths >1.5 km, inhibited degassing and fragmentation of ascending magma and thus erupted lava. This lack of degassing and fragmentation permitted effusive eruptions during the latter stages of volcanism. Our models for volcano growth in the deep submarine realm demonstrate the power of using 3D seismic data when investigating the internal structure and total volume of deep-water volcanoes.