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

Abstract

Abstract. Submarine volcanism accounts for ca. 75\u2009% of the Earth s volcanic activity.\nYet difficulties with imaging their exteriors and interiors mean that the\nextrusion dynamics and erupted volumes of deepwater volcanoes remain poorly\nunderstood. Here, we use high-resolution 3-D seismic reflection data to\nexamine the external and internal geometry and extrusion dynamics of two\nlate Miocene–Quaternary deepwater (>\u20092\u2009km emplacement depth)\nvolcanoes buried beneath 55–330\u2009m of sedimentary strata in the South China\nSea. The volcanoes have crater-like bases, which truncate underlying strata\nand suggest extrusion was initially explosive, and erupted lava flows that\nfeed lobate lava fans. The lava flows are >\u20099\u2009km long and contain\nlava tubes that have rugged basal contacts defined by ∼ 90 ± 23 \u2009m high erosional ramps. We suggest the lava flows eroded down\ninto and were emplaced within wet, unconsolidated, near-seafloor sediments.\nExtrusion dynamics were likely controlled by low magma viscosities as a\nresult of increased dissolved H2O due to high hydrostatic pressure and\nsoft, near-seabed sediments, which are collectively characteristic of\ndeepwater environments. We calculate that long-runout lava flows account\nfor 50\u2009%–97\u2009% of the total erupted volume, with a surprisingly minor\ncomponent ( ∼3 \u2009%–50\u2009%) being preserved in the main volcanic\nedifice. Accurate estimates of erupted volumes therefore require knowledge\nof volcano and lava basal surface morphology. We conclude that 3-D seismic\nreflection data are a powerful tool for constraining the geometry, volumes,\nand extrusion dynamics of ancient or active deepwater volcanoes and lava\nflows.