Controls on lava morphology and volcano growth in submarine and subglacial environments

Submarine and subglacial lava flows share common morphological characteristics derived from underwater eruption. Rapid cooling in water produces a wide range of lava morphologies that provide a remarkably good record of emplacement style and corresponding eruption dynamics. Submarine lava morphology has been shown to be sensitive to changes in local lava flow rate resulting from a combination of vent effusion rate, seafloor slope, and lava viscosity. Subaqueous flows that are slow-moving or efficiently cooled tend to form nearly spherical pillow lavas, while faster flows with less efficient cooling become flatter thus tending to produce a higher proportion of lobate and sheet flows. Recent advances in mapping submarine lava morphology at a regional-scale by applying remote-sensing methods to high-resolution sonar data allow us to infer the relative flow rates within individual eruptive units as well as understanding regional emplacement style from comprehensive maps of lava morphology for entire ridge segments. Recent studies at the hotspot-influenced Galapagos Spreading Center reveal a strong correlation between long-term magma supply and lava surface morphology. Eruptions at lower magma supply predominantly form pillow lavas, which tend to build steep-sided mounds and ridgelines. With increasing magma supply, lava morphologies tend toward higher effusion rate lobates and sheets, producing low-relief flow fields and smoother ridge topography. This observation seems to hold irrespective of spreading rate, and is associated with a shift in eruptive style from focused point-source eruptions in low magma supply areas to dominantly fissure-fed eruptions at higher magma supply. Complications arise in subglacial and shallow submarine environments, where low confining pressures result in phreatomagmatic activity and produce hyaloclastic deposits. Variable water/magma ratios during subglacial eruption can cause abrupt changes between effusive and explosive activity, often producing more complex lithological sequences than observed in deep submarine environments.. Nevertheless, fundamental relationships between volcano growth and magma supply seem to hold for both environments. We compare submarine and subglacial volcanic styles from seamounts and tuyas to sheet flows and moberg sheets, their lava morphology, and relationship to long-term magma supply.