Abstract
Magmas erupted in deep-water environments (> 500 m) are subject to physical constraints that are very different to those for subaerial eruptions, including hydrostatic pressure, bulk modulus, thermal conductivity, heat capacity and the density of water mass, which are generally orders of magnitude greater than for air. The exsolved volatile content of the erupting magma will generally be lower because magmas decompress to hydrostatic pressures orders of magnitude greater than atmospheric pressure. At water depths and pressures greater than those equivalent to the critical points of H2O and CO2, exsolved volatiles are supercritical fluids, not gas, and so have limited ability to expand, let alone explode. Gas overpressures are lower in deep submarine magmas relative to their subaerial counterparts, limiting the degree of explosive fragmentation. Explosive intensity is further minimised because of the higher bulk modulus of water, relative to air. Higher retention of volatiles makes subaqueously erupted magmas less viscous, and more prone to fire fountaining eruption style compared with compositionally equivalent subaerial counterparts. The high heat capacity and thermal conductivity of (ambient) water makes the effusively (and/or explosively) erupted magmas more prone to rapid cooling and quench fragmentation, producing non-explosive hyaloclastite breccia. Subaqueous eruption columns or plumes form above both explosive and non-explosive eruptions, consisting of heated water, supercritical fluid and gas, and both can entrain pyroclasts and pumice autoclasts upwards. The height of such plumes is limited by the water depth and will show different buoyancy, dynamics, and height and dispersal capacity compared with subaerial eruption columns. Water ingress, condensation and dissolution erosion of gas bubbles will be major factors in controlling column dynamics. Autoclasts and pyroclasts with an initial bulk density less than water can rise buoyantly, irrespective of plume buoyancy. Dispersal and sedimentation of clasts in water is affected by the rate at which buoyant clasts become water-logged and sink, and by wind, waves, and oceanic currents, which can produce very circuitous dispersal patterns in floating pumice rafts. Floating pumice can abrade by frictional interaction with neighbours in a floating raft, and generate in transit, post-eruptive ash fallout unrelated to explosive activity or quench fragmentation.
Highlights
Understanding the physical processes controlling the dynamics, style and intensity of volcanic eruptions has historically relied upon observations of recent subaerial eruptions (e.g., 1980 Mt St Helens, Lipman and Mullineaux, 1981; 1991 Mt Pinatubo, Newhall and Punongbayan, 1996; 1982 – present Hawai’i, Poland et al, 2014; Soufriere Hills, Montserrat, 1995–2012, Druitt and Kokelaar, 2002) and well-preserved subaerial eruption deposits (e.g., Askja, Carey et al, 2010; Colli Albano, Giordano et al, 2010; Vesuvius, Shea et al, 2011; Santorini, Druitt et al, 1999; Simmons et al, 2016; Tenerife, Marti and Geyer 2009, Edgar et al, 2017)
Using data from Carey et al (2018) and Manga et al (2018) for rhyolite samples erupted at water depths of 900 m at the wholly submarine Havre volcano in the Kermadec arc, Southwest Pacific in 2012 (9 MPa confining hydrostatic pressure; pre-eruptive water content of 5.8 wt%; eruption temperature of ∼850◦C), we calculate that the residual water content after
This review has focused on scenarios involving gas over-pressures relative to hydrostatic pressures in subaqueous settings
Summary
Understanding the physical processes controlling the dynamics, style and intensity of volcanic eruptions has historically relied upon observations of recent subaerial eruptions (e.g., 1980 Mt St Helens, Lipman and Mullineaux, 1981; 1991 Mt Pinatubo, Newhall and Punongbayan, 1996; 1982 – present Hawai’i, Poland et al, 2014; Soufriere Hills, Montserrat, 1995–2012, Druitt and Kokelaar, 2002) and well-preserved subaerial eruption deposits (e.g., Askja, Carey et al, 2010; Colli Albano, Giordano et al, 2010; Vesuvius, Shea et al, 2011; Santorini, Druitt et al, 1999; Simmons et al, 2016; Tenerife, Marti and Geyer 2009, Edgar et al, 2017).
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