Why Deep-Water Eruptions Are So Different From Subaerial Eruptions

Abstract

Magmas erupted in deep-water environments (>500 m) are subject to physical constraints 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. Generally, the exsolved volatile content of the erupting magma will 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 H 2 O and CO 2 , exsolved volatiles are supercritical fluids, not gas, and so have limited ability to expand, let alone explosively. Gas overpressures are lower in deep submarine magmas relative to subaerial counterparts, limiting explosive expansion of gas bubbles to shallower waters. Explosive intensity is further minimized by 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 effusively (and/or explosively) erupted magmas more prone to rapid cooling and quench fragmentation, producing non-explosive hyaloclastite breccia. Gaseous subaqueous eruption columns and hot water plumes form above both explosive and non-explosive eruptions, and these can entrain pyroclasts and pumice autoclasts upward. 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 and condensation 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, which they cannot do in the atmosphere. Dispersal and sedimentation of clasts in water is affected by the rate at which buoyant clasts become waterlogged 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 neighbors in a floating raft, and generate in transit, post-eruptive ash fallout unrelated to explosive activity or quench fragmentation.

Stratigraphy of Architectural Elements of a Buried Monogenetic Volcanic System and Implications for Geoenergy Exploration

“…Some rock-types recovered from Resolution-1 have textures that indicate intense material fragmentation (rocks composed of very-fine broken crystals and glassy shards with platy and cuspate shapes), and deposits that commonly contain limestone and sandstones lithics, potentially sourced from the root zone of the MVS diatremes (Figure 12; Figure 13), which maybe represent of fallout material formed by submarine pyroclastic eruptions in the MVS (Bischoff, 2019). Cas and Simmons (2018) suggest that subaqueous effusive eruptions can produce fallout deposits of ash size autoclastic vitric material, similar to typical deposits of subaqueous pyroclastic eruptions. This autoclastic process could explain the large (ca 5 km) seismically detected limits of our ring plain and overspill wedge architectural elements, without necessarily requiring large explosive eruptions, but cannot explain the pit craters excavated into the PrErS horizon.…”

“…Fiske et al, 1998;Bonadonna et al, 2002;White, 2000;Deardoff et al, 2011;Cas and Giordano, 2014; Di Capua and Groppelli, 2016). Rafts of highly vesiculated pumice can travel long distances carried out by currents (Rotella et al, 2013;Cas and Giordano, 2014;Cas and Simmons, 2018). When saturated in water, these fragments sink and can form deposits with size varying from ash to blocks (e.g.…”

“…). 2 This stratal relationships are commonly observed for deposits formed by explosive eruptions in both subaerial and submarine environments(Cas et al, 1989;White, 1996;White, 2000;Kereszturi and Németh, 2013),3 These textures alone are not diagnose for explosivity, but could also represent submarine auto-brecciation (e.g White and Valentine, 2016;Cas and Simmons, 2018)…”

Submarine giant pumice: a window into the shallow conduit dynamics of a recent silicic eruption

“…Similar techniques have been, and can be, applied to deposits in the submarine environment to reveal the effect of high hydrostatic pressure from the overlying ocean, which provides a fundamental control on the production of pumiceous clasts (Allen et al 2010;Barker et al 2012;Cas and Giordano 2014;Cas and Simmons 2018;Head and Wilson 2003;Jones et al 2018;Murch et al 2019;Rotella et al 2015;Schipper et al 2010aSchipper et al , 2010bSchipper et al , 2010cVon Lichtan et al 2016;White et al 2015). Explosive fragmentation of magma in conduits is thought to occur at very shallow-depth subaqueous vents or at sufficiently high decompression and strain rates (Cashman and Scheu 2015;Manga et al 2018a).…”

Deep sea explosive eruptions may be not so different from subaerial eruptions

“…In addition, the dynamics of submarine explosive eruptions may be highly variable: observations at Mata (Tonga Trench) evidence degassing, lava flow emissions, Vulcanian-and Strombolian-like eruptions from the same vent area 11 . Our knowledge on how the deposits of this type of eruptions vary with the distance from the vent, the amount of gas in the emitted magma, the fragmentation mechanisms, the settling times of the pyroclasts, the height of volcanic plumes, and the traveling distance of the generated volcanoclastic flows is, however, poorly constrained 13,14 . Results from theoretical and analogue models show that jets and plumes can raise several hundreds of meters above vents and are, in principle, able to generate gravity currents [2][3][4][5][6][7][8][9][10][11][12][13][14][15] ; however, the results of these models have not been confirmed from field data, with the possible exception represented by nowadays emerged Precambrian pyroclastic deposits 13 .…”