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Abstract

Water influences volcanic eruption processes as a magmatic volatile, and in subaqueous and subglacial eruptions, as an ambient medium. Its role depends on its fluid state, which can be supercritical fluid, gas, liquid or solid (ice), and in some cases several of these. Magmatic explosions can occur if the magmatic water volatile content and vapour pressure are high enough to generate expandable gas bubbles. Exsolved fluid bubbles are initially filled with supercritical water, not gas, and will only transform into gas at a depth where the magmastatic pressure is less than the critical pressure (CP) for water at magmatic temperatures. For H₂O the CP is 22 MPa and for dense felsic magma (density ~ 2,200 kg m^-3) a magmastatic pressure of 22 MPa occurs at a depth of ~1,000m in the magma column. However, explosive expansion of gas filled bubbles will only occur when the gas develops an overpressure that exceeds the magmastatic pressure which occurs at magmastatic pressures considerably and less and shallower than the CP, between the exsolution depth and the fragmentation surface in the magma column. During subaqueous eruptions ambient hydrostatic pressure constrains volatile exsolution and the state of superheated ambient water at the interface with erupting magma. At water depths approaching the CP equivalent pressure (2,200 m), magmatic, phreatic and phreatomagmatic explosive eruptions and quench fragmentation are suppressed because exsolved water and superheated water at the magma-water interface are supercritical fluid. Both submarine explosive eruptions as well as submarine quench fragmentation are depth limited.