| Phreatomagmatic eruptions occur when rising magma mixes with shallow surface water or groundwater to produce highly explosive eruptions. The prevalence of water in the crust and the hazards of such eruptions offer compelling reasons to understand their dynamics and range of possible behaviors. Detailed field studies at Narbona Pass Maar (NPM), New Mexico, and the Table Rock Complex (TRC), Oregon, revealed the eruptive dynamics and emplacement mechanisms for each field site. These are compared to previous studies to elucidate groups of depositional characteristics that can be used to infer relative water-to-magma ratios and explosivity, and to explore the conditions that produce the range of activity in mafic hydrovolcanic eruptions (MHE).;The combination of fine-grained ash and lack of evidence for liquid water at the time of deposition suggests that the external water is effectively vaporized at the site of magma-water interaction, and the melt efficiently fragmented to produce a highly explosive eruption, as was the case at NPM. In contrast, evidence for abundant liquid water at the time of deposition typically indicates excess water at the location of magma-water interaction, producing a less efficient conversion of energy, and thus less efficient fragmentation and a less explosive eruption than the former scenario, as was the case for the tuff cone eruption at TRC. Dynamic flow conditions for base-surge currents produced during the maar-style eruption at TRC were quantified using depositional features, and indicate initial surge velocities up to 110 m/s, and corresponding column collapse heights up to 4 km. These values significantly exceed what has been previously documented for eruptions of this type, and expand the range of conditions that can be expected for MHE. In this eruption, it is likely that the external water was efficiently vaporized, but later condensed during rise and collapse in the eruption column to produce "wet" deposits.;Conditions that control the explosivity of a MHE are a function of the availability of external water and confining pressures, which are strongly controlled by subsurface geology, magma composition and mass flux, and the influence of magmatic volatiles. The eruption of NPM is a particularly well-documented case which emphasizes both the controls of subsurface geology and the role of magmatic volatiles. This work improves the ability of volcanologists to interpret the deposits of MHE, and provides a better understanding of eruption mechanisms, pyroclastic emplacement dynamics, and potential hazards. |
