Energetics of Surtseyan Eruptions: Insights From the Fluid Dynamics and Phenomenology of Underwater Chemical Explosions

Abstract:

Perhaps the most distinctive feature of Surtseyan eruptions is the “tephra jet”. The subaerial eruptive column comprises an aggregate of such jets, commonly with a volcanic bomb at the tip moving on a ballistic trajectory. Yet “tephra jet” seems an odd name for a phenomenon that was first described as follows in relation to the 1952 Myojinsho eruption:

The sea surface…suddenly domed up nearly to 20 m in height...A number of blocks followed by dark water tails, mixtures of water and rock fragments, broke through the crest of the dome like plumes. These numerous plumes of dark mixture of water and rocks were ejected to the heights of 400 m or more….(Morimoto & Ossaka, 1955, Bull. Earthq. Res. Inst. Tokyo Univ., vol. 33, pp. 221–250).

It is implausible that trails of water tens to hundreds of m long can be dragged in the wake of a volcanic bomb as it passes through the water surface. Moreover, it is easy to show that an individual volcanic bomb ejected from an underwater vent cannot pass through water and continue on a subaerial ballistic trajectory unless hydrodynamic drag is radically reduced from normal by presence of a continuous vapor film. Yet such a film tends to be unstable and collapse. Nor is a tephra jet likely to arise from a submarine vapor/pyroclast jet except for very shallow eruptive vents, as the vapor will condense. As an alternate explanation, I propose that Surtseyan eruption phenomena be interpreted by analogy with observations of the effects of detonating an underwater explosive (UNDEX). In this context, watery tephra jets are understood as hydrodynamic phenomena associated with discrete phreatomagmatic explosions, and UNDEX physics and phenomenology are used to make inferences about the energetics of Surtseyan explosions. Data for tephra-jet characteristics from subaqueous basaltic eruptions can be brought into agreement with UNDEX data only if the enthalpy release during the volcanic explosions is at most a few percent of the available enthalpy of the erupted magma. Comparable “inefficiency” has been found in a range of experiments on molten fuel/coolant interactions. UNDEX studies also provide a framework for understanding some perplexing sedimentological aspects of explosive subaqueous eruptions, such as the presence of clasts that seem to have been deposited subaqueously in a dry state.