Havre 2012 Submarine Eruption: The Role of Hydrostatic Pressure on the Morphology of a Silicic Lava Flow

Abstract:

The challenge of the undersea environment limits research on submarine rhyolitic lavas. In particular, research on the influence of hydrostatic pressure on magma properties, lava rheology and morphology. The 2012 submarine eruption of Havre volcano, Kermadec Arc New Zealand, included the effusion of 14 lavas along caldera wall fault structures. Lava morphologies, volumes and effusion rates can be calculated from pre- and post-eruption seafloor stratigraphic relationships. One of the lavas, Lava G, has distinct morphological features such as pressure ridges with amplitudes reaching 2.5m that suggest a lower viscosity for the lava than would be expected. Knowledge of eruption depths permit the relationship between hydrostatic pressure with volatile degassing, lava viscosity and morphology to be explored. Analysis of microlitic textures and abundancies within the five samples through SEM confirmed post-cooling wall collapse on the northern margin of Lava G. We observed an unusual texture in two samples where large bubbles were rimmed by microlites separated by void space. We suggest two theories of formation – viscosity variances between the melt and the microlites, and mechanical fracturing during lava emplacement. We present FTIR data from the carapace of a single glassy lava sample erupted at 900 mbsl to determine H₂O volatile contents and speciation. Total H₂O contents are higher than those of published subaerial rhyolitic lavas (Pinkerton & Wilson, 1993), averaging 0.9 wt.%. Incorporating this result into Giordano’s equation (Giordano et al., 2008) for silicic lava viscosities we calculated a viscosity comparable to a subaerial-erupted andesitic lava of between log₁₀ 10⁷ and 10⁹ Pa s, potentially contributing to Lava G’s morphological and textural characteristics.