The Effects of Hydrostatic Pressure on the Deep Submarine Silicic Lavas - Havre 2012 Eruption

Abstract:

Since the depth-dependent variation in vesicularity was reported for submarine basaltic lava in Hawaii (Moore, 1965), many authors have argued that deep submarine lavas might show increased mobility as the large hydrostatic pressure suppresses the exsolution of the H₂O (Jones, 1969; Cas, 1978) which keeps the viscosity of the melt low (Wysoczanski and Tani, 2006; Giordano et al. 2008). However, the idea has not been tested on a modern silicic lava because few submarine silicic lavas have been identified and sampled.

The Havre 2012 eruption was the first deep submarine silicic eruption ever recorded (Carey et al. 2014; Jutzeler et al. 2014). The high-resolution map and seafloor observations obtained by the MESH cruise (Soule and Carey, 2015) revealed that the eruption produced fourteen silicic lavas of 0.24±0.08 km³ in total volume along the southern rim of the Havre caldera. Differences in lava volume (<0.001~0.12 km³), length (0.002~1.35 km), composition (69~71 wt.% SiO₂), morphology (domes or channelized lobes), and depth (1500 ~ 900 mbsl) provide an excellent opportunity to investigate the influence of environment vs magma properties on lava morphology in the deep submarine setting.

Our morphological analysis on 1-m gridded bathymetry suggests that the lobate lavas have the multiple levee structures which allow calculation of the apparent yield strength of the lava flow approximating it to a Bingham fluid (Hulme, 1974). The initial calculations for lava lobes emplaced on a gentle slope (< 10°) demonstrate yield strength of <10⁵ Pa which is comparable to lavas of andesitic, or even basaltic-andesitic composition. This calculation together with measurements of 0.9 wt.% residual H₂O in glass derived from FTIR (Fourier Transform Infrared Spectroscopy) analysis on a single lava (Mundana, 2016) supports the hypothesis that hydrostatic pressure can influence the viscosity (and hence morphology) of submarine lavas through the suppression of volatile exsolution.