Mid-Ocean-Ridge Rhyolite (Morr) Eruptions On The East Pacific Rise Lack The Fizz To Pop

eruptions is the subject of ongoing debate due to limited steam expansion and volatile exsolution in magma under high pressure (Schipper and White, 2010; Rotella et al., 2013). If deep-marine volcanism can be explosive despite these limitations, understanding the dynamics of magma ascent and volatile degassing may influence models of eruptions at atmospheric pressure (Cashman and Sparks, 2013). Here, we determine the eruption styles of the only known deep-marine mid-ocean-ridge rhyolite (MORR) and the accompanying intermediate to mafic volcanic suite to evaluate the potentialEruptions on the Alarcon Rise segment of the northern East Pacific Rise (23.55?N, 108.42?W) at 2500?2200 m below sea level (mbsl) produced the most compositionally diverse volcanic suite found along the submarine mid-ocean-ridge (MOR) system, offering an opportunity to compare mafic through silicic eruption styles at the same abyssal depth. Eruption styles that formed evolved volcanic rocks on the submarine MOR have not been studied in detail. The prevalence of lava flows along the MOR indicates that most eruptions are nonexplosive, but some volcaniclastic characteristics suggest that explosive styles also occur. Higher viscosities in intermediate (103?5 Pa?s) versus mafic (101 Pa?s) lavas on Alarcon Rise correspond with larger, more brecciated pillows, while highly viscous rhyolite lavas (106?7 Pa?s) formed rugged domes mostly composed of autoclastic breccia. Although high H2O contents (1.5?2.1 wt%), abundant volcaniclasts, and vesicularities up to 53% in rhyolite might imply eruption explosivity, limited fine-grained ash production and dispersal indicate an effusive origin. Higher viscosities of MOR rhyolite (MORR) magma and small eruption volumes, compared to MOR basalt (MORB), limit bubble coalescence and rapid magma ascent, two likely prerequisites for deep-marine eruption explosivity. This idea is supported by widespread dispersal of basaltic ash, but very limited production and dispersal of silicic ash on Alarcon Rise.