A four-dimensional X-ray tomographic microscopystudy of bubble growth in basaltic foam

摘要

Understanding the influence of bubble foams on magma permeability and strength is criticalto investigations of volcanic eruption mechanisms. Increasing foam porosity decreasesstrength, enhancing the probability of an eruption. However, higher porosities lead to largerpermeabilities, which can lessen the eruption hazard. Here we measure bubble size and wallthickness distributions, as well as connectivity, and calculate permeabilities and tensile strengthsof basaltic foams imaged by synchrotron X-ray tomographic microscopy during bubble growthin hydrated basaltic melts. Rapid vesiculation produces porous foams whose fragmentationthresholds are only 5–6 mPa and whose permeabilities increase from approximately 1×10~(-10)to 1×10~(-9)m~2between 10 and 14s despite decreasing connectivity between bubbles. These resultsindicate that basaltic magmas are most susceptible to failure immediately upon vesiculationand at later times, perhaps only 10’s of seconds later, permeability increases may lessen thehazard of explosive, basaltic, Plinian eruptions.