SIMULATING THE UNSEEN – ANALOGUE EXPERIMENTS ILLUMINATE SUBMARINE ERUPTIVE PROCESSES

Abstract:

Subaerial volcanic eruptions are among Earth's most spectacular events. Tephra plumes, lightning, and ash clouds down mountainsides are visible from far away, and with foresight and caution much reliable information can be extracted from direct observation and measurements. This information can underpin physical analysis of eruptive processes. In contrast, the vast majority of submarine eruptions occur unseen, not only because there are no observers at the site, but also because water's optical properties will always prevent visual observation from large distances. Water is much more dense and viscous than air, with much greater ability to absorb thermal energy, so we cannot simply "scale" subaerial eruptive phenomena into the submarine environment by changing pressure terms. To visualise, and to properly parameterise, processes of submarine eruptions, analogue experiments are needed. These will range from exploratory '1^st order' investigations to dynamically scaled ones yielding quantitative information for modelling. Major phenomena of explosive submarine eruptions to be investigated are the interaction of magma with water, the behaviour of submarine explosions and jets, and the evolution of initially hot density currents shed from an eruption. (1) Subaqueous magma-water interaction has so far been addressed by pouring basaltic melt into water, injecting water into melt, by explosively expelling magma into water-sprayed chambers, and by gas-driven fragmentation of water-flooded rhyolite. (2) Submarine explosions have the growth and collapse of gas bubbles as a fundamental behaviour. More volcano-similar preliminary experiments using gas to eject particles upward into the water column confirm the importance of opening and collapse, and provide a first glimpse of how this controls release of erupted particles and their deposition around the eruption site. Future work will use water vapour as the driving gas to investigate condensation effects, and should also develop submarine particle-laden jets to investigate behaviour of sustained eruptions. (3) Effects of eruptive heat on aqueous currents generated from submarine eruptions include reduction in current velocity and runout for hot density currents, and segregation of fines into buoyant plumes while bedload is deposited nearer source.