H 2 O-content and temperature limit the explosive potential of rhyolite magma during Plinian eruptions

FORTE, PABLO; CASTRO, JONATHAN M.

EARTH AND PLANETARY SCIENCE LETTERS

ELSEVIER SCIENCE BV

Año: 2019 vol. 506 p. 157 - 167

0012-821X

Recent rhyolite eruptions on Earth have demonstrated their capacity to produce a multitude of hazards, including ash formation lasting months and impacting the large reaches of the southern hemisphere. Nevertheless, the underlying mechanisms driving these eruptions are not yet fully understood. Magmatic volatiles, especially H 2 O, dictate whether volcanic eruptions proceed explosively or effusively. Experimental evidence for the role played by H 2 O in driving explosive fragmentation is rare, in particular in the eruption of rhyolitic magma. Here we show that when hydrous rhyolitic obsidians from Chaitén Volcano (Chile) are experimentally heated above their glass transition temperatures at ambient 1 atm-conditions, two different behaviors result, depending on starting H 2 O concentration and temperature: obsidians vesiculate to stable or quasi-steady state foams when H 2 O is ≤1 wt.% for a wide range of temperatures (728-1032 °C), but will explode within just tens of seconds (874 °C. Explosive activity occurs above Chaitén´s estimated eruption temperature but, for lower temperatures, only foaming occurs. Whether a foaming sample remains coherent or explodes depends on two interrelated factors, the Peclet number (Pe), a dimensionless ratio of diffusive and viscous timescales, and the timescale or rate of decompression, which is dictated in part by the H 2 O-vapor pressure gradient between bubbles and atmosphere. At or above 1.4 wt.% H 2 O, and for a range of permissible Chaitén eruption temperatures (∼780-825 °C), Pe is large (>10), meaning viscous deformation aiding vapor expansion is the dominant mode of growing bubbles. Consequently, vesiculation can proceed rapidly due to high initial overpressure and low melt viscosity, until the point is reached that the melt deforms at a rate greater than its relaxation rate, resulting in fragmentation. Below 1.4 wt.% H 2 O and for temperatures equal to or lower than Chaitén´s eruption, decompression and deformation rates, inferred from experimental clast expansion and bubble growth behavior, are at least an order of magnitude smaller than the explosive cases, which is insufficient for critical melt-rupturing. Pe estimations highlight the first order influence of temperature and H 2 O content on physical degassing behavior. Since the starting H 2 O content and temperature dictate many the parameters that govern bubble growth, pressure gradient, melt viscosity, and chemical diffusivity, our experiments shed light on what limits fragmentation in natural eruptions and offer an alternative explanation for H 2 O contents measured in Plinian fall deposits from Chaitén and other rhyolite centers. In essence, pyroclastic obsidian H 2 O contents could reflect temperature and P-H 2 O limitations on fragmentation. Furthermore, since foam expansion under low-P, high-T-H 2 O conditions can foster strain rates in excess of the melt´s relaxation rate and thus drive fragmentation, we contend that, in addition to rapid magma ascent (and high magma supply rates), rapid clast expansion under low pressure conditions may also be important for explosive magma fragmentation.