Dynamic observations of vesiculation reveal the role of silicate crystals in bubble nucleation and growth in andesitic magmas

Bubble nucleation and growth control the explosivity of volcanic eruptions, and the kinetics of these processes are generally determined from examinations of natural samples and quenched experimental run products. These samples, however, only provide a view of the final state, from which the initial conditions of a time-evolving magmatic system are then inferred. The interpretations that follow are inexact due to the inability of determining the exact conditions of nucleation and the potential detachment of bubbles from their nucleation sites, an uncertainty that can obscure their nucleation location \u2013 either homogeneously within the melt or heterogeneously at the interface between crystals and melts. We present results of a series of dynamic, real-time 4D X-ray tomographic microscopy experiments where we observed the development of bubbles in crystal bearing silicate magmas. Experimentally synthesized andesitic glasses with 0.25\u20130.5 wt% H2O and seed silicate crystals were heated at 1 atm to induce bubble nucleation and track bubble growth and movement. In contrast to previous studies on natural and experimentally produced samples, we found that bubbles readily nucleated on plagioclase and clinopyroxene crystals, that their contact angle changes during growth and that they can grow to sizes many times that of the silicate on whose surface they originated. The rapid heterogeneous nucleation of bubbles at low degrees of supersaturation in the presence of silicate crystals demonstrates that silicates can affect when vesiculation ensues, influencing subsequent permeability development and effusive vs. explosive transition in volcanic eruptions

Controls on explosive-effusive volcanic eruption styles

One of the biggest challenges in volcanic hazard assessment is to understand how and why eruptive style changes within the same eruptive period or even from one eruption to the next at a given volcano. This review evaluates the competing processes that lead to explosive and effusive eruptions of silicic magmas. Eruptive style depends on a set of feedbacks involving interrelated magmatic properties and processes. Foremost of these are magma viscosity, gas loss, and external properties such as conduit geometry. Ultimately, these parameters control the speed at which magmas ascend, decompress and outgas en route to the surface, and thus determine eruptive style and evolution

Thermochemical dynamics of magma chambers: a simple model

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The Effect of Ascent Rate on the Kinetics of Bubble Nucleation in a Rhyolitic Melt.

In order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma, we performed three series of experiments decompressed at 1000, 167, or 27.8 kPa/s. The experiments were carried out in an externally heated pressure vessel at 800°C; the starting material was a crystal-free and bubble-free rhyolitic glass containing 7.0 wt.% H2O. In the three decompression series, homogeneous bubble nucleation began at 90 MPa, that is, ≡ 150 MPa below the water saturation pressure of the silicate liquid, 240 MPa. After a short nucleation event, the nucleation rate dropped and the bubble number density N reached a stationary value that was strongly sensitive to decompression rate: 6.8 mm 3 at 27.8 kPa/s, 470 mm 3 at 167 kPa/s, and 5800 mm 3 at 1000 kPa/s. This behaviour was dictated by a competition between nucleation and diffusive bubble growth, which depleted in water the surrounding liquid and so reduced the degree of volatile supersaturation. With increasing N, the length scale of diffusion of water molecules to growing bubbles decreased: nucleation stopped when N attained a critical value, at which the degree of volatile supersaturation in the liquid was everywhere below the value required for nucleation and no longer increased with decreasing pressure. The strong correlation between bubble number density and decompression rate has fundamental volcanological implications. If we extrapolate the experimental data to the typical ascent rates of silicic magmas, we obtain bubble number densities that are orders of magnitude smaller than those measured in most natural pumices. We propose that the large values of N in silicic pumices may be due to two successive nucleation events: (1) a first event, which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles; and (2) a second nucleation event, yielding a very large number of small bubbles, and presumably related to the dramatic increase of decompression rate that precedes fragmentation