Why Are Andesitic Eruptions Often More Violent Than Basaltic Ones? -- Insights from 4D X-ray Tomographic Microscopy

Abstract:

Basaltic eruptions often are relatively quiescent, producing lava flows and small strombolian eruptions. On the other hand, andesitic eruptions often are much more violent, sometimes plinian. In both cases the eruptions are typically driven by the exsolution of volatiles, primarily H₂O, CO₂, S and halogens. We seek to understand the origin of these differences in the “average” eruption styles of basaltic and more-silicic magmas. We hypothesize that the topological properties of bubble foams created during vesiculation play an important role in controlling the explosivity of eruptions. As the bubble fraction increases during vesiculation a competition develops between the falling strength of the magma and its increasing permeability. If the permeability increases sufficiently to release trapped magmatic gases before the fragmentation threshold is reached the eruption will be quiescent, whereas if the permeability does not reach a critical threshold value the magma can fragment, producing a violent eruption. To test this hypothesis, we performed 4-D (x, y, z, and time) X-ray microtomography on the TOMCAT beamline at the Swiss Light Source during 1-atm. vesiculation of previously hydrated melts of basaltic, andesitic and dacitic compositions at temperatures reaching 1200 °C. We find that bubble inter-connectivity is significantly higher in basaltic melts than in andesitic and dacitic melts. This higher inter-connectivity in basaltic systems results in higher permeabilities than in andesitic magmas at similar porosities and reduces the probability of violent basaltic eruptions. We attribute the higher bubble inter-connectivity in basaltic melts to their low viscosities, which in some cases are orders-of-magnitude below those of more-silicic melts. We suggest that characterization of the inter-connectivity of a volcano's previous eruptive products (e.g., scoria, pumice, etc.) may provide insight into the explosivity of its future eruptions.