Abstract
Magma from Plinian volcanic eruptions contains an extraordinarily large numbers of bubbles. Nucleation of those bubbles occurs because pressure decreases as magma rises to the surface. As a consequence, dissolved magmatic volatiles, such as water, become supersaturated and cause bubbles to nucleate. At the same time, diffusion of volatiles into existing bubbles reduces supersaturation, resulting in a dynamical feedback between rates of nucleation due to magma decompression and volatile diffusion. Because nucleation rate increases with supersaturation, bubble number density (BND) provides a proxy record of decompression rate, and hence the intensity of eruption dynamics. Using numerical modeling of bubble nucleation, we reconcile a long-standing discrepancy in decompression rate estimated from BND and independent geospeedometers. We demonstrate that BND provides a record of the time-averaged decompression rate that is consistent with independent geospeedometers, if bubble nucleation is heterogeneous and facilitated by magnetite crystals.
Highlights
Magma from Plinian volcanic eruptions contains an extraordinarily large numbers of bubbles
By accounting for the aforementioned time-dependent feedback during magma decompression, we find that the timeaveraged decompression rates estimated from bubble number density (BND) under heterogeneous nucleation are consistent with independent geospeedometers
Bubble nucleation is the formation of molecular clusters that are larger than a critical size and stable to grow into bubbles
Summary
Magma from Plinian volcanic eruptions contains an extraordinarily large numbers of bubbles. Bubble number density provides a record of this feedback Reconstruction of these processes and reliable estimation of magma decompression rate require quantitative models of bubble nucleation that are calibrated with experiments. The conventional estimates of decompression rate based on homogeneous nucleation are unrealistically high (~100 MPa s−1), do not correlate with magma discharge rate, and are substantially greater than estimates from independent geospeedometers, which are ≤1 MPa s−1 (Fig. 1c). To resolve these discrepancies, Shea[11] hypothesized that bubble nucleation is perhaps heterogeneous and facilitated by abundant pre-existing crystals. Helens 1912 Novarupta 1991 Pinatubo 1.8 ka Taupo (b) No overlap 1016
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