Crystals Connect Bubbles in Explosive Magmas

Hydrous silica-rich magmas can degas through connected bubble pathways when as little as 20% crystals are present, influencing transitions from explosive, Vulcanian-style eruptions to lava effusion.

SOURCE: Geophysical Research Letters

X-ray tomography images of experimental magmas captured at different crystal contents (courtesy of the University of Texas High-Resolution X-Ray Computed Tomography Facility). Greyscale contrast shows how increasing crystal content (light grey and white shades) increases coalescence of bubbles (vesicles, black) in the magma (light grey, here quenched to glass). Credit: deGraffenried et al. [2019], Figure 2a

Volcanic eruptions of hydrous and silica-rich (intermediate-felsic) magmas can cycle between Vulcanian-style explosions, lava dome extrusions, and dome collapse. The processes through which magmas degas during ascent depend on many factors, but the connectivity of bubble pathways ultimately controls how rapidly pressurized gas escapes. The critical threshold of gas permeability (the percolation threshold) in ascending intermediate to felsic magmas can be influenced by mineral crystallization.

Through laboratory experiments on crystal-bearing synthetic magmas, deGraffenried et al. [2019] show that at as little as 20 per cent crystallization can reduce the percolation threshold to just over 50 per cent porosity of the magma volume, creating connected bubble pathways that enhance degassing. That lowered threshold likely plays an important role in transitions between effusive and explosive eruption styles in hydrous, crystal-bearing magmas.

Citation: deGraffenried, R. L., Larsen, J. F., Graham, N. A., & Cashman, K. V. [2019]. The influence of phenocrysts on degassing in crystal‐bearing magmas with rhyolitic groundmass melts. Geophysical Research Letters, 46. https://doi.org/10.1029/2018GL081822

—Steven Jacobsen, Editor, Geophysical Research Letters

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