| Summary: | Abstract Bubbles in magmas drive explosive volcanic eruptions. The spatial distribution of bubble nucleation sites in an ascending, decompressing, and supersaturating magma is one of the primary controls on ash morphologies and volcanic hazards. The mechanism of bubble formation is important because it ultimately determines the spatial distribution of bubbles in the fragmenting magma. The initial nucleation of bubbles in a homogeneous magma is problematical because excessive surface tension pressure in very small, nascent bubbles should drive exsolved volatiles back into the melt. This thermodynamic barrier to bubble viability confounds understanding of homogeneous bubble nucleation, yet very small bubbles form, grow, and ultimately drive explosive volcanic eruptions. We refer to this as “the tiny bubble paradox.” Classical nucleation theory typically explains bubble formation and growth, but we propose that a spectrum of bubble‐forming mechanisms may include both homogeneous nucleation and spinodal decomposition (the spontaneous unmixing of phases by uphill diffusion) as end‐member processes. As spinodal decomposition progresses, regularly sized and regularly spaced quasi‐spherical zones form with increasingly high concentration of dissolved water at the centers. Bubble formation occurs as the concentration of water in the interior of the water‐rich zones approaches 100% and the concentration of melt approaches zero. The presence of a broad, diffuse, concentration gradient of water rather than a narrow water‐melt interface means that there is no surface, per se, for surface tension to arise. This is the crux of the solution of the tiny bubble paradox.
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