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.

中文翻译：

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爆炸性火山喷发和旋节线分解：破译微小气泡悖论的另一种方法

岩浆中的气泡驱动着爆发性的火山爆发。上升，减压和过饱和岩浆中气泡成核位点的空间分布是灰分形态和火山灾害的主要控制之一。气泡形成的机制很重要，因为它最终决定了破碎岩浆中气泡的空间分布。气泡在均质岩浆中的初始成核是有问题的，因为在很小的新生气泡中过大的表面张力压力应将溶解的挥发物驱回熔体中。这种对气泡生存能力的热力学障碍混淆了对均匀气泡成核的理解，但很小的气泡会形成，生长并最终推动爆炸性火山喷发。我们将其称为“微小的泡沫悖论。经典的成核理论通常可以解释气泡的形成和生长，但是我们建议，一系列气泡形成机制可能包括均相成核和旋节线分解（上坡扩散使相自发解混）作为最终成员过程。随着旋节线分解的进行，规则大小和规则间隔的准球形区域形成，并且中心的溶解水浓度越来越高。当富水区内部的水浓度接近100％，熔体浓度接近零时，就会形成气泡。水的浓度分布较宽且分散，而不是水与熔体界面较窄，这意味着表面本身并不存在表面张力。这是解决微小泡沫悖论的关键。