Volcanic eruptions are driven by bubbles that form when volatile species exsolve from magma. The conditions under which bubbles form depend mainly on magma composition, volatile concentration, presence of crystals, and magma decompression rate. These are all predicated on the mechanism by which volatiles exsolve from the melt to form bubbles. We critically review the known or inferred mechanisms of bubble formation in magmas: homogeneous nucleation, heterogeneous nucleation on crystal surfaces, and spontaneous phase separation (spinodal decomposition). We propose a general approach for calculating bubble nucleation rates as the sum of the contributions from homogeneous and heterogeneous nucleation, suggesting that nucleation may not be limited to a single mechanism prior to eruption. We identify three major challenges in which further experimental, analytical, and theoretical work is required to permit the development of a general model for bubble formation under natural eruption conditions. ▪ We review the mechanisms of bubble formation in magma and summarize the conditions under which the various mechanisms are understood to operate. ▪ Bubble formation mechanisms may evolve throughout magma ascent as conditions change such that bubbles may form simultaneously and sequentially via more than one mechanism. ▪ Contributions from both homogeneous nucleation and heterogeneous nucleation on multiphase crystal phases can be captured via a single equation. ▪ Future work should focus on constraining macroscopic surface tension, characterizing the microphysics, and developing a general framework for modeling bubble formation, via all mechanisms, over natural magma ascent pathways.

中文翻译：

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岩浆中气泡的形成

火山喷发是由挥发性物质从岩浆中溶解时形成的气泡驱动的。气泡形成的条件主要取决于岩浆成分、挥发物浓度、晶体的存在和岩浆减压速率。这些都是基于挥发物从熔体中溶出形成气泡的机制。我们批判性地回顾了岩浆中气泡形成的已知或推断机制：均质成核、晶体表面上的异质成核和自发相分离（旋节线分解）。我们提出了一种计算气泡成核率的通用方法，将其作为均质和异质成核贡献的总和，这表明成核可能不限于喷发前的单一机制。我们确定了三个主要挑战，需要进一步的实验、分析和理论工作，以开发自然喷发条件下气泡形成的通用模型。 ▪ 我们回顾了岩浆中气泡形成的机制，并总结了各种机制运作的条件。 ▪ 随着条件的变化，气泡形成机制可能会在岩浆上升过程中演变，使得气泡可能通过不止一种机制同时且依次形成。 ▪ 均相成核和异相成核对多相晶相的贡献可以通过单个方程获得。 ▪ 未来的工作应侧重于约束宏观表面张力、表征微观物理，并开发一个通过所有机制、在自然岩浆上升路径上模拟气泡形成的通用框架。