Diffusion is due to thermally activated atomic-scale random motion of particles (atoms, ions and molecules) in minerals, glasses, melts, fluids, and gases (Fig. 11). The random motion leads to a net flux when the concentration (more strictly speaking, the chemical potential) of a component is not uniform. Even though diffusion is a microscopic process, it can lead to macroscopic effects. For example, the initial phase of explosive volcanic eruptions (or more commonly encountered champagne eruptions) is powered by bubble growth, which in turn is controlled by diffusion that brings gas molecules into bubbles. This chapter provides a brief...

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