INTRODUCTION

Once magma has been intruded or extruded, it begins to cool and crystallize. To cool, the magma must lose heat to its surroundings, which it does mainly by conduction. Conduction can be thought of as the diffusion of heat. The loss of heat from the magma causes solidification and crystallization of the magma and the growth of metamorphic minerals in rocks that are heated by the magma. The growth of crystals in the magma and in the metamorphic rocks first involves the formation of nuclei, and then the components needed for crystal growth must diffuse to these nuclei. This occurs through the liquid in the case of magma and through solid rock or films of grain-boundary fluid in the case of metamorphic rocks. As magma crystallizes, the composition of the residual liquid is continuously changing and the composition of crystals must change their composition if they are to remain in equilibrium with the melt. For early formed phenocrysts to adjust their composition, components need to diffuse in and out of the crystals. As the temperature of metamorphic rocks changes, the minerals must adjust their compositions to remain in equilibrium and this again involves diffusion. Diffusion, consequently, is one of the most important processes in the formation of rocks.

To appreciate diffusion processes, it is necessary to understand some simple mathematical relations. We will develop these relations by discussing the cooling of igneous bodies. The same mathematical relations will then be applied to the diffusion of chemical components.