What is the process called when magma reacts with the surrounding wall rock?

The process of magma reacting with the surrounding wall rock is known as assimilation. This phenomenon occurs when the heat from the magma causes the surrounding rock to melt partially or fully, and the resulting materials are incorporated into the magma. This interaction can lead to changes in the composition of the magma itself and is significant in the petrology of igneous rocks. As the surrounding wall rock breaks down and mixes with the magma, it alters the chemical and mineralogical characteristics of the molten rock, which can influence the types of minerals that crystallize as the magma cools and solidifies.

Fusion pertains to the melting of solid rock to form magma, but it does not involve the interaction with surrounding materials. Differentiation refers to the process by which different mineral phases separate from a magma as it cools, and incorporation generally relates to the inclusion of solid materials that could be pieces of rock within the magma. These terms describe different aspects of igneous processes but do not capture the specific interaction between magma and wall rock that assimilation does.