For many years, volcanic and igneous processes were explained by the storage of magma in a melt-rich magma body, where chemical differentiation is controlled by fractional crystallisation. However, this idea for the storage and chemical differentiation of magma in the Earth’s crust has been challenged by data from natural magmatic systems. Evidence suggests that melt-rich magma bodies are most likely a transient feature, and it is now thought that magma storage occurs in a vertically extensive magmatic system, mainly comprising of high-crystallinity ‘mush’. Mush is defined as closely packed crystals which create a crystalline framework, where melt survives within the pore space. A recent numerical model showed how a high-crystallinity magma reservoir can form in response to the intrusion of parental magma. The reservoir mainly consists of mush, but forms low-crystallinity evolved magma at the top of the reservoir, with chemical differentiation being mainly controlled by reactive melt flow. In this thesis, the recent numerical model for the formation and storage of magma in the crust is extended to investigate observed behaviours of magmatic systems unexplained through the context of high-crystallinity magma reservoirs. These behaviours include: the eruption of large volumes of low-crystallinity magma, the formation and development of a vertically extensive magmatic system and the presence of multiple heat sources in the crust to create and grow the reservoir. Results from the extended model suggest the size, frequency, and composition of eruptions to be controlled by the buoyancy-driven evacuation of magma from a source magma reservoir. The numerical model also shows the magma to be stored as discrete mush bodies in a vertically extensive magmatic system, rather than a transcrustal mush; with the chemical diversity of the system being dependent on whether partial melting of the crust occurs. Furthermore, the presence of multiple heat sources in the model reduces the magma flux needed to form a magma reservoir and recreates long-lived magma storage observed in natural systems and not currently reproduced by numerical magmatic models.