We will use pressure as a probe of the network-forming versus network-modifying role of Mg in MgSiO3 glass, one of a family of geophysically important silicate materials that form a significant component of the Earth's mantle. Motivation is provided by the capability of Mg to act as a network former in magnesium silicate glasses, a role that stems from the ability of Mg to form structural motifs with a small Mg-O coordination number of ~4 that can fit into a silicate network with minimal disruption. With increasing pressure, however, Mg will transform into a network-modifying role as the Mg-O coordination number increases. Pressure can therefore be used as a tuneable probe of the network-forming versus network-modifying role of this additive, an issue that is important for understanding the fundamentals of modified network glass-forming materials. The changes to the network topology will be modelled by using molecular dynamics simulations with a polarisable ion model.