Thermal Constraints on the Longevity, Depth, and Vertical Extent of Magmatic Systems

Magmatic regions are typically viewed as vertical systems, with magma coming in the bottom and either freezing in the crust to form plutons or erupting to form volcanic rocks. Maintaining an upper‐crustal magmatic system for 10 5 –10 6  Ma timescales requires establishing and maintaining a strong perturbation in the geotherm via continued input of magma from below. Thermal models indicate that establishing such a system requires the energy content of a vertical stack of magma tens of km thick, and maintaining the system requires continual supply of tens of km per million years. These rates of magma addition into the upper crust would require rapid crustal thickening and surface uplift, neither of which are observed at the required scale. Mass and energy balance issues are ameliorated if large magmatic systems are primarily lower‐crustal phenomena without long‐term upper‐crustal storage and if horizontal space‐making processes (extension), rather than vertical inflation, focus magmatism, and induce decompression melting.