Ultra-rapid formation of large volumes of evolved magma

We discuss evidence for, and evaluate the consequences of, the growth of magma reservoirs by small increments of thin (⋍ 1–2 m) sills. For such thin units, cooling proceeds faster than the nucleation and growth of crystals, which only allows a small amount of crystallization and leads to the formation of large quantities of glass. The heat balance equation for kinetic-controlled crystallization is solved numerically for a range of sill thicknesses, magma injection rates and crustal emplacement depths. Successive injections lead to the accumulation of poorly crystallized chilled magma with the properties of a solid. Temperatures increase gradually with each injection until they become large enough to allow a late phase of crystal nucleation and growth. Crystallization and latent heat release work in a positive feedback loop, leading to catastrophic heating of the magma pile, typically by 200 °C in a few decades. Large volumes of evolved melt are made available in a short time. The time for the catastrophic heating event varies as Q− 2, where Q is the average magma injection rate, and takes values in a range of 105–106 yr for typical geological magma production rates. With this mechanism, storage of large quantities of magma beneath an active volcanic center may escape detection by seismic methods.