Abstract
The deposits of large volcanic eruptions provide the sole record of the architecture of magmatic plumbing systems in the moments when large pools of crystal-poor, eruptible magma are present in the crust. It is widely accepted that silicic magmas form by segregation of melt-rich, crystal-poor magma from a crystal-rich source; however, the depths at which segregation takes place and the distribution of the magma within the crust are not well constrained. We present a new approach to calculate pressures at which crystal-poor, eruptible magma is extracted from a crystal-rich source (i.e. mush). We apply the approach to a sequence of large (>50 km3) eruptions from the Taupo Volcanic Zone (TVZ), New Zealand, which were part of a volcanic flare-up. We compare the calculated extraction pressures with pre-eruptive storage pressures for the same units. Our results show that storage and extraction pressures do not always coincide. Instead, eruptible magma can be completely segregated from the crystal-rich source, and stored at shallower levels in the crust prior to eruption. In the case of the TVZ flare-up, repeated input of material and heat – probably coupled with tectonic extension – gradually conditioned the crust and allowed extraction of eruptible magma over a growing range of pressures with time. Our approach has the potential to reveal important information on the structure and distribution of magmatic systems within the shallow crust.
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