Dynamics of magma ascent in the volcanic conduit

This chapter presents the various mechanisms and processes that come into play within the volcanic conduit for a broad range of effusive and dry explosive volcanic eruptions. Decompression during magma ascent causes volatiles to exsolve and form bubbles containing a supercritical fluid phase. Viscous magmas, such as rhyolite or crystal-rich magmas, do not allow bubbles to ascend buoyantly and may also hinder bubble growth. This can lead to significant gas overpressure and brittle magma fragmentation. During fragmentation in vulcanian, subplinian, and plinian eruptions, gas is released explosively into the atmosphere, carrying with it magma fragments. Alternatively, high viscosity may slow ascent to where permeable outgassing through the vesicular and perhaps fractured magma results in lava effusion to produce domes and flows. In low viscosity magmas, typically basalts, bubbles may ascend buoyantly, allowing efficient magma outgassing and relatively quiescent magma effusion. Alternatively, bubbles may coalesce and accumulate to form meter-size gas slugs that rupture at the surface during strombolian eruptions. At fast magma ascent rates, even in low viscosity magmas, melt and exsolved gas remain coupled allowing for rapid acceleration and hydrodynamic fragmentation in hawaiian eruptions.