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The effect of microlites on the extensional rheology and fragmentation of basaltic magma

Ceri Allgood , Thomas Jones

Abstract

Basaltic magma typically erupts in an effusive manner, or sometimes with mild explosivity associated with lava fountaining or Strombolian activity. Pyroclast formation in these conditions is governed by fluidal or ductile fragmentation, as opposed to brittle fragmentation which occurs in eruptions of more viscous magma. However, some basaltic eruptions are known to have been more explosive, even Plinian, making them comparable with more viscous eruptions. One potential cause of explosive basaltic behaviour is the crystallisation of microlites, which increase the magma's viscosity, allowing it to fragment in a brittle manner under plausible conduit conditions. Here, we present results from laboratory experiments exploring the extensional rheology and fragmentation of particle suspensions analogous to microlite-bearing magmas. The rheology of particle suspensions in simple shear has been studied extensively, but extensional rheology has received much less attention. Extensional conditions occur at a range of scales during basaltic eruptions; for example, as pyroclasts separate within lava fountains, or as bubbles expand during magma decompression within the conduit. We explore how the volume fraction of solid particles within a Newtonian fluid phase influences the extensional rheology, breakup dynamics and fragmentation efficiency of the bulk suspension. Lastly, we evaluate the propensity for these particle/microlite induced dynamics to occur during the eruption of basaltic magmas.