Tridymite formation as a direct consequence of explosive volcanism – evidence preserved in tuffisite veins and concurrently erupted volcanic ash

ABSTRACT The mineralogy of volcanic rocks is usually considered as a consecutive crystallisation path related to a liquid line of decent. Tephra and pyroclastic rocks hold a special status as their formation includes the physical disruption of the magma during an explosive eruption. We describe tuffisite samples (captured veins of pyroclastic material in coherent lava bombs) and concurrently ejected volcanic ash from the ongoing eruption of Ebeko Volcano, Russia. Our samples show that tridymite (a mineral, not present in the original phase assemblage of the magma) forms as a consequence of explosive eruptive activity, volatile exsolution and fluid flow following decompression and magma fragmentation. Vapour phase crystallisation (VPC) of cristobalite is well known from highly evolved, dome‐forming eruptions due to gas flux through highly permeable and porous glassy dome rocks. At Ebeko volcano, increased permeability develops in the volcanic conduit during magma fragmentation and transient tephra storage. Magmas erupted at Ebeko are less evolved than those of typical (dome‐forming) andesitic – dacitic Peléan eruptions and show comparably higher eruption temperatures favouring the formation of tridymite over cristobalite during VPC.