Exhumed conduit records magma ascent and drain-back during a Strombolian eruption at Tongariro volcano, New Zealand

Field evidence from a basaltic-andesite dyke preserved in the eroded wall of a scoria cone at Red Crater, Tongariro volcano, New Zealand, records a history of up-conduit magma flow during a Strombolian eruption, subsequent drain-back and final cessation of flow. The dyke intrudes pre-Strombolian andesite lavas, and the overlying proximal basaltic-andesite scoria deposits associated with contemporaneous lavas, which are, in turn overlain by laminated lapilli-tuff and large blocks. Textural and kinematic evidence of ductile shear recorded in basaltic andesite at the dyke margins records magma deformation imposed by bypassing movement of magma up the centre of the conduit during the eruption, whereas the basaltic andesite occupying the central part of the lowermost exposures of the dyke preserves ductile flow-folds with the opposite (down-flow) shear sense. The evidence indicates that the downward magma flow followed the eruption, and this draining left the central part of the dyke empty (unfilled) at uppermost levels. We discuss the kinematic constraints in the context of the criteria for up-flow of mafic magma and present the factors most likely to result in a final drain-back event. With reference to experimental and numerical work, we propose a draining model for the end of this eruption, and that magmatic drain-back may feature commonly during closing stages of Strombolian eruptions at mafic volcanoes. Drain-back which leaves large cavities in a volcanic edifice could result in hazardous structural instabilities.