Eruption and emplacement processes of the Pleistocene Barriball Road Tuff Ring, South Auckland Volcanic Field, New Zealand

Phreatomagmatic eruptions in basaltic monogenetic volcanic fields are strongly influenced by their geological and environmental settings. Barriball Road volcano exemplifies the eruption processes associated with South Auckland Volcanic Field (SAVF), New Zealand. Stratigraphy and petrography reveal the complex eruptive history of this small-volume phreatomagmatic volcano. An initial phreatomagmatic phase formed two overlapping tuff rings from successive vents, and excavated lithics from a shell-rich Pliocene age aquifer (~170 m depth). The first tuff ring was constructed mainly through pyroclastic fall and the second is dominated by pyroclastic surge (dilute pyroclastic density current) deposits. Transition to a terminal magmatic phase produced a nested scoria cone. Vent migration between the eruption of the tuff rings may have been induced by collapse of the soft substrate, restricting water and magma supply to the first vent. Regional block faulting is inferred to have strongly influenced magma ascent and vent alignment, as seen at many SAVF and other monogenetic field volcanoes.