Ground penetrating radar imaging of Auckland volcanic surge deposits

The city of Auckland sits on an active volcanic field, which comprises approximately 50 volcanoes that have erupted over the last ∼250 kyr. The most recent eruption, Rangitoto, occurred ∼750 years ago. Many of the volcanic centres in the Auckland Volcanic Field (AVF) had initial phreatomagmatic eruptions, with pyroclastic density currents that left behind surge deposits. Because the AVF encompasses much of the Auckland urban area, there is little physical exposure. Accurate source locations and runout distances of the surge deposits are poorly known. Consequently, it is difficult to model the hazard and risk associated with the initial explosive phases of a future AVF eruption. As part of the project for DEtermining VOlcanic Risk in Auckland (DEVORA), ground penetrating radar (GPR) imaging was completed in association with coastal cliff mapping, calibration with quarry exposures, and limited other outcrops at the Crater Hill, Maungataketake and Waitomokia volcanic centres. The GPR images of the surge deposits could help to estimate pyroclastic current directions, the volcanic surge origins, and the proximal and distal deposit geometries for the purposes of modelling the flow dynamics and the consequent risk. Coastal cliff exposures indicated that even subtle features in the GPR profiles could be correlated with geological features, lending greater confidence that interpretations of the profiles will provide crucial information when outcrops and cliff exposures are not available.