Reconstruction of Subaqueous Eruption‐Fed Current Characteristics and Source Location Using Anisotropy of Magnetic Susceptibility (AMS) at Otapahi, Dunedin Volcano, New Zealand

Information about birth and early eruptions of composite volcanoes is scarce, but erosion has revealed very early deposits of the Dunedin Volcano, and we apply anisotropy of magnetic susceptibility (AMS) techniques to retrieve information about the earliest known, submarine, eruptive centre. Although AMS has been used to recover information from subaerial eruptions, and to examine turbidites, this is its first use in investigating depositional processes and the source location for submarine volcaniclastic deposits. AMS fabrics of the deposits, emplaced by submarine eruption‐fed density currents, show basal portions of the currents were decoupled from more‐buoyant upper parts. Triangulation of AMS vectors and orientations yields a vent site location, with some imprecision reflecting the chaotic nature of deposits rapidly aggraded within 500 m of the source vent. This method elucidates depositional processes of subaqueous volcanic currents as well as subaerial ones, while still yielding sufficient consistency for triangulation of flow source sites. Plain Language Summary The early histories of volcanoes are difficult to reconstruct, as rocks from the earliest eruptions are buried by later eruptions. The Dunedin Volcano (active 16–11 million years ago), has been eroded back so that deposits from early eruptions are exposed along the coasts. We used a rock‐magnetic technique, anisotropy of magnetic susceptibility (AMS) on a submarine eruption deposit, to locate the vent of origin active early in the life of the Dunedin Volcano. This is the first time this technique has been applied to rocks formed by submarine eruptions. AMS fabrics suggest that the source vent was located