Coupled eruptions in the Auckland Volcanic Field: are we underestimating the volcanic threat to our city?

Abstract

Of the 53 volcanic centres identified in the Auckland volcanic field, twenty are defined as “coupled”. This term is here defined are eruptions that have occurred close in space (≤ 1 km apart) and time (≤ 1 kyrs apart, but generally ≤ 100 yrs apart). The geochemical relationship between these twenty couplets was investigated using major, trace and isotope chemistry to determine if their coupled natures is linked to the mantle source processes. Using the geochemical characteristics of the two centres in a couplet, the couplets are initially split into four groups, which is then reduced to two groups during investigations. Group 1 shows a geochemical evolution from a less evolved source in the first eruption, to a more evolved source for the second eruption. Group 2 shows no geochemical variation between the first and second eruptions. Hypotheses for mantle mechanisms that could form both of these types of eruption scenarios are proposed and tested. Our results suggest that neither fractional crystallisation or crystal assimilation on ascent are responsible for the geochemical signatures identified. However, partial melting of a heterogeneous mantle source could be used to explain the signatures seen for both the Group 1 and Group 2 geochemical signatures.

Additionally, we identify two overprinted structural controls on the ascent dynamics of the rising melt. These include 1) the interaction of the Dun Mount Ophiolite Belt (DMOB), with perpendicular E-W trending faults controlling the location of the volcanic field, and 2) the shallow crustal NNE-SSW trending faults that dictate the locations of the coupled volcanic centres themselves.