QUANTIFICATION OF GROUNDMASS CRYSTALLINITIES IN AUCKLAND VOLCANIC FIELD BASALTS AS INDICATORS OF FLOW EMPLACEMENT RATES

The Auckland Volcanic Field (AVF) is a basaltic intraplate volcanic field made of 53 small basaltic cones that erupted approximately 193 ka- 500 ya [1]. The rate of volcanism has increased in the last 60 ka and activity in the AVF could be “in its infancy” [2], which poses potential hazards for the city of Auckland. Our aim is to understand lava flow emplacement at two volcanic centers in the AVF, Rangitoto and Te Kōpuke/Mount St. John. Rangitoto is 504 ya [4] and the youngest volcano in the AVF, located 13 km from the city center. Te Kōpuke/Mount St. John is 75 ka [2], is located within Auckland city limits and erupted the longest flow in the AVF (11 km [3]). Basalts from both centers contain primarily olivine with subordinate clinopyroxene phenocrysts [1]. The basalt of Rangitoto contains 35% phenocrysts and 65% groundmass [3], composed of plagioclase, minor olivine, pyroxene, and Fe-Ti oxides. The basalt of Te Kōpuke/Mount St. John has 20% phenocrysts and 80% groundmass [3], with plagioclase, clinopyroxene and Fe-Ti oxides. We examined groundmass crystallinities along the length of ‘a‘ā lava flows at both volcanoes to understand crystal growth during lava flow emplacement. The groundmass of distal samples at both volcanoes (2 km from the vent at Rangitoto and 9.3 km from the vent at Te Kōpuke/Mount St. John [3], have higher % plagioclase crystallinities, larger crystal sizes, and blockier crystals (lower aspect ratios) compared to samples collected closer to the vents. Ongoing research will use cooling experiments to replicate these crystallinities, helping to determine the cooling rates of the lavas during emplacement. Results may assist in estimating emplacement timelines, which could support civil defense and emergency management in planning evacuation strategies from potential future eruptions.

[1] Hopkins et al., 2020; [2] Leonard et al., 2017; [3] Rhodé, 2016; [4] Needham et al., 2011