Integrating LiDAR to unravel the volcanic architecture and eruptive history of the peralkaline Tūhua (Mayor Island) volcano, New Zealand

Abstract

Tūhua (Mayor Island), located off New Zealand's Bay of Plenty coastline, is a compound, shield-like volcano dominated by peralkaline rhyolite lavas and pyroclastic rocks. Previous studies established a chronostratigraphic framework for the island, yet detailed geomorphic interpretation has been hindered by the rugged, shrub covered terrain. Here we present a revised geological map, stratigraphic framework and geological history based on existing work, a 1 m digital elevation model, 100 m bathymetry and new field observations. Three caldera/crater structures are recognised; the 36 ka C1 caldera, 9.2 ka C2 caldera and a newly identified 7.6 ka C3 crater. Our analysis suggests that the 7.6 ka Tūhua eruption is not related to collapse and formation of the C2 caldera, as interpreted by previous studies. Instead, we attribute the Tūhua eruption to the C3 crater and associated ejecta ring identified in this study. Geomorphic assessment, field observations and volume estimates suggest the C2 caldera collapse event occurred at c. 9.2 ka. The upper Taumou Pa and 9.2 ka spatter-fed lava flows are both observed to be flowing into and outward of the C2 caldera, suggesting they were both active during the collapse event. The lack of volumetrically significant eruptions at the time of the two collapse events is leading us to attribute caldera collapses to magma inflation and drawdown at Tūhua. Coastal exposures include thick cone building pyroclastics and deposits of pyroclastic density currents alternating with lava flows exhibit the dominance of hydrovolcanism for explosive activity. Consequently, explosive interaction between magma and water is the most probable scenario for the initial stage of future eruptions.