Abstract
Alkaline intraplate volcanoes tend to quickly develop complex plumbing systems in which magma fractionates and mixes to produce a wide range of lithologies and eruptive behaviours, making them unpredictable and potentially dangerous. To better understand the processes that construct them, this study applies novel technologies in the reconstruction of volcanic sequences at Heyward Promontory, a section of the extinct Dunedin Volcano/Rakiriri. This volcano experienced a prolonged eruptive history between 16-11 Ma and its edifice exceeded 25 km in diameter. Much of its history is preserved in cliffs and caves along the northern coast of Heyward Promontory, where numerous lavas, tuffs, paleosols, dikes and autobreccias indicate a complex development of the edifice over time.
We have created a digital 3D model of the promontory using aerial photogrammetry techniques supplemented by regional elevation data. Photogrammetry was focused on well-exposed outcrops at Long Beach, Pilot Point and Aramoana, where verticality has obstructed traditional mapping methods. Field, petrographic and geochemical data from these outcrops have enabled us to recolour the model to display ~30 distinct lithological units that formed between ~13.4-11.7 Ma.
We have shown that two major ‘kaiwekite’ flows exist where previously only one was recognised. The older ‘kaiwekite’ erupted near Aramoana, and the younger in the Long Beach area. The Aramoana flow is comparatively more ‘evolved’ in composition, containing more Na-rich plagioclase feldspars, K-rich alkali feldspars and Fe-rich pyroxene phenocrysts on average. Phonotephrites at Aramoana and Pilot Point have been positively correlated and used to constrain the area’s stratigraphy, as have shoshonites at Pilot Point and Purakaunui. Members of a suite similar to the Waitati Phonolite have been correlated at the top of the Heyward Point and Purakaunui sequences on the basis of bulk major and trace element analyses.
The final model is available online as a virtual field trip (VFT) in which a guided tour systematically introduces volcanic processes in relation to visible lithologies and structures. Specific processes that are highlighted include lava extrusion, dike intrusion, autobrecciation, magma mixing and diatreme formation. Our approach to lithological reconstruction is applicable to other landforms across the globe. We have shown that photogrammetry is time- and cost-efficient, allowing researchers to quickly map geological structures in three dimensions. This technology is useful for monitoring active hazards, such as erupting volcanoes, landslides and regions affected by earthquakes. Digital lithological models illuminate processes that continue to shape active volcanoes, including Kīlauea and Cumbre Vieja, which erupted recently and present significant risk to people and the environment. As the techniques involved in their creation become more powerful in the future, such models could become standard components of geological studies, and we hope that the approach developed here will serve as a useful foundation.