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Te Pouhawaiki Volcano and pre‐volcanic topography in central Auckland: Volcanological and hydrogeological implications

Abstract Te Pouhawaiki Volcano in the Auckland Volcanic Field was identified on the basis of a small scoria cone, but whether this cone marked the location of a significant eruption centre has been unknown. Volcanic stratigraphy in the central Auckland isthmus is complex, with older deposits (possibly entire volcanic centres) obscured by younger deposits. The distribution of lava flows in the central Auckland isthmus was strongly influenced by the pre‐volcanic topography, and is a major control on present‐day groundwater flow regimes. Detailed gravity data from the central Auckland isthmus are used here to model the thicknesses of volcanic deposits and hence determine the pre‐volcanic topography. The site of the former Te Pouhawaiki scoria cone is shown to correlate with a distinct positive gravity anomaly (c. 6 μN.kg‐1) interpreted in terms of a lava‐filled depression in the Waitemata surface, surrounded by a tuff ring. This inferred explosive eruption centre is similar in both size and eruption style to a number of others in the Auckland Volcanic Field and suggests that the Te Pouhawaiki scoria cone may have been the surface manifestation of a substantial eruption centre which also produced phreatomagmatic deposits and lavas. The gravity model also defines the location and geometry of the paleotopographic divide between the ancestral Waitemata and Manukau River systems, showing it to be a complex ridge system. These buried ridges peak at c. 10–20 m depth (60–70 m a.s.l.) with a saddle in an eastern limb of the ridge which may have allowed lava from One Tree Hill Volcano to flow north of this divide. The configuration of the pre‐volcanic Waitemata surface indicates that the present‐day groundwater flow regime is likely to be complex and divergent away from the ridge system, controlled in some areas by narrow paleovalleys. Within the ridge complex, an area in which groundwater flow is likely to be convergent has been defined which correlates with the location of occasional surface flooding.