Puketutu Island is a significant geological feature of the Auckland Volcanic Field. It is a compound volcanic centre that erupted on land approximately 32,000 years ago, when sea levels were much lower and the Manukau Harbour was a river valley. The formation of the island involved a sequence of different eruption styles. The earliest activity was wet and explosive, resulting from magma interacting with groundwater, which built a tuff ring made of hardened volcanic ash and rock fragments. This was followed by a shift to drier fountaining activity that created a cluster of scoria cones in the centre of the island, and finally, fluid basalt lava flows that spread out to form an apron around the base of the cones.
While parts of the volcanic edifice, including several scoria cones, were extensively quarried in the mid-20th century for construction fill (notably for the nearby Auckland Airport), remnants of the island’s complex geology remain highly valued. Puketutu is one of only a few volcanoes in the Auckland field where the different phases of eruption – tuff ring remnants, scoria cones, and lava flows are preserved, making it an Outstanding Natural Feature. Of particular interest is a unique intrusive lava flow on the western side, where a finger of basalt lava pushed its way under the overlying tuff and soft sediment, baking the sediment into natural brick columns and pushing up the tuff into a rare anticline (an upward fold).
There are also a number of small lava caves on the island.
