Grafton Volcano

First recognised in 2010,[1] it includes the Outhwaite Park scoria cone that was first mapped by Hochstetter (1864) and inferred by later geologists to be a late phase vent of adjacent Pukekawa Volcano.

Borehole drilling and building excavations in the Grafton-Auckland Domain area during the 1990s and 2000s provided new subsurface geological information that allowed geologists to recognise the buried Grafton Volcano.

[2] The central and western parts of this Grafton Volcano comprise a tuff ring arc surrounding a 600-metre (2,000 ft) diameter explosion crater filled with a solidified lava lake (basalt) at least 50 metres (160 ft) thick, which underlies and surrounds scoria cones that erupted from two vents within the crater (at Outhwaite Park and the east end of Auckland Hospital).

[2] Most of Grafton Volcano is buried beneath 2–10 metres (7–33 ft) of volcanic ash that forms the western sector of the adjacent Pukekawa tuff ring.

The fractures and rubble within the solidified lava lakes of the Grafton and Pukekawa volcanoes now form a significant groundwater reservoir utilised by Auckland Hospital and the Domain.