Grafton Volcano is a buried volcano in New Zealand'sAuckland volcanic field that underlies much of the Auckland suburb of Grafton. First recognised in 2010, it includes the Outhwaite Park scoria cone that was first mapped by Hochstetter and inferred by later geologists to be a late phase vent of adjacent Domain 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.
Structure
The central and western parts of this Grafton Volcano comprise a tuff ring arc surrounding a diameterexplosion crater filled with a solidified lava lake at least thick, which underlies and surrounds scoria cones that erupted from two vents within the crater. Most of Grafton Volcano is buried beneath of volcanic ash that forms the western sector of the adjacent Auckland Domaintuff ring.
Neighbouring Domain Volcano
Subsurface information suggests that the Domain Volcano probably erupted 5–100 years after the Grafton Volcano from a separate batch of magma that rose most of the way up the same conduit. The Domain explosion crater erupted east of Grafton Volcano and blasted through and destroyed the eastern arc of Grafton Volcano’s tuff ring and basalt-filled crater floor, creating its own diameter explosion crater and surrounding tuff ring. Widespread cobble-sized chunks of basalt within the Domain's tuff deposits are probably shattered parts of the Grafton crater floor. The ages of the Grafton and Domain volcanoes were until recently not known precisely, although a tree under the Domain tuff ring was radiocarbon dated at over 50,000 years old.. Recent studies have shown them to be approximately 100,000 years old. The fractures and rubble within the solidified lava lakes of the Grafton and Domain volcanoes now form a significant groundwater reservoir utilised by Auckland Hospital and the Domain. Whether the Grafton and Domain volcanoes are recognised as separate volcanoes or merely two halves of a more complex single volcano is a matter yet to be resolved and agreed upon by scientists.
