Grafton Volcano

Grafton Volcano is a buried volcano in New Zealand's Auckland 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 (1864) 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.

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

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 500 metres (1,600 ft) 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 600-metre (2,000 ft) 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 are not known precisely, although a tree under the Domain tuff ring was radiocarbon dated at over 50,000 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.

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