Grafton Volcano is a buried volcano in New Zealand's

Auckland volcanic field The Auckland volcanic field is an area of monogenetic volcanoes covered by much of the metropolitan area of Auckland, New Zealand's largest city, located in the North Island. The approximately 53 volcanoes in the field have produced a diverse a ...

that underlies much of the

Auckland Auckland (pronounced ) ( mi, Tāmaki Makaurau) is a large metropolitan city in the North Island of New Zealand. The List of New Zealand urban areas by population, most populous urban area in the country and the List of cities in Oceania by po ...

suburb of Grafton. First recognised in 2010, it includes the Outhwaite Park scoria cone that was first mapped by

Hochstetter The family of Höchstetter (also rendered Hechstetter or Hochstetter), from Höchstädt in western Bavaria near the banks of the Danube, were members of the fifteenth and sixteenth-century mercantile patriciate of Augsburg. For a time, these i ...

(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.Hayward, B.W., Kenny, J.A., High, R., France, S., 2011, ''Grafton Volcano.'
Geocene 6
12–-17.

Structure

The central and western parts of this Grafton Volcano comprise a

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

ring arc surrounding a diameter explosion crater filled with a solidified

lava lake Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified (sometim ...

(basalt) at least thick, which underlies and surrounds

scoria cone Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...

s 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 of volcanic ash that forms the western sector of the adjacent Pukekawa tuff ring.

Neighbouring Domain Volcano

Subsurface information suggests that the Pukekawa 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 Pukekawa 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 Pukekawa tuff deposits are probably shattered parts of the Grafton crater floor. The ages of the Grafton and Pukekawa volcanoes were until recently not known precisely, although a tree under the Pukekawa tuff ring was

radiocarbon date Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was dev ...

d 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 Pukekawa volcanoes now form a significant groundwater reservoir utilised by

Auckland Hospital Auckland City Hospital is a public hospital located in Grafton, Auckland, New Zealand. It is the largest hospital in New Zealand,Largest hospital in New Zealand...' - News-Medical.Net, Tuesday 29 June 2004 as well as one of the oldest medical fac ...

and the Domain. Whether the Grafton and Pukekawa 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.

References

*''Volcanoes of Auckland: The Essential Guide''. Hayward, B.W., Murdoch, G., Maitland, G.; Auckland University Press, 2011. *''Volcanoes of Auckland: A Field Guide''. Hayward, B.W.; Auckland University Press, 2019, 335 pp. . {{Auckland volcanic field Auckland volcanic field