Grafton Volcano
Encyclopedia
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 metres (1,968.5 ft) diameter explosion crater filled with a solidified lava lake
(basalt) at least 50 metres (164 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 m (6.6–32.8 ft) of volcanic ash that forms the western sector of the adjacent Auckland Domain tuff ring.
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.
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
's Auckland volcanic field
Auckland Volcanic Field
The Auckland volcanic field is a monogenetic volcanic field in the North Island of New Zealand. Basaltic in nature, it underlies much of the metropolitan area of Auckland....
that underlies much of the Auckland
Auckland
The Auckland metropolitan area , in the North Island of New Zealand, is the largest and most populous urban area in the country with residents, percent of the country's population. Auckland also has the largest Polynesian population of any city in the world...
suburb of Grafton
Grafton, New Zealand
Grafton is a suburb of Auckland City, New Zealand. The suburb is named for the Duke of Grafton, a patron of the first Governor of New Zealand, William Hobson, and was once known as 'Grafton Heights', denoting it's history as a well-off suburb in Auckland's earliest decades...
. First recognised in 2010, it includes the Outhwaite Park scoria cone that was first mapped by Hochstetter
Ferdinand von Hochstetter
Christian Gottlieb Ferdinand Ritter von Hochstetter was a German geologist.He was born at Esslingen, Württemberg, the son of Christian Ferdinand Friedrich Hochstetter , a clergyman and professor at Bonn, who was also a botanist and mineralogist...
(1864) and inferred by later geologists to be a late phase vent of adjacent Domain Volcano
Auckland Domain
The Auckland Domain is Auckland's oldest park, and at 75 hectares one of the largest in the city. Located in the central suburb of Grafton, the park contains all of the explosion crater and most of the surrounding tuff ring of the 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.
Structure
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
ring arc surrounding a 600 metres (1,968.5 ft) diameter explosion crater filled with a solidified lava lake
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...
(basalt) at least 50 metres (164 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 m (6.6–32.8 ft) of volcanic ash that forms the western sector of the adjacent Auckland Domain tuff 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 500 metres (1,640.4 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 metres (1,968.5 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.