Auckland is a monogenetic volcanic field. Monogenetic volcanic fields form by numerous single episode eruptions at independent vents. After each eruption, the magma conduit is blocked by solidifying magma. The magma finds a new path to the surface and forms a new volcano. Eruptions tend to be short-lived and small volume (less than 0.1 cubic km).
The Auckland Field is made of at least 49 vents that cover 260 square km. Eruptions began about 60,000 years ago. The most recent eruption was 800 years ago. Lava flows, tuff rings and scoria cones are common. The Southern Auckland field has at least 70 vents over an area of 190 square km. Over half of these vents are cones and tuff rings. The tuff rings range in diameter from 0.5 to 2.5 km and are commonly clustered or nested. The Southern Auckland field was active from 1.56 million to 510,000 years ago. The Ngatutura field consists of 16 vents and was active from the late Pliocene to early Pleistocene (Johnson and others, 1989).
Rangitoto was formed by the most recent eruption about 800-600 years ago. With a volume of 2 cubic km and an area of 23 square km, it is the largest volcano in the Auckland Field. Eruptions at Rangitoto probably continued for several hundred years. Smith and Allen (1993) suggested that eruption volume has been increasing over the history of the volcanic field.
The Auckland Field is about 220 miles (350 km) north of the subduction zone that produces the Taupo Volcanic Zone and thought to be the result of intraplate volcanism (Johnson and others, 1989).
Sources of Information:
Heming, R.F., and Barnet, P.R., 1986, The petrology and petrochemistry of the Auckland volcanic field: Roy. Soc. New Zealand Bull., v. 23, p. 64-75.
Johnson, R.W., Knutson, J., Taylor, S.R., eds., 1989, Intraplate volcanism in eastern Australia and New Zealand: Cambridge, England, Cambridge University press, 408 p.
Johnston, D.M., Nairn, I.A., Thordarson, T., and Daly, M., 1997, Volcanic impact assessment for the Auckland volcanic field: Auckland Regional Council, Technical Publication Number 79.
Kermode, L.O., 1992, Geology of the Auckland urban area: Institute of Geological & Nuclear Sciences map 2.
Searle, E.J., 1981, City of volcanoes: a geology of Auckland (2nd ed.): Auckland, Longman Paul, 195 p.
Smith, I.E.N., and Allen, S.R., 1993, Volcanic hazards at the Auckland Volcanic Field: Ministry of Civil Defense Volcanic Hazard Information Series No. 5.
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