The South Auckland volcanic field, also known as the Franklin Volcanic Field, is an area of extinct

monogenetic volcano A monogenetic volcanic field is a type of volcanic field consisting of a group of small monogenetic volcanoes, each of which erupts only once, as opposed to polygenetic volcanoes, which erupt repeatedly over a period of time. The small monogeneti ...

es around

Pukekohe Pukekohe is a town in the Auckland Region of the North Island of New Zealand. Located at the southern edge of the Auckland Region, it is in South Auckland, between the southern shore of the Manukau Harbour and the mouth of the Waikato River. ...

, the

Franklin Franklin may refer to: People * Franklin (given name) * Franklin (surname) * Franklin (class), a member of a historical English social class Places Australia * Franklin, Tasmania, a township * Division of Franklin, federal electoral d ...

area and north-western

Waikato Waikato () is a local government region of the upper North Island of New Zealand. It covers the Waikato District, Waipa District, Matamata-Piako District, South Waikato District and Hamilton City, as well as Hauraki, Coromandel Peninsul ...

, south of the Auckland volcanic field. The field contains at least 82 volcanoes, which erupted between 550,000 and 1,600,000 years ago.

Features

The field extends from Pukekiwiriki east of Papakura in the north and

Pukekawa Pukekawa is a town in the Lower Waikato River area of New Zealand's North Island, 66 km south of central Auckland. The area's fertile soils are used to grow a range of vegetables, including onions, potatoes and carrots. The town was in ...

in the south. The field contains at least 82 volcanoes, and is older than the Auckland volcanic field to the north. The youngest volcanoes are likely the

Bombay Hills The Bombay Hills are a range of hills to the south of Auckland, New Zealand. Though only a small and seemingly insignificant range of hills, they lie at the southern boundary of the Auckland region, and serve as a divide between Auckland and th ...

shield volcano, which erupted an estimated 600,000 years ago, and Pukekohe Hill, the largest shield volcano of the field, which erupted an estimated 550,000 years ago. The largest tuff ring in the field is Onewhero maar which is in diameter and was formed 880,000 years ago.The field can be divided into three broad geographic areas: the north-eastern section, which consists of eroded remnants of lava flows and scoria cones, some of which are found in the south-western

Hunua Ranges The Hunua Ranges is a mountain range and regional park to the southeast of Auckland city, in the Auckland and Waikato regions of New Zealand's North Island. The ranges cover some and rise to 688 metres (2255 ft) at Kohukohunui.

, the southern section to the south of the

Waikato River The Waikato River is the longest river in New Zealand, running for through the North Island. It rises on the eastern slopes of Mount Ruapehu, joining the Tongariro River system and flowing through Lake Taupō, New Zealand's largest lake. It th ...

, which features many of the more recent and better preserved scoria cones, and the central section, which forms much of the low-lying land of the

area, between the

Manukau Harbour The Manukau Harbour is the second largest natural harbour in New Zealand by area. It is located to the southwest of the Auckland isthmus, and opens out into the Tasman Sea. Geography The harbour mouth is between the northern head ("Burne ...

and Waikato River.

Context

Relations to other volcanic activity

Other basaltic volcanic fields that are also now thought to represent

intraplate volcanism Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of p ...

active in the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

are adjacent from the south in a more recent to the north trend. As already mentioned the younger Auckland volcanic field is to its immediate north. To the south west is the older

Ngatutura volcanic field The extinct Ngatutura volcanic field that was active between 1.54 and 1.83 million years ago is one of four volcanic fields in an intraplate back arc relationship with the still active Hauraki Rift and the presently dormant Auckland volcanic f ...

which was active between 1,830,000 and 1,540,000 years ago and these locations fit with the south north trend being related to the opening of the

Hauraki Rift The Hauraki Rift is an active NeS-to NWeSE-striking rift valley system in the North Island of New Zealand that has produced the Firth of Thames and the Hauraki Plains. It is approximately wide and long. Geology The rift valley in the north a ...

in the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

or fracturing of the lithosphere. To its west, are the even older volcanoes associated with the Northland-Mohakatino

volcanic belt A volcanic belt is a large volcanically active region. Other terms are used for smaller areas of activity, such as volcanic fields. Volcanic belts are found above zones of unusually high temperature () where magma is created by partial melting ...

(Mohakatino Volcanic Arc) which are of a subduction-related origin but which include the still active

Mount Taranaki Mount Taranaki (), also known as Mount Egmont, is a dormant stratovolcano in the Taranaki region on the west coast of New Zealand's North Island. It is the second highest point in the North Island, after Mount Ruapehu. The mountain has a seco ...

at the southern end of this belt. To the south and east, visible on the horizon from the

are back arc volcanoes. These include the volcanoes of the

Taupō Volcanic Zone The Taupō Volcanic Zone (TVZ) is a volcanic area in the North Island of New Zealand that has been active for the past two million years and is still highly active. Mount Ruapehu marks its south-western end and the zone runs north-eastward thro ...

to the south which have now been continuously active for over 2 million years. This was also the time that activity ceased in the extinct volcanoes of the

Coromandel Peninsula The Coromandel Peninsula ( mi, Te Tara-O-Te-Ika-A-Māui) on the North Island of New Zealand extends north from the western end of the Bay of Plenty, forming a natural barrier protecting the Hauraki Gulf and the Firth of Thames in the ...

in the Coromandel Volcanic Zone to the east.

Tectonics

Many of the volcanoes are related to known fault structures. The Auckland region lies within the

Australian Plate The Australian Plate is a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when India broke ...

, about west of its plate boundary with the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

. The volcanoes are located south of the Auckland volcanic field. The structure of these Auckland regional faults and the resulting fault blocks is complex but like the volcanic field their locations can be postulated to be related to gravitational variations and where the Stokes Magnetic Anomaly passes through this section of the North Island. A north south line of central volcanoes is orientated along the Drury Fault. These extend from Papakura through and beyound the

. While the western margin of the belt is defined by the north-south Wairoa North Fault the eastern margins have at least three east-west fault lines. The unnamed most northern of these was presumably followed by the basaltic extrusion that extended all the way to the Morley Road tuff rings. The volcanoes along the lines of the Waiuku Fault and to the south the Waikato Fault that is also followed by the mouth of the

suggest that there has been strong volcano-tectonic relationships during the fields historic eruption activity.

Identification

The volcanic nature of the

Tuakau Tuakau ( mi, Tūākau) is a town in the Waikato region at the foot of Bombay hills, formerly part of the Franklin district until 2010, when it became part of Waikato District in the North Island of New Zealand. The town serves to support local ...

and

areas was first identified by

Ferdinand von Hochstetter Christian Gottlieb Ferdinand Ritter von Hochstetter (30 April 1829 – 18 July 1884) was a German-Austrian geologist. Career Having received his early education at the evangelical seminary at Maulbronn, Ferdinand proceeded to the University o ...

in 1859, however the first volcanic cones only began to be identified in the mid-20th century.

List of volcanoes

References

{{Auckland Monogenetic volcanic fields Geography of Auckland Landforms of the Auckland Region * Volcanism of New Zealand Geology of New Zealand Volcanoes of Waikato Volcanoes of the Auckland Region