The Alexandra Volcanic Group (also known as Alexandra volcanic lineament or Alexandra Volcanics) is a chain of extinct

calc-alkalic The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic mag ...

basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

stratovolcano A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and per ...

es that were most active between 2.74 to 1.60 million years ago but is now known to have had more recent activity between 1.6 to 0.9 million years ago. They extend inland from Mount Karioi near Raglan with

Mount Pirongia Mount Pirongia is an extinct stratovolcano located in the Waikato region of New Zealand's North Island. It rises to 959 metres and is the highest peak in the Waikato region. It was active in the late Pliocene and early Pleistocene times. ...

being the largest, with Pukehoua on the eastern slopes of Pirongia,

Kakepuku Kakepuku (Kakipuku-o-kahurere) is a volcanic cone which rises from the plain between the Waipā and Puniu rivers, about NW of Te Kawa and SW of Te Awamutu in the Waikato region of New Zealand's North Island. Geology The 'Geology of the Wa ...

Te Kawa Te Kawa is a rural community in the Ōtorohanga District and Waikato region of New Zealand's North Island. It lies just to the south of the volcanic hills of Kakepuku and Te Kawa. Until the swamp was drained in the 1900s, Te Kawa was well know ...

, and Tokanui completing the definitive lineament. The associated, but usually separated geologically basaltic monogenetic Okete volcanic field (also known as the Okete Volcanic Formation or Okete Volcanics), lies mainly between Karioi and Pirongia but extends to the east and is quite scattered.

Geology

The chain extends in the Alexandra volcanic lineament, an alignment striking north-west to south-east over in length and is an example of backarc, intraplate basaltic volcanism that is very rare on land. This is because the arc basalts are in a very close relationship to a basaltic intraplate

monogenetic volcanic field 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 monogenetic ...

, the Okete which also erupted in late Pliocene times (2.7-1.8 million years ago). The separation of the two fields because of the different basalt composition was first proposed in 1983. The arc-type lavas of the Alexandra Volcanic Group are mainly

ankaramite Ankaramite is volcanic rock type of mafic composition. It is a dark porphyritic variety of basanite containing abundant pyroxene and olivine phenocrysts.https://dialnet.unirioja.es/descarga/articulo/281059.pdf Luis Enrique Ortiz Hernández, ''An ...

, a type of basalt found typically in some South Pacific Ocean Islands and not within continental crust. There are at least 27 vents in the Okete volcanic field, with most being in the northwest near the eastern flanks of Karioi. Only a few sites globally have island arc basalt and intraplate ocean island basalt so associated. The first stage of activity that finished about 1.9 million years ago produced all the volcanoes of both the Alexandra volcanic lineament and the monogenetic Okete volcanic field. Karioi is the oldest at 2.48 to 2.28 ± 0.07 million years ago on unmodified chronology. Pirongia has at least six edifice-forming vents separated by features including those resulting from large volume collapse events. The second stage was confined to Pirongia and consisted of basaltic eruptions between 1.6 to 0.9 million years ago during the period that the

South Auckland volcanic field The South Auckland volcanic field, also known as the Franklin Volcanic Field, is an area of extinct monogenetic volcanoes around Pukekohe, the Franklin area and north-western Waikato, south of the Auckland volcanic field. The field contains at l ...

and

Mangakino caldera complex The Mangakino caldera complex (other names are Mangakino volcanic center, Mangakino Caldera) is the westernmost and one of oldest extinct rhyolitic caldera volcanoes in the Taupō Volcanic Zone of New Zealand's North Island The North Island, ...

were active. The arc basalt volcano remnants at Tokanui are a small mound that rises about within higher rolling hills of the Puketoka and Karapiro Formations. There has been much progress over the last decade in characterising

Karioi Karioi or Mount Karioi is a 2.4 million year old extinct stratovolcano SW of Raglan in the Waikato region of New Zealand's North Island. It was the earliest of the line of 6 calcalkalic volcanoes, the largest of which is Mount Pirongia (the oth ...

Pirongia Pirongia is a small town in the Waipa District of the Waikato region of New Zealand's North Island. It is 12 kilometres to the west of Te Awamutu, on the banks of the Waipā River, close to the foot of the 962 metre Mount Pirongia, which lies in ...

and a separate arc basaltic centre at Pukehoua incorporated into the eastern slopes of Pirongia. The small basaltic centre at Kairangi is likely the furthest east point of the Okete volcanic field, but there is the possibility from drill sampling in the Hamilton Basin that other basaltic volcanoes exist that are subsurface now.

Tectonics

To its west, under the

Tasman Sea The Tasman Sea (Māori: ''Te Tai-o-Rēhua'', ) is a marginal sea of the South Pacific Ocean, situated between Australia and New Zealand. It measures about across and about from north to south. The sea was named after the Dutch explorer Abe ...

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 secon ...

at the southern end of this belt. The Taranaki Fault is between the two sets of volcanoes. To the south east are more back arc volcanoes including now 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 ...

which have now been continuously active for over 2 million years. Between Karioi and Pirongia the highland terrain of the Karioi horst block is interrupted by the mongenic volcanoes of Okete volcanic field. The lineament then extends into the Hamilton Basin, a major rift-related depression bound by the Waipa Fault Zone with the arc basaltic volcanoes of Pukehoua,

, Tokanui. Kairangi is the furtherist to the east and has been dated at 2.62 ± 0.17 million years ago.

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 in a more recent to the north trend from the Alexandra Volcanic Group through to the

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, the

which erupted between 550,000 and 1,600,000 years ago, and the very recently active but presently dormant younger

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 ...

. These locations fit with the 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 ...

and/or fracturing of the lithosphere. At the same approximate time the Alexandra Volcanic Group was initially active to its east in

Zealandia Zealandia (pronounced ), also known as (Māori) or Tasmantis, is an almost entirely submerged mass of continental crust that subsided after breaking away from Gondwanaland 83–79 million years ago.Gurnis, M., Hall, C.E., and Lavier, L.L., ...

the Tauranga Volcanic Centre was active.

Details of some volcanoes

More age data is accessible for individual basalts/vents by enabling mouseover in the interactive map of the field in the infobox.

References

{{Reflist Monogenetic volcanic fields Geography of Waikato Landforms of Waikato Volcanism of New Zealand Geology of New Zealand Volcanoes of Waikato

Geology

Tectonics

Relations to other volcanic activity

Details of some volcanoes

See also

References