Location and extent of the Auckland volcanic field. Clicking on the map enlarges it, and enables panning and mouseover of volcano name/wikilink. Please also see for age and geographical
relationships to other North Island surface volcanism
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[2] in the field have produced a diverse array of
maars (explosion craters),
tuff rings,
scoria cones, and
lava flows. With the exception of Rangitoto, no volcano has erupted more than once,[3][4] but the other eruptions lasted for various periods ranging from a few weeks to several years.[5] Rangitoto erupted several times[4] and recently twice; in an eruption that occurred about 600 years ago, followed by a second eruption approximately 50 years later.[6] The field is fuelled entirely by
basalticmagma, unlike the explosive
subduction-driven
volcanism in the central North Island, such as at
Mount Ruapehu and
Lake Taupō.[7]
The first vent erupted at
Pupuke 193,200 ± 2,800 years ago.[1] The most recent eruption (about 600 years ago[8] and within historical memory of the local
Māori) was of Rangitoto, an island
shield volcano just east of the city, erupting 0.7 cubic kilometres of
lava. The last volcano to erupt was much bigger than all others, with Rangitoto making up 41 per cent of the field's entire volume of erupted material[9] with characteristics as to slope and symmetry around the eruptive vents seen in basaltic
shield volcanoes as might be expected in a volcano, that may have buried other volcanoes, and now known to have a 1000-year odd eruptive history.[4][3] The field's other
volcanoes are relatively small, with most less than 150 metres (490 ft) in height.
Lake Pupuke, on the North Shore near
Takapuna, is a volcanic explosion crater. A few similar craters such as
Ōrākei Basin are open to the sea.
The field has produced voluminous lava flows that cover much of the
Auckland isthmus. One of the longest runs from
Mt Saint John northward, almost crossing the
Waitematā Harbour to form
Meola Reef.[10] More than 50
lava tubes and other lava caves have been discovered, including the 290-metre (950 ft)-long
Wiri Lava Cave.[11] There can be an association with lava caves and the formation of
rootless cones due to their mechanism of formation and a rootless cone was suggested to exist at Wiri being
Matukutūreia.[12] This may not be quite the case even though at least one steam only driven eruption occurred close to Matukutūreia.[13] The second-longest individual cave in the Auckland field, some 270 metres (890 ft) in total length, is the Cave of a Thousand Press-ups to the east of
Maungakiekie/One Tree Hill.[14] Two impressive depressions caused by lava cave collapses are the Puka Street Grotto and the nearby Hochstetter Pond, also known as Grotto Street Pond, in Onehunga.[15][16]
For most of the 200,000 years that the field has been erupting, the planet has been in
glacial periods (ice ages) where sea levels were much lower due to water being locked up as ice, and the Waitemata and
Manukau Harbours were dry land. All the volcanoes probably erupted on land except for Rangitoto, which erupted during the current
interglacial (warmer) period.[17]
Tectonic relationships
Map of faults and
Quaternary volcanoes in the
Auckland region. To show well the volcanoes in relation to fault lines you have to click on the map to enlarge and then zoom and pan. This also enables mouse over of the volcano and fault names. Definite active faults are shown in red. Well characterised inactive fault segments are in dark grey and other faults are shown in grey. A number of faults characterised by sea floor studies off the west coast of North Island are not shown. The type of volcanic eruption (some are composite) is indicated by basalt shield type eruption (black), scoria cone (red), or phreatomagmatic eruptions tuff ring (red-brown) and maar (purple). Volcanoes from both the Auckland volcanic field and
South Auckland volcanic field are displayed.
Miocene volcanoes are not shown.
The
Auckland region lies within the
Australian Plate, about 400 kilometres (250 mi) west of its plate boundary with the
Pacific Plate.[18] The volcanoes are located south of a geological region called the
Northland Allochthon, and with the northern volcanoes located over early
Miocene sedimentary deposits of the
Waitematā Group of rocks and the southern volcanoes over post Miocene sediments.[18] A large proportion of the volcanoes in the field, particularly those with cone structures, lie within 500 metres (1,600 ft) of inferred or known faults, with the qualification that these are inactive historic faults and unlike in many other volcanic fields it is rare for volcanoes to be actually on the fault line.[19] 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.[18]
The field is part of the
Auckland Volcanic Province which comprises four volcanic fields with intra-plate basaltic volcanism starting in the south, at
Okete, near
Raglan in late Pliocene times (2.7-1.8 Ma).[20] Activity has since moved north through the
Ngatutura,
South Auckland and Auckland fields since then.[21]
Human context
Mythology
Tāmaki Māori myths describe the creation of the volcanic field as a creation of
Mataaho (the guardian of the earth's secrets) and his brother
Rūaumoko (the god of earthquakes and volcanoes), made as punishment against a tribe of
patupaiarehe, supernatural beings living in the
Waitākere Ranges, who used deadly magic from the earth to defeat a war party of patupaiarehe from the
Hunua Ranges.[22][23] In some traditions, the fire goddess
Mahuika creates the volcanic field as a way to warm Mataaho, after his wife leaves and takes his clothing.[24][25] Because of their close association to Mataaho, the volcanic features can be collectively referred to as Nga Maunga a Mataaho ("The Mountains of Mataaho"),[22] or Ngā Huinga-a-Mataaho ("the gathered volcanoes of Mataaho").[25] Many of the volcanic features of
Māngere can be referred to as Nga Tapuwae a Mataoho ("The Sacred Footprints of Mataoho"), including
Māngere Lagoon,
Waitomokia,
Crater Hill,
Kohuora,
Pukaki Lagoon and
Robertson Hill.[24][26] Many of the
Māori language names of volcanic features in the field refer to Mataaho by name, including Te Pane o Mataaho (
Māngere Mountain), Te Tapuwae a Mataoho (Robertson Hill) and Te Kapua Kai o Mataoho (the crater of
Maungawhau / Mount Eden).
Usage
Many of the maunga (mountains) were occupied by substantial
Māoripā (fortifications) before
Pākehā settlement, and many terraces and other archeological remnants are still visible.[27] Many of the cones have been levelled or strongly altered, in small part due to the historical Māori use, but mostly through relatively recent quarrying of construction materials (especially
scoria). However many of the remaining volcanoes are now preserved as landmarks and parks.[8]
The warmer northern sides of the mountains were also popular among early
Pākehā settlers for housing.[27] In the 1880s,
Takarunga / Mount Victoria and
Maungauika / North Head were developed as military installations due to
fears of a Russian invasion.[27] The cones are also protected by a 1915 law, the Reserves and Other Lands Disposal and Public Bodies Empowering Act 1915, which was passed due to early concern that the distinctive landscape was being eroded, especially by quarrying. While often ignored until the late 20th century, it has amongst other things minimised severe changes to Mount Roskill proposed by
Transit New Zealand for the
Southwestern Motorway.[28]
In March 2007, New Zealand submitted the volcanic field, with several specifically named features, as a
World Heritage Site candidate based on its unique combination of natural and cultural features.[8] At that time, only 2 per cent of more than 800 World Heritage Sites worldwide were in this "mixed" category.
In the 2014
Treaty of Waitangi settlement between the Crown and the Ngā Mana Whenua o Tāmaki Makaurau collective of 13 Auckland
iwi and
hapu (also known as the Tāmaki Collective), ownership of the 14 Tūpuna Maunga (ancestral mountains) of Tāmaki Makaurau / Auckland, was vested to the collective. The legislation specified that the land be held in trust "for the common benefit of Ngā Mana Whenua o Tāmaki Makaurau and the other people of Auckland". The Tūpuna Maunga o Tāmaki Makaurau Authority or Tūpuna Maunga Authority (TMA) is the
co-governance organisation established to administer the 14 Tūpuna Maunga. Auckland Council manages the Tūpuna Maunga under the direction of the TMA.[27][29][30]
Dangers
Since the field is not extinct, new volcanic events may occur at any time, though the usual period between events is, on average, somewhere between hundreds to thousands of years. There has been at least one eruption in every 2,500 years over the last 50,000 years.[20] However, the effects of such an event—especially a full-scale eruption—would be substantial, ranging from
pyroclastic surges to
earthquakes,[31]lava bombs,
ash falls, and the venting
volcanic gas, as well as
lava flows. These effects might continue for several months, potentially causing substantial destruction and disruption, ranging from the burial of substantial tracts of residential or commercial property, to the mid-to-long-term closures of major parts of the country's infrastructure such as the
Port of Auckland, the
State Highway network, or the
Auckland Airport.[32] It is possible that several volcanoes could erupt simultaneously. There is strong evidence that eight erupted within a span of 3000 years or so, between 31,000 and 28,000 years ago.
Most eruptive events in the field have been small volume, very constrained in time, typically involving less than 0.005 km3 (0.0012 cu mi) of magma making its way to the surface.[20] However the same amount of magma can have an order of magnitude different impact. An underwater eruption which is more likely to be explosive resulted in the formation of the 0.7 km (0.43 mi) wide
Ōrākei crater that destroyed an area of 3 km3 (0.72 cu mi) by crater formation and base surge impact. This contrasts with the about 0.5 km (0.31 mi) diameter cone produced by the same amount of upwelling magma that might be expected to destroy an area of 0.3 km3 (0.072 cu mi) if there is no ground water interaction.[20] Modelling has suggested that the next eruption in the volcanic field is likely to be associated with water and in the area extending from the central city to its north and northeast suburbs surrounding and including the
Waitemata Harbour.[33] Within New Zealand the volcanic hazard of the field is graded below that of
Taupo Volcanic Zone volcano's but is likely to be perceived by the population affected as a greater potential nuisance if it occurs[34]
Various operative structures, plans and systems have been set up to prepare responses to volcanic activity within the urban areas, mainly coordinated in the Auckland Volcanic Field Contingency Plan[32] of the
Auckland Regional Council, which provides a framework for interaction of
civil defence and
emergency services during an eruption. Auckland also has a seismic monitoring network comprising six
seismometers—including one 250 metres (820 ft) deep at
Riverhead—and three repeaters within the region that will detect the small
tremors likely to precede any volcanic activity.[35] This is likely to give between a few hours and several days' warning of an impending eruption, and its approximate location.[31]
Auckland War Memorial Museum, itself built on the crater rim of
Pukekawa, has an exhibition on the field, including the "Puia Street multi-sensory visitor experience", which simulates a grandstand view of an eruption in Auckland.[36]
^
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^Needham, A.J.; Lindsay, J.M.; Smith, I.E.M.; Augustinus, P.; Shane, P.A. (April 2011). "Sequential eruption of alkaline and sub-alkaline magmas from a small monogenetic volcano in the Auckland Volcanic Field, New Zealand". Journal of Volcanology and Geothermal Research. 201 (1–4): 126–142.
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^David Lomas (Winter 2006).
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^Rout, D.J.; Cassidy, J.; Locke, C.A.; Smith, I.E. (1993). "Geophysical evidence for temporal and structural relationships within the monogenetic basalt volcanoes of the Auckland volcanic field, northern New Zealand". Journal of Volcanology and Geothermal Research. 57 (1–2): 71–83.
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^"Visit Volcanoes". Auckland War Memorial Museum. Retrieved 28 September 2017.
^Lindsay, J.M.; Leonard, G.S.; Smid, E.R.; Hayward, B.W. (December 2011). "Age of the Auckland Volcanic Field: a review of existing data". New Zealand Journal of Geology and Geophysics. 54 (4): 379–401.
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