Tectonic relationships

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]

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 Māori pā (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.

For most of Auckland's post-1840 history, the mountains have been administered variously by the New Zealand Crown, the Auckland Council (or its former bodies including the Auckland City Council and Manukau City Council) or the Department of Conservation.^[27]

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]