Much of the volcanic activity in the northern portions of the North Island of New Zealand is recent in geological terms and has taken place over the last 30 million years. This is primarily due to the North Island's position on the boundary between the Indo-Australian and Pacific plates, a part of the Pacific Ring of Fire, and particularly the subduction of the Pacific plate under the Indo-Australian plate. The activity has included some of the world's largest eruptions in geologically recent times and has resulted in much of the surface formations of the North Island being volcanic as shown in the map.
Current activity The active or dormant volcanoes extend from Northland through the "City of Volcanoes", Auckland to Taranaki on the west coast and Bay of Plenty on the East with the North Island Volcanic Plateau hosting multiple active volcanoes. There has been activity within the last 5000 years in all these areas, with most activity on North Island Volcanic Plateau where the Taupō Rift is widening by as much as 19 mm (0.75 in)/year, much faster than other continental intraarc rifts. The Taupō volcanoes most recent major eruption, the Taupō or Hatepe eruption, took place around 232 CE, and ejected some 120 cubic kilometres (29 cu mi) of material (rating 7 on the VEI scale) This included pyroclastic deposits and ash that impacted much of the North Island which was not settled by man at the time.
Geology
The large ignimbrite sheets of the North Island Volcanic Plateau extend, under later sedimentary and volcanic deposits, from Hawkes Bay all the way to Auckland. This is as the Kidnappers eruption of a million years ago (1 Ma) produced 1,200 km3 (287.9 cu mi) of ignimbrite deposits, the most widespread on Earth, being over 45,000 km2 (17,000 sq mi) in area. This eruption had an estimated VEI of 8 and has been assigned a total eruption volume (not just tephra) of 2,760 km3 (662.2 cu mi). The basaltic volcanism of the far north is tens of millions of years older than this but towards the south by the Bay of Islands we have basaltic volcanoes that erupted only shortly before human colonisation, being Te Puke at between 1300 and 1800 years ago. Also associated with the 20 million year old Kaikohe-Bay of Islands volcanic field are andesitic and rhyolitic centres. The slightly older 8 million to 250,000 year ago mix of basaltic and dacitic Whangārei volcanic field volcanoes is adjacent to the andestic and dacitic remnants of 22 to 18 million years ago activity of a large stratovolcano off Whangārei Heads Further south is the basaltic Rangitoto Island off Auckland which last erupted about 600 years ago It is part of the a line of monogenetic volcanic fields extending southward as they get older along the west coast from the dormant Auckland volcanic field to the South Auckland volcanic field, then Ngatutura volcanic field and finally the Okete volcanic field. This last is situated on the slopes of the calc-alkalic basaltic stratovolcanoes of the Alexandra Volcanic Group. While considering stratovolcanoes, by Auckland on its west coast is the Waitākere Ranges, which are the andesitic conglomerate remnants of the 23 to 15 million year old Waitākere volcano. Much further south on the west coast is the potentially active andesitic Taranaki group of stratovolcanoes that have a history of collapse events. To the east of Auckland is the dacite stratovolcano of Little Barrier Island that is between 3 and 1.2 million years old and older Great Barrier Island at 18.5 to 12.3 million years. Great Barrier Island leads on to the Coromandel Volcanic Zone which towards its south last erupted about 5 million years ago but has similar timings to Great Barrier to the north. The Coromandel Volcanic Zone contains andesitic and rhyolitic eruptives mainly. Mayor Island just off the Coromandel east coast is a rhyolitic caldera that last erupted only about 6,340 years ago. The Tauranga Volcanic Centre on the east coast was active with andesitic and rhyolitic events between 2.95 and 1.9 million years ago. Inland from the Coromandel and Tauranga are a line of andesitic volcanoes such as Maungatautari that are located on the far side of the Hauraki Rift and have ages of about 1.8 million years. The Hauraki Rift intercepts the Taupō Rift creating the most active area of silicic volcanism on Earth in the Taupō Volcanic Zone. Its northeastern and southeastern portions are active andestic volcanoes while the central calderas erupt rhyolitic ignimbrite with later basaltic eruptions.
Volcanic areas
Northland The Northland region contains two recently active volcanic fields, one centred around Whangarei, and the other the Kaikohe-Bay of Islands volcanic field. The latest activity in the Kaikohe-Bay of Islands field, around 1300 to 1800 years ago, created four scoria cones at Te Puke (near Paihia).
Earlier, during the Miocene, a mainly andesitic volcanic arc ran through Northland and neighbouring regions (including the Three Kings Ridge and northern Coromandel Peninsula), with western and eastern belts active 25–15 million years ago and 23–11 million years ago respectively. Although this produced substantial volcanic edifices, including New Zealand's largest known stratovolcano, the Waitakere volcano, most of these have been eroded away, buried, or submerged, especially in the west, where a series of volcanoes buried offshore stretches south almost to New Plymouth. This is called the Northland-Mohakatino Volcanic Belt. Remnants of these two ancient volcano belts are still exposed in many places, including Whangarei Heads, the Hen and Chickens Islands, around Whangaroa Harbour, Waipoua forest, and the Waitākere Ranges.
Auckland volcanic field
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