The Waikato Plains ( the alternative name Waikato Basin is an ambiguous term as it can refer to the entire river catchment ) form a large area of low-lying land in the northwest of the North Island of New Zealand. They are the alluvial plains of the Waikato River, the country's longest river with a length of 425 km (264 mi). [1]
The plains can be divided roughly into the Middle Waikato Plain (also Middle Waikato Basin [2] Hamilton Basin [3] ), extending in all directions around the city of Hamilton, and the Lower Waikato Plain, nearer the river's mouth. [4] The two are broken by the rough, low-lying hills of the Hakarimata Range, between Ngāruawāhia and Huntly, and the Taupiri Range. [3]
The region is heavily populated by New Zealand standards, with many living in Hamilton towards the center of the plains. [1]
The plains are an area of once swampy land, much of which was drained by the early settlers, such as the Morrin brothers in the late 18th century [5] and is now intensively farmed. Dairy cattle, sheep, grain and maize are all farmed here, but it is dairy farming that is the staple of the local economy. The lower plain is also known for vineyards, and the middle plain has some of the southern hemisphere's top thoroughbred stables, notably around the towns of Cambridge and Matamata.
A considerable amount of the land is peaty, and significant sections especially in the north east are still undrained swamp. [2] Dozens of shallow riverine lakes lie at the central and southern end of the lower plain, notably Lake Waikare. [6]
This part of the Wailkato watershed is currently managed as the Middle Waikato Management Zone [7] and the Lower Waikato Management Zone. [4]
There is geological evidence that the Hamilton Basin portion was formed as a rift valley. [8] The normal faulting associated with this may have become inactive before 350,000 years ago, [9] although in lake sediments there is evidence of significant local earthquake activity as recently as 7600 years ago, but epicenters may relate to known current active fault systems to the north east of the plains. [6]
The basement is Waipapa Morrinsville Terrane greywacke formed in the Late Jurassic to Early Cretaceous (160-120 Ma).
The Hamilton Basin originally formed under water so marine Pliocene beds underlie Quaternary deposits that date from about 1.8 million years ago. [2] The oldest deposits are called the Puketoka Formation which contains clays, sands, breccias and ignimbrite sheets from pyroclastic flow reaching the basin. [2] The younger Waerenga Gravels are composed of weathered greywacke debris presumably deposited in fans extending from the surrounding ranges. [2] Even younger, and so closer to the surface usually, is the Karapiro Formation with rhyolitic sands and gravels which can be weathered to clay. [2]
The Waikato River over, even the last 1800 years, has changed course many times in the basin. It has changed course even more spectacularly on at least four occasions in the last 100,000 years, flowing northeast from the region of the current Lake Karapiro, and exiting near Thames in the Hauraki Gulf leaving drainage of the basin to the Waipa River which has not always been a tributary to the Waikato River. [2] Between 65,000 years to at most 25,000 years ago it drained the Waikato Plains but then drained through the Hauraki Plains for 6,000 years returning to drain the Waikato Plains from around 19,000 years ago. [2] So only in recent times,again, has it flowed northwest to empty into the Tasman Sea near Port Waikato. [2] This means the recent sedimentary deposits near the river are mainly altered volcanics washed down from the North Island Volcanic Plateau and broken down volcanic soils but there are layers of tephra/breccia from the many significant rhyolitic eruptions to the south where recent river flooding has not been disruptive. [6] Layers of the fifteen layer Kauroa Ash Formation are found within both the Puketoka and Karapiro Formation depending upon where you are in the basin. The ash beds can be many meters thick, although thin out north of Hamilton, and include the eight layer 3 to 5 m (9.8 to 16.4 ft) thick Hamilton Ash Formation deposited between 350,000 and 100,000 years ago. [2] There are also tephras derived from the andesitic stratovolcanoes of the Tongariro and Taranaki regions, as well as from Mayor Island/Tuhua. [2] The airfall tephra layers of the last 50,000 years vary from between 0.5 to 1.5 m (1 ft 8 in to 4 ft 11 in) thick. [2]
Mount Ruapehu is an active stratovolcano at the southern end of the Taupō Volcanic Zone and North Island volcanic plateau in New Zealand. It is 23 km (14 mi) northeast of Ohakune and 23 km (14 mi) southwest of the southern shore of Lake Taupō, within the Tongariro National Park. The North Island's major ski resorts and only glaciers are on its slopes.
The Waikato River is the longest river in New Zealand, running for 425 kilometres (264 mi) 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 then drains Taupō at the lake's northeastern edge, creates the Huka Falls, and flows northwest through the Waikato Plains. It empties into the Tasman Sea south of Auckland, at Port Waikato. It gives its name to the Waikato region that surrounds the Waikato Plains. The present course of the river was largely formed about 17,000 years ago. Contributing factors were climate warming, forest being reestablished in the river headwaters and the deepening, rather than widening, of the existing river channel. The channel was gradually eroded as far up river as Piarere, leaving the old Hinuera channel through the Hinuera Gap high and dry. The remains of the old course are seen clearly at Hinuera, where the cliffs mark the ancient river edges. The Waikato's main tributary is the Waipā River, which converges with it at Ngāruawāhia.
Kikai Caldera is a massive, mostly submerged caldera up to 19 kilometres (12 mi) in diameter in the Ōsumi Islands of Kagoshima Prefecture, Japan.
The Oruanui eruption of New Zealand's Taupō Volcano was the world's most recent supereruption.
The Hatepe eruption, named for the Hatepe Plinian pumice tephra layer, sometimes referred to as the Taupō eruption or Horomatangi Reef Unit Y eruption, is dated to 232 CE ± 10 and was Taupō Volcano's most recent major eruption. It is thought to be New Zealand's largest eruption within the last 20,000 years. The eruption ejected some 45–105 km3 (11–25 cu mi) of bulk tephra, of which just over 30 km3 (7.2 cu mi) was ejected in approximately 6–7 minutes. This makes it one of the largest eruptions in the last 5,000 years, comparable to the Minoan eruption in the 2nd millennium BCE, the 946 eruption of Paektu Mountain, the 1257 eruption of Mount Samalas, and the 1815 eruption of Mount Tambora.
The volcanism of New Zealand has been responsible for many of the country's geographical features, especially in the North Island and the country's outlying islands.
The geology of New Zealand is noted for its volcanic activity, earthquakes and geothermal areas because of its position on the boundary of the Australian Plate and Pacific Plates. New Zealand is part of Zealandia, a microcontinent nearly half the size of Australia that broke away from the Gondwanan supercontinent about 83 million years ago. New Zealand's early separation from other landmasses and subsequent evolution have created a unique fossil record and modern ecology.
The Waitoa River is a major river of the Waikato Region of New Zealand's North Island. It flows initially northeast from its origins at Piarere, before veering north through the Hinuera Gap and across the Hinuera Plains to pass to the west of Matamata, Walton and Waharoa before running through the settlement of Waitoa and reaching the southern edge of the Hauraki Plains. It converges with the Piako River in the Kopuatai Peat Dome wetland, approximately 15 kilometres north of Morrinsville.
The Auckland Region of New Zealand is built on a basement of greywacke rocks that form many of the islands in the Hauraki Gulf, the Hunua Ranges, and land south of Port Waikato. The Waitākere Ranges in the west are the remains of a large andesitic volcano, and Great Barrier Island was formed by the northern end of the Coromandel Volcanic Zone. The Auckland isthmus and North Shore are composed of Waitemata sandstone and mudstone, and portions of the Northland Allochthon extend as far south as Albany. Little Barrier Island was formed by a relatively isolated andesitic volcano, active around 1 to 3 million years ago.
The Waikato and King Country regions of New Zealand are built upon a basement of greywacke rocks, which form many of the hills. Much of the land to the west of the Waikato River and in the King Country to the south has been covered by limestone and sandstone, forming bluffs and a karst landscape. The volcanic cones of Karioi and Pirongia dominate the landscape near Raglan and Kawhia Harbours. To the east, the land has been covered with ignimbrite deposits from the Taupo Volcanic Zone. Large amounts of pumice from the Taupo Volcanic Zone have been deposited in the Waikato Basin and Hauraki Plains.
Lake Taupō, in the centre of New Zealand's North Island, is the caldera of the Taupō Volcano, a large rhyolitic supervolcano. This huge volcano has produced two of the world's most powerful eruptions in geologically recent times.
Whitikahu is a settlement scattered along Whitikahu Rd in the Waikato District and Waikato region of New Zealand's North Island.
Kaihere is a dispersed Waikato rural settlement on SH27, overlooking the Hauraki Plains. It has a school, hall, domain a rest area and is the starting point for the Hapuakohe Walkway.
The Maroa Caldera is approximately 16 km × 25 km in size and is located in the north-east corner of the earlier Whakamaru caldera in the Taupo Volcanic Zone in the North Island of New Zealand.
The Mangakino caldera complex is the westernmost and one of oldest extinct rhyolitic caldera volcanoes in the Taupō Volcanic Zone of New Zealand's North Island. It produced about a million years ago in the Kidnappers eruption of 1,200 km3 (287.9 cu mi), the most widespread ignimbrite deposits on Earth being over 45,000 km2 (17,000 sq mi) and was closely followed in time by the smaller 200 km3 (48.0 cu mi) Rocky Hill eruption. The Kidnappers eruption had a estimated VEI of 8 and has been assigned a total eruption volume of 2,760 km3 (662.2 cu mi).
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 25 kilometres (16 mi) wide and 250 kilometres (160 mi) long.
The Ōkareka Embayment is a volcanic feature in Taupo Volcanic Zone of New Zealand. It most significant recent volcanic eruption was about 15,700 years ago and this deposited the widespread Rotorua tephra that reached beyond Auckland.
The Alexandra Volcanic Group is a chain of extinct calc-alkalic basaltic stratovolcanoes that were most active between 2.74 and 1.60 million years ago but is now known to have had more recent activity between 1.6 and 0.9 million years ago. They extend inland from Mount Karioi near Raglan with Mount Pirongia being the largest, with Pukehoua on the eastern slopes of Pirongia, Kakepuku, Te Kawa, and Tokanui completing the definitive lineament. The associated, but usually separated geologically basaltic monogenetic Okete volcanic field, lies mainly between Karioi and Pirongia but extends to the east and is quite scattered.
Omanawa Caldera is inferred by an area of magnetic anomaly that exists to the north-west of the Rotorua Caldera. It is also located to the north west of the present boundary of the modern Taupō Volcanic Zone but its existence would be compatible with activity in the area of intersection of Taupo Rift and Hauraki Rift before 1.9 million years ago. The area of the caldera is covered by Mamaku Ignambrite from the Mamaku eruption of 240,000 years ago that formed the Rotorua Caldera. Eruptions from the Omanawa Caldera would explain formations such as the Waiteariki ignimbrite which covers much of the Bay of Plenty and forms the bulk of the Whakamarama Plateau. This would date the major caldera formation to 2.1 million years ago. However, there are at least eight large eruptions that occurred in the Tauranga Volcanic Centre between 2.4 and 1.9 million years ago and at this time which ones relate to this caldera can not be definite. However to date there is no other obvious closer inferred volcanic structure to assign a super volcanic eruption to, so at least some of these eruptions are likely to be associated with the Omanawa Caldera magnetic anomaly.
The Coromandel Volcanic Zone (CVZ) is an extinct intraplate volcanic arc stretching from Great Barrier Island in the north, through the Coromandel Peninsula, to the Kaimai Range in the south. The area of transition between it and the newer and still active Taupō Volcanic Zone is now usually separated and is called the Tauranga Volcanic Centre. Its volcanic activity was associated with the formation and most active period of the Hauraki Rift.