An allochthon is land that formed elsewhere and slid on top of other land in other words, the material of an enormous landslide. Much of the land of Northland and East Cape were created in this manner.
Northland-East Cape was an undersea basin. Much of the land that now forms Northland-East Cape was higher land to the Northeast composed of rocks formed 90—25 Ma. Layers of rocks were peeled off the higher land, from the top down, and slid Southwest under the influence of gravity, to be stacked the right way up, but in reverse order. Most of the material to slide were sedimentary rocks, however, the last rocks to be slid across were slabs of oceanic crust ophiolites , mainly basalt. Widespread volcanic activity also occurred 23—15 Ma , and is intermixed with the foreign rocks.
Sedimentary basins formed on the allochthons while they were moving. East Cape was later separated from Northland and moved further south and east to its present position. Volcanism is recorded in New Zealand throughout its whole geological history. Most volcanism in New Zealand, both modern and ancient, has been caused by the subduction of one tectonic plate under another; this causes melting in the mantle , the layer of the earth below the crust.
This produces a volcanic arc , composed of mainly basalt , andesite and rhyolite. Basaltic eruptions tend to be fairly placid, producing scoria cones and lava flows, such as the volcanic cones in the Auckland volcanic field , although Mount Tarawera 's violent eruption was an exception.
Andesitic eruptions tend to form steep stratovolcanoes , including mountains such as Ruapehu , Tongariro and Taranaki , islands such as Little Barrier , White and Raoul Islands , or submarine seamounts like Monowai Seamount. Rhyolitic eruptions with large amounts of water tend to cause violent eruptions, producing calderas , such as Lake Taupo and Lake Rotorua. New Zealand also has many volcanoes which are not clearly related to plate subduction including the extinct Dunedin Volcano and Banks Peninsula , and the dormant Auckland Volcanic Field.
The South Island has no currently active volcanoes. However, in the late Cretaceous —65 Ma , there was widespread volcanic activity in Marlborough, West Coast, Canterbury and Otago; and in Eocene times 40 Ma , there was volcanic activity in Oamaru.
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The most well known Miocene volcanic centres are the intra-plate Dunedin Volcano and Banks Peninsulas. The Dunedin Volcano which later eroded to form Otago Peninsula near Dunedin was built up by a series of mainly basaltic intra-plate volcanic eruptions in Miocene times 16—10 Ma. Southland's Solander Islands were active around 1 to 2 million years ago.
There are also minor volcanic from a similar time period throughout Canterbury, Otago and also on the Chatham Islands. Intra-plate basaltic volcanic eruptions also occurred in the North Island, near the Bay of Islands in Northland, in the Late Miocene 10 Mya , and again more recently 0. The South Auckland volcanic field was active in Pleistocene times 1. The Auckland volcanic field started erupting around , years ago. It includes around 50 distinct eruptions, with most of the prominent cones formed in the last 30, years, and the most recent eruption, which formed Rangitoto Island , around years ago.
The field is currently dormant and further eruptions are expected. Over time the volcanic field has slowly been drifting northwards. Volcanism in the North Island has been dominated by a series of volcanic arcs which have evolved into the still active Taupo Volcanic Zone. Over time, volcanic activity has moved south and east, as the plate boundary moved eastward.
This started in Miocene times 23 Ma when a volcanic arc became active to the west of Northland, and gradually moved South down to New Plymouth , where Taranaki is still active.
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It produced mainly andesitic strato-volcanoes. Notable visible volcanoes in the Waikato include Karioi and Pirongia 2. The volcanoes off the West coast of the North Island, together with Taranaki and the Tongariro Volcanic Centre, are responsible for the black iron sand on many of the beaches between Taranaki and Auckland. Shortly after 18 Ma , a volcanic arc developed further east to create the Coromandel Ranges and undersea Colville Ridge. The initial activity was andesitic but later became rhyolitic 12 Ma. In the Kauaeranga Valley , volcanic plugs remain, as does a lava lake that now forms the top of Table Mountain.
Active geothermal systems , similar to those that now exist near Rotorua , were present around 6 Ma, and produced the gold and silver deposits that were later mined in the Coromandel gold rush. Later 5—2 Ma , volcanic activity moved further south to form the Kaimai Range. Activity was initiated around 2 Ma, and continues to this day. The Tongariro Volcanic Centre is composed of andesitic volcanoes, while the areas around Taupo and Rotorua are largely rhyolitic with minor basalt.
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Early eruptions between Taupo and Rotorua around 1. Every so often, there are swarms of earthquakes within an area of the Taupo Volcanic Zone, which last for years. These earthquake swarms indicate that some movement of magma is occurring below the surface. While they have not resulted in an eruption in recent times, there is always the potential for a new volcano to be created, or a dormant volcano to come to life. The Tongariro Volcanic Centre developed over the last , years and contains the active andesitic volcanic cones of Ruapehu, Tongariro, and Ngauruhoe really a side cone of Tongariro.
Ruapehu erupts about once a decade, and while the eruptions cause havoc for skiers, plane flights and hydroelectric dams, the eruptions are relatively minor. However, the sudden collapse of the crater wall caused major problems when it generated a lahar in , that destroyed a rail bridge, and caused deaths at Tangiwai.
The last significant eruption was — Ngauruhoe last erupted — Taranaki is a perfectly formed andesitic strato-volcano, that last erupted in Lake Taupo , the largest lake in the North Island, is a volcanic caldera , responsible for rhyolitic eruptions about once every 1, years. The most recent eruption, around AD was also a major event, the biggest eruption worldwide in the last 5, years. The eruption caused a pyroclastic flow that devastated the land from Waiouru to Rotorua in 10 minutes. The Okataina volcanic centre, to the East of Rotorua, is also responsible for major cataclysmic rhyolitic eruptions.
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The last eruption, of Tarawera and Lake Rotomahana in , was a relatively minor eruption, which was thought to have destroyed the famous Pink and White Terraces , and covered much of the surrounding countryside in ash, killing over people. In researchers rediscovered the locations of the Pink and White Terraces using a forgotten survey from For example, Lake Rotorua erupted around 13, years ago. A line of undersea volcanoes extends out along the Kermadec Ridge. White Island , in the Bay of Plenty, represents the southern end of this chain and is a very active andesitic volcano, erupting with great frequency.
It has the potential to cause a tsunami in the Bay of Plenty, as does the dormant Mayor Island volcano. The Taupo Volcanic Zone is known for its geothermal activity. For example, Rotorua and the surrounding area have many areas with geysers , silica terraces , fumaroles , mud-pools , hot springs , etc. Geothermal energy is used to generate electricity at Wairakei , near Taupo. The list has been compiled from various sources, and does not purport to be free from omissions. Of the glaciers the largest is the Tasman, which, with others of comparable size, rises in the more elevated area surrounding Mount Cook.
On the western slope of the range, owing to the greater snow precipitation, the glaciers are more numerous and descend to lower levels, while the steeper slope gives them a more rapid rate of flow. As will be realized, these glaciers are an important tourist attraction, and as such have definite economic significance.
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Moreover, those glaciers on the eastern slopes which feed rivers utilized for irrigation and hydro-electric purposes are valuable in that they help to ensure a steady volume of water throughout the year. Moreover, owing to the high relief of the country, they are mostly swift-flowing, while, as mentioned previously, nearly all are obstructed at their mouths by bars.
For the purpose of internal communication, therefore, they are of little economic utility, and only in two or three isolated instances have they been thus consistently used. With improved roading conditions, however, their traffic has become negligible even in these cases. As sources of hydro-electric power New Zealand rivers are of considerable importance, since their rapid rate of flow and dependable volume of ice-free water make them eminently suitable for this purpose. The characteristics just mentioned are also important for purposes of irrigation, but, owing to the country's reliable rainfall, there are few areas other than in Canterbury and Otago where the rivers are so utilized.
In the Year-Book appears an account of the rivers of New Zealand, and below is given a list of the more important ones. For purposes of uniformity, the length of a river is taken to be the distance from the mouth to the farthest point in the system, whether this should happen to bear the same name or that of an affluent, and is inclusive of the estimated course of a river flowing into and emerging from any lake in the system. The discovery in that the beds of numerous rivers in the South Island contained extensive deposits of alluvial gold was of considerable importance in the economic development of the country.
Not only did it lead to an increase in population and in wealth, but, through the following of the numerous streams to their sources, it also led to the rapid exploration of large tracts of remote country. The exploitation of these deposits has been carried on with varying degrees of success up to the present time by both manual and mechanical means, but the amount of gold now extracted is comparatively small.
A further factor in connection with the rivers is that, owing to the very successful acclimatization of fresh-water fish, notably trout, many of them now provide exceptionally fine fishing. Surrounded by extremely rugged country the larger lakes of the South Island are distinguished by the grandeur of their alpine settings, while those of the North Island, situated on a volcanic plateau, are of interest by reason of the neighbouring thermal activity. Owing to the excellence of their fishing, the North Island lakes possess an added tourist attraction.
In both Islands the larger lakes are situated at high altitudes, and their consequent remoteness renders them unsuitable as a means of communication.
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In their functions as reservoirs the lakes of both Islands are of vital importance for the maintenance of the streams draining them and as a means of flood prevention. More especially is this the case where hydro-electric schemes are involved, Lakes Waikaremoana and Taupo in the North Island, and Lakes Coleridge. A series of narrow man-made lakes have been produced in connection with hydro-electric development along some of the rivers.