After the Oligocene submergence earth movements became more vigorous; many ridges rose from the sea as islands, and sank or were worn down again; sea basins formed and rapidly filled with sediments. New Zealand's late Tertiary environment has been described as follows: "The pattern of folds, welts and troughs that developed was on a finer scale than in the Mesozoic.
Tuatara Vol. Very late in the Cenozoic era—in the Pliocene and Pleistocene periods—one of the greatest episodes of mountain building in New Zealand's history took place. Earth movements became intense, and slowly pushed up the Southern Alps and other main mountain chains, and determined the general shape and size of the present islands of New Zealand.
Much of the movement during this mountain building period the Kaikoura Orogeny took the form of displacement of blocks of the earth's crust along fractures called faults. The total movements of the earth blocks adjacent to major faults amounted to thousands of feet; it must have been achieved very slowly, probably by innumerable small movements, each of a few inches or feet. The blocks adjacent to "transcurrent'' faults moved not only vertically but also laterally along the faults.
The New Zealand landscape today in some regions shows well preserved tilted fault blocks bounded by fault-scarps—steep faces hundreds or even thousands of feet high.
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Fault movements continue to the present day, and have accompanied several major earthquakes of the past century. Many minor but revealing landscape features such as scarplets, fault ponds, and shutter ridges show where movement has been occurring in recent centuries. Erosion during this time has eaten into the major landscape forms that the earth movements have built, carving the detailed landscape pattern of peaks, ridges, valleys, and gorges, and has built up alluvial plains, shingle fans, and other constructional forms; at the coast, waves have driven back the headlands, and built beaches, spits, and bars.
The Pleistocene period was the time of the Ice Age, and in the high mountains of the South Island glaciers carved deep valleys and carried huge loads of rock, dumping them as moraines. The late Pleistocene glaciers carved the fiords of Fiordland and the basins occupied by most South Island lakes; there were small glaciers also on Ruapehu, where remnants survive, and on Mount Egmont and the Tararua Range.
Volcanic activity of the past few million years has played an important part in making the rocks and shaping the landscape of parts of the central and northern North Island; Banks Peninsula, a twin volcanic dome in Canterbury, achieved much of its growth then, too. The largest volcanic outpourings of late geological times in New Zealand have been in the region between Tongariro National Park and the Bay of Plenty Coast: andesite lava, scoria, and ash were erupted in the Pleistocene period and later to build the huge volcanoes, Ruapehu, Tongariro, and Ngauruhoe; more than 2, cubic miles of molten rhyolitic magma was erupted in the form of ignimbrite, pumice, and rhyolite lava, building up the Volcanic Plateau.
The Geological Survey, Department of Scientific and Industrial Research, has built up this body of geological knowledge. Geological Maps —The geological maps show the present distribution of major rock groups in New Zealand, brought about by the events and processes that have been summarised in previous paragraphs. These maps were originally prepared for the New Zealand Encyclopaedia.
Older Rocks —Much of the late Cretaceous and Tertiary sedimentary rock that once; must have covered a large part of the South Island has been worn off by erosion since the Kaikoura Orogeny, and the "undermass'' of old rocks has been exposed. The oldest of these rocks lie to the west: Fiordland is made up mainly of metamorphic diorite, granite, and coarse schist, gneiss, and marble, with Ordovician graptolite-bearing slates in its south-west extremity; greywackes and argillites of possibly pre-Cambrian age occur in Westland and south-west Nelson, and further north in Nelson there are large areas of complexly folded Cambrian and Ordovician sedimentary and metamorphic rocks.
Large granitic masses, hundreds of square miles in area, occur in Westland, Nelson, and Stewart Island. These ancient rocks terminate with an abrupt boundary which shows clearly even on this small-scale map extending from Milford Sound along the western margin of the Southern Alps. This boundary is the Alpine Fault, a great fracture that divides the South Island into two areas of dissimilar geological structure: to the west of it, and in Fiordland, are the granites and other ancient rocks just described; to the east, the undermass rocks are predominantly the sedimentary and marine volcanic rocks of the New Zealand Geosyncline, and their metamorphosed forms, of later Paleozoic and Mesozoic age.
From Marlborough, through Canterbury to North Otago, the map shows an almost continuous expanse of these rocks; here they are mainly sparsely fossiliferous greywackes and argillite strata of Triassic and Jurassic age.
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On the western flanks of the Southern Alps, and in Otago, these sedimentary rocks merge gradually with schist and gneiss. Those of the Southern Alps show on the map as a very narrow belt, cut off by the Alpine Fault, but the southern schists form a belt some 60 miles wide extending for about miles across Otago. To the south, also, this schist mass merges gradually with sedimentary rocks of the New Zealand Geosyncline, here of Permian age: these strata, mainly tuffs and tuffaceous greywackes forming a belt that stretches across Southland, are the northern limb of a major downfold or syncline; Triassic and Jurassic strata occupy its core, making up much hill country of Southland.
The southern limb rocks include much marine volcanic rock, and in the core of the syncline in western Southland a belt of dunite and serpentine is intruded and is well exposed in the Olivine and Red Hill ranges. A sequence of rocks very like that of Southland is found also in eastern Nelson; here, fossiliferous Triassic rocks and Permian sedimentary and volcanic rocks closely resembling those of Southland are found, and a belt of dunite and serpentine the Nelson "mineral belt'' intrudes them.
It has been suggested that the Nelson and Southland rocks, which terminate abruptly at the Alpine Fault and its continuation as the Wairau Fault, were originally joined, and have been displaced some miles by lateral movement at the fault. Younger Rocks —On the eastern side of the South Island, upper Cretaceous and Tertiary strata survive only as small patches, the remnants of a once fairly complete cover of younger rocks.
Thick geosynclinal Cretaceous strata are found in the Clarence and Awatere Valleys of Marlborough, but elsewhere in the eastern South Island the upper Cretaceous and lower Tertiary strata are thin. They include foraminiferal limestone, greensands, sandstones, and other shell deposits formed during slow transgression by the sea.
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Oligocene limestone remnants mark the period of maximum transgression. On the western side of the South Island the younger rocks are more widespread, and include some thick sequences that were formed in rapidly sinking basins.
The oldest are the coal measures, mainly Eocene in age. They are overlain in some areas by thick lower Tertiary marine strata.
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The map shows some large areas of Pleistocene to Recent terrestrial deposits in the South Island. The largest forms the Canterbury Plains, and consists of old shingle deposits of unknown thickness washed from the Southern Alps during the Pleistocene glaciation.
Others occupy the Moutere depression of Nelson, and form Southland Plains, and intermontane basins, such as the McKenzie Plains, in the main mountain chains. Thick Pleistocene moraines form the main surface rocks of South Westland. Banks Peninsula is the only large mass of young volcanic rocks in the South Island; there are smaller areas at Timaru, Oamaru, and in the Dunedin district. Older Rocks —Unlike the South Island, the North Island has no large expanses of granite or of metamorphic rocks: the undermass rocks are almost wholly complexly folded and faulted greywackes and argillites of the New Zealand Geosyncline, predominantly Mesozoic in age.
The largest expanse of these hard rocks forms the main mountain backbone of the North Island, extending from Cook Strait to the East Cape area. Smaller areas of them are exposed between north Taranaki and Auckland; they include the richly fossiliferous strata of the Kawhia Syncline, a major downfold of the undermass rocks. In North Auckland, deeply weathered undermass rocks, in part of Permian age, form low hill country in the east, particularly between Whangaroa and Whangarei harbours.
Younger Rocks —Over most of the North Island the older rocks are hidden by Cretaceous, Tertiary and Quaternary sedimentary rocks and by young volcanic rocks. In some areas the younger sedimentary rocks are thin and patchy; in others they are many thousands of feet thick over hundreds of square miles. The bulk of them are poorly consolidated sandstones, and grey mudstones to which the colloquial name "papa rock'' is often applied.
The main areas with thick sequences of these young strata are the Taranaki - Wanganui - Rangitikei district, and the region east of the main ranges, including most of the Gisborne, Hawke's Bay, and Wairarapa districts. In both areas intensive oil prospecting of recent years has greatly added to knowledge of the structure. In the Taranaki - Wanganui - Rangitikei district the strata dip gently south, so that increasingly young rocks are exposed in this direction, the lower Tertiary being seen only in the north.
However, recent oil bores drilled to depths of about 13, ft at Kapuni in Taranaki, passed through a full sequence of strata from Pleistocene through all Tertiary stages, finally reaching Eocene coal measures. In the eastern North Island the structure of the younger rocks is much more complex than in the western area. Upper Cretaceous strata are followed by Tertiary in many sedimentary basins large and small, with many unconformities. The southern part of the region is broken by many transcurrent faults, and hard lower Cretaceous greywacke piercement bodies project from the younger rocks.
Younger rocks of South Auckland do not form such large basins as those just described. The oldest of these strata are the Eocene coal measures of the Waikato region.
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Upper Cretaceous strata, mainly mudstones, are the most widespread of the younger rocks of North Auckland. Young volcanic rocks are widespread in the North Island. The largest area of them is the Central Volcanic District: north of the three great andesite volcanoes, Ruapehu, Ngauruhoe, and Tongariro, lies the "volcanic plateau'', an expanse of some 10, square miles made up of several thousand cubic miles of ignimbrite, rhyolite lava, and pumice. This is one of the largest and youngest accumulations of acid volcanic rocks in the world. Most of it has been erupted in late Pliocene and Pleistocene times.
The belt of most recent activity in the Central Volcanic District is known as the Taupo Volcanic Zone; it contains all this country's active volcanoes, many inactive ones, and all the geysers and boiling springs. Mount Egmont is a huge, conical, andesite volcano, with the remnants of two other volcanic cones nearby; all are of Pleistocene age.
In the Waikato there are eroded Pleistocene cones of approximately basic andesite composition; the largest is Pirongia, some 3, ft high. Auckland city and the area immediately to the south has been the scene of many eruptions of basalt lava and scoria in late Pleistocene and Holocene times; many small scoria cones are seen at Auckland city. Late Tertiary and Quarternary basaltic eruptions in North Auckland have built lava plateaus and many young scoria cones. New Zealand Geological Survey Bulletin 66, The Geological Map of New Zealand, ,, , is a lengthier summary with a more detailed geological map and cross sections.
Such disturbed regions, of which New Zealand is one, are evidently the site of some kind of development affecting the outer shell of the Earth. Little is yet known about the internal processes that give rise to these geophysical disturbances, nor are the relations connecting them understood in any detail. Formerly earthquakes were believed to be caused by volcanic activity, but it is now recognised that volcanic earthquakes are restricted to small shocks in the immediate vicinity of the volcanism.
In New Zealand, tremors of this kind are experienced in the zone of active volcanism that extends from Mount Ruapehu to White Island. In some places geological faulting at the surface gives visible evidence that a major earth movement has occurred. Occasionally movement on a fault has been observed to occur simultaneously with an earthquake in the same vicinity.
New Zealand provided one of the earliest examples of this to become generally known, when movement took place on the Wairarapa Fault at the time of the great Wellington earthquake of Such events as this have led to the idea mat earthquakes in general are caused by fault movements, but it has proved difficult to find convincing evidence in support of this theory.