By contrast with the ancient stable mass of Australia, New Zealand is part of the unstable zone of mountains (the so-called Mobile Zone) that contains the European Alpine system, the Himalayas, the Indonesian Islands, and New Guinea. Only the eastern edge of Australia has been caught up in the mountain-making (orogenic) movements that have raised great ridges above the floor of this part of the Pacific Ocean. The oceanographer's chart, compiled from soundings of ocean depths, shows how the main islands of New Zealand rise from a wide submarine shelf, and now two main arcs of the South-west Pacific Mobile Zone meet in the middle of the North Island. One of these comes in from New Guinea by way of New Caledonia to the Auckland Peninsula; the other from Fiji and Tonga to the East Cape – Cook Strait ranges. Consequently, New Zealand is a long, narrow, mountainous country; its islands are the unsubmerged parts of complex mountain ridges separated from Australia by the wide and deep trough containing the Tasman Sea.
The grain of the Auckland Peninsula is generally north-west to south-east, though its mountain ridges are relatively low (up to 2,750 ft in Coromandel). The East Cape – Cook Strait ranges trend north-east to south-west and are much higher (5,753 ft in Hikurangi). Where these two mountain lines meet is concealed beneath the volcanic accumulations of the central plateau. The most prominent of the volcanic mountains are in this North Island zone: Ruapehu, Tongariro, and Ngauruhoe in the interior, and Mt. Egmont in the west.
The north-east to south-west trend continues in the South Island beyond Cook Strait until, in the vicinity of Mt. Aspiring, it swings abruptly to the south-east. It is on this general framework that the mountainous islands of New Zealand are built, the highest, steepest, and most extensive ranges being in the South Island (up to 12,349 ft in Mt. Cook). Indeed, some two-thirds of the South Island is hard-rock highland of one kind or another.
Most of the hard-rock mountains of the North Island and the South Island east are made of tough fine-grained sedimentary sandstones and argillites, ranging in age from Triassic to Cretaceous. The Great Divide in the South Island is the Alpine fault; west of this the highland is mainly made of granites and ancient crystallines, while in Central Otago it is of metamorphic schist. But with all their diversity of kind these rocks possess the common property of hardness.
The history of the making of the highland may be briefly stated thus:
In a Mesozoic sea (Triassic and Jurassic), vast deposits of fine grained sands and silts were laid, to be tightly compressed and raised to form a complex highland in the early Cretaceous. This highland was worn down to its roots by erosion through most of the Tertiary period, submerged and covered with a thick mantle of sediments conveniently called the covering beds. These were mainly silts and muds with occasional harder bands of limestone or sandstone embedded in them. Then in the later Tertiary and, continuing well on into the Pleistocene in some places, came the great uplift, the Kaikoura Orogeny, a period of vigorous mountain-making activity that gave New Zealand its present general form and outline.
The new land arched up by tremendous compressive forces was broken into massive blocks by extensive faults. From the rapidly rising mountain blocks, the weak-rock cover was quickly stripped away and the hard-rock masses themselves were carved and wasted by vigorous erosion. So the mountains of today are the product of the orogenic processes and the erosion to which the emerging highland would be immediately subjected. As erosion proceeds, the hard-rock ridges stand out more and more boldly, while in the South Island erosion by ice gives a special character to the ranges and valleys of the high country.
Thus the high-standing mountains of New Zealand are made of the hard rocks of the understructure, and are best developed in the South Island, much of the North Island being weak-rock hill country mainly made of mudstone, locally known as papa.
The most prominent single feature of the North Island is the hard-rock mountain core that extends from Cook Strait to the Bay of Plenty, a topographic barrier between east and west breached only by the Manawatu Gorge. This continuous mountain core, known locally by a variety of names – the Rimutaka, Tararua, Ruahine – is an assemblage of hard-rock crust blocks bounded in the east by major faults; in the general direction of these is the almost continuous valley lowland between Hawke and Palliser Bays. Between this and the east coast, some minor ridges of hard rock project through the cover but none is of considerable height.
Similarly in the Auckland Peninsula, hard-rock ridges project through the cover in many places though none is as high as 3,000 ft. North of the Egmont-Ruapehu line, however, are most of New Zealand's diverse volcanic forms. From its broad base Ruapehu itself rises to 9,175 ft, with Ngauruhoe (7,515 ft), and Tongariro (6,517 ft) just to the north of it. The North Island west is dominated by the single cone of Egmont (8,260 ft). Banks Peninsula is the only notable volcanic form in the South Island, but this is much older, has been much more reduced by erosion, and is only 3,014 ft in maximum height.
The South Island, almost wholly mountainous, is a complex assemblage of crust blocks or slabs raised high in the Kaikoura orogeny and vastly modified by subsequent erosion. Perhaps the best example of such a mountain landscape is that of Nelson and Marlborough, made of a series of north-east to south-west trending fault-bounded block mountains. Of these faults, those defining the valleys of the Wairau, Awatere, and Clarence Rivers are perhaps the most sharply defined, and the two Kaikoura ranges themselves the most striking examples of this unit crust block type of mountain range.
In North Canterbury, similar fault-bounded crust blocks enclose wide basin plains such as those of Culverden and Hanmer, but at the “narrow waist” of the island the mountain zone is tightly compressed to a width of some 50 miles. In the west it is bounded abruptly by the Alpine fault marking the base there of the Southern Alps proper.
Again, in South Canterbury and in Central Otago, the mountain country opens out into the range and basin type, the Mackenzie and the Maniototo being the largest intermontane basins. In the highland of Canterbury, western Otago, and Fiordland, ice sculpture has left a special mark in mountain ridges and valleys alike. Fiordland is an ancient hard-rock plateau showing now a remarkable uniformity of summit level of the ridges into which it has been carved by running water and, intermittently, by ice.