The Pacific Ocean is immensely deep: over millions of square miles the monotonous expanses of ocean floor lying 2–3 miles below the surface are interrupted only where great volcanoes have grown by the eruption of lava on to the bed of the ocean; the largest of these volcanic masses break the surface as Hawaii, Samoa, and other islands of the mid-Pacific. In its south-western part the Pacific is shallower and its floor is more irregular — it is diversified not by volcanoes but by deep trenches, broad ridges, and swells, where the earth's crust has been thrown into huge folds. One vast complex of ridges, rises, and plateaux lies about a thousand miles east of the continent of Australia, separated from it by the deep waters of the Tasman Sea. Much of this great system of folds is submerged, but a small part of it has risen and been shaped into a group of mountainous islands known today as New Zealand.
For hundreds of millions of years there have been land areas of slowly changing size and shape in this part of the Pacific: at times this land must have been large, at times small; it may even have altogether disappeared beneath the sea. A suggestion of the changing form of ancestral “New Zealand” during the later chapters of its long history is given in diagram 7. The evidence for these changes is recorded by the rocks of which this small portion of the earth's crust is built. New Zealand, unlike the wholly volcanic islands of the central Pacific, consists of rocks of a wide variety of types and ages. The intense folding and shearing that they display suggests that this country has long been one of the earth's “mobile belts” — part of a region where the outer part of the earth's crust has been buckling and breaking at a geologically rapid rate. The rocks are cut by innumerable great fractures, called faults, which have displaced them thousands of feet; even young marine sedimentary rocks have been involved in these movements and raised thousands of feet above the level of the sea. Many of the faults have broken the present land surface, showing that the crust blocks adjacent to them have been moving during the past few centuries or thousands of years (diagram 8–9). Even in the short time that Europeans have populated New Zealand, there have been many movements of the crust: shore platforms rose 5 ft from the sea at Wellington during the severe earthquake of 1855; a sudden fault movement raised one part of a road 12 ft above the other at Murchison in 1929, and 5 sq. miles of land rose from the sea at Napier during the Hawke's Bay earthquake of 1931. The presence of active volcanoes is further evidence that New Zealand remains a part of the circum-Pacific mobile belt.
Below, a summary is given of what geologists have learned of New Zealand's geological history: here, as in every part of the world, there is seen in the geological record “no vestige of a beginning, no prospect of an end”. New Zealand's rocks have been formed and its landscape shaped by the continuous interplay during millions of years of the universal geological processes of earth movements, erosion, sedimentation, metamorphism, and igneous activity. The earth's crust in the New Zealand region, yielding to great pressure, has constantly been warping and breaking. Some areas have sunk, others have risen to form land which rain and wind, waves and glaciers have attacked, carrying mud and gravel and sand to the lowlands and out to the sea where this debris has accumulated as thick deposits of sandstone, mudstone, conglomerate, and other sedimentary rocks. These in turn have been raised to form new land.
At times parts of New Zealand were the sites of huge, slowly sinking basins of deposition, called geosynclines, in which sedimentary rock layers accumulated to thicknesses of tens of thousands of feet. In time the geosynclines were compressed and their fill of sedimentary rocks folded, sheared, and raised, to be sculptured by erosion into mountain ranges. The deeper rocks of the geosynclines, subjected for millions of years to great pressure, temperature, and shearing stress, were transformed to slate, schist, gneiss, marble, and other metamorphic rocks: Huge bodies and tongues of molten rock, formed perhaps by melting of deeper rocks of the geosynclines, invaded the roots of the growing mountain ranges, cooling there to form large masses of granite, diorite, gabbro, and other intrusive igneous rocks; volcanoes at times poured out lava and ash and other volcanic rocks, both on to the land and on to the sea floor.
Erosion has constantly been at work removing the outer, younger rocks, revealing again older sedimentary strata and the ancient intrusive and metamorphic rocks that once lay thousands of feet below. The present distribution of New Zealand's rocks, the product of this complex interplay of geological processes during a vast span of time, is summarised by the accompanying small-scale geological maps and by those showing geological cross sections.