THE GEOLOGY OF NEW ZEALAND

Introduction

The following account is a simple description of the geology of the New Zealand region. This includes North, South and Stewart Islands, and also the surrounding continental shelf, an area of sea floor which slopes gently to about 200 metres below sea level. The shelf is an integral part of the New Zealand region, a part which at present is below sea level, but in the past has been land. The land and continental shelf areas together, termed "Zealandia", have had a long and varied geological history; a history of see-sawing distribution of land and sea, of quiescent periods ended by powerful mountain building phases, of vast sediment-collecting ocean troughs and of small isolated basins. They have also undergone many climatic changes, varying from tropical heat to chilling ice ages. The distribution and nature of the rocks in New Zealand at the present-day reflect the events of the past. The detailed study of the rocks enables us to build up a geological history; but there remain many gaps in our knowledge and the rocks still hold their secrets.

Geological setting

The New Zealand region lies in the southwest of the Pacific Ocean astride a distinct belt of volcanic and earthquake activity that surrounds the Pacific Ocean. This is the Pacific Mobile Belt or "Ring of Fire" and the activity results from the structure of the Earth's crust. The crust is made up of a number of segments called plates, which move relative to one another in response to forces deep within the Earth. The plates may rub past one another, one may be forced down below another, or they may buckle at the edges as they meet head on. Wherever there is a plate boundary there is geological activity of a volcanic or tectonic nature. New Zealand straddles the boundary between the Pacific and Indian-Australian plates. To the north of New Zealand and beneath the eastern North Island, the thin, dense, Pacific plate moves down beneath the thicker, lighter Indian-Australian plate in a process known as subduction; within the South Island the plate margin is marked by the Alpine Fault and here the plates rub past each other horizontally; while south of New Zealand the Indian-Australian plate is forced below the Pacific plate. Plate movement results in volcanic activity in the North Island and in earthquakes that are felt throughout the country.

Topography and its relation to geology

Northwest Nelson and Fiordland are rugged mountainous areas formed of very hard rocks, some of which are among the oldest in New Zealand. Their present mountainous form results not only from recent mountain building, but also from their greater resistance to erosion. The Southern Alps and the axial ranges of the North Island form the "backbone" of New Zealand. These mountains are mainly composed of hard sandstone and mudstone, collectively known as "greywacke", of Mesozoic age, but the southern and western parts of the Southern Alps are formed of schist. The uplift of these ranges began about 15 million years ago and has accelerated in the last few million years. The total amount of uplift in that period has been estimated to be in the order of 20 000 metres, but continuing erosion is responsible for the present height (up to 4000 m) as well as the dissected nature of the country.

A feature of the Central Otago area is the flat, even-topped, rather subdued, schist topography, commonly with rocky outcrops (tors). About 70 million years ago this part of New Zealand was reduced by erosion to a nearly level plain (peneplain) close to sea level. This level surface has been particularly well preserved as it has been only gently uplifted and tilted rather than complexly deformed.

A great proportion of the southern part of the North Island is formed of rather soft Tertiary rocks. The rocks are very similar in character throughout the area: blue-grey sandstone or mudstone, commonly known as "papa". They form a very characteristic topography of steep slopes and sharp ridges, reflecting the easily erodible nature of the rock. Harder rock types, such as limestone, stand out prominently, particularly in Hawkes Bay and the Wairarapa.

Limestone may have a dramatic effect on topography, commonly forming steep bare bluffs. It is soluble in water which enables erosion to take place along joints in the rock resulting in strange-looking "karst" landforms. Sinkholes occur where streams disappear below the surface to join extensive underground drainage systems, or where collapse of cave roofs has resulted from dissolving the rock. Caves, of course, are common in limestone country. Some systems continue for many kilometres. The best examples of limestone formations are near Te Kuiti in the North Island, and in northwest Nelson and south Canterbury in the South Island.

The cone-like silhouettes of volcanoes in Taranaki and Tongariro National Park dominate the topography, while further north equally remarkable flat-topped plateau areas are formed by ignimbrite flows. Volcanic cones are also prominent in the Auckland region, Bay of Plenty, and in some areas of Northland. Much of the topography in the northern half of the North Island has been modified by deposits of ash from repeated volcanic activity in the past million years. Many lakes in the central volcanic region are in the craters of previously active volcanic centres.

The climate of the last two million years (a time of successive cooling and warming) has had a major effect on present-day topography. Glaciers carved out huge U-shaped valleys in the mountains, and fans of alluvial detritus eroded from the bare mountain slopes built up during the colder times. The Canterbury Plains, for example, were built from glacial outwash. During warmer periods sea level rose and cut coastal terraces in rocks and alluvial debris; successive rises and falls of the sea to different levels formed the flights of terraces seen in the river valleys and around the coast. When sea level was low, wide areas of sand exposed to the wind were blown into dunes. They form the extensive dune country of the west coast of the North Island and Northland.

The Marlborough Sounds, Fiordland and eastern Stewart Island are examples of drowned topography. River and glacier valley systems formed at times of lower sea level were flooded by the sea when it rose to its present level.

Classes of rock

Rock units are divided into three major classes based on their mode of formation.

Sedimentary rocks result from deposition and consolidation of particles mostly eroded from an adjacent land area. Different sources of material and depositional environments result in different types of sedimentary rock, such as sandstone, mudstone, and limestone. Most New Zealand sedimentary rocks are mudstone and sandstone that were deposited beneath the sea. A sedimentary rock derived from ejected volcanic fragments (e.g., ash) is called tuff. Some sedimentary rocks, such as chert and certain limestones, are chemical precipitates.

Igneous rocks form when molten rock (magma) from deep within the Earth's crust cools after being intruded into existing rocks or sediments, or after being extruded on to the surface. The rocks are classified as acid, intermediate, or basic, according to the amount of silica they contain. Acid (silica-rich) igneous rocks are light in colour, whereas the basic (silica-poor) igneous rocks are darker. There is a secondary two-fold division of igneous rocks into: plutonic rocks which are intruded at depth, cool slowly, and are coarse grained (granite is an example of an acid plutonic rock, diorite is intermediate, and gabbro is basic); and volcanic rocks which are extruded on to the land surface, or beneath the sea, and cool quickly to form fine grained and sometimes glassy rocks. Acid volcanic rocks include rhyolite, ignimbrite, and pumice, the last two being formed when the lava was particularly gaseous and explosive in nature. Common intermediate and basic lavas are andesite and basalt respectively.

Metamorphic rocks such as schist and gneiss, have been recrystallised under conditions of high temperature, pressure, or both. They vary depending on the nature of the parent rock (either sedimentary or igneous) and the intensity and type of metamorphism. Characteristic minerals form under certain conditions of temperature and pressure. For example, chlorite tends to crystallise in rocks metamorphosed at low to moderate temperatures and pressures, and biotite and garnet at somewhat higher temperatures and pressures. Gneiss forms at deep levels in the crust under conditions of high temperature and pressure.

Geological processes

The processes of rock formation are never-ending. The rock cycle involves particles being continually eroded, transported, deposited, and cemented to form new rock that may later be uplifted so that erosion starts the cycle again. These uplift and erosional episodes make it impossible for any one area to have a rock sequence representing the whole of geological time. Only when an area is submerged beneath the sea can there be continuous sedimentation providing a complete time record, and even then there may be gaps (unconformities) resulting from removal of sediments by strong bottom currents, or periods of non-deposition. When an area is above sea level, sporadic terrestrial deposits such as fluvial sediments and coal measures, and igneous rocks may be the only representatives of that time.

Compressional and tensional forces produce movement of the Earth's crust (tectonism) causing it to buckle, warp, or crack. Long-continued downwarping may occur and form very extensive troughs or basins in which many thousands of metres of sediments can accumulate. The high temperature and pressure deep within the sedimentary pile can cause metamorphism. In contrast, slight downwarping allows the sea to extend over the land, producing shallow shelves close to the land and deeper basins further offshore. When compression forces the crust and overlying sediments upward, mountain chains are formed often accompanied by severe folding and fracturing of the rocks and intrusion of magma. The effect of mountain building (orogeny) varies from place to place, and locally the rocks may be only gently tilted.

Geological time scale and its relation to New Zealand

The geological time scale which is used worldwide divides the last 600 million years into several periods of differing lengths of time. The time scale was originally established on the rock sequences of Europe. Obvious breaks in sedimentation or change in rock type marked the limits of the periods. Unfortunately these breaks and changes do not occur worldwide, and it is sometimes difficult to apply the scale away from Europe. One way of correlating rock ages is to use the fossils contained within rock units. This is reasonably satisfactory as long as the animal or plant groups achieved a worldwide distribution. However, many groups did not, particularly if they preferred a particular climate or environment. Also some groups were in existence for such a long time that they do not characterise one particular geological period.

Another method of dating rocks is isotopic, where the proportions of particular isotopes of elements in the rocks are measured. Certain elements begin to decay radioactively as soon as the mineral crystallises, slowly changing their isotopic character. The decay rate is in effect a "geological clock". By measuring the amount of the parent and daughter elements and isotopes, and then applying the decay rate, the age of the rock can be calculated. This method is generally applied to igneous or metamorphic rocks, and even then may not be accurate if the rocks have undergone more heating and deformation after their formation. Another way of estimating geological time is to apply known rates of present-day sedimentation to the thickness of rock sequences.

It can be seen from the above that there is no way of absolutely relating all New Zealand rocks to the international time scale, as the age is inferred from global correlations, or from the relationship of one set of rocks to another whose age is known. The isolated position of New Zealand has made correlation with some other parts of the world difficult, while limited laboratory resources have meant that only a small number of isotopically determined ages are available. Thus the age of many New Zealand rocks is a not well known and is constantly under review.

Outline of geological history

Distribution of land and sea has varied greatly during the geological past. The present-day shape of New Zealand is shown on many maps, but millions of years ago the relative positions of land and sea were quite different. Some hundreds of millions of years ago a super-continent (Gondwanaland), which included the present-day continents of South America, Africa, Australia, India, and Antarctica, existed in the southern hemisphere surrounded by sea. The New Zealand area was situated on the edge of Gondwanaland. Since that time, movements from within the Earth have caused the constituent continents to break away from one another and move to their present positions - a process which is still continuing. The original super-continent was not stationary; it too responded to forces from within the Earth so that it was in different positions with respect to the Earth's poles at different times. Thus at various times the fossil record and the rocks may show evidence of either cold, temperate, or tropical climate.

The fossil record in the older rocks in New Zealand, and their composition, show their affinity with rocks of Australia and Antarctica of similar age. The relationship apparently ended between 100 and 80 million years ago when New Zealand broke away from Gondwanaland and started to move toward its present position, with the accompanying formation of the Tasman Sea. Since that time New Zealand has had its own geological history and developed a unique flora and fauna.

The very oldest sedimentary rocks in New Zealand were deposited in basins lying offshore from the landmass of Gondwanaland. Subsequently the sediments were disrupted by tectonic movements and pushed up to form land that eventually became parts of Australia, Antarctica, and New Zealand. Later, an extensive series of depositional troughs developed off-shore, which collected sediment eroded from adjacent continents for nearly two hundred million years. Here the "greywacke" rocks that now make up the main ranges of New Zealand were formed. This era came to a close about 110-120 million years ago when tectonic plate movements uplifted the sediments to form new land. A period of quiescence followed when erosion reduced much of the mountainous land to a low-lying, almost level plain. It was during this time that the split between Australia and New Zealand occurred.

As the land was reduced in height, low-lying swampy areas developed, which are now the sites of major coalfields. Eventually the sea started to cover the land, firstly depositing sediments in marginal basins, and later over most of the New Zealand area. Then, about 15 million years ago, the mainly quiet period ended, and New Zealand once again experienced tectonic activity, mountain building and widespread volcanic activity. Several basins developed, filled with sediment, and were uplifted as land. In more recent geological times, the effects of rises and falls of sea level, due to alternating glaciations and warmer intervals, were superimposed on the tectonic events.

It must be remembered that New Zealand is still involved in a continuing cycle of geological events, and the level of tectonic activity remains high. Offshore basins receiving sediment may, one day in the future, become land, while other areas onshore, being depressed, will be invaded by the sea. Our great mountains are being continually eroded - just look at the debris on their flanks and in the river valleys. Each major earthquake has an effect on the land; in 1855 the coastline of Wellington Harbour was uplifted 1.5 metres. Nothing is permanent in terms of geological time.