Islands adrift: N.Z.’s long, slow voyage through the centuries

Press, 10 April 1980, Page 21

Islands adrift: N.Z.’s long, slow voyage through the centuries

Report from the Department of Lands and Survey

The islands of New' Zealand . are like rafts of cracking clay on an ocean of viscous, semi-molten rock. They heave and tilt, bend and fracture, as they are carried on a journey lasting thousands of centuries across the surface of the earth.

The earth was thought by the ancients to be solid and immovable beneath their feet. Evidence of violence and upheaval all around them was not understood and the occasional earthquake or volcanic eruption was seen as a sign of the wrath of some subterranean god. : W h e n • early in-

vestigators at last saw the fossil record that high mountains had once been beneath the sea, they thought the movement was restricted to the upward thrust of the mountain chains. They believed that the earth’s crust had

been squeezed into mountainous folds by the slow shrinkage of a cooling jvorld.

They likened the mountain ranges to the wrinkled skin of a baked apple fresh from the oven. They were sure that the

continents had remained forever in their primordial positions.

There were those who thought differently. As long ago as 1620 Sir Francis Bacon was struck by the jig-saw fit of the coasts of Africa and South America. He speculated that they may once have been pari of a super-conti-nent, now split into two widely separated pieces. The theory' of continental drift was first put into formal shape by the German scientist, Alfred Wegener, who pointed out in 1915 that it would explain many geological mysteries and the puzzling distribution of many species of plants and animals.

The closely-related family of flightless birds, including ostriches, emus, moas. kiwis, and rheas, are found in countries as far apart as South Africa, Australia, New Zealand, and Argentina.

There are similar examples in the plant world. Podocarp species are found in New Zealand, Australia, South Africa, and South America; and New Zealand shares close-ly-related species of southern beech (nothofagus) with Chile.

Most orthodox geologists ridiculed Wegener’s theories. They knew of no mechanism that could explain how or why continents could have moved from their original positions. But some geologists,

notably Alexander du Toit of South Africa and Lester King of New Zealand, believed otherwise. They remained convinced that the southern continents had all been joined together in a super-continent in some remote geological time. This now-dismembered continent had been named Gondwanaland after a province in India by the nineteenth-century Swiss scientist, Edward Suess, another believer in the drift theory. In the 1950 s and 1960 s evidence began to pour in supporting the theory of drifting continents. Studies

of the magnetic alignment of belts of rock on the floor of the Pacific were made by the research ship Glomar Challenger and these overturned the conventional view.

They showed beyond , doubt that molten material ’ from within the earth is surfacing - continually in the mid-Pacific and hardening into an immense north-south ridge dividing the eastern Pacific from the western Pacific.

This constant up-welling of new rock is forcing the already hardened sea-floor of both sides of the midocean rise further and further part. Two plates, or

sections of the earth’s crust, are drifting like icebergs, one eastward and one westward, into the lands on either side. The south-eastern Pacific, known as the Nasca plate, is being forced under South America, lifting the Andes as 1 it does so. The western Pacific plate is being forced under the chain of islands stretching from New Zealand through the Philippines and Japan to the Kurile Islands and the Kamchatka Peninsula. To complicate the picture, the Indian plate is

carrying the floor of the Indian Ocean, Australia, and part of New Zealand in a north-westerly direction and squeezing Indonesia against the mainland of Asia. A new Himalyan range is in the making.

The movement of this plate has shifted the Westport-Collingwood-Nel-son region some 500 km in the last 160 million years from its Original position near Fordland.

New Zealand is caught at the junction of two plates, one tearing us apart horizontally and the other thrusting under us from the east. The eastern side of the country is

being forced upwards as the floor of the Pacific dives beneath us and returns to the molten interior of the earth. Sometimes this movement is barely perceptible; at other times it is frighteningly rapid. The Napier earthquake of 1931 lifted the Hawke’s Bay shoreline by about two metres in as many minutes. The 1855 Wellington earthquake raised the western side of Wellington Harbour by three metres in as short a time.

Earthquakes happen with terrifying suddenness and cannot be predicted with any reliability. Chinese scientists have had some success in predicting earthquakes during the last 10 years,- but they were caught out badly when they failed to predict an earthquake which struck the Tangshan province in 1976, causing tremendous loss of life.

Many scientists now believe that by measuring the bending and shifting of the earth’s crust Over a long period they may gain an insight into how and when sudden movements along earthquake fault lines take place. In a project that will last for several decades, American scientists arcusing laser techniques to measure movement along the San Andreas fault which parallels the coast of California. It was movement along this fault which caused the San Francisco earthquake of 1906.

Surveyors from the Department of Lands and Survey are making a similar study of levels and positions along a broad band across the North Island from Castlepoint to Taranaki. It is part of a land deformation survey which will eventually cover the greater part of New Zealand. In most cases measurements of the required accuracy are being made now for the first time. Although much of New Zealand was surveyed during the last century, scientists cannot be certain how accurate these measurements were.

Our first surveyors had different aims from those of today. They were concerned mainly with defining property boundaries and did not need, the high degree of precision neces-

sary for the present survey. A link to the mile was the required standard of accuracy. The equipment being used by the Lands and Survey team in its present land deformation study is accurate to within one second in an arc, or 0.0003 of a degree. This is equivalent to only a few millimetres in every kilometre.

Further measurements in about 10 years’ time will show if the land has moved and by how much. The surveyors hope that deformations measured on the earth’s surface will reveal stress patterns in the deeper rocks below. This knowledge, together with information from other sources, may one day enable scientists to say where, and even when, sudden earth movemens may occur.

“It is not likely that New Zealand will be able to predict earthquakes for some time,” the SurveyorGeneral, lan Stirling, says. “The scientific method calls for the accumulation of evidence over a long period before a hypothesis can be proved or discarded.”