Satellite derived gravity data, originally intended to guide missiles during the Cold War, but publicly released over recent years, is revolutionising knowledge of the Earth’s structure in oceanic areas including the vast ocean area around New Zealand.
Commercial fishing operators have been quick to target possible new fishing grounds highlighted on the maps of the gravity data produced by a team of marine geophysicists at the Institute of Geological and Nuclear Sciences (IGNS).
The gravity data is derived from satellite radar altimeters which measure the sea surface height beneath a satellite orbiting the Earth.
Where there is a dense body beneath the sea surface, such as a volcano, the gravitational pull of that body attracts the sea water from adjoining areas, locally elevating the height of the sea surface.
Above comparatively low-density sediment-filled basins or deep ocean trenches, the pull of gravity is less at the sea surface. As a result, water flows to adjacent regions, and the water height is reduced. The difference between high and low areas of sea varies from a few tens of centimetres to as much as a metre or two.
Sea surface height data was partly classified by the United States military until four years ago, when non-military scientists began gaining access to it.
Scientists at research institutions in the U.S. have corrected this sea-surface height data for distorting effects including ocean tides, currents and wave motion. This seemingly impossible task is simplified by the fact that every time the satellite passes overhead, more measurements are made, allowing temporal variables to be averaged out.
The resulting gravity values, combined with known water depths from ship soundings, permit predictions of water depths over unsurveyed areas of the ocean to be made.
Scientists at IGNS have developed techniques for producing maps from all this gravity data, and the resulting maps show, in never before-realised detail, structures that have evolved over the last 100 million years as Earth’s surface has been deformed by the movements of tectonic plates.
Not only have the maps been a boon to geophysicists trying to decipher the history of New Zealand’s continental area, but they have also revealed previously unknown seamounts and ridges. These previously unfished shallow-water havens are prime targets for fishermen, particularly if they lie outside the jurisdiction of the New Zealand fish quota management regime. IGNS has been commissioned to produce maps of the data for areas as far away as the middle of the North Atlantic.
The flood of data has been so rapid that scientists are still interpreting the structures revealed. The prominent line of seamounts called the Louisville Ridge, (in the north-east of the map reproduced on the opposite page), has been recognised and interpreted for some years by scientists as a hot-spot trail a sort of burn mark on the Earth’s surface created by the motion of the Pacific Plate over a fixed hot-spot in the Earth’s interior. (The Hawaiian Islands are the result of the same phenomenon.) The volcanic chain is particularly well defined by the gravity data. However, crossing it at right angles and trending south-west towards the Chatham Islands is a structure that has only become apparent with this release of data perhaps a 100 million-year-old extinct spreading ridge.
A similar spreading ridge where new ocean crust is created as the Earth’s plates pull apart is active today in the mid-Pacific. Scientists are still debating the impact such a structure would have had on New Zealand’s formation, as well as on plate motion recon
structions worldwide. A survey involving a U.S. research vessel and scientists in cooperation with N.Z. scientists has been proposed to help resolve the nature and age of the structure.
The long extinct New Zealand-Australia spreading ridge highlighted by darker green at the west of the map is a relic from when New Zealand and Australia stopped moving away from each other 58 million years ago. The gravity map shows previously unrecognised details of the spreading process.
The deep ocean crust to the east of this ridge is bordered by the shallower continental crust of the Challenger Plateau-Lord Howe Rise. This area of the Tasman Sea, which shows in the gravity map many of the signs of stretching prior to splitting apart, is currently being surveyed by IGNS and NIWA scientists as part of the Law of the Sea investigations between Australia and New Zealand.
The deep blue (gravity low) areas to the north-east and south-west of New Zealand mark the Kermadec and Puysegur Trenches associated with the bending down of the Pacific and Indian Plates, which are diving into the Earth’s interior in opposite directions at either end of New Zealand.
The gravity lows visible north of East Cape, east of Kaikoura and in the Wanganui Bight are produced by sedimentary basins full of low-density sediment rather than deep water a reminder that this is a gravity map, and that although depth is the main factor in determining gravity, there are anomalies such as these. The basins are principally formed by buckling of the Indian and Pacific Plates as they scrape past each other The deep Vanganui Taranaki basin has proved a good foundry for the generation of most of New Zealand’s petroleum resources. IGNS has just published three satellite-derived gravity anomaly maps at a scale of 14,000,000.