Like most land masses, New Zealand is surrounded by a continental shelf. Fishermen have long known that there is a gently sloping seabed that drops away (at a point called the shelf break), usually at a depth of 100–160 metres. The shelf extends for only a few hundred metres off Fiordland, but is over 100 kilometres in western Cook Strait.
The shelf seabed is of great interest to New Zealand: it supports near-shore fisheries and is the most accessible area to exploit marine minerals.
Effect of the ice ages
During the last ice age, when much of the world’s water was frozen in polar ice-caps, the sea was about 120 metres below its present level. What is now the continental shelf was, in the ice age of 20,000 years ago, a bleak windswept plain that was smoothed by wave action. The shelf grew outward as rivers dumped sediment there.
Nearer to the present-day shoreline, as sea levels rose due to melting ice, parts of the continental shelf were cut by waves. In places where the land has since risen, old wave-cut platforms and cliff lines were preserved as terraces, and can now be seen in many places on the New Zealand coast.
It is about 7,000 years since the sea stopped rising at the end of the last ice age. The continental shelf, including old river valleys such as the Marlborough Sounds, is now drowned.
Two meanings of ‘continental shelf’
Seafarers have long understood the continental shelf to be the flat seabed extending from the shore to where the sea floor drops away more steeply onto the continental slope. This is sometimes confused with a more recent legal use of the same term.
Since the 1980s international lawyers have worked to clarify a nation’s jurisdiction of the seabed that surrounds the land. The legal concept of an ‘extended continental shelf’ encompasses the whole area underlain by continental crust, and includes not only the shelf and the slope, but also some distance across the deep ocean floor. Most of the continent of Zealandia is submerged, enabling New Zealand to have jurisdiction over a very large offshore area.