A drowned continent
The land area of New Zealand is a small part of a continent of nearly 4 million square kilometres (almost half the size of Australia, or about the size of western Europe). However, 93% of the New Zealand continent, sometimes called Zealandia, is underwater.
The continent is unusually long and thin. It stretches from latitude 19° south (north of tropical New Caledonia) to 56° south (south of New Zealand’s bleak subantarctic islands).
Continents and the deep ocean basins around them represent the two main levels that make up the earth’s surface:
- continents are generally close to sea level
- abyssal ocean basins are usually about 5 kilometres deep.
These two levels are separated by a steep zone known as the continental slope.
The rocks that make up continents and the rocks that make up ocean basins are different in origin and composition.
The floors of deep ocean basins are made up of heavy, basalt volcanic rocks. These form on spreading ridges in the middle of oceans. Spreading ridges have huge cracks in the earth’s crust where molten rock is squeezed out. The cooled rock spreads outward on both sides of the ridge, moving away from it and across the basin floor as if on a conveyor belt.
At the speed a fingernail grows, the rocks move on the conveyor belt towards the edges of the oceans. They become older, heavier and colder the further they are from the ridge. When they reach a plate boundary, ocean floor rocks may sink back (subduct) into the earth’s interior.
Around the edge of the Pacific Ocean, subduction occurs in many places, marked by deep ocean trenches. Parallel to trenches are lines of volcanic activity where water-rich ocean floor melts, and the magma moves up to the surface. The circle of volcanoes around the ocean is known as the Pacific Ring of Fire.
Compared to ocean basins, continents are very stable. When the conveyor belt of the ocean floor reaches a continent and sinks (subducts) into the earth’s interior, the sediment that has formed on the floor over millions of years is scraped off onto the continent. Continental rocks can be extremely old – ten times older than the oldest oceanic rocks. This is because continents have grown over hundreds of millions of years, from oceanic sediments that are added to the edges.
Continents are generally 25–30 kilometres thick, and are light and buoyant. They float on the earth’s mantle (the layer of semi-molten rock beneath the earth’s crust). Because of their buoyancy and thickness, continents ride high on the mantle, generally above the level of water in the oceans.
The restless seabed
Pushed by the ever-moving ocean basins, much of the shallower seabed around New Zealand is also moving. Submarine faults rip the seabed apart, leaking hot mineral-rich fluids, or they push it together to form new land, or tear it sideways. Fault movements generate earthquakes that trigger submarine landslides hundreds of times bigger than those on land.
A continent of two ridges
The New Zealand continent is principally made up of two almost parallel ridges. These are largely underwater and trend north-west through the southern Pacific Ocean. The western ridge includes Lord Howe Rise and the Campbell Plateau. The narrower eastern ridge forms New Caledonia, Norfolk Ridge, the Northland peninsula of New Zealand and the Chatham Rise.
Both ridges form sea floor 1,000–1,500 metres deep, with occasional rocky islets rising above the water. These islands help define the edge of New Zealand’s huge Exclusive Economic Zone. The ridges are continental rock, but are deeper than ordinary continents because they are thinner than normal (only about 20 kilometres thick), and therefore float lower on the earth’s mantle. These continental rocks pulled away from the edge of the great southern continent of Gondwana when the Tasman Sea opened, between 60 million and 85 million years ago.