Whether it is the gentle wash of sand-bearing waters against estuarine plants, or storm waves attacking crumbling cliffs, constructive and destructive forces are constantly at play on the coast. Some parts of the coast are being eroded by waves. Others are growing as the sea deposits sediment on beaches or in subtidal bars. It is convenient to divide the coast into two sections, based on these different processes:
- rocky coasts that are eroding
- beaches developing from deposited material.
Much of the New Zealand coastline is steep cliffs. Some, composed of soft mudstones and jointed sandstones, are eroding rapidly. Dramatic examples are to be found on the east coast of the North Island. As the waves cut away at the cliff face, it retreats landward, leaving wide tidal platforms and rockfalls.
In contrast, the resistant granite and gneiss rocks of Fiordland in the deep south have been little affected by the regular pounding of rough seas. Steep cliffs plunge directly into the sea, with no beaches or only narrow shore platforms.
Sea levels in New Zealand have risen on average 1.6 millimetres each year since the start of the 20th century. This amounts to a strip of land about 15 centimetres wide being lost from around the coastline since 1900. Scientists predict the rise in sea level will be two or three times greater this century as global warming increases.
About a third of New Zealand’s coastline consists of beaches. Sandy beaches are well represented on the North Island’s west and north-east coasts, at the top and bottom of the South Island, and on Stewart Island. Mixed sand and gravel beaches make up nearly a quarter of the coastline and are typical of the Canterbury and Hawke’s Bay coasts.
The composition of the beach reflects the energy of the waves depositing the material. Mixed shingle and gravel beaches form along exposed coasts where waves have enough energy to throw up sizeable stones. Fine silty muds accumulate in sheltered bays with gentle waves. Occasionally, storm waves may deliver large stones and boulders high up the beach.
How beaches form
The sediment that builds up beaches comes from three sources:
- Eroding coastal cliffs. For example, the Nelson Boulder Bank has formed from rocks falling from Mackays Bluff, some 13.5 kilometres from the bank’s southern tip. Powerful currents transport eroded rock material southwards.
- Rivers. Glaciers and rivers carry huge amounts of sediment to the coast. Some is carried offshore and deposited beyond the reach of breaking waves, and some is swept up in coastal currents and transported onto beaches. The latter process has produced the distinctive black-sand beaches on the west coasts of both main islands. Rivers flowing from the volcanic rocks of Mt Taranaki (Mt Egmont) and the central plateau volcanoes supply black ironsand (titanomagnetite) onto the west coast beaches of the North Island. The South Island’s black sand is mainly ilmenite, carried down rivers from schist rocks in the Southern Alps.
- Offshore sediments. During the last glacial period, about 20,000 years ago, sea level was 120 metres below the present-day level, and more of the continental shelf around New Zealand was exposed. Rivers, glaciers and wind carried eroded debris from the hills and mountains onto the shelf. These ancient deposits have been swept ashore for thousands of years by prevailing currents, creating sandy beaches and barriers. Farewell Spit, a 24-kilometre sandbar at the north-western tip of the South Island, is a prime example.
Large earthquakes have shaped parts of the coast in a dramatic fashion. A series of uplifted beaches is preserved at Turakirae Head on the southern coast of the North Island. The youngest was raised 6.4 metres in January 1855.