Coasts made of solid rock, or sediment deposits with tightly packed or cemented grains, are known as hard coasts and are relatively resistant to erosion. When hard coasts erode they form cliffs, which is evidence of constant wearing away over thousands of years.
The erosion of rocky coasts is not reversible, and loss is permanent. However, the rate of erosion is less than for sandy shores. Hard coasts erode at a rate predominantly controlled by the strength of the rock; waves play only a minor role.
Some people think that the rise in sea level associated with global warming has caused coastal erosion. But erosion occurs naturally whether sea levels are rising, falling or static. However, if sea levels continue to rise as they have in the last century it is likely that vulnerable coastal areas will continue to erode, possibly at an accelerating rate. And coasts as yet unaffected by erosion may begin to experience it.
The strength of a rocky coast depends mainly on how many breaks there are between the blocks of rock. The spacing between breaks, their angle, length and width, and what material (if any) they are filled with all influence how susceptible cliffs are. Lots of breaks close together result in increased erosion that can form sea caves, arches, and eventually rock pillars known as stacks. The rate and magnitude of erosion is also affected by the strength of intact blocks, the amount of water present within them, and the degree to which they have been weakened by weathering.
The stability of steep slopes, and the climate – particularly rainfall – are factors in erosion. Earthquakes can trigger slope failures.
Some cliffs made of soft sedimentary rocks, such as papa (mudstone) can retreat inland up to 1–2 metres each year, while hard rock erosion rates can be imperceptible over a year. Cliffs of soft sedimentary rock are particularly vulnerable to waves. The cliffs retreat as waves undermine the base, causing rock faces to slip into the sea.
The rate of cliff erosion along most of the New Zealand coast is very slow, and its cliffs have been shaped by processes that occurred before the sea reached its present level, about 7,200 years ago. For example, glaciers carved the fiords of Fiordland, the East Cape’s headland bays are the result of valleys flooding as sea levels rose, and the Marlborough Sounds are a combination of inundated river valleys and land subsidence.