The main agent of erosion at headlands, cliffs and beaches is moving water,
principally in the form of waves. As a rule the stronger the waves the more erosion will take place.
However, the impact of waves is moderated by the geology of the cliff.
Hard rocks erode more slowly than soft rocks.
We call this differential erosion.
Therefore part of the coastline made of harder volcanic
rocks like basalt or andesite tend to jut out as headlands, while softer sandstones
and mudstones just erode back, eventually forming bays.
Over time the coastline would completely straighten if not for the fact that land building processes of
volcanism, folding and faulting are a dynamic force
that is constantly changing the shape of the coastline.
Normal ocean waves result from the frictional drag of wind on the surface of the water.
Waves have a crest and a trough.
The distance between crests being known as the wavelength.
Waves travel in groups called a wave train.
Ocean waves can travel hundreds of kilometers before they reach the land.
The distance of water over which the wind has blown is called the fetch.
A long fetch will result in big waves,
and big waves carry with them a tremendous amount of energy.
Storms create even bigger waves.
Thus a lot of erosion takes place during storms.
Even on calm days it is possible to get big waves as these ones have been generated in a storm somewhere else some time ago.
Waves that have been generated somewhere else are called swell.
Swell waves have a long wavelength and are considered desirable by surfers compared
to the choppy locally produced waves which have short wavelengths.
The prevailing wind direction on the west coast of the North Island is southwesterly.
The Tasman sea has very few islands in the way and therefore a very long fetch.
That is why waves on the west coast are so frequent and so large.
When large waves are constantly hammering away at the coastline the
processes of hydraulic action and abrasion can cause cliffs to retreat.
Hydraulic action also known as quarrying is when waves smashing against cracks and
joints in the rock compress the air in those cracks.
The pressure created by this can eventually cause fragments of rock to break off.
Abrasion occurs when wave action causes rocks to rub against each other.
As this happens the original rocks reduce in size to smaller and rounder fragments.
The waves cause more erosion by also hurling the small rocks against the cliff face.
As these processes take place, a notch can form at the base of a cliff
This notch gets progressively bigger until a cliff eventually collapses.
Then the whole process starts all over again.
Understanding the workings of erosion processes such as cliff retreat is very
important for people, as they impact heavily on any human coastal development.
As a result of a collapsing cliff face there is a lot of debris made up of large boulders.
Walking below cliffs can be a dangerous activity.
Here at Maori Bay you can see the evidence of cliff retreat
in the form of this pile of debris at the base of the cliff.
Another feature that is formed by the process of cliff retreat is called a shore platform.
These are also known as wave cut platforms.
These are broad flat areas of rock that are often found at the base of cliffs and headlands.
Many short platforms also have a notch which is overhang by a visor
like this one at Fisherman's Rock at the end of Otakamiro Point.
At the edge of the platform there is an escarpment, which at Fisherman's Rock is over 2 meters.
They form because the forces of waves is only eroding the rock between the high and low water mark.
Thus the rock below the low-water mark remains.
In these places the debris from the collapsed cliffs is broken down by
the waves and transported out to sea.
As I mentioned earlier the West Coast beaches have a mixed geology
which leads to differential erosion resulting in the formation of a variety of coastal features
such as arches
caves
blow holes
and stacks.
For caves to formed there must be weaknesses in the rock of a cliff
or headland such as soft rock types or an existing crack.
Over time hydraulic action makes these cracks bigger and bigger until a cave forms.
Sometimes a cave roof will partially collapse to form as a blowhole such as this one here on Fisherman's Rock.
Other times the waves will erode the cave right through to the other side of the headland.
Eventually the ceiling of the cave will completely collapse causing
part of the headland to detach
and form a vertical column of rock in the sea which we know as a stack.
This sea cave at Maori Bay is formed by the processes of differential erosion.
As you can see, the softer rock has eroded first, leaving the hard
volcanic rock to form the walls of the cave.
That stack there is Motutara Island.
It used to be part of the headland and some somewhere along the line it collapsed.
It really stinks here it. That is because of that white stuff on those rocks.
Another peculiar type of erosion feature found on rocky shores around the
world is called honeycomb weathering. For example, these rocks here at Muriwai.
This unusual but common coastal formation is caused by salt crystals
getting into cracks in the rock.
Sea spray and wind are the usual ways for
salt water to make its way into these places.
When the water content evaporates salt crystals are left behind.
The rocks go through a wetting and drying process
as they are often found in the intertidal zones.
When they dry out from the Sun the crystals heat up and expand, which puts more pressure on the confining rock.