When wave energy is high coastal erosion takes place resulting in
a variety of coastal features. The rate of erosion is moderated by the mixed
geology of the coastline. Hard rocks such as basalt are more resistant and thus
erode much slower than soft sedimentary rocks such as limestone. We call this
differential erosion. Coastlines that have an alternating geology of harder and
softer rocks results in the formation of headlands and bays as can be seen here
in Port Macquarie. The places where the rock is harder has headlands jutting out.
Where the rock is softer more erosion takes place and bays form. Over time the
headlands become more exposed to the erosive powers of waves due to rain
refraction. This occurs because the waves slow down first in shallower water
forcing the refracting parts of the wave to break first at the headland,
while the other part of the wave is traveling faster towards the beach in
deeper water. The erosion at headlands results in the formation of spectacular
coastal features such as cliffs, wave cut platforms, arches,
caves, blowholes
and stacks.
When large amounts of water are constantly hammering away at the coastline the
processes of hydraulic action and abrasion can cause cliffs to retreat and
create a feature called a wave cut platform. Wave cut platforms are broad, flat
areas of rock that are often found that the base of cliffs and headlands.
At the edge of the platform there is an escarpment. Hydraulic action is when waves
smashing against cracks and joints in the rock compress the air those cracks.
The pressure created by this can eventually cause fragments of rock to
break off. Abrasion occurs when wave action transports sediment which hits the
cliff and gradually wears it away. Attrition also takes place where the
waves cause rocks to rub against each other. As this happens the original rocks
reduce in size to small and rounder fragments. As these processes take place
a wave cut notch can form at the base of the cliff. The notch forms because the
force of waves is only eroding the rock between the high and low water
mark. This causes the rock below the low-water mark to remain. Overtime this
notch gets progressively bigger until the cliff eventually collapses. As a
result of collapsing cliff faces there is a lot of debris at the base made up
of large boulders. In these places the debris from collapsed cliffs is broken
down by waves. The backwash carries the debris towards the sea forming the wave
cut platform. Then the whole process repeats and the cliff continues to
retreat. Understanding the workings of erosion processes such as cliff retreat
is very important for people as they impact heavily on any human coastal
development. For caves to forum there must be a
weakness in the headland's rock, such as a soft rock type 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 blow hole such as
this one here. Other times the waves will erode the cave
right through to the other side of a headland
to form an arch. 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 known as a stack. Another peculiar type of erosion feature found
on rocky shores around the world is called honeycomb weathering. This
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 intertidal
zones. When they dry from the Sun the crystals heat up and expand which puts
pressure on the confining rock.