erosion is accomplished in three main ways, with each of the three ways
having subsidiary methods, as detailed below;
Wave quarrying affects previously unconsolidated rock. A wave
exerts considerable impact, or shock pressure as many tonnes of
water hit the rock face. Since energy is proportional to height, storm
waves do large amounts of damage in a short space of time. Some storm
waves have been recorded up to 50 kg/cm3. This high pressure
exists for a brief instant, but can act along fault, joint and bedding
planes. It is similar to hydraulic action in rivers. If air is trapped,
then pneumatic pressure may assist in loosening blocks, which can extend
the erosion above the water line.
abrasion is the most effective type of wave erosion. The wave contains
debris of varying sizes, which is hurled against the cliff. This wears
away the rock. In hard rock areas, the action is slow, and produces smoothed
areas. Differential erosion between hard and soft rock may accentuate
the layering of the rock.
attrition affects the debris within the wave. The particles reduce
in size and angularity as they collide with each other within the wave.
Most of this takes place within the breaker zone. The range of attrition
varies with the tides.
Crystallisation is effective where there is a high evaporation potential,
and the growth of chloride slats derived from the salt in sea-water attacks
a wide variety of rocks. It loosens fragments of rocks for erosion
on the coast is notable on limestones. Sea-water is often saturated with
calcium carbonate, and it is hard to see why it is so effective. This
is particularly true if the tropics, where CaCO3 levels decrease
with temperature increase. It is possible that photosynthesis in plants
oxygenates the water during daylight, but releases carbon dioxide in to
the water at night, increasing the acidity and effectiveness of solution.
The morphological effect is to produce sharp fretted pinnacles of limestone
called lapies, low down in the inter-tidal zone. When wave attack
is less prevalent, solution may cut notches at the edge of pools and leave
overhanging lips. In tropical seas with microtidal or mesotidal ranges,
these features become very large and are called visors.
activity assists solution. Other rock types besides limestone are
attacked by secretions, particularly from the blue-green algae which live
between the tide lines. Seaweed firmly attaches itself to rocks, which
effectively increases the exposed surface area.
a wave approaches a headland, it must refract around the headland. This
has several effects. The first of these is that the wavelength shortens.
The wave depth decreases, and height increases. The energy is focused
on to specific points either side of the headland, and it is in these
points that caves often begin to form. The concentration of energy is
accompanied by a rise in wave height. Since energy is proportional to
wave height, then the power of the waves is greater on the headland. As
waves approach the shoreline and enter progressively shallower water,
the speed at which they move no longer depends on the size of the wave,
but becomes entirely dependent on the depth of water. In the shallower
water commonly found off headlands, a wave crest moves more slowly than
it would in the deeper water of adjoining bays. The wave crest in the
bay, therefore, moves ahead of the wave crests approaching the headlands
on either side, and the wave appears to be bent into the bay. This phenomenon
is known as refraction. Because of refraction, waves commonly approach
the coast with their crests closely paralleling the line of the shore,
although the actual plan shape of the wave will depend on the contours
of the seabed close to the shore. Wave refraction also
tends to concentrate wave energy on headlands, at the expense of adjoining
bays; material eroded from the headlands washes into the bays.