Erosional coastal landforms

Question 1

How do wave-cut platforms form?

Answer 1

When waves break against the foot of a cliff, erosion (hydraulic action, cavitation, and corrasion) tends to be concentrated close to the high-tide line in the intertidal zone (between low and high tide). The creates a wave cut notch.

As the notch gets bigger, the cliff is undercut and the rock above it becomes unstable, eventually collapsing and causing the cliff to retreat. This aided by weathering process above such as freeze thaw.

As the erosional processes are repeated, the notch migrates inland and the cliff retreats leaving behind a gently sloping wave-cut platform, which is usually only completely exposed at low tide. The platform is often marked by rock pools created due to abrasion and smoothed by the action of sediment rolling over the rock.

Question 2

What are the factors affecting erosion?

Answer 2

Rock type - softer rocks eg. clay will erode at a faster rate than harder rocks eg. limestone

Geological structure - cracks, joints or faults in a rock front will be more susceptible so erosion

Wave type - destructive waves have more energy so are more likely to erode the rock

Presence of a beach - beaches create friction between the wave and sediment, which decreases velocity and therefore decreases erosional power

Coastal management - eg. sea walls deflect wave energy to erosion decreases. Groynes decrease erosion at specific points of the coastline however increase erosion downdrift

Sub-aerial processes - eg. weathering and mass movement act together and weaken the cliff making it more susceptible to erosion

Question 3

Why is erosion concentrated on headlands?

Answer 3

When wave action has eroded the
coastline into bays and headlands,
headlands receive a concentration of wave energy.

This is due to the fact that the headland is exposed on three sides to the sea.

The process of wave refraction then
exposes the headland to further, and more concentrated erosion, which creates a process of negative feedback

Question 4

How do geos and caves form?

Answer 4

• Rock fractures at the intertidal zone of the headland become enlarged due to hydraulic action, abrasion and corrasion.
• Fissures and cracks are also widened by weathering processes such as salt crystallisation, carbonation and oxidation.
• As they become bigger they form a Geos (a long, narrow, steep-sided cleft) and crevices.
• Developing crevices may enlarge into small caves with a larger surface area exposed to weathering and erosional processes, increasing the rate of rock removal.

Question 5

How are blowholes formed?

Answer 5

When a cave is formed, the power of the waves creates high pressures within the cave. Occasionally the pressure of air, compressed in the caves by the waves weakens the roof along a major joint. The air trapped inside the cave tries to escape through any vertical opening or weakness. This opening is called blowhole. It is usually found at the top of the headland above the cave.

Question 6

How do arches form?

Answer 6

• Wave refraction means the lateral
  sides of the headlands are exposed to
  oncoming destructive waves, not just
  the cliff face orientated directly out to
  sea
• Caves developing on opposite
  sides of a head land may erode
  deeper as a result of wave refraction
  until they meet, coalesce, and form
  an arch

Question 7

How do stacks form?

Answer 7

• The arch roof becomes narrower,
  weathered by sub-aerial processes
  from above and corrosion, salt
  crystallisation, carbonation and
  oxidation by sea spray from below. It
  collapses.

Question 8

How do stumps form?

Answer 8

• The resultant stack is weathered by
  sub-aerial processes at its top, and by
  marine processes at its base. It becomes narrow and shorter until all that remains is a stump.
• The stump is eventually eroded
  entirely as it resides in the intertidal
  zone of constant exposure to erosion
  and weathering.

Question 9

How are headlands and bays formed?

Answer 9

Headlands are formed when the sea attacks a section of coast with alternating bands of hard and soft rock. The bands of soft rock such as sand and clays are eroded more quickly than resistant rocks such as chalk. This leaves a section of land jutting out called a headland. The area where the softer rock has eroded away next to the headland are bays

Question 10

What are concordant and discordant coastlines?

Answer 10

Discordant - bands of soft and hard rock running at right angles to the coastline

Concordant - bands of soft and hard rock running parallel to the coastline