3.4 Coastal Erosion- Processes And Landforms Flashcards
What are the ways in which the foot of a cliff is eroded by waves?
Abrasion
Hydraulic action
Corrosion
What is abrasion?
Also known as corrasion
When waves advance, they pick up sand and pebbles from the seabed. Then, when they break at the base of the cliff, the transported material is hurled at the cliff foot- chipping away the rock.
What is hydraulic action?
When a wave advances, air can be trapped and compressed (either in joints in the rock forming the cliff, or between the breaking wave and the cliff). Then when the wave retreats, the compressed air expands again. This continuous process can weaken joints and cracks in the cliff- causing pieces of rock to break off.
How is hydraulic action different when there’s high velocity waves?
At high velocities, when bubbles form in the water and then collapse, they erode by hammer-like pressure effects
What is corrosion?
When cliffs are formed from alkaline rock- carbonate rocks (such as chalk or limestone)- or alkaline cement bonds rock particles together, solution by weak acids in seawater can dissolve them
What type of erosion is most dominant during storms?
Hydraulic action and abrasion
What is attrition?
The process acts on already eroded sediment
As sediment is moved around by waves, the numerous collisions between particles slowly chip fragments off the sediment
The net result is that sediment gets smaller and more rounded over time
How does lithology influence erosion on a small scale?
Any weaknesses such as joints, bedding planes or faults are eroded more quickly, which can result in the formation of landforms.
Bands of more resistant rock between weaker joints and cracks erode more slowly. This selective erosion of areas of weakness is called differential erosion
How does lithology influence erosion on a large scale?
Areas of resistant rock, generally form headlands and areas of weaker rock form lowland areas with bays and inlets.
What are stacks and stumps?
These are isolated pillars of rock that lie just off the coast and are surrounded by water. The stumps are completely covered at high tide
What is an example of stacks and stumps?
The twelve apostles on the Great Ocean Road in Melbourne, Australia
What are shoreline platforms?
Flat rocky platforms that extend out from the coast and surround the isolated stacks and stumps
What is an example of a wave cut notch and shoreline platform?
Flamborough Head in Yorkshire
How do wave cut notches form?
When waves break against the foot of a cliff, erosion tends to be concentrated close to the high-tide line. This creates a wave cut notch, which begins to undercut the cliff. As the wave cut notch gets bigger, the rock above becomes unstable and eventually the upper part of the cliff will collapse
How do shoreline platforms form?
As wave cut notches continually undercut the cliff and erosional processes are repeated, the notch migrates inland and the cliff retreats, leaving behind a shoreline platform
What are the features of a shoreline platform?
It has a gentle slope of less than 4 degrees, which is normally only completely exposed at low tide
They rarely extend more than a few hundred metres, because their width means that a wave will break earlier and it’s energy will be dissipated before it reaches the cliff- thus reducing rates of erosion and limiting further growth of the platform
Where are the steepest cliffs found?
Where strata are vertical or horizontal, or have almost vertical joints
Where are the gentlest cliffs found?
Where rock dips towards or away from the sea
How are caves and blowholes formed?
When joints and faults are eroded by hydraulic action and abrasion, this can create a cave. If the overlying rock then collapses, a blow hole will develop as the cave opens up at ground level. During storms and high tides, seawater can be blown out of these blowholes with considerable force
How are arches formed?
Once the cave has been formed, if two caves on either side of a headland join up, or a single cave is eroded through a headland, an arch is formed. The gap is then further enlarged by erosion and weathering, becoming wider at the base.
How are stacks formed?
After an arch has been formed, the top of the arch will become unstable and collapse, leaving a stack
How are stumps formed?
After a stack has been created, the stack will continue to be eroded by the sea. As it collapses and is eroded further, it may only appear above the surface at low tide, and is now known as a stump
When does most erosion occur?
-when waves are at their largest, which is influenced by wind speed and fetch
-when waves approach the coast at 90 degrees to the cliff face
-when the tide is high
-when there’s heavy rainfall and so percolation of water through permeable strata and surface runoff weakens the cliff
-when debris from previous erosion has had time to be removed from the cliff foot, as debris can protect against waves
What is an example of a stack in the UK?
The Old Man of Hoy is the UK’s tallest stack at 137m high. In 1750, the location was a headland and by 1820, an arch had formed. Today only a stack remains and future erosion means it will inevitably collapse
What are examples of igneous rocks and what’s it’s erosion rate like? Why?
Granite
Basalt
Very slow recession rates
Igneous rocks are very crystalline; the interlocking crystals make for strong, hard, erosion-resistant rock.
Igneous rocks have few joints, so there are limited weaknesses the erosion can exploit
What are examples of metamorphic rocks and what’s it’s erosion rate like? Why?
Slate
Schist
Marble
Slow recessional rate
Crystalline metamorphic rocks are resistant to erosion
Many metamorphic rocks exhibit a feature called foliation, where crystals are orientated in one direction, which produces weaknesses
Metamorphic rocks are often folded and heavily fractured, which are weaknesses that erosion can exploit
What are examples of sedimentary rocks and what’s it’s erosion rate like? Why?
Sandstone
Limestone
Shale
Moderate to fast recessional rates
Most sedimentary rocks are clastic and erode faster than crystalline igneous and metamorphic rocks
The age of sedimentary rock is important; geographically young rocks tend to be weaker
Rocks with many bedding planes and fractures such as shale are often most vulnerable to erosion
What’s the difference between permeable and impermeable rocks and what are examples of each?
Permeable rocks allow water to flow through them, and include sandstone and limestones
Impermeable rocks don’t allow water through and include clays, mudstones and most igneous and metamorphic rocks
How does permeability of strata influence cliff profiles?
Permeability is important because groundwater flowing through rock can weaken layers by removing the cement that binds sediment together.
It can also create a high pore water pressure within cliffs which affects stability.
Surface run off can weaken the cliff profile
Groundwater flow and pore water pressure can also cause slumping in the unconsolidated material on the cliff top. This is because the water can flow through permeable sands but not impermeable material below. Water therefore flows along the sand/clay interface, weakening the material and leading to slumping.
