2B.4b Flashcards
State 4 different types of erosion
Hydraulic action, attrition, abrasion (corrasion), corrosion
What are erosion processes influence by
Wave type, size and lithology
Describe hydraulic action
- Air trapped in cracks and fissures of coastal rocks is compressed by the force of waves crashing against the cliff face.
- Considerable pressure (around 50 kg/cm3) from the force of the water opens the cracks, meaning more air is trapped for the next cycle of compression.
- When the wave retreats, the pressure is released explosively, which can shatter the rocks around these lines of weakness.
- This process dislodges blocks of rock from the cliff face or over time smaller fragments of rock break away
Describe corrosion
- Sand and pebbles are picked up by the sea from an offshore sediment sink or temporal store and hurled against the cliffs at high tide, causing the cliffs to be eroded.
NB: he shape, size, weight and quantity of sediment picked up, as well as the wave speed, affects the erosive power of this process.
Describe abrasion
- perhaps the most effective wave erosion process, and occurs when destructive waves pick up loose particles of sand and pebbles (sometimes boulders) and scrape
them against the cliff face as they break. - This gradually wears down the rock, like sandpaper, by removing small rock fragments.
State the rocks eroded the most quickly by abrasion
Soft sedimentary rock such as chalk, mudstones, and clays, and unconsolidated material, e.g. boulder clay.
When is abrasion most effective
High-energy destructive waves with a large wave height hurl load items with greater force, resulting in faster rates of erosion by abrasion
Describe attrition
- Occurs when already eroded rock particles (boulders, pebbles or sand) are continually moved around in the breaker or surf zone. As these sediments move, they collide with one another.
- The collisions result in protruding angular corners being knocked off so that the particles become more rounded and smaller over time.
- This process thus supplies more material surface area for abrasion.
What rocks are most eroded by attrition
soft rocks (e.g. poorly cemented sandstones, chalk and clay) are broken down quickly by attrition into silt and sand grains
Describe corrosion
- Seawater and salt spray from waves may react with rock minerals and actively dissolve
them. - Rock minerals are then carried away in solution by backwash.
- Some rocks, such as carbonates (e.g. Limestone) are more vulnerable to this corrosion
process.
When is corrosion most effective
Constructive waves, as the force of impact is not relevant, and the spilling wave increases the time for the chemical reaction to occur. They are slow, and with a long wavelength (longer the better) it prolongs the contact of rock with the water
Rock eroded most quickly by corrosion
: carbonate rocks like limestones (e.g. chalk, Jurassic limestone and carboniferous limestone) and sedimentary rocks with calcite sediment/cemen
How are erosional processes influenced by type
- They are most effective during high energy storm events with large destructive waves.
- Most erosion (in the UK) occurs in the winter, in high energy storms.
- It’s faster when the wind is blowing directly onshore
- It’s faster when the tide is high (bringing deeper water closer to the cliff so less energy is lost to friction before impact
How are erosional processes influenced by rock type
However, even coastlines composed of soft, unconsolidated sediment (e.g. boulder clay of Holderness Coast in Yorkshire), experience little erosion under normal conditions.
