1. Erosional Processes.
Marine erosion
Wave action is the most important erosive force acting in the coastal zone. Its effect varies with wave energy and with the coastal geology. When steep cliffs plunge straight down into relatively deep water, waves tend not to break before they reach the cliff. In this case, hardly any erosion occurs because there is little forward movement of water and the waves reflect back from the cliff. It is more common to find coastlines where waves break and some of the energy is used to erode the coast (Figure 2.1). Marine erosion consists of the following processes:
● hydraulic action
● abrasion
● attrition
Hydraulic action
Hydraulic action includes processes that result from the movement of water without the involvement of rock particles. The impact of waves causes variations in pressure due to the weight of the water and its movement. Rock that is alter- nately covered by water and then exposed to the air becomes weakened by the fluctuating forces acting on it. Added to these forces are pneumatic stresses associated with moving air. Water and air can be trapped and compressed between forward moving water and a cliff face. This is particularly the case when the cliff is made of well-jointed rock. As the wave retreats, the pressure is suddenly released, which weakens the cliff face. In addition, the impact of a mass of water can dislodge fractured and loose rock.
The term wave quarrying is used when loose blocks of rock are eroded. If the waves are extremely large, cavitation occurs. Air bubbles, at great pressure within the wave, collapse. This generates shock waves that erode rock surfaces with a similar effect to hammer blows. Small-scale features such as flutes and grooves can also result from cavitation.
Abrasion
Breaking waves pick up and carry sediment such as sand, gravel and pebbles. As the moving water drags the sediment over rock and as sediment is thrown at a rock face, a scouring action takes place, known as abrasion (also called corrasion). The effectiveness of abrasion depends on wave energy and on the availability of sediment. Larger sediment (boulders and pebbles) are moved only when the water has plenty of energy, for example in an intense storm. Smaller sediment (sand and gravel) is moved more frequently and by lower- energy waves. For example, along a short stretch of the North Yorkshire coast near Whitby, cliff erosion increases by a factor of 15 where sediment is available.
Attrition
Individual sediment particles collide with each other as they are moved around by water. When this happens, fragments are broken off, which reduces the size of the particles. Only smaller sediment is carried as suspended load in the water, with the larger particles being rolled up and down the beach as successive waves break, run up the beach and then run back down towards the sea. This movement produces smoothed and rounded sediment — a process known as attrition. Sediment is also broken and rounded when it is involved in abrasion.
2. Weathring
Mechanical Weathering:
Freeze-thaw – in latitudes where temperatures fluctuate above and below freezing, freeze thaw action is common, especially where there is a ready supply of water. Water enters cracks and freezes as temperatures remain below 0°C. As it freezes the water expands by almost 10%, meaning the ice occupies more space and exerts pressure on the surrounding rock. As the process repeats and continues, the crack widens and eventually pieces of rock break off.
Salt weathering – (haloclasty) Physical changes produced by salt crystallization (the growth of salt crystals within rock under drier conditions), salt hydration or thermal expansion of salts. (Leads to small scale breakdown)
Wetting and drying- rock in the intertidal zone (and some above, within wave and spray reach) may alternate between wet and dry. Some rocks such as shale expand when they are wet and contract when they are dry and this can contribute to fissures developing.
Exfoliation- dry rock in sunshine may absorb considerable heat, then be cooled rapidly by contact with much colder sea. Repeated expansion and contraction may lead to out layers of rock fracturing.
Biological weathering:
Solution: the dissolving of rock minerals such as halite (rock salt)
Oxidation: causes rocks to disintegrate when the oxygen dissolved in the water reacts with some minerals, forming oxides and hydroxides. It especially affects ferrous, iron rich rocks, and is evident by a brownish or yellowish rock. This can lead to disintegration.
Carbonation: coastlines composed of chalk or limestone may be dissolved by acidic rainwater or seawater. The rain/sea absorbs CO2 from the atmosphere creating a weak carbonic acid. This converts solid calcium carbonate to soluble calcium bicarbonate and the rock dissolves.
Freeze-thaw – in latitudes where temperatures fluctuate above and below freezing, freeze thaw action is common, especially where there is a ready supply of water. Water enters cracks and freezes as temperatures remain below 0°C. As it freezes the water expands by almost 10%, meaning the ice occupies more space and exerts pressure on the surrounding rock. As the process repeats and continues, the crack widens and eventually pieces of rock break off.
Salt weathering – (haloclasty) Physical changes produced by salt crystallization (the growth of salt crystals within rock under drier conditions), salt hydration or thermal expansion of salts. (Leads to small scale breakdown)
Wetting and drying- rock in the intertidal zone (and some above, within wave and spray reach) may alternate between wet and dry. Some rocks such as shale expand when they are wet and contract when they are dry and this can contribute to fissures developing.
Exfoliation- dry rock in sunshine may absorb considerable heat, then be cooled rapidly by contact with much colder sea. Repeated expansion and contraction may lead to out layers of rock fracturing.
Biological weathering:
- Roots from vegetation growing into the bedding planes or extending into joints/ Animals burrowing into cliffs.
- Molluscs and other marine organism producing secretions which cause a chemical breakdown of rocks. E.g. Limpets cause the disintegration of chalk.
- Water running into decaying vegetation becomes acidic, which leads to increased chemical weathering.
Solution: the dissolving of rock minerals such as halite (rock salt)
Oxidation: causes rocks to disintegrate when the oxygen dissolved in the water reacts with some minerals, forming oxides and hydroxides. It especially affects ferrous, iron rich rocks, and is evident by a brownish or yellowish rock. This can lead to disintegration.
Carbonation: coastlines composed of chalk or limestone may be dissolved by acidic rainwater or seawater. The rain/sea absorbs CO2 from the atmosphere creating a weak carbonic acid. This converts solid calcium carbonate to soluble calcium bicarbonate and the rock dissolves.
3. Transportation
Watch the video below to learn more about Longshore Drift
4. Deposition
Deposition is the geological process in which sediments, soil and rocks are added to a landform or land mass. Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.
Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and friction, creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from organically derived matter or chemical processes. For example, chalk is made up partly of the microscopic calcium carbonate skeletons of marine plankton, the deposition of which has induced chemical processes (diagenesis) to deposit further calcium carbonate. Source: Wikipedia
Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and friction, creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from organically derived matter or chemical processes. For example, chalk is made up partly of the microscopic calcium carbonate skeletons of marine plankton, the deposition of which has induced chemical processes (diagenesis) to deposit further calcium carbonate. Source: Wikipedia