Erosion within a river

Erosion Processes

Vertical erosion: This form of erosion deepens channels, aided by weathering mass movement and soil creep. Characteristics of a channel undergoing vertical erosion include large bed load comprising coarse hard particles. Potholes and deep narrow gorges are common.

Lateral erosion: This process increases a river’s width. A large sediment load has to be entrained for this process to work most effectively. It is responsible in conjunction with the processes of slope transport and mass movement for valley widening, meander migration and river cliff formation.

Headward erosion: This increases the length of a river. This process is most active in the source area of a river or where a bed is locally steep. It causes accelerated erosion and is commonly associated with waterfall formation.

Abrasion: Smaller material, carried in suspension, rubs against the riverbanks and wears it away.

Attrition: When bed load is moved downstream, boulders collide with other material and the impact break the rock into smaller pieces. In time, angular rocks become increasingly rounded in appearance.

Corrosion/ Solution: This occurs continuously and is independent of river discharge or velocity. When acids in the river dissolve rocks, which form the river’s bed/ bank. It is related to the chemical composition of the water e.g. the concentration of carbonic acid and humic acid.

Hydraulic action: The sheer force of the turbulent current hits riverbanks, pushes water unto cracks. The air in the cracks is compressed, pressure is increased and over time the back will collapse. Cavitation is a rare form of hydraulic action and the sudden and violent implosion of gas bubbles caused by this process shatters banks extremely rapidly. The resultant shockwaves hit and slowly weaken the banks. This is the slowest and least effective process.

Corrasion: Corrasion occurs when the river picks up material and rubs it along its bed and banks, wearing them away by abrasion. This process is most effective during times of flood and is the major method by which the river erodes both vertically and horizontally. If there are hollows in the riverbed, pebbles are likely to become trapped. Turbulent eddies in the current can swirl pebbles around to form potholes. This form of erosion occurs most often during times of higher river flow, bed load being used as an abrasive agent, scratching and scraping of the solid bedrock. 

Peltier Diagram *AS*

Peltier Diagrams

Physical Weathering

•      Frost shattering is important in a climate where temperatures fluctuate at around 0^oC but if a climate is too cold, or too warm, or too dry, or too wet (covered by vegetation) it will not operate.

Chemical Weathering

•       
This increases as temperature and rainfall totals increase. The rate of chemical weathering (around about) doubles (Increase by about 2 1/2 times) with every 10⁰C temperature increase.

•       Recent theories suggest that in humid tropical areas, direct removal by solution may be a major factor in the lowering of landscape, due to the continuous flow of water through the soil.

•       Chemical weathering is strong in warm moist climates e.g. rainforests.

Weathering Regions

·      Peltier constructed this diagram as an attempt to predict weathering at a place in the world by the mean annual rainfall and mean annual temperature. Physical and chemical weathering operates together at the same time and at the same place, but usually one process is more significant than the other. 

chemical weathering *AS edition*

Chemical Weathering - AND yes you should attempt to know the formulas.

Chemical Weathering: The decomposition of rock caused by a chemical change in the rock. It produces changed substances and soluble, and usually forms clay. It is more likely to occur in areas in warm moist climates where there is associated vegetation on rocks. It tends to attack certain minerals selectively and occur in zones of alternate wetting and drying (where the level of the water table fluctuates). It tends to occur mostly on the base of the slope where there tends to be wetter and warmer. These processes are more likely to occur in conjunction with another.

Hydrolysis: Hydrogen in water reacts with minerals in the rock; there is a combination of H+ and OH- ions in the water and ions of mineral (combines rather than dissolves the mineral).  It affects mostly granite (igneous rock – crystallised magma underground), which is composed of Feldspars (aluminium and potassium silicates). Feldspars (pink-grey rock forming mineral) + water à kaolinites (soft clay that is the residual weathering products) + potassium + silica oxide (Potassium and silica oxide are soluble and are washed away). The kaolinites represents the decomposition of feldspar, and the chemical weathering of granite by hydrolosis produces a chemical change in the rock. It occurs mostly in the tropics. The rate of hydrolysis depends on the amount of H+ ions, which in turn depends on the composition of the air and water in the soil, the activity of organisms, the presences of organic acids and the cat ion exchange.

Carbonation - solution: Rainwater contains carbon dioxide in solution, which produces carbonic acid (H₂CO₃). The weak acid reacts with rocks that are composed of calcium carbonate, such as limestone/ chalk and rocks that have calcareous rock. The limestone dissolves and is removed in solution by running water. Carboniferous limestone is well jointed and bedded, which results in the development of a distinctive group of landforms. Carbonation = CaCo₃ + H₂Co₃ (rainwater) à Ca (HCo₃)₂. The calcium bicarbonate is the weathered product, and is soluble (thus washed away).

Oxidation: This occurs when rocks are exposed to oxygen in the air or water. An example of this is when iron rusts. The rock or soil, which may have been blue or grey, is discoloured into a reddish-brown colour – in a process called rusting. Oxidation causes rocks to crumble more easily and occurs in iron rich rocks. In water logged areas oxidation operates in the reverse and the amount of oxygen in the soil is reduced in a process called reduction. Ferrous oxide + water à Ferric oxide. FeO + H₂O à Fe₂O₃. Sandstone is most affected by oxidation.

Hydration: Certain rocks, especially those containing salt minerals, are capable of absorbing water into their structure, causing them to swell (about 0.5%) and to become vulnerable to future breakdown. This process is most active following successive periods of wet and dry weather and is important in forming clay particles. Anhydrite + water à Gypsum. CaSo₄ + H₂O à (CaSo₄ 2H₂O) powder form. Hydration is in fact a physio-chemical process as the rocks may exert pressure as well as changing their chemical structure.

Solution: Some minerals are soluble in water and simply dissolve in situ. The rate of solution can be affected by acidity since many minerals can become more suitable as the pH of the solvent increases.

Organic Weathering/ Chelation: It requires a bio agent e.g. plants (chelates/ organic acid) and animal excretion. The decomposition of minerals in the rock leads to the crumbling of rock. Humic acid, derived from the decomposition of vegetation (humus), contains important elements such as calcium, magnesium and iron. The action of bacteria and the respiration of plant roots tend to increase carbon dioxide levels which helps accelerate solution processes, especially carbonation. Lichen can also extract iron from certain rocks through the process of reduction. High lichen and algae help in the development of the lithosphere. 

Physical Weathering *AS edition*

Physical Weathering

Freeze thaw shattering: Occurs in rocks that contain crevices and joints (e.g. joints formed in granite as it cooled, bedding planes found in sedimentary rocks, and pore spaces in porous rocks), where there is limited vegetation cover and where temperature fluctuates around 0 ⁰C. In the daytime, when it is warmer water enters the joints, but during cold nights it freezes. The process of shattering of rock is due to frost cycles i.e. fluctuating above and below 0⁰C. The process occurs with climates with rapid frost cycles, rocks with joints and rainfall.  Frost leads to mechanical breakdown in two ways:

1.     As ice occupies 9% more volume than water, it exerts pressure within the joints.

2.     When water freezes within the rock it attracts small particles of water, creating increasingly large ice crystals.

In either case the process slowly widens the joints and, in time, causes process of rock to shatter (or disintegrate) from the main rock. Where the block disintegration occurs on steep slopes large angular rocks collect at the foot of the slope as scree; if the slopes are gentle large blockfields tend to develop.

Salt crystallisation: If water entering the pore spaces or joints in rocks is slightly saline then, as it evaporates, salt crystals are likely to form. As the crystals become larger, they exert stresses upon the rock, causing it to disintegrate. This process occurs in deserts and coastal areas (areas contains sodium sulphates, magnesium sulphates and calcium chloride) where capillary action draws water to the surface and where rock is sandstone. Individual grains of sand are broken off by granular disintegration. This process also occurs on the coast with a constant supply of salt. During the day water enters the rock and is heated, water evaporates leaving salt crystals. These are large in volume and put pressure on rocks by expansion and eventually will disintegrate.

Spheroidal Weathering: In jointed rock, the weathering and heating/cooling takes place along all joints so this temperature change produces rounded boulders.

Exfoliation: Occurs in hot arid and desert climates where diurnal ranges can range up to 50⁰C (below zero to 40). It also occurs in places of high altitudes in low latitudes. These rocks are usually heated via conduction. Because the outer layers of the rock warm up faster (and expand) and cool more rapidly (and contracts) than the inner ones, stresses were set up that would cause the outer thickness to peel off (or flake off) like layers – the process of exfoliation. Changes in temperature will also cause different minerals within a rock to expand and contract at different rates. It is also theorised that water is needed for the process to be stimulated or accelerated.

Pressure Release: Many rocks have developed under considerable pressure. The confining pressure increases the strength of the rocks. If these rocks are exposed to the atmosphere, then there will be a substantial release of pressure.  The release of pressure weakens the rock allowing other agents to enter it and other processes to develop. When cracks develop parallel to the surface, a process called sheeting causes the outer layers of the rock to peel away. This process is responsible for the formation of large round rocks called exfoliation domes.

Wetting and drying: Affects less resistant rocks such as clays. The clay is porous and has the ability to absorb. When these rocks are wet they expand and when dry is contracts. Over time they disintegrate the rocks. 

Biological weathering: When tree roots penetrate and widen weaknesses in the rock until blocks of rocks become separated. 

Weathering 101 - basic introduction

Weathering: The disintegration and decomposition of rock in situ (in their place of origin). There are two types of weathering: Mechanical (Physical) or Chemical

Physical Weathering: The disintegration of rocks into smaller pieces caused by physical processes without any change to the chemical compound of the rock. It occurs on bare rock that lacks vegetation. Physical weathering usually produces sand.

Chemical Weathering: The decomposition of rock caused by a chemical change in the rock. It produces changed substances and soluble, and usually forms clay. It is more likely to occur in areas in warm moist climates where there is associated vegetation on rocks.

River Processes: Transportation

This is for all levels:

Rivers can either deposit, erode or deposit material.

Transportation

Load is either transported through suspension, solution or bed load (traction & saltation). For sediment to move resisting forces have to overcome, competent velocity has to be achieved (this is the lowest velocity at which particles of a particular size are set in motion), and critical tractive force must be achieved (This is when drag and embedded particle inertia is overcome and the particle begins to move).

Traction: Traction occurs when the largest cobbles (100-1000mm) and boulders (bed load) roll or slide along the bed. The largest of these may only be moved during times of extreme flood (high discharge).

Saltation: Bed load is either moved through saltation or traction. Saltation occurs when pebbles (1-100mm), sand (0.1-1mm) and gravel are temporarily lifted by the current and bounced along the bed in a hopping motion.

Solution: If the bedrock of the river is readily soluble, it is constantly dissolved in flowing water and removed in solution. Except in limestone areas, the material in solution forms only a relatively small proportion of total load.

Suspension: Very fine particles of clay and silt (0.001-0.1mm) are dislodged and carried by turbulence in a fast-flowing river. The greater the turbulence and velocity, the larger the quantity and size of particles which can be picked up. The material held in suspension usually forms the greatest part of the total load; it increases in amount towards the river’s mouth, giving the water its brown or black colour. 

How did mid years go?

Hope you are all pleased with your results of all your exams from the may/june exams. Hope you did extra well in geography. I hope to get more AS Geography notes and posts up by the end of year exams. Will also try to do more physical geography posts. Hope this helps ^_^.