AS Geography - Physical Geography
Terms in this set (189)
• Occurs where two plates diverge, or move away, from each other and new crust is created on a boundary. This process is known as sea floor spreading and occurs on the Mid-Atlantic Ridge.
• As the plates diverge, molten rock or magma rises from the mantle to fill any possible gaps between them and, by doing so, create new oceanic crust.
• The magma initially forms submarine volcanoes, which may in time grow above sea level e.g. Surtsey, Iceland.
• The Atlantic Ocean did not exist some 150 million years ago and is still widening by about 2-5cm per year.
• Where there are lateral movements along the mid-ocean ridges, large cracks called transform faults are produced at right-angles t the plate boundary.
• Where continental and oceanic plates converge
• Continental plates surround the Pacific Ocean, which extends over five oceanic plates.
• At the Nazca Plate, made up of oceanic crust, which cannot override continental crust, is forced to dip downwards at an angle to form a subduction zone with an associated deep-sea trench. As oceanic lithosphere descends, the increase in pressure can trigger major earthquakes. While dehydration of the sub ducted oceanic crust, caused by the increase in pressure, results in the release of water into the overlying mantle that promotes partial melting and the generation of magma. Being less dense than the mantle, the newly formed magma will try to rise to the Earth's surface. Where it does, volcanoes will occur.
• These volcanoes are likely to form a long chain of fold mountains e.g. Andes or, if eruptions take place offshore, an island arc e.g. Japan, Caribbean.
Collision Zones/ margins
• Formation of fold mountains (orogenesis)
• Fold Mountains occur where oceanic crust is subducted by continental crust.
• A second, though less frequent, occurrence is when two plates composed of continental crust move together.
• The Indian subcontinent, forming part of the Indo- Australian Plate is shown to have collided with Eurasian Plate.
• Because the continental crust cannot sink, the subsequent caused the intervening sediments to be pushed upwards to form the Himalayas.
• These plates are moving towards each other at a rate of 5.8cm per year.
• The main effects of a conservative plate boundary are earthquakes, which can be fairly violent and frequent.
• Two plates move parallel to each other, without creating or destroying any crustal rocks.
• As they move past each other they often get stuck, building up great pressure until finally they jolt past each other. This sudden movement is what causes earthquakes.
• The boundary between the two plates is often characterized by pronounced transform faults.
• The best-known example of a conservative plate boundary is the San Andreas Fault, where the North American and Pacific plates are actually moving in the same direction, but at a different speed.
Shockwaves begin to travel faster here - indicating a change in structure. The Moho Discontinuity is the junction of the Earth's crust and the mantle where seismic waves are modified.
Ocean Crust (Sima)
Thickness 6-10Km on average
Age of Rocks Very young, mainly under 200 million years
Weight of Rocks Heavier, with an average density of 3.0
Nature of Rocks Dark in colour; many contain silica and magnesium; few types, mainly basalt
Continental Crust (Sial)
Thickness 35-40 km on average, reaching 60-70 km under mountain chains
Age of Rocks Very old, mainly over 1500 million years
Weight of Rocks Lighter, with an average density of 2.6
Nature of Rocks Light in colour; many contain silica and aluminium; numerous types, granite is most common
The lithosphere is the oceanic crust, continental crust and upper mantle.
The plates move due to convection currents in the mantle. These are hot currents of molten rock that slowly move within the mantle and cause the plates above them to move, usually by as little as a few centimeters each year.
The disintegration and decomposition of rock in situ (in their place of origin). There are two types of weathering: Physical or chemical
Erosion that lowers the landscape in known as denudation.
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.
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.
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 0C. 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.
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.
In jointed rock, the weathering and heating/cooling takes place along all joints so this temperature change produces rounded boulders.
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.
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.
When tree roots penetrate and widen weaknesses in the rock until blocks of rocks become separated.
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 → 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) → Ca (HCo3)2. The calcium bicarbonate is the weathered product, and is soluble (thus washed away).
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 → Ferric oxide. FeO + H2O → Fe2O3. Sandstone is most affected by oxidation.
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 → Gypsum. CaSo4 + H2O → (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.
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.
Factors that influence weathering: Climate
• Greatest rates of weathering are found in equatorial regions, because it's humid all year round. Chemical weathering needs water and is the active weathering agent in weathering.
• For every 10 degrees rise in temp. weathering rates increase by 2 1/2 times.
• In colder higher lats and mid lats, weathering is predominantly physical weathering.
• There is anomaly to this relationship. There is chemical weathering in high lats. beneath snow and there are large abounts of CO2 dissolved in the water. This creates carbonic acid from carbonic solution.
• Glacial climates: Susceptibility to frost increases wih increasing grain size. In Taiga claimates fairly high soil leaching, and low rates of organic matter decompose. In Tundra areas low precipitation, low temps and permafrost - moist conditions, slow organic production and breakdown. May have slower chemical weathering. Bacterial weathering may occur. Granular disintegration occurs. Hydration weathering common.
• Temperate climates: Precipitation and evaporation generaly fluctuate. Both physical and chemical weathering occur. Organic content moderate to high, breakdown moderate. Silicate clays formed and altered. In deciduous forest areas biological weathering occurs as well as organic weathering.
• Arid/ semi-arid climates: Here evaporation exceeds precipitation rainfall is low, temps are high and organic content low. Mechanical weathering, salt weathering, and granular disintegration is domiant indriest areas. Low organic input relative to decomposition. Slight leaching CaCO3 in soil. Sulphates and chlorides may accumulate in driest areas. Increased precipitation an decreased evaporation toward semi-arid areas and steppes yield thick organic layers, moderate leaching and CaCo3 accumulation.
• Humid tropical climates: High rainfall often seasonal. Long periods of high temperature. Moisture availability is high. Weathering products are removed or accumulate to create red/ black clay. Organic content high but decomposition high. Usually intense deep weathering.
Factors that influence weathering: Vegetation
• Weathering via humic and organic acids (chelates)
•Equatorial regions/ low latitudes: Dense vegetation in Tropical Rainforests. Constant leaf fall leads to rapid decomposition. Humus and organic acids with constantly infiltrating rainwater. This accelerates chelation, and causes a very deep regolith (up to 30m).
• High latitudes: Little vegetation only mosses and lichens grow on bare rock. A small amount of chelation is associated with shallow regoliths (slow growth of vegetation).
• Mid latitudes: Tree roots and physical weathering
Factors that influence weathering: Geology (Rock type/ Structure)
• Structure is to do with joints, fractures, horizontal bedding planes, cracks and openings.
• Water can penetrate via joints and the opening provides a greater surface area for weathering to take place.This can cause chemical weatheirng via hydrolysis and cause block disintegration in granite.
Factors that influence weathering: Rock type is the composition and texture
• This Hydrolysis can cause granular disintegration in granite. the breakdown into particles due to differential hydrolysis e.g. Kaolinite clay
• In limestone (a calcareous rch rock that have both vertical and horizontal joints. Rainwater enters via joints. Carbonation solution will result in the solubility of calcium bicarb along the openings and cause block disintegration. THe remaining blocks are known as clints.
• Porous rocks - the water in the rocks in the pore spaces breaks down the rock via chemical weathering into individual grains (granular disintegration). This affects predomiantly clays and sandstones. A large surface area long the grains provides a surface for weathering.
Factors that influence weathering: Relief
• Altitude affects climate and vegetation and thus the type of weatheirng.
• Higher altitudes carbonation occurs.
• The steeper the slope the less the infiltration the greater the overland flow because there is no time for the water to infiltrate because of the force of gravity.
• In the southern hemispheres north facing slopes are warmer than south facing slopes. On south facing slopes an acccumulation of snow and ice can lead to a greater rate of carbonation and thus a greater rate of weathering.
Factors that influence weathering: Time
• Ice ages reduce length of time weathering has been operating at current rates e.g. the last ice age concluded 12,000 years ago in the northern hemisphere e.g. the UK where weathering in this temperate latitude has produced relatively shallow regolith whereas large continents e.g. Africa weathering has been taking place for millions of years contributing to depths of regolith between 30-60m.
Factors that influence weathering: Human Activities
A deep pit
lager surface area
Collect water - increasing chem weather
Toxic waste washed into rivers - water pollution thus increasing chemical weathering
• Air Pollution
Carbon dioxide, carbon monoxide, nitrous oxide, sulphur dioxide and methane.
These are created from industry, cars, and agriculture.
These pollutants produce acid rain (sulphur dioxide and nitrous oxide). This accelerates the processes of weathering. It can fall as wet (rain) or dry (dust) fall. Carbonation solution is especially created and occurs predominantly in cities on limestone sandstone buildings.
Peltier Diagram - Types of weathering in different climates
This is a diagram showing the relationship between climate and the rate/ amount of weathering.
• Frost shattering is important in a climate where temperatures fluctuate at around 0oC but if a climate is too cold, or too warm, or too dry, or too wet (covered by vegetation) it will not operate.
• This increases as temperature and rainfall totals increase. The rate of chemical weathering doubles with every 100C temperature increase (Van t' Hoff).
• 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.
• 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.
• Mechanical and chemical weathering operates together at the same time and at the same place, but usually one process is more significant than the other.
• The weakest weathering regions are in deserts (cold and hot) where rainfall is low and temperatures extreme.
• The strongest chemical weathering is in equatorial regions where temperatures and rainfalls are high.
Plants decompose and produce humus and adds an organic component known as regolith in the soil. Deep regolith is caused by weathering. High rates of weathering occur in the equatorial/ tropics area due to the lack of seasons and that it rains all year round.
Limestone is a rock containing at least 80% calcium carbonate and is formed primarily over four geological periods.
• Hard, grey, crystalline and well-jointed
• Contains fossils such as coral, crinoids and brachiopods
• The rock must have been formed on the bed of a warm clear ocean
• It has developed it's own unique landscape known as 'Karst' scenery
• Contains a high proportion of magnesium carbonate
• Similar scenery to that of chalk
• It's pure, soft, well-jointed limestone
• It's believed to be composed of the remains of small marine organisms which lived in clear shallow waters
Karst Scenery example: Li Valley, South China
• Limestone covers 300,000 km2 of China
• The limestones that outcrop near Guilin have formed a unique karst landscape
• The massively bedded, crystalline rock, which in places is 300m thick, has been slowly pushed upwards from its seabed origin through tectonic movements formed on the Himalayas and Tibetan Plateau.
• Heavy monsoon rain, exceeding 2000mm, has led to rapid fluvial erosion by the Li River.
• The availability of water together with the high sub-tropical temperatures (Guilin is at 25oN) encourages high active chemical weathering (solution and carbonation).
Carboniferous Limestone develops its own unique scenery for three main reasons:
1. It is found in thick beds separated by almost horizontal bedding planes and with joints at right angles.
2. It is pervious but not porous, meaning the water can pass along the bedding planes and down joints but not through the rock itself.
3. Thirdly, Calcium Carbonate is soluble. Carbonic acid in rainwater together with humic acid from moorland plants, dissolve the limestone and widen any weaknesses in the rock. Acid rain also speeds up carbonation and solution. As there is little surface drainage and breakdown of bedrock to form soil, the vegetation cover tends to thin or absent. In winter, this allows frost shattering to form scree at the foot of steep cliffs.
Carboniferous limestone can classify into four types:
1. Surface features caused by solution - Limestone pavements are flat areas of exposed rock. They are flat because they represent the base of dissolved bedding plan, and exposed because the surface soil may have been removed from glacial activity and never replaced. Where joints reach the surface, they may be widened by acid rainwater to leave deep gashes called grikes. Between the grikes are flat topped yet dissected blocks called Clints. In time, grikes widen and the Clints are weathered down until the lower bedding plane is exposed and the process of solution-carbonation is repeated.
2. Drainage features - Rivers that have a source on surrounding impermeable rocks may disappear down swallow holes or sinks as soon as they reach the limestone. The streams flow underground finding a pathway down enlarged joints, forming potholes, and along bedding planes. Where solution is more active, underground caves form mostly above the water table (vadose caves). Corrosion often widens the caverns until part of the roof collapses, providing the river with angular material that is ideal for corrasion. Heavy rainfall quickly infiltrates downwards, so caverns and linking passages may become water filled. Resurgence occurs when the river reappears on the surface at the junction of permeable and impermeable rock.
3. Surface features resulting from underground drainage - Steep sided valleys are likely to have been formed as rivers flowed over the surface of the limestone, when permafrost acted as an impermeable layer. When the rivers were able to revert to their subterranean passages, the surface valleys were left dry (steep gorges). If the areas above an individual cave collapses, a small surface depression called a doline is formed.
4. Underground depositional features - Groundwater may become saturated with calcium bicarbonate, which is formes by the chemical reaction between carbonic acid in the rainwater and calcium carbonate in the rock. However, when the hard water reaches the cave, much of the carbon dioxide bubbles out of solution back into the air. Helped by the loss some moisture through evaporation, calcium carbonate crystals are subsequently precipitated. Water dripping from the ceiling of the cave forms, over time, stalactites. As water drips onto the floor, further deposits of calcium carbonate form more rounded stalagmites that may join the stalactites to form a pillar.
• Granite was formed when magma was intruded into the Earth's crust.
• Having been formed at a depth and under pressure, the rate of cooling was slow and this enabled large crystals of quartz, mica and feldspar to form. As the granite continued to cool, it contracted and a series of cracks were created vertically and horizontally, at regular intervals. These cracks may have been further enlarged by pressure release as overlying rocks were removed.
• The coarse-grained crystals render the rock non-porous but, although some believe granite is impermeable, water can make its way along many cracks making some areas permeable. Despite this, most granite areas usually have a high drainage density and, as they occur upland, they are often covered by marshy terrain.
• Although hard, granite is susceptible to both physical and chemical weathering. The joints, which can hold water, are widened by frost shattering, while the different rates of expansion and cooling of the various minerals within the rock cause granular disintegration.
• The feldspar and mica can be changed chemically by hydrolysis. This means that calcium, potassium, sodium, magnesium and, if the pH is less than 5.0, iron and aluminium, are released from the chemical structure. Where the feldspar changes near the surface it forms white clay called kaolinite. When the change occurs at a greater depth it produces kaolin. Quartz is not affected by chemical weathering and remains as loose crystals.
• Tors form in temperate climates and inselbergs in tropical climates.
• Indeed controversy has surrounded their origin.
If we define a tor as a residual mass of rock capping hills or high ground then many theories can explain their origin. They are not just restricted to granite regions and appear to be found in more than one climatic region of the world. Theories centre on weathering and erosion.
• Consider the granite tor, so characteristic of Dartmoor, Linton's theory advocates deep chemical weathering as the exponent, suggesting that where joints in the rock were closer together the rock would be more deeply weathered and so easily removed by later erosion. He saw a prolonged chemical weathering under tropical conditions as the main factor in tor genesis.
• A second theory favoured by arctic workers suggests mechanical weathering during the ice age was responsible. King's theory advocates tors to be nothing more than the residual remains of sub aerial erosion surfaces.
Is the movement of weathered material down a slope under gravity.
The slope is a system with inputs, outputs and flows. It has exogenetic factors (external factors) and endogenetic factors (internal factors).
The slope as an open system.
Classifications of mass movements
The types of processes can be classified in a number of different ways:
- Speed of movement
- Water content
- Type of movement: flows, slides, slumps
The mechanism of mass movement
Rock particles on slopes are held on the slope by friction in a state of dynamic equilibrium. Their steady state (not moving) represents a balance between the internal (within/ between the particles known as internal or shear strength) and external forces (known as external/shear stress). When shear strength = shear stress = no movement. If one is greater than the other = movement.
Water as a factor in changing the dynamic equilibrium of the slope:
Water is a very important factor in influencing slope stability. Particles in the soil stick together if it rains, the rain infiltrates via the pores and lubricates the weathered material therefore reduces friction and makes the weathered material easier to move down the slopes. Water may also increase external stress because it adds weight to the slope (because of an increase in pore pressure).
What keeps a slope in place
Vegetation (Binds soil thus stabilising slopes)
Friction (will vary with the weight of the particle and slope angle, and can be overcome with the help of water)
Cohesive force (An act to bind the particles of the slope and prevalent is water-less clay)
Pivoting (occurs in debris layers that contain material embedded in the slope).
What factors lead to increasing stress and decreasing shear resistance
What factors lead to increasing stress and decreasing shear resistance:
Weathering and the type of material can reduce resistance.
Shear Stress can be increased by:
Stress can be increased by:
Steepening of a slope
Undercutting of a slope
Addition of a mass of regolith
Dumping of mining waste
Sliding from higher up the slope
Vibrational shock and earthquakes.
Factors that contribute to increased shear stress:
• Removal of lateral support through undercutting or slope steepening - Erosion by rivers, glaciers, wave action, faulting, previous rock falls or slides.
• Removal of underlying support -Undercutting by rivers, waves, sub-surface solution, loss of strength by extrusion of underlying sediments.
• Loading of slope - Weight of water, vegetation, and accumulation of debris.
• Lateral pressure - Water in cracks, freezing in cracks, swelling and pressure release.
• Transient stresses - Earthquakes and movement of trees in the wind.
Factors that contribute to reduced shear strength:
• Weathering effects - Disintegration of granular rocks, hydration of clay materials, dissolution of cementing minerals in rock or soil.
• Changes in pore water pressure - Saturation or softening of material
• Changes in structure - Creation of fissures in shales and clays, remoulding of sand and sensitive clay.
• Organic effects - Burrowing of animals and decaying tree roots.
Types of mass movements
- Sub-aerial weathering e.g. physical weathering by changes in temperature wetting/ drying → block disintegration at the top of the slope
- Rockfalls occur on slopes exceeding 400.
- Blocks at the top become loose and fall vertically down the free face (90 degrees) due to gradual weathering processes such as freeze thaw and/ or tectonic activity.
- Weathered rock at the foot of the slope (scree or talus) at a 45 degree angle and is a boulder controlled concave slope.
- Causes: Reduction in shear strength due to weathering → loose blocks. These fall under the influence of gravity (900) → base of the slope to form a new gentler slope segments (450).
- Effects and movement - imperceptible and hardly visible
- Most widespread - most occurring
- Associated with relatively fine materials e.g. silt, unconsolidated material
- Occurs at high lats. And altitudes in mid lats.
- Water freezes during winter and produces a lens of ice beneath the particles because the particles conduct the cold. The cold, ice expands by 9% and pushes outwards.
- Mass movement process
- Occurs on slopes of about 50 and produces terracettes
- The impact on the slope can be direct (microfeatures include terracettes (tiny ripples under the grass due to a accumulation of soil in very small ridges)).
- The impact can be indirect where fences can break, bases of trees can turn downwards, telegraph pole tilt, cracks can form in the road and soil can accumulate behind walls.
- Can cause a down slope breaking of bed rock or a rock outcrop.
- This process meaning 'soil flow' is a slightly faster movement averaging between 5cm to 1m per year.
- Often takes place under periglacial conditions where vegetation cover is limited.
- During winter both the bedrock and regolith are frozen. In summer, the surface layer thaws but the underlying layer remains frozen and acts like impermeable rock. Because surface water cannot infiltrate downwards and temperatures are too low for effective evaporation, any topsoil will become saturated and will flow as an active layer over the frozen layers.
- This process produces solifluction sheets and lobes (rounded tongue like features), and heads (a mixture of sand and clay formed in valleys and at the foot of sea cliffs).
- Fast, need well lubricated material
- The material behaves like a viscous fluid
- Material size - large boulders - small grains
- The debris avalanche (large boulders) is the fastest of the flows
- Some other types of flows include earthflows and debris flows (small grain sized slows).
- Occurs because there is a decrease in internal/ shear strength. Heavy rain infiltrates the regolith - lubricates the material by filling the pores thus increasing pore pressure. Shear strength < external stress i.e. gravity
- Flows can be triggered by earthquakes would increase shear stress.
- The most important point about flows is that there is a decrease in movement with depth. The top middle moves the fastest and the front extends the furthest (an area known as the 'toe/lobe'). The internal deformation of material down the slope and as the material goes down the slope there is a decrease in velocity. A scar is left at the top of the slope where the flow began. This is a steeper section of the slope.
- The overall impact of the slope: Scar, gentler gradient at the base of the slope and material may spread widening slope foot.
- Mudflows: Rapid movements, occurring on steeper slopes, exceeding 1km/hr. They are most likely to occur following periods of intensive rainfall, where both volume and weight are added to the soil giving it a higher water content than an earthflow
- Earthflows: When the regolith slopes 5-150 becomes saturated with water, it begins to flow downhill at a rate varying between 1 and 15 km per year. The movement of material may produce short flow tracks and small bulging lobes or tongues, yet may not be fast enough to break the vegetation.
- Occurs on a slide/ slip plane or what is known as a failure surface, which is lubricated by rain water which had infiltrated along this major line of weakness.
- Slides may be rotational or translational (planar).
- In the planar slide, the weathered rock moves downhill leaving behind it a flat rupture surface.
- Where rotational movement occurs, a process sometimes referred to as slumping, a curved rupture surface is produced. These are mostly slides, usually along more than one slip plane, which is curved. These have a variety of names but are essentially the same process. The more resistant/ permeable rock e.g. limestone remains, while the clay material slumps into blocks on slip planes.
- Impacts on slopes: Stepped uneven profile, scars, slumped block and a toe of material at the base.
Example of a (accelerated by man) Landslide in a MEDC: Abbotsford Landslide, Dunedin, NZ 1979 - Causes
• 1978 families noticed cracks appearing in their homes.
• 1979 workmen discovered that a leaking water main had been pulled apart. Geologists discovered that water had made layers of clay on the hill soft, and the sandstone above it was sliding on this slippery surface.
• Construction of the nearby Dunedin southern motorway, an earthquake that occurred in the area in 1974, deforestation (reduced evapotranspiration), increased urbanization (involved cutting into slope and infilling) and quarrying activity on the toe of the slope in the decades before may have further affected the land's stability.
Example of a (accelerated by man) Landslide in a MEDC: Abbotsford Landslide, Dunedin, NZ 1979 - The landslide
• On July 27th the slide began to accelerate.
• Early warning system was put in place by Civil defense and a civil emergency was declared on the 6th of August 1979. This was not thought to have been necessary, as geologists believed the slope would only move slowly.
• However on the 9th of August a 7 ha section of East Abbotsford started moving down the slope at a rate of 3m per minute, taking houses with 17 people inside.
• It was essentially a block of sandstone resting on a bed of weaker clay. Displacement of 50m took place in about 30 minutes, leaving a small rift 30m deep in the head of the slope. In addition the slope was on an angle of 70. Water collected in the impermeable clay, reduced its strength and cohesion, and caused the sandstone to slip along the boundary of the two rocks.
• The sandstone involved 5.4 million m3 of material. At first the land moved as slow as soil creep, followed by a rapid movement with speeds of 1.7 m per minute.
Example of a (accelerated by man) Landslide in a MEDC: Abbotsford Landslide, Dunedin, NZ 1979 - Impacts
• Nobody was killed but 69 homes were destroyed or damaged and 200 people were displaced. The total cost from the destruction of the homes, infrastructure and relief organization amounted to £7 million ($10-13 million NZ today). In total 18 ha was affected.
• Insurance schemes and government relief to cope with such disasters meant that residents were compensated for any damage.
• However other impacts such as depressed housing prices, trauma and the cost of a prolonged inquiry were not immediately appreciated.
• Lessons on landslide preparedness, and the affect human activity has on slopes can be learnt from this.
Case Study on a Physical Landslide in a LEDC: Vargas State, Venezuela - 1999 - Causes
• First two weeks of December 1999 saw an unusually high amount of precipitation (40-50% above normal rainfalls).
• Political corruption - allowing shanty-towns to be built on steep slopes surrounding Caracas. The slopes around the region were changed to accommodate vast squatter settlements.
Case Study on a Physical Landslide in a LEDC: Vargas State, Venezuela - 1999 - The Landslide
• 15-16th December the slopes of the 2000m Mt Avila began to pour forth rock and mud burying 300 km stretch of the central coast.
• Rains triggered mudslides, landslides and flash floods in between the mountains and the Caribbean Sea.
• Search and rescue were deployed to search for survivors but very few were found in the first few days.
Case Study on a Physical Landslide in a LEDC: Vargas State, Venezuela - 1999 - The Impacts
• Rains triggered mudslides, landslides and flash floods which claimed the lives of 10,000 -50,000 (unknown accurately as most people were buried under mud or swept to sea) in between the mountains and the Caribbean Sea.
• 150,000 were left homeless by landslides and floods in the states of Vargas and Miranda.
• Slum dwellings were often buried by mudslides (8-10m deep) or swept out to sea. This is why fatalities are unknown as many went missing and entire families went unreported as missing.
• Bridges, roads, factories, crops, telecommunications and the tourism industry (in the immediate future) were destroyed. The international airport in Caracas was closed.
• Containers at the seaport of Maiqueita were damaged. Harzardous material leaked out of these containers. Operations at the port were halted and hampered efforts to bring in emergency supplies. The economic damage was estimated at $3billion.
• 70% of Venezuelan population was living in this small coastal area. The government then made a plan to move some of the population to inland areas.
• As a result of these landslides a plan to rebuild 40,000 homes was created for Vargas. A $100 million extension was planned for the international airport. The country's main seaport in Vargas, was also planned to be modernized. Tourist destinations in Macuto and Camuri Chico were also rebuilt. Towns such as Carmen de Uria were not rebuilt, and instead created into parks & bathing resorts.
• These improvements reduced the number of fatalities to 14 in the next 2005 mudslides in the region.
Impacts of Human Activity on Slopes
• In order to build a horizontal base plus reasonable access for roads, a cut-and-fill technique is used and a small level terrace with an over-steepened slope at both ends.
• The steep slope now devoid of vegetation and soil, is potentially less stable than the former natural slope, and in times of high rainfall can be susceptible to landslips.
A Drainage basin is an area of land drained by a river and its tributaries.
A Drainage basin is known as an open system.
A ridge of high land beyond which any precipitation will drain into its adjacent basins marks its boundary.
Where water is lost in the system.
Overland flow (Hortonian and saturated)
It's the physical process by which moisture is lost directly into the atmosphere from water surfaces, excluding vegetation and the soil, caused by the effects of air movement and the sun's heat. It is the energy lost through the process of water vapor turning into gas.
Evaporation rates depend of the temperature, wind speed, humidity, hours of sunshine and other climatic factors.
Transpiration is a biological process which water is lost from a plant through the minute pores in its leaves. Transpiration rates are affected by the time of year, amount of vegetation, the availability of moisture and length of the growing season. Potential evapotranspiration is high in deserts because the amount of moisture that can be lost is greater than the water actually available. Actual Evapotranspiration is when the amount of water available exceeds the amount of evapotranspiration that takes place. Evoptranspiration is the total water lost.
Interception: Interception is the catching and storing of incoming precipitation through the canopy. It is greater in woodland areas than grass. In light precipitation the water may never reach the ground and could be lost in the system. It is estimated that 30% of precipitation is lost through interception.
If rainfall persists, water may reach the ground. A way in which it reaches the ground is through stem flow. This is through water flowing down the trunk.
If rainfall persists, water may reach the ground. A way in which it reaches the ground is through fall. This is through water dropping off the leaves.
Infiltration is when water flows from the surface through the soil in a vertical (downwards) direction. Soil will gradually admit water from the surface, if the supply rate is moderate, allowing it slowly to infiltrate vertically through the pores in the soil. The maximum rate in which water can pass through soil is called its infiltration capacity (mm/hr). The rate of infiltration depends upon the amount of water already in the soil, structure of the soil, nature of the soil surface (ploughed, cracked ect.), type of vegetation, amount of vegetation and seasonal changes in vegetation cover. Some of the water will flow laterally through the soil as through flow. During drier periods, some water may be drawn towards the surface by capillary action.
In most environments, overland flow (this is water running off or across the surface and eventually develop in rills, channels and eventually rivers (surface runoff)) is rare except in urban areas (impermeable) and during heavy rains. When rainfall is greater than the infiltration capacity it is known as Hortonian overland flow. Saturated overland flow is when the surface is saturated (every single pore is filled).
As water reaches the underlying soil or rock layers, which tend to be more compact, its progress is slowed. This constant movement is known as percolation (the vertical movement of water from soil to rock) and creates groundwater storage.
Water eventually collects above an impermeable rock layer, or may fill all pore spaces, creating a zone of saturation. The upper boundary of the saturated material (the upper surface of the groundwater) is known as the water table (the level of water below the ground surface below which the soil or rock is saturated and vary seasonally). If the water table reaches the surface it means the ground is saturated, and surface runoff may occur.
Water may then be slowly transferred laterally through rock as base flow (or groundwater flow).
Recharge: Percolating water adds to the groundwater store raising the water table. Recharge is the replacement of water during drier months.
The point at which subsurface water emerges at the surface or the point at which the water table reaches the surface.
A water-bearing porous rock
Porous vs. Non porous rocks
Porous: Water is held in pores between rock/soil particles. Porous rocks have the ability to hold water. They do not allow water to pass through them e.g. clay.
Non-Porous: Does not have pores and cannot hold water within the rock
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 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).
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.
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.
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.
When velocity of a river begins to fall, it has less energy and no longer has the competence (maximum size of material being transported) or the capacity (the total load actually transported) to carry its load. So starting with the largest particles, material begins to deposit.
It occurs when:
• Discharge is reduced following low precipitation
• Velocity is lessened on entering the sea or lake
• Shallower water occurs on the inside of a meander
• The river overflows
• The load is suddenly increased
Sediment in a river comes from a variety of sources. It may be from outside the river (exogenetic) that includes, mass movement, rill and gully erosion and sheet wash. Or from within the rivers channel itself (endogenetic) that could be material from the streambed and banks, which is influenced by the power of erosion and the resistance of material to erosion.
It is essential to understand the relationship between velocity and particle size in order to appreciate the processes at work in the river channel. This is because velocity overwhelmingly determines whether a particle can be eroded from a riverbed.
Rivers either transports, erode, or deposit sediment (load). The relationship between the size of particles (sediment) and water velocity required eroding, transport and deposit it is shown on the Hjulstrom curve below. The critical erosion curve shows the minimum velocity required to lift a particle of a certain size. The critical deposition curve shows the maximum velocity at which a river can be flowing before a particle of a certain size is deposited. The zone in-between is the zone of transport. The velocities to transport are lower than erosion, because it takes more energy to lift sediment than to maintain it in transport. Also it takes more energy to erode the smallest particles because, clay particles are strongly bonded together (stick together electrochemically) and therefore require a lot of energy to be eroded.
Competence is the maximum size of material a river can transport
Capacity is the total load actually transported
Erosion in a river
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.
Smaller material, carried in suspension, rubs against the riverbanks and wears it away.
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.
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.
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 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.
Volume of a river
Most streams and rivers obtain their water from rainfall and other forms of precipitation. This precipitation evaporates, soaks in or contributes to the run-off or drainage of the land surface. As rivers flow from high (source areas) to low areas (the mouth, usually the sea, unless the river has entered an arid basin), their volume increases as contributions from other parts of the drainage basin via tributaries are added. There can be variations in a river's volume relating to seasonality of rainfall (in monsoon areas), the contribution of snowmelt and of springs and groundwater.
Velocity is more or less constant along the length of a river. It is true that steeper slopes do encourage higher velocities, but the larger channels of the lower course exert relatively less friction than the small channels of the upper course, causing an increase in velocity and allowing the river to become much more efficient.
Small streams carry a greater quantity of fine material than coarse material. Conversely, large rivers with more available energy carry larger/ coarser material.
Energy of a river
Volume of water carried + velocity of this water = energy of the river
Discharge: Discharge is defined as the volume of water passing a particular point in the river in a unit of time, expressed as m3/s-1, or cumecs. Q=A x V (A= cross sectional area (width x depth) and V = velocity)
Flow patterns: Tubulent flow
Irregularities in the riverbed cause eddying. This causes the river to appear rough with a white foamy surface.
Flow patterns: Laminar flow
Laminar: Occurs because the riverbed is completely smooth. The water flows in parallel lines.
Flow Patterns: Helicoidal flow
Associated with a meandering river in a corkscrew motion. The thalweg goes down one side of the channel, up the other side and across the channel.
Braided: If a stream has a high proportion of bed load in relation to its discharge, it deposits much of its load as sand and gravel bars/ islands/ eyots (which shift as discharge changes and are stabilized by roots on vegetation) in the streambed. These channels are alluvial and are not formed on solid rock (unstable). These channels have a high capacity, a low competence, a variable discharge and a gentle gradient. They also have incoherent banks which are rapidly eroded which add to the load. The stream flows in multiple interweaving strands that split and rejoin around the bars to give a braided appearance to the channel. This is known as a Braided river e.g. Canterbury Plains in NZ. The channel pattern develops in settings that provide considerable amounts of loose sand and gravel bed load to the stream system and is therefore common in arid (deserts with flash floods) and flood plains in glacial regions.
Straight: Straight channels can exist under natural circumstances for short distances usually at fault lines, rock joints or steep gradients. However most straight channels with parallel linear banks are artificial channels.
The most common channel pattern in humid climates display broad sweeping river bends known as meanders. Over time these sinuous meandering channels also wander from side to side across their low-gradient floodplains widening the valley by lateral erosion on the outside (highest velocity) of the meander bends and leaving lateral accretion deposits on the inside of the meander bend.
Meanders form due to the greater volume of water carried by the river in lowland areas which results in lateral (sideways) erosion being more dominant than vertical erosion, causing the channel to cut into its banks forming meanders. Pools and riffles initiate meanders. The thread of maximum velocity is diverted towards the riverbank. Here the deeper water and fastest flow = the greatest rate of erosion. So gradually a bend develops. The outside of the bend is steeper known as a river cliff. On the inside of the bend where the water is shallower, the thread of maximum velocity (thalweg) is reduced in speed and material is deposited on a gentle slope known as a slip-off slope. The deposited material is known as a point bar.
A river is meandering when its sinuosity is above 1.5.
A meander is asymmetrical in cross-section. It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition).
Sinuosity = actual channel length / straight-line distance.
Circular depressions, sometimes found on the riverbed. They are found on rivers flowing across bedrock. It is usually located in the source zone. Potholes can cause eddying of the water and turbulent flow. These depressions can cause the load of the river to grind against the bed as the water eddies in the depression. Potholes are usually formed due to abrasion.
Waterfalls: Waterfalls form when a river, after flowing over relatively hard rock (bed rock), meets a band of less resistant rock or where it flows over the edge of a plateau. As the water approaches the brink of the falls, velocity increases because the water in front of it loses contact with its bed and so is unhampered by friction. The underlying softer rock is worn away as waterfalls onto it. In time, the harder rock may become undercut and unstable and may eventually collapse. Some of the collapsed rock may be swirled around at the foot of the falls by turbulence to grind to bed (abrasion), usually at times of high discharge, to create deep plunge pool. As this process is repeated, the waterfall retreats upstream leaving a deep steep-sided gorge. The Niagara Falls retreats on average 1m per year. A waterfall is a vertical wall or curtain of water found on a river channel in the source zone usually on bedrock. The formation is initiated by a change in gradient in the riverbed as a red result of a change in rock type. Undercutting of the falls is due to splash back from the water falling and is cut through the process of cavitation.
Rapids: Occurs where layers of hard rock and soft rock are very thin, and so obvious break of slope develops as a waterfall.
V-Shaped valleys: Any spare energy possessed by a river near to its source will be used to transport large boulders along its bed. These results in the river cutting rapidly downwards, a process called vertical erosion. Vertical erosion leads to the development of steep sided narrow valleys shaped like the letter V. Valleys are steep due to soil and loose rocks being washed away. Interlocking Spurs: The River itself is forced to wind its way around protruding hills. These hillsides restrict the view up and down the valley
Thalweg: This is the line of fastest flow in a stream and is usually exaggerated variation of the stream channel shape that crosses to the outside of each meander at the point of inflection. Because erosion is greatest where the stream flow is fastest, the thalweg is also the deepest channel in the stream. It is found in the top middle of a straight channel because this is where the water is the deepest and is where there is the least friction.
Pools and Riffles
Riffle and pool sequence: River channels have irregularities in the bed, which cause the thalweg to shift from the middle. These are known as 'pools' and 'riffles'. In a flowing stream, a riffle-pool sequence (also known as a pool-riffle sequence) develops as a stream's hydrological flow structure alternates from areas of relatively shallow to deeper water. This sequence is present only in streams carrying gravel or coarser sediments. Riffles are formed in shallow areas (the shallow points of inflection) by coarser materials such as gravel deposits on river with a turbulent flow with a lower velocity. Pools are deeper and calmer areas of laminar flows with higher velocities, whose bed load (in general) is made up of finer material such as silt. Streams with only sand or silt-laden beds do not develop the feature. The sequence within a streambed commonly occurs at intervals of from 5 to 7 stream widths. Meandering streams with relatively coarse bed load tend to develop a riffle-pool sequence with pools in the outsides of the bends and riffles in the crossovers between one meander to the next on the opposite side of the stream. The pools are areas of greater erosion where the available energy in the river builds up due to a reduction in friction. The material eroded tends to be deposited in the riffle area between pools as energy is dissipated across the riffle area. Pools and riffles are responsible for the initiation of a meander. The pools are areas of high velocity and the thalweg is fast in a pool. Its energy is reduced and diffused (spread out) as it crosses the riffles. This is because the water is shallower; the bed is covered with bed load, is rough and creates turbulent flow. Therefore in order to overcome, these obstacles the river uses up more energy become slower.
Point bars: On a meander, material deposited on the convex inside of the bend may take the form of a curving point bar. Material is deposited here where velocity is at its lowest round a bend.
Ox Bow Lakes
Ox Bow lakes: Continual erosion on the outside bends, results in the neck of the meander getting narrower until the river undercuts through the neck and shortens the coarse. The current will take the path of least resistance, giving it renewed energy. The faster current will now be flowing in the centre of the channel and deposition is more likely next to the banks. The original meander will now be blocked off to leave a crescent shaped ox bow lake with a meander core in the centre. The lake will slowly dry up except during heavy rain. This lake can also be become filled with alluvium over time (marshland).
It's a flat wide expanse of alluvium covering the valley floor formed due to deposition when the river is in overbankful. As the river floods, the river slows down, loses energy and consequently deposits its large (capacity) load of small material (competence) usually silt (alluvium). Rivers have the most energy at their bank full stage. Should the river continue to rise, and then the water will cover any adjacent flat land. The land susceptible to flooding in this way is known as the floodplain. As the river spreads over its floodplain, there will be a sudden increase in both the wetted perimeter and the hydraulic radius. These results in an increase in friction, a corresponding decrease in velocity and the deposition of material (alluvium) previously held in suspension. The thin veneer of silt, deposited each flood, increases the richness of the soil, while each successive flood causes the floodplain to increase in height. The floodplain may also be made up of material deposited as point bars on the inside of meanders and can be widened by the lateral erosion of the meanders. Prominent slopes known as bluff lines often mark the edge of the flood plain. These bluff lines can change as the flood plains become wider and more sinuous as they migrate downstream - which in turn widens the valley.
Levees: When a river overflows its banks, the increase in friction produced by the contact with the floodplain causes material to be deposited. The coarsest material is dropped first to form a small, natural embankment (levee) alongside the channel. During subsequent periods of low discharge, further deposition will occur within the main channel causing the bed of the river to rise and the risk of flooding to increase. To try to contain the river, the embankments are sometimes artificially strengthened and heightened. Some rivers flow above their floodplains so if levees increase the river can cause serious damage to properties.
River Terraces: They are the remnants of former floodplains which, following vertical erosion caused by rejuvenation, have been high and dry above the maximum level of present day flood plains. If a river cuts rapidly into its floodplain, a pair of terraces of equal height may be seen flanking the river and creating a valley-in-valley feature. However, more often than not, the river cuts down relatively slowly, enabling it to meander at the same time. The result is that the terrace to one side of the river may be removed as the meanders migrate downstream. If the uplift of land continues, the river may cut downwards to form incised meanders. There are two types of incised meanders. Entrenched meanders have a symmetrical cross-section and a result from either a very rapid incision by the river, or valley sides being resistant to erosion. Ingrown meanders occur when the uplift of the land, or incision by the river, is less rapid, allowing the river time to shift laterally and to produce an asymmetrical cross-valley shape. As with meanders in the lower course, incised meanders can also change their channels to leave an abandoned meander with a central meander core.
Deltas: Deltas are usually composed of fine sediment, which is deposited when a river loses energy and competence as it flows into an area of slow-moving water such as a lake or the sea. When the river meets the sea the meeting produces an electric charge, which causes clay particles to coagulate and to settle on the seabed, a process called flocculation (larger coagulated particles carried out into the shallow water offshore and deposited, and the river loses energy on meeting the sea water). The water flows into a delta via distributaries. They are usually highly populated, not very navigable and have a great risk of flooding. Crops are usually grown on these deltas and are usually staple crops e.g. Rice. Deltas are named after the fourth letter in the Greek alphabet (∆). Yet Deltas range in geomorphology into three main types:
• Arcuate: (Wave dominant) Having rounded, convex outer margins. They also have smooth coastlines and have well developed beaches/ sand dunes. Lagoons form near coastal areas e.g. the Nile Delta.
• Cuspate: (Tide dominant) Where material brought down by a river is spread out evenly on either side of the channel. It is tide dominant and is covered by the high tide and left dry at low tide e.g. the Bangladesh Delta.
• Bird's foot: (River Dominant) Where the river has many distributaries bounded by sediment and which extend out to sea like the claws of a bird's foot. The river has a large load from a huge drainage basin, a low energy river into the Gulf of Mexico, and a small tidal range e.g. Mississippi Delta.
Alluvial Fans: In order to form, alluvial fans require a flat or a gently sloping plain near the foot of a hill or plateau, where a stream carrying sediment emerges abruptly from a mountain front and spreads out. As the stream reaches the flat plan, known as the piedmont, its velocity slows and it loses competence to carry sediment load. The coarse sediment is therefore deposited at the junction of the hill and the piedmont, and a fan-shaped deposit builds up. Arid environments are well suited to alluvial fan development because they are prone to flash flooding. Furthermore, they have hill slopes that erode easily and therefore provide alluvial material suitable for deposition. Alluvial channels are considered disconnected from the channel.
Zones of a river: Source zone
• Steep gradient
• Little discharge
• Rapid velocity
• Limited amount of load
• Large sized load e.g. boulders
• Channel is bedrock
• Small narrow channel
• Dominant process is erosion
• Rapids, waterfalls, and potholes form here
Zones of a river: Transfer Zone
• Gentler gradient
• Increase in discharge
• Velocities vary
• Increase in amount of load
• Decrease in size of load
• Channel may be bedrock and alluvium
• Dominant process is transportation
• Meanders, ox bow lakes, and flood plains form here
Zones of a river: Accumulation Zone
• Very gentle gradient
• Increase in discharge - because the addition of water from tributaries and the catchment and subsurface storage of water.
• Slight increase in velocity
• Increase in the amount of load
• Increase in the size of the load
• Channels are alluvial
• Dominant process is deposition
• Braided channels, meandering channels, and deltas are formed her
A Hydrograph is a graph showing the discharge of a river at a given point (a gauging station) over a period of time. A storm hydrograph shows how a river responds to a given input of rainfall (a particular storm). When a storm begins, discharge does not increase immediately as only a little of the rain will fall directly into the channel. The first water to reach the river will later be supplemented by water from through flow. The rising limb shows the increase in discharge. The gap between the peak rainfall and the peak discharge is called the lag time. A short river lag time and a high discharge are more likely to flood than a river with a lengthy lag time and a low discharge.
Measurements on a hydrograph
Velocity is the speed of the river. It is measured in meters per second
Volume is the amount of water in the river system. It is the cross sectional area of the river's channel measured in square meters
Discharge is the velocity of the river times its volume. It is the amount of water in the river passing a given point at a given time, measured in Cumecs. Discharge depends on the river's velocity and volume.
The Lag time is the length of time between peak rainfall and peak discharge
Peak Discharge is the highest discharge on the hydrograph.
The line of flooding occurs when the river overflows its banks. The river is thus said to be overbank full.
The rising Limb
The Rising limb is the increase in discharge from the beginning of the rainfall to the point of peak discharge
The Recession Limb is the discharge from the peak to the base flow level in the river at the end of a storm after overland and through flow has occurred.
Base Flow is the water that flows laterally through the rock. It is the slowest of flows.
Basin watershed: A drainage basins boundaries, marked by a ridge of higher land is called a watershed. A watershed separates one drainage basin from the neighboring one.
Density: The total length of all the streams, and rivers in a drainage basin divided by the total area of the drainage basin. The greater the density, the greater the risk of flood.
Factors influencing Hydrographs
- Climate (amount & intensity of rainfall) (Temperature (affects evaporation rates))
- Rock type (Permeable: allows water in) (Impermeable: Will produce through flow and overland flow) (Porous: Holds water but may not transmit water storage - fewer flows equals more storage)
- The storage of water in reservoirs (this increases evaporation and regulating discharge)
- Deforestation (This decreases interception & evapotranspiration and increases overland flow) Increased use of irrigation and tapping of groundwater on farms
- Urbanization (Impervious tarmac surfaces prevent infiltration and increase overland flow - which results in a shorter lag time and higher discharges).
Flashy Hydrographs reasons
Flashy Hydrographs reasons:
• Impermeable rock
• Steep Hydrograph
• Steep slope
• Small round drainage basin
• No forest Catchment
• High density drainage basin
• Flat, slightly curved hydrograph
• Lower flatter later peaked hydrographs
• Sometimes known as attenuated hydrographs
• Light rain over a prolonged period
• Porous and permeable rock
• Relief possibly gentle
• Large drainage basin
• Forest catchment with high interception
• Low levels of urbanization
• Surface storage e.g. reservoirs, dams
• Response to two periods of rainfall (each has a relatively short lag time)
• Could be different rock types in one drainage basin
• Clover leaf drainage basin
How factors affect hydrograph shape:
How factors affect hydrograph shape:
o Circular basins produce water at the gauging station at the same time from over the whole catchment area. The rivers have about the same distance to travel. (Flashy Hydrographs).
o Elongated basins, the tributary water enters gradually at several points down stream producing a flatter hydrograph.
o A small basin produces flashy hydrographs because there is a smaller distance of the catchment over which the water must reach the gauging station (point at which river discharge is measured).
• Deforested catchment
o Interception is reduced
o Rain reaches channel very quickly across bare soil surfaces.
• Rock type
o Impermeable rock do not allow water to infiltrate because they have few openings e.g. Granite
• Drainage density
The nature of the storm hydrograph is the result of:
The nature of the storm hydrograph is the result of:
• The nature and amount of the input of precipitation
• The amounts of water already dry in the system. This is known as antecedent moisture and is the result of the rain that has previously fallen. If it had been dry, there will be relatively little, therefore base-flow will be below. Base-flow is the flow that contributes to river discharge even when there has been no rain, keeping the river flowing all year round.
• A combination of catchment factors e.g. vegetation, land use, relief, rock type/geology and human activities.
Hydrograph Case study: River Ribble
Hydrograph Case study: River Ribble
The combination of catchment factors that influence the discharge of the river. The river is 121km, and is in a drainage basin that has an area of 2182m2. It rises in the hills known as the Pennines, which are up to 442m above sea level.
o Quite large and elongated
o Rainfall takes time via rivers to reach the gauging station, therefore there is a longer lag time
o Intense rainfall especially in summer
o Produces a flashy hydrograph in winter saturated ground
o River floods (not coastal floods which are the result of high winds, huge waves and surges).
o Grassland removal of vegetation because too many sheep are grazing
o This leads to rapid run-off, short lag times, and high peaks on storm hydrographs.
• Soil type
o Upper part of the basin has peat vegetation. It is spongy and holds moisture.
o This means the lag time increases and the peak lowers.
• Rock type
o Permeable rock such as limestone (infiltration and percolation) and impermeable rock mixture (increased overland flow).
A river is in flood when it overtops its banks. This is known as overbankful discharge. This water will spread out across the flood plain.
Causes of floods
Causes of floods:
1. Physical Causes
b. Snow melt
2. Human Causes - also influence flooding, however they don't really cause flooding and just intensify floods.
c. Dam/ reservoir failure
d. Agricultural practices
i. Over cultivation = loss of vegetation = rapid runoff
f. Human induced climate change
i. Temperature increase = increased evaporation and warm air can hold more water vapor than cold air = potential for higher rainfall
ii. More extreme weather events
Case Study Physical and human intensification of floods: Mississippi River
Case Study Physical and human intensification of floods: Mississippi River
• Huge river system - 3800 Km long - 1400 million m3 per day discharge
• Every year there is either flooding or severe drought
• Engineering of the channel to control floods, but this exacerbated problems.
• Natural levees were heightened, but this heightening channeled fast flowing water into the deepened river. The water could not be accommodated in the confined space - discharge levels caused more flooding by overtopping new levees and breaks in the new man made embankments.
• Source is a small glacial lake, Lake Itasca, in Minnesota at 480m above sea level. It takes an average of 90 days for the water in the Mississippi to flow to the Gulf of Mexico.
• Just north of St Louis, Missouri the Mississippi River is joined by the Missouri River. The confluence of the river is now doubled.
• The river is a major transport link for grain from the Midwest and petrochemicals from the Gulf. To help these boats use the river, many dams and locks have been constructed. Also the river had been deepened at least 3m and up to 4m in some places.
• Dykes have been built to prevent the riverbanks from eroding.
• The lower Mississippi has huge sweeping meanders. Some of the channels have been changed to divert the river from this course, to shorten the journey for boats.
• The city of New Orleans is built on a delta about 160 Km from the Gulf. A hurricane in 2005 devastated the city when levees protecting the city had broken.
This is the changing and modifying of the river
1. Straighten and widen the channel. This will lead to a shorter channel width with a greater capacity to accommodate surface runoff. Water is controlled via sluice gates.
a. Advantages: There will be faster direct flow allowing more water to be discharged.
b. Disadvantages: May lead to flooding downstream
2. Deepen the channel. This is to dredge it out or to raise the riverbank.
a. Advantages: Increased channel capacity
b. Disadvantages: Unaesthetic and very expensive. Also the increased channel capacity can lead to flooding in narrow channels such as parts of the Mississippi.
a. Increased interception therefore the rain takes longer to reach the channel (flatter hydrographs, longer lag times, lower peak).
2. River restoration
a. Conversation measures to the channel bank/ flood plains
b. This is revolutionary in flood management. It is sustainable and encourages biodiversity
c. This includes lowering flood plains and fitting banks with non-artificial concrete. Also re-meandering the course of straightened rivers.
• The hydrological cycle accounts for 1% of the total water on the planet
• The hydrological cycle is a closed system because water is neither added nor lost
• Over the last 300 years the world's population has increased x7 and demand for water has increased x40.
• Severe water stress is experienced by 1.1 billion people in 80 countries
Reliability of rainfall
• Few homes in LEDC's have piped water
• Few developing countries have the money or the technology to build dams to store water. If they have it was mostly like built with foreign aid
• Torrential downpours give insufficient time to infiltrate the ground. Instead surface runoff create flash floods
• The most vulnerable areas are desert margins and tropical interiors where average annual rainfall is low and rainy seasons are short
• Many countries just experience wet/ dry seasons. If rain fails one year, the result of produce can be disastrous
• Deforestation decreases interception rates and increases evaporation rates
• Climate change can also have an effect on length of droughts
• Increased use of water for irrigation, demand for water for home use (population increase) and for manufacturing also can intensify droughts.
• 1.1 billion people lack clean water
• Rural areas use local rivers for drinking as well as washing and sewerage disposal
• Shanty towns lack proper drainage for sewerage which may pollute water ways
• Droughts: Animals die, crops whither, human dehydrate,
• Villagers can help themselves out though by: building wells to reach a permanent supply, lining wells with concrete, using pumps and teaching about proper hygiene, building stone walls, reducing amount of trees cut down, education on droughts, reducing reliance on irrigation, Using more drought resistant HYV plants, changing cultivation techniques, recycling water from showers, baths and washing, water tanks to use during droughts and save water during rainy seasons, de-salination kits.
The hour-by-hour state of the atmosphere. It is short term and can be localized in relatively small areas.
It's the average weather conditions of a place taken over 30 years. It is expected, rather than actual conditions.
Contains almost all the moisture in the atmosphere, and is where 'weather occurs'. As altitude increases within the troposphere, temperature decreases.
Where most of ultraviolet radiation from the sun is absorbed.
Solar energy travels in the form of radiant energy or radiation.
Radiation can be distinguished by its wavelength. This wavelength depends on the temperature of the radiating body. The hotter the body, the shorter this wavelength is. The sun is around 6000oC and therefore has significantly short wavelengths.
Solar radiation can pass through a clear and cloudless atmosphere with little or no interruption towards the Earth's surface. This radiation is absorbed or reflected by the Earth's surface. The sun does not heat the Earth directly - the warmed Earth surface does when in contact with air. The Earth is thus heated from below.
The vertical movement of a parcel of air, which is at a different temperature than its surroundings. Any portion of the atmosphere at a different temperature from the air around it will tend to remain separate rather than mix with its surroundings - 'The conservative behavior of air masses theory'.
Clouds' roles in radiation exchanges
Plays an important role in regulating the Earth's radiation exchanges, and reflecting the sun's radiation back into space. The influence depends on the cloud type and its height. In low clouds e.g. Stratus Clouds most shortwave radiation (incoming solar radiation) is reflected back into space, and radiate long wave radiation into space. This provides a cooling effect on Earth. In high Cirrus clouds transmit most incoming solar radiation, but absorb and delay losses of outgoing long wave radiation.
Long wave radiation
The Earth gives out long wave radiation, which is absorbed and delayed before it returns to space (Without this delay and absorption the atmosphere would be too cold for humanity to exist). However this form of radiation cannot pass as easily through the atmosphere.
• The long wave energy budget totals - 69 units
• Radiative long wave radiation accounts for 49 units
o 3 units are UV being returned to space by the ozone layer
o 21 units are returned from the atmosphere
o 8 units directly returned from the Earth's surface
o 14 units are returned through the greenhouse effect
• The greenhouse effect involves large long wave energy exchanges.
o A total of 110 units from the ground are absorbed by the atmosphere
o 96 units are returned the Earth's surface
o A balance of 14 units are returned to space
• Non-radiative involves air movements and the properties of water and accounts for the remaining 23 units of the long wave budget
Shortwave radiation budget
Shortwave radiation budget:
• In the stratosphere 3 units of radiation (ultraviolet) is absorbed by the ozone. This prevents too much UV entering the Earth's surface.
• 25 units of solar radiation pass directly through the atmosphere to the Earth's surface. This amount can vary depending on the cloud cover.
• Dust, gases and impurities absorb 18 units.
• Clouds absorb 3 units and 21 units are reflected back by them. 10 units continue through to the surface.
• Albedo: In total 31 units are reflected back into space, forming a substantial proportion of what is known as the Earth's albedo (total reflectivity).
The process whereby a solid is converted into vapour with no liquid state intervening. This process can be responsible for the formation of ice crystals.
The change in state from vapour to ice, which are directly from air containing water vapour. It is usually associated with hoar frost.
Radiation Cooling (Horizontal air movements)
Occurs on calm cool evenings, when the ground looses heat rapidly through terrestrial radiation and the air in contact with it is then cooled by convection. If the air is moist, vapour will condense to from radiation fog and if it is under 00C it will form hoar frost.
Warm moist air moving over cooler land or ocean surfaces. Advection fogs are formed when warm air from over the land drifts over cold offshore ocean currents.
Orographic/ frontal uplift
Warm moist air is forced to rise as it crosses a mountain barrier or meets cold air at a front.
Convective/ adiabatic cooling
Air warmed during the daytime rises in pockets of thermals. As this air expands it uses energy so the temperature in these pockets drops and sinks back to the ground.
Measure of water vapour content in the atmosphere
Mass of water vapour in a given volume of air. This is measured in g/m3.
This is measured in g/kg. The mass of the water vapour in the air per 1kg of air.
Amount of water vapour in the air at any temperature expressed as % of the maximum amount of vapour the air can hold.
Humidity depends on:
Temperature of the air and the limit of water the air can hold.
Energy budget variation with latitude
Energy budget variation with latitude
• Occurs in 100 strips
• In terms of radiation received and emitted by each strip, those between 0-400 gain more than they lose, while from 40-900 more is lost than gained.
• A transfer mechanism must work to maintain the balance between them.
• The black line has been added to show how much energy needs to be transferred pole wards at each latitude in order for the balance to be maintained, otherwise the tropics would be unbearable to live in.
• In the 0-100 strip the excess amount of energy will be matched by the other side of the equator.
• Further away from the equator in the 20-300 strip the total excess energy to be transferred will consist of the excess of the strip itself plus the excesses that have reached it from the two strips nearer the equator.
• Between the 30-400 strip (greatest energy transfer) all excess energy from latitudes nearer to the equator must pass on passage to the poles.
• Beyond 400 in latitude the energy flow is reduced since there are deficits of radiant energy in each latitude strip.
• These transfers are achieved through ocean currents, general circulation (winds) and weather systems.
How is energy transferred around the world?
1) General circulation
• Tri-cellular model
o Hadley Cell
• Moves anticlockwise in the northern hemisphere in convection currents
• Moves between 0-30 degrees and is associated with cumulonimbus cloud formations
o Ferrell Cell
• Associated with warm south westerlies
• Circulates up to 12Km in height and between latitudes 30-600 and in a clockwise direction in the northern hemisphere
o Polar cell
• Circulates in an anti-clockwise direction in the northern hemisphere
• Associated with easterlies
• Circulates between latitudes 60-90 degrees
• Up to 9-10 Km in height
• A mechanism for excess heat to be transferred
o Coriolis force
• Refers to the direction that the water spins in the northern hemisphere compared with the southern hemisphere.
• It is due to the Earth's rotation
• In the northern hemisphere water spins a clockwise direction and in the southern it spins anticlockwise.
o Trade winds
• Pick up latent heat as they cross warm tropical oceans
• North East in the northern hemisphere and south easterly in the southern hemisphere
• Gentle variable winds
o Inter-tropical convergence zone
• This is the zone where trade winds meet
• Jet Streams
o Helps in a rapid transfer of energy
o Narrow bands of extremely fast moving air
• Rossby Waves
o Rossby waves are high altitude, fast moving westerly winds, which often follow an irregular path. The path that they take changes throughout seasons.
2) Ocean currents
• The ocean has a greater specific heat capacity than land
• Warm currents carry water pole-wards and raise the temperature of the maritime environment. Cold currents also carry water towards the equator and lower the temperature of coastal areas there.
3) Weather systems
Factors affecting Isolation - Long-term factors:
• Height above sea-level
o The atmosphere is not warmed directly by sun but by heat radiated from the Earth surface and is distributed via convection and conduction.
o As height increases, it produces a decreasing surface area e.g. mountains, from which to heat surrounding air.
• Altitude of the sun
o As the angle of the sun in the sky decreases, the land area heated from a ray and the depth of the atmosphere increases.
• Land and sea effect
o Land and sea differ in their ability to absorb, transfer and radiate heat energy. The sea is more transparent than land, and more capable of absorbing heat. The sea also has a greater specific heat capacity (energy required to 1kg by 10C).
• Prevailing winds
o The temp of the wind is determined by its area of origin and by the characteristics of the surface over which it subsequently blows. A wind blowing from the sea tends to be warmer in winter, but cooler in the summer.
• Ocean currents
o Warm water is carried pole ward by currents, and raise the temp of the maritime environment. Cold currents also carry water towards the equator and lower the temperature of coastal areas there.
Short-term factors affecting isolation:
• Seasonal changes
o Summer (due to the Earth's tilt) the sun is overhead at the tropics, the hemisphere experiencing 'summer' will receive maximum insolation.
o In spring/ autumn when the sun is directly over the equator, insolation is distributed equally between both hemispheres.
• Length of day and night
o Insolation is only received during daylight hours
o There is no seasonal variation at the equator so day/ night is the same all year round.
o In contrast the arctic circle area receive no insolation during winter when there is continuous darkness.
o North facing slopes in the southern hemisphere is at a higher angle to the sun's rays, therefore these slopes are warmer.
• Cloud cover
o The presence of cloud reduces both incoming and outgoing radiation.
o In the daytime clouds reduce temps, however in the night they act as an insulator.
o Urban heat islands
6 factor model (Daytime budget)
•Incoming solar radiation
Main energy input
Strongly influenced by amount and type of cloud
Dependent on the angle of the sun
•Sensible heat transfer
The movement of parcels of air to or from the point at which the enrgy budget is assessed
•Reflected solar radiation
Expressed as a % or as a fraction (albedo of the surface).
The fraction of solar radiation reflected will very greatly wit the nature of the surface
Energy arriving at the surface is available to heat the surface
•Long wave radiation
Emitted by the surfaces and passes into the atmosphere
• Latent heat evaporation
Turning liquid into vapour consumes a considerable amount of energy
Energy available at suface
Energy available at surface = Solar radiation receipt - [reflected solar radiation + surface absorption + latent heat + Sensible heat transfer + Long wave radiation]
4 factor model (Night time)
•Latent heat (condensation)
•Sensible heat transfer
Warm air at a given point contribute energy and keep temps up.
If cold air moves in energy levels will fall
•Long wave radiation
Weather phenomena associated with local energy budgets: Mist and Fog
• Droplets of water in the atmosphere, which are so small they fall only very slowly to the surface under gravity. This phenomenon is known as mist.
• Fog droplets are denser than mist droplets; therefore visibility is lower than mist.
• Mist visibility is reduced to between 1-2km
• Fog visibility is reduced to less than 1km
• Dense Fog is reduced to less than 200m
• Mist usually occurs when a large body of cold air passes over a warm body of ocean. Humidity is around 95% for this to occur.
• Air can only hold so much water in vapour form, once this capacity is reached, the air is described as saturated and vapour will condense into droplets. The amount of water held depends on the temperature of the air.
• The temperature this saturation occurs is called dew point
• Fog will not always form when the saturation point is reached. On occasion's small particles of dust, salt and other matter (condensation nuclei) is needed in the air to encourage droplet formation. This is why fog is more common in cities.
• Condensation on a surface
• Occurs when air in contact with the surface has dropped sufficiently.
• It may occur when more moisture is introduced into the air from onshore breezes.
• Air is warmed and becomes less dense than surrounding air, so it rises.
• The parcel of air will cool at a fixed rate. Nothing changes that rate.
• The surrounding air presents more of a problem since temperature at any given height would have developed as a response to heating and cooling experienced at that height.
• Temperature changes with altitude. The rate of temperature change with height of surrounding air is known as the Environmental Lapse rate (ELR).
• Usually once outside the zone very close to the ground, the environmental temperature of the atmosphere decreases steadily with height, because the atmosphere is heated from below. Under these conditions a warm parcel of air moving up from the surface will continue to rise as long as it cools at a rate that ensures that it will remain warmer than the air around it.
• Occasionally the air does not cool steadily with height but contains a zone where temperature actually increases. This zone is called the inversion layer. Under this, a rising parcel of air from warm factories may arrive at the layer at the same time or a lower temperature than its surrounding air. When this occurs factory air will accumulate at the level of inversion and, if it contains enough pollutant particles, it may be thick enough to block the sun's rays and give a 'lid' effect.
A wind that carries high-density air from a higher elevation down a slope under the force of gravity. It is very cold. The 'barber wind' that blows over Greymouth is an example of a katabatic wind.
These winds are winds that blow up a steep slope or mountainside, driven by heating of the slope through isolation.
Environmental Lapse Rate:
The decrease in temperature is usually expected with an increase in height in the troposphere. The ELR is 6.50C per 1000m but varies with local air conditions. It may vary due to height (the ELR is lower the lower to the ground), different surfaces and different air masses.
Adiabatic Lapse rate:
Describes what happens when a parcel of air rises and the decrease in pressure is accompanied by an associated increase in volume and a decrease in temperature.
• If upward the movement of air does not lead to condensation, energy used by expansion will cause the temperature of the parcel of air to fall at the Dry Adiabatic Lapse Rate. The DALR is the rate in which unsaturated air parcels cool as it rises and as it descends. Remains consistently at a decrease of 9.80C per 1000m.
• The upward movement of air is sufficiently prolonged to enable the air to cool at its dew point temperature - condensation occurs and the losses of temperature will height be partly compensated by a release in latent heat. Saturated air (at a slower rate than unsaturated air) loses heat and is known as the Saturated adiabatic lapse rate. The SALR can vary because warm air holds lots of moisture and so a greater amount of latent heat is released following condensation. The SALR varies from 40C per 1000m to 90C per 1000m.
Stability: Linked with anticyclonic weather, high pressure systems and calm dry conditions. A sate of stability is achieved when a rising parcel of unsaturated air cools more rapidly than air surrounding it. If there is nothing to force the parcel of air to rise e.g. a mountain range or a front, it will sink back to its starting point. Air is atble as dew point may not have been reached and the only clouds that may of developed would be shallow flat-topped clouds that do not produce precipitation.
This is linked with thunderstorms and the formation of cumulonimbus clouds. Instability arises on warm days when localized heating of the ground warms adjacent air by conduction, creating higher lapse rates. The resultant parcel of rising unsaturated air-cools less rapidly than the surrounding air. Rising air remains lighter than surrounding air. If moist and de point is reached, then upward movement may be accelerated to produce towering cumulus or cumulonimbus clouds.
It occurs when the ELR is lower than the DALR but higher than the SALR. Rising pockets of air cooling at the DALR, become cooler than surrounding air, and should sink down to the ground. However it may be forced to rise because of a rise in relief. This may cause the air to cool to its dew point. Once saturation occurs, condensation takes place. Thus the air begins to cool at the SALR. If it becomes warmer than the surrounding air, it will continue to rise. The air is unstable on the condition that dew point is reached, and it cools at the SALR. Weather is usually fine in areas at altitudes below condensation level, but cloudy and showery above.
Weather Phenomena - Resultant weather
• Form when air cools to dew point and vapour condenses into water droplets and/or ice crystals.
• There are many different types of clouds and they form under different conditions.
• Forms under similar conditions to rain except dew point is below 00C
• Sublimation occurs
• Ice crystals form if hydroscopic nuclei is present
• Mixture of ice and snow formed when upper air temperature is below 0, allowing snowflakes to form, and the lower air temperature is around 2-40C, which allows there partial melting.
• Frozen water droplets which exceed 5mm in diameter
• Forms in cumulonimbus clouds from an uplift of convection currents
• Quite common in warmer climates
Global warming and the Greenhouse effect
The greenhouse effect
It is the process by which certain gases absorb outgoing long-wave radiation from the earth, and return some back to Earth. In all greenhouse gases such as CO2, Methane, CFC's, nitrous oxides and water vapour, raise the Earth's temperatures by 330C. Without this effect the Earth would be too cold to exist. These gases can contribute to global warming.
• The concern about global warming is the build-up of gases within the atmosphere.
• Deforestation has meant that the extra CO2 that has been produced, can not be absorbed by trees and converted in oxygen.
• An increase in greenhouse gases has meant that there is an increase in long-wave radiation being absorbed by these gases. This has meant the earth has gotten warmer.
Impacts of global warming
Impacts of Global warming
o Increased storm activity
• Tornadoes in the mid west
o Temperature increases
• With no action, temperatures will increase by 2.50C in the next 50 years.
o Reduced rainfall
• Leads to droughts
o Rise in sea temperatures
o Forest fires
• Increase of fires because of dry forests
o Coral bleaching
• From a rise in sea temperatures
• For example reefs on the Great Barrier Reef
o Water shortages
• 4 billion could face water shortages if temperatures increase 20C.
o Changes in agriculture
• Samoln fishing could become obselte
• A 35% decrease in yields if temperatures increase 30C
• However there could be an increase in timber yields
• Increase in growing seasons in temperate and alpine areas
• By 2100 an overall 1m rise in sea levels if no action has occurred.
• Flooding will occur in Delta areas more frequently e.g. New Orleans.
• 4 million km2 is threatened
• 200 million could be at risk of loosing their homes from floods by 2050.
o Changes to Tourism
• Longer tourist seasons for summer tourist destinations
• Winter tourism to ski fields and glaciers may decline.
o Soil erosion
• Especially in areas such as the amazon that is damaged by slash and burn practices
o Spreading of disease
• A 20C rise could increase the number infected by Malaria by 60 million
o Extinction of wildlife
• If temperatures increase by 20C, 40 % of species will become extinct
• Habitats could decrease in range e.g. Polar Bears
• Range of Species could increase e.g. Butterflies in the UK