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Geography AS Level full revision notes

Geography notes Hydrology, Atmosphere, Weathering, Population and Migration
Casestudies aren't included - sorry. Hope these are helpful. Good luck everyone with your exams.

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Geography AS Level full revision notes

  1. 1. H Y D R O L O G Y A N D F L U V I A L G E O M O R P H O L O G Y Geography AS Level
  2. 2. Introduction  Water enters and continually cycles around the earth through the global hydrological cycle, it is a closed system with no inputs or outputs. The hydrological cycle refers to the cycle of water between atmosphere, lithosphere and biosphere.  The Drainage basin system however is an open local system as it has inputs, outputs and transfers of energy and matter into and from the system.  A rivers drainage basin is the area of land drained by a river and its tributaries - also called the rivers catchment  The drainage basin is surrounded by higher land - this boundary of the drainage basin is called the watershed- any precipitation falling beyond the watershed enters a different basin. The watershed separates different drainage basins.
  3. 3. Drainage Basin Features Source: The beginning of a river. A river may have multiple sources. The source of a river is normally found in upland mountainous areas. Mouth: The end of a river. A river may end in a lake, but more normally in the sea. Tributary: A small river that flows into a larger river. Confluence: Where two rivers meet. Watershed: The border between two drainage basins. Estuary: The tidal section of a river near the mouth. Channel: The physical confines of the river, encompassing two banks and a bed.
  4. 4. The Hydrological Cycle  Water evaporates from water bodies such as rivers, lakes and seas, and from plants and trees. The water vapour rises, cools and condenses to form clouds. Rain falls from the clouds . The rain water is intercepted by plants , seeps into the ground before reaching surface streams, or runs off the land surface into streams and rivers. The rivers enter lakes or seas. The hydrological cycle is then repeated.
  5. 5. The Drainage Basin Processes
  6. 6. Drainage Basin Processes Explained  Precipitation: Any moisture that falls from the atmosphere. The main types of precipitation are rain, snow, sleet, hail, fog and dew.  Evaporation: The process of water turning from a liquid into a vapour. Evaporation only takes place from a body of water e.g. a lake, puddle or the sea.  Transpiration: The evaporation of moisture from vegetation's stomata.  Evapotranspiration: The combined action of evaporation and transpiration  River discharge via channel flow: Water entering the sea and leaving a drainage basin. A very small amount of water also enters the sea via through flow and groundwater flow (base flow).  Through fall: Precipitation that drips from vegetation to the ground  Stem flow: Precipitation that flows down plant stems to the ground.  Surface store: Precipitation lying on the ground (puddles)  Overland flow: movement of water along ground surface to a river.  Infiltration: Process whereby water enters soil layer.  Through flow: is the horizontal occurs downslope along well-defined lines of (percolines) or above impermeable layer.  Percolation: process by which water drains to the water table  Ground water flow: is deeper level of gravitational flow in downslope direction through rock to feed rivers and springs  Channel flow: Water flowing in a river.  Phreatic Zone: The permanently saturated zone within solid rocks and sediments. The upper layer of this is known as the water table.  Vadose zone: Zone of temporary saturation
  7. 7. Types of Surface Runoff/Overland Flow
  8. 8. Inputs, outputs, stores and transfers. Inputs: When water is added to a drainage basin. Outputs: When water leaves a drainage system • Precipitation • Evaporation • Transpiration • Evapotranspiration • River runoff Stores: When water is stationary and not moving in a drainage basin. Transfers: When water is moving within a drainage basin. • Interception • Ground water store • Soil water store • Vegetation Store • Channel Store • Through fall • Stem Flow • Overland Flow • Infiltration • Through flow • Percolation • Ground Water Flow • Channel Flow
  9. 9. Definitions  Saturated: Ground where the pores are full and can contain no more water.  Unsaturated: Ground where there is still space between the pores.  Water table: The border between saturated and unsaturated ground. The water table may go up or down.  Permeable: Surfaces that allow water to pass through them.  Impermeable: Surfaces that do not allow water to pass through them.  Pores: Gaps between soil and gravel that water can fill.  Aquifer: Rock that can hold water.  Aquiclude: Rock that can not hold water.  Porous: Rock with pore spaces and cracks in it.  Non-porous: Rock with no pore spaces or cracks in it.  Condenses: When water vapour turn into water droplets. Water can only condense around condensation nuclei  Antecedent Moisture: Amount of water in the soil before additional precipitation  Topography: The shape of the land
  10. 10. Aquifer (rocks that can hold water)  Using examples , explain how geology can define if a rock is an effective aquifer (8m)  The porosity and permeability of the rock under the ground decides whether it will be an effective aquifer  High porosity and permeability rocks with many pores and big gaps between the pores allow water to transfer well in them to make a good aquifer  An example of this type of rock is sandstone  Low porosity rocks do not make good aquifers as water can not pass through them  An example of this type of rock is glacial till  Some rocks have high porosity but are impermeable and so do not act as a good aquifer  An example of this type of rock is clay
  11. 11. Movement of water – The Water table  Water is infiltrated at the surface and then is percolated under gravity through pores, joints and bedding planes to reach an area of permanent saturation where all pores, joints, etc are full of water.  This may be seasonal or permanent depending upon the nature of the rock and the level of input.  The water table will generally follow the surface topography and hence water will flow under gravity and by the hydraulic gradient to a point where it will emerge as a spring or base flow of a river. It may also be abstracted by wells or boreholes.
  12. 12. Drainage Basin Shapes
  13. 13. The Water Balance  The water balance is worked out from inputs and outputs and affects how much water is stored in the basin. The general water balance in the UK shows seasonal patterns:  in wet seasons, precipitation exceeds evapotranspiration creating water surplus. the ground stores fill with water so there's more surface runoff and higher discharge- river levels rise  in drier seasons, precipitation is lower than evapotranspiration. ground stores are depleted as some ware is used and flows into the river channel but isn't replaced by precipitation  at the end of a dry season there's a deficit of water in the ground. the ground stores are recharged in the next wet season.
  14. 14. River Discharge  River Discharge = is defined as the volume of water passing a measuring point or gauging station in a river in a given time. It is measured in cubic metres per second (cumecs)  precipitation- more precipitation, higher the discharge  hot weather- higher temperature, lower the discharge because evaporation is higher  removal of water from the river- reduces discharge Discharge can be illustrated using hydrographs. These can show annual patterns of flow ( the river regime) in response to climate. Short-term variations in discharge are shown using a flood of storm hydrograph.
  15. 15. Flood(storm) Hydrographs Discharge= Q=AxV Q= Discharge A=Cross sectional area V=Velocity
  16. 16. Flood storm hydrograph  The starting and finishing level show the base flow of a river. The base flow is the water that reaches the channel through slow through flow and permeable rock below the water table. As storm water enters the drainage basin the discharge rates increase. This is shown in the rising limb. The highest flow in the channel is known as the peak discharge. The fall in discharge back to base level is shown in the receding limb. The lag time is the delay between the maximum rainfall amount and the peak discharge.  The shape of a hydrograph varies in each river basin and each individual storm event.
  17. 17. Definitions and Explanation of Terms involved in hydrographs.  lag time- is the delay between peak rainfall and peak discharge- delay happens because it takes time for the rainwater to flow into the river  rising limb- part of the graph up to the peak discharge- increases as rainwater flows into the river increase in discharge after start of precipitation event.  falling limb- part of the graph after the peak- discharge is decreasing because less water is flowing into the river – decline in discharge after the precipitation event.  Hydrograph – a graph that shows river discharge and rainfall over time.  Base flow – represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel.  Storm flow – Water that reaches the steam via overland flow and through flow  Bankfull discharge – the maximum discharge that a particular river channel is capable of carrying without flooding.  Peak discharge – the point on a flood hydrograph when river discharge is at its greatest.  Peak rainfall – the point on a flood hydrograph when rainfall is at its greatest.
  18. 18. Factors affecting flood storm hydrographs  larger drainage basins can catch more precipitation so have a larger peak discharge, smaller basins generally have shorter lag times  steep-sided drainage basins have shorter lag times because water flows more quickly downhill  circular basins- more likely to have flashy hydrograph because all points on the watershed are roughly the same distance from the point of measurement means a lot of water will reach the measuring point at the same time  basins with lots of streams drain quickly so have shorter lag times  the amount of water already present in the drainage basin affects lag time: if ground is already waterlogged then infiltration is reduced and surface runoff increases, surface runoff is much faster than throughflow or baseflow so rainwater reaches the river more quickly-reducing lag time
  19. 19. Factors affecting storm hydrograph  Rock type- affects lag time and peak discharge: impermeable rock types don't store water or let water flow through them reduces infiltration and increases surface runoff, reducing lag time  Soil type- affects lag time and peak discharge. Sandy soils allow a lot of infiltration but clay soils allow little. low infiltration rates increases surface runoff, reducing lag time, increasing peak discharge  Vegetation- affects lag time and peak discharge. Intercepts precipitation and slows its movement- increasing lag time. The more vegetation there is, the more water is lost before it reaches the river channel, reducing peak discharge
  20. 20. Factors affecting flood storm hydrograph  Precipitation- affects peak discharge. Intense storms will generate more precipitation and so greater peak discharges than light rain showers  the type of precipitation affects lag time e.g. snow thats fallen in a winter storm can melt and flow into the river in spring, giving a very long lag time  Seasonal variation  temperature- affects lag time and peak discharge  hot, dry conditions and cold, freezing conditions both result in hard ground- reduces infiltration and increases surface runoff- reducing lag time and increasing peak discharge  high temperatures can increase evapotranspiration, so less water reaches the river channel, reducing peak discharge
  21. 21. Factors affecting storm hydrographs (human)  in urban areas- much of the soil is covered with man- made impermeable materials like concrete  water can't infiltrate into the soil, which increases surface runoff, so water flows more quickly into the river making lag time short and increases peak discharge  man-made drainage systems affect the hydrograph in a similar way. water flows down drains into the river before it can evaporate or infiltrate into the soil, causing a shorter lag time and increased peak discharge  Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
  22. 22. River Channel Processes and Landforms  Rivers - Source to Mouth Having understood the basics of a Drainage Basin we now need to consider the journey that a river within a Drainage Basin takes from its beginning to its end.  The path the river follows from its source to mouth is known as the river's course.  When studying rivers we often divide it into 3 main sections, the upper course; middle course and lower course.  Each part of the river has distinctive features which form and the characteristics of the river and its surrounding valley change downstream.
  23. 23. River Profile
  24. 24. Processes in different stages of river profile:  Processes in the Upper Course In the upper course, the river has a lot of gravitational potential energy so it has a lot of energy to erode vertically. The bed of the river is eroded greatly while the banks aren’t eroded as much. The river mainly transports large pieces of angular rock and does so by traction because it doesn’t have enough kinetic energy to move the load in any other way. This increases erosion of the bed by abrasion as a result of the load being dragged along the bed of the river. Vertical erosion is further increased by the rough nature of the channel in the upper course which increases the water’s turbulence and its ability to erode. Erosion and transportation only takes place in large quantities in the upper course when the river’s discharge is high after periods of heavy precipitation. When the river’s discharge falls the river stops transporting the large boulders its transporting and deposits them.  Processes in the Middle Course In the middle course, the river has less gravitational potential energy and more kinetic energy so erosion shifts from vertical to lateral erosion. Abrasion is still the main erosive process as large particles are transported by saltation. The average load size has decreased in the middle course, so more load is being transported in suspension. In the middle course, the river can flood and in doing so, it deposits gravel and sand sized particles onto its flood plain.  Processes in the Lower Course In the lower course, the river has next to no gravitational potential energy so erosion is almost exclusively lateral. There isn’t much erosion though because the channel is smoother resulting in less turbulent flow. The main place where erosion takes place is where the river meanders. The average particle size is very small now, another reason for the reduction in erosion. The river’s load is mainly composed of silts and clays and it is transported in suspension or even solution. Like in the middle course, when the river floods it deposits its load but deposition now also takes place at the mouth of the river where the river meets the sea or a stationary body of water.
  25. 25. River Channel Processes  As a river flows along its course it undertakes 3 main processes which together help to shape the river channel and the surrounding valley. These are  Erosion  Transportation  Deposition  At any one time the dominant process operating within the river depends on the amount of energy available.
  26. 26. Erosion  River erosion is the wearing away of the land as the water flows past the bed and banks. There are four main types of river erosion. These are: 1. Abrasion (corrosion): This is the scraping and rubbing action of material carried along by a river (bed load). Rivers carry rack fragments in the flow of water or drag them along the bed, and in doing so wear away the banks and bed of the river channel. Abrasion is most effective in short turbulent periods when the river is at Bankfull or in flood. During times when river levels are low, the load consists of small particles such as sand grains, and these tend to smooth the surface of the river. 2. Hydraulic Action: this is where the water in the river compresses air in cracks in the bed and banks (Cavitation – force of air exploding) This results in increased pressure caused by the compression of air, mini 'explosions' are caused as the pressure is then released gradually forcing apart parts of the bed and banks 3. Solution: Is most active on rocks that contain carbonates such as limestone and chalk. The minerals in rocks are dissolved by weak acids in the river water and are carried away in solution. 4. Attrition: Is the reduction in the size of fragments and particles within a river due to the collision of boulders with one another as they move down the river. The fragments strike one another as well as the river bed, therefore, becoming smoother, smaller and rounder. Consequently larger, more angular fragments tend to be found upstream, which smaller, more rounded fragments are found downstream.
  27. 27. Erosion  Rivers erode because they have energy. Their total energy depends on 3 main factors… 1. The weight of the water: The greater the mass of the water, the more energy it will posses due to the influence of gravity on its movement. 2. The height of the river above its base level: this gives it a source of potential energy and the higher the source of the river the more gravitational potential energy it has. 3. The steepness of the channel: this controls the speed of the river which determines how much kinetic energy it has.
  28. 28. Erosion
  29. 29. Transportation:  The type of transport taking place depends on...  (i) the size of the sediment and  (ii) the amount of energy that is available to undertake the transport. • There are four types of transportation: 1. Traction: Large rocks and boulders are rolled along the river bed by water moving downstream. This process operates only at times of high discharge. 2. Solution: When dissolved minerals invisible to the naked eye are transported within the mass of moving water. 3. Saltation: Small stones bounce along the channel bed in a skipping motion. This process is associated with relatively high energy conditions. Small particles land then dislodge other particles upwards causing more bouncing movements to take place. 4. Suspension: Very small particles of sand and silt are carried along by the flow of a river.
  30. 30. Transportation
  31. 31. Deposition:  When a river loses energy and therefore velocity, deposition occurs. This is because the river doesn’t have enough energy to carry the material it is transporting.  This could happen in an estuary when the river meets the sea and slow down, depositing its load and creating a delta.  The main factors leading to deposition are: 1. Low rainfall reducing precipitation 2. A river entering the sea or a lake – reducing velocity 3. Water becoming shallower 4. Increase in load 5. River overflows its bank, depositing material on a flood plain.  May result in the formation of features such as slip off slopes (on the inner bends of meanders); levees (raised banks) alluvial fans; meanders; braided streams and the floodplain.  Remember - it is the largest material that will be dropped first as it requires the most energy to be transported. Eroded material carried in suspension and solution will be dropped last.
  32. 32. River Capacity  =the total amount of material it can carry  capacity is the total volume of the load  the load of a river can be divided into different categories according to particle size- which varies from fine silt and clay to big boulders  the competence describes the maximum particle size that a river is capable of transporting at a given point
  33. 33. Hydraulic Radius  The efficiency of a channel can be quantified as the channel’s hydraulic radius. The hydraulic radius shows you how efficient a channel is. The larger the hydraulic radius, the higher the channel’s efficiency. The hydraulic radius can be calculated using the following formula:  Rh=AP  Where Rh is the hydraulic radius, A is the cross-sectional area of the channel and P is the wetted perimeter of the channel. The wetted perimeter is the length of the river’s bed and banks that is in contact with the water.  A large hydraulic radius is more efficient because it means that a smaller proportion of the river’s water is in contact with the bed & banks so there is less friction. The ideal channel shape for a large hydraulic radius would be a narrow and deep channel. Wide and shallow channels are less efficient and have a smaller hydraulic radius.
  34. 34. Hjulstrom Curve  -The capacity of a stream refers to the largest amount of debris that a stream can carry  -The competence refers to the diameter of the largest particle that can be carried  -The critical erosion velocity is the lowest velocity at which grains of a given size can be moved  -The relationship between these variables is shown by means of the Hjulstrom Curve. • Most of the time, larger particles such as boulders, need a higher velocity for them to be picked up because of their large size • However, the exception to this rule is clay and silt, as even though the particles are very small , the particles tend to stick together, making them hard to pick up. • Higher velocities are needed for picking up (entrainment) than just for transporting. • When velocity falls below a certain level (settling velocity), particles are deposited
  35. 35. Patterns of Flow in a river:  Turbulent Flow: provides upward motion in the flow that allows the lifting and support of fine particles which will contribute to depositional landforms further down the river. The flow is a series of erratic eddies, both vertical and horizontal in the downstream direction.  conditions necessary for turbulent flow to occur are: 1. complex channel shapes such as meandering channels and alternating pools and riffles 2. high velocities 3. cavitation in which pockets of air explode under high pressure • Laminar flow it is common in groundwater and in glaciers but not in rivers although it can occur in the bed in the lower course of a river. This is the horizontal movement of water where the water moves at uniform velocity with one layer of water molecular sliding over the next without mixing. It cannot support solid particles in suspension. best condition are: • shallow channels • smooth straight channels • low velocities
  36. 36. Helicoidal Flow  A corkscrew movement in a meander. It is responsible for moving material from the outside of one meander bend and depositing it on the inside of the next bend.
  37. 37. Channel Types  Braided stream: Occurs when the river is forced to split into several channels separated by islands/eyots.  It is a feature of rivers that are supplied with large loads of sand and gravel and they are most likely to occur when a river has variable discharge. The banks formed from sand and gravel and unstable and easily eroded and as a consequence, the channel becomes very wide in relation to its depth.  Streams with high sediment loads that encounter a sudden reduction in flow velocity generally have a braided channel. In a braided stream, the main channel divides into a number of smaller, interlocking or braided channels. Braided channels tend to be wide and shallow because bedload materials are often coarse (sands and gravels) and non-cohesive.
  38. 38. Channel Types
  39. 39. Landforms formed by fluvial erosion
  40. 40. Landforms formed by fluvial erosion:  Riffles: Areas of shallow water, due to deposition of coarse material.  Pools: Areas of deeper water between rifles
  41. 41. V-Shaped Valleys  V-Shaped valleys are found in the upper course of the river and are a result of both erosion by the river and weathering. V-Shaped valleys are deep river valleys with steep sides that look like a letter V when a cross section of them is taken, hence the name. They’re found in the upper course because this is where the river has the greatest gravitational potential energy and so the greatest potential to erode vertically. It does so during periods of high discharge. When the river’s discharge is high, it is able to transport its large bedload by traction eroding the river’s bed and valley by corrasion, deepening it. Not much lateral erosion takes place so the channel and valley remains relatively narrow.  As the channel and valley deepens the sides of the valley are exposed and become susceptible to weathering. The valley’s sides also undergo mass movements resulting in large volumes of material falling into the river’s channel, adding to its erosive power and causing the valley sides to take up a V shape. The steepness of the valley sides and whether the valley actually looks like a V is dependent on the climate, vegetation and rock structure among things. In cold, wet climates, freeze thaw weathering is abundant and rainwater can act as a lubricant, aiding mass movements. Vegetation can impede mass movements because it will help bind the soil. If the valley is composed of hard rock the valley sides will be very steep because they won’t be weathered easily.  The steepness of Valley sides depends on factors such as: 1. Climate: valleys are steeper where there is sufficient rainfall for mass movement. 2. Rock Structure Resistant, permeable rocks such as limestone produce vertical sides 3. Vegetation: helps to bind soil together and keep hill slope more stable.
  42. 42. How does a V-Shape Valley Form  1. Vertical erosion (in the form of abrasion, hydraulic action and solution) in the river channel results in the formation of a steep sided valley  2. Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley  3. Gradually mass movement of materials occurs down the valley sides, gradually creating the distinctive v-shape.  4. The material is gradually transported away by the river when there is enough energy to do so.  As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs). As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of creating interlocking spurs.  Interlocking Spur - spurs are ridges of more resistant rock around which a river is forced to wind as it passes downstream in the upper course.  Interlocking spurs form where the river is forced to swing from side to side around more resistant ridges
  43. 43. Waterfall Formation Waterfalls develop when a change of lithology (rock type) takes place along the river’s course resulting in differential erosion. When the rock type of the river’s channel changes from a resistant rock to a less resistant one (e.g. granite to limestone), the river erodes the less resistant rock faster producing a sudden drop in the gradient of the river with the resistant rock being higher up than the less resistant rock. As the river flows over the resistant rock, it falls onto the less resistant rock, eroding it and creating a greater height difference between the two rock types, producing the waterfall. When water flows over the waterfall it creates a plunge pool at its base and the splashback from the falling water undercuts the resistant rock. The unsupported rock is known as the cap rock and it eventually collapses into the plunge pool causing the waterfall to retreat upstream. Over thousands of years, the repeated collapse of the cap rock and retreat of the waterfall produces a gorge of recession
  44. 44. Rapids and Potholes  Potholes: Potholes are cylindrical holes drilled into the bed of a river that vary in depth & diameter from a few centimetres to several metres. They’re found in the upper course of a river where it has enough potential energy to erode vertically and its flow is turbulent. In the upper course of a river, its load is large and mainly transported by traction along the river bed. When flowing water encounters bed load, it is forced over it and down cuts behind the bed load in swirling eddie currents. These currents erode the river’s bed and create small depressions in it. • Rapids: Rapids are sections of a river where the gradient of the river bed is relatively steep resulting in an increase in the river’s turbulence, velocity and therefore erosive power. They form where the gradient of the river is steep and the bed is composed mainly of hard rocks
  45. 45. Floodplains and Levee’s  Floodplains are large, flat expanses of land that form on either side of a river. The floodplain is the area that a river floods onto when it’s experiencing high discharge. When a river floods, its efficiency decreases rapidly because of an increase in friction, reducing the river’s velocity and forcing it to deposit its load. The load is deposited across the floodplain as alluvium. The alluvium is very fertile so floodplains are often used as farmland.  The width of a floodplain is determined by the sinuosity of the river and how much meander migration takes place. If there’s a lot of meander migration, the area that the river floods on will change and the floodplain will become wider. Levees are natural embankments produced when a river floods. When a river floods, it deposits its load over the flood plain due to a dramatic drop in the river’s velocity as friction increases greatly. The largest & heaviest load is deposited first and closest to the river bank, often on the very edge, forming raised mounds. The finer material is deposited further away from the banks causing the mounds to appear to taper off. Repeated floods cause the mounds to build up and form levees. Levees aren’t permanent structures. Once the river’s discharge exceeds its bankfull discharge1, the levees can be burst by the high pressure of the water. Levees increase the height of the river’s channel though, so the bankfull discharge is increased and it becomes more difficult for the river to flood
  46. 46. Exempler Answer (floodplain)  River transportation is an essential process in the formation of a floodplain. At this stage, the river will carry a large load, by solution and suspension and also by saltation and traction. When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. The shallower depth of water flowing over the surface results in frictional drag and a reduction in velocity (speed) of flow. As the floodwater loses energy, the capacity and competence of the flood-water is reduced, leading to deposition. The heaviest materials (bedload) are deposited first nearest the channel, as these require the most energy to be transported and therefore build up around the sides of the river forming raised banks known as levees. Finer material such as silt and fine clays continue to flow further over the floodplain before they are deposited (alluvium). Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain. The slopes of the river valley border the edge of the floodplain. These slopes are known as the “bluff line”.
  47. 47. Delta  Deltas are depositional landforms found at the mouth of a river where the river meets a body of water with a lower velocity than the river (e.g. a lake or the sea). For a delta to develop, the body of water needs to be relatively quiet with a low tidal range so that deposited sediment isn’t washed away and has time to accumulate.  When a river meets a stationary body of water, its velocity falls causing any material being transported by the river to be deposited. Deltas are made up of three sediment beds that have been sorted by the size of the sediment. The bottom most bed, the bottomset bed, is composed primarily of clay and some other fine grained sediments. Clay is the main constituent because when clay meets salt water a process called flocculation takes place where clay & salt particles clump together (flocculate) due to an electrostatic charge developing between the particles. This makes the clay particles sink due to their increased weight producing the bottomset bed. The bottomset bed stretches a fair distance from the mouth of the river as the fine sediments can be transported a reasonable distance from the river’s mouth.  The foreset bed lies on top of the bottomset bed. The foreset bed is composed of coarser sediments that are deposited due to a fall in the river’s velocity and aren’t transported very far into the stationary body of water that the river flows into. The foreset bed makes up the majority of the delta and is dipped towards deep water in the direction that the river is flowing in.  The topset bed is, as the name suggests, the topmost bed of the delta. It too is composed of coarse sediment but, unlike the foreset bed, the topset bed doesn’t dip, it’s horizontally bedded
  48. 48. Delta Structure
  49. 49. There are three types of Deltas 1. Arcuate: Have rounded, convex outer margins e.g. Nile River. 2. Cuspate: Where material brought down by a river is spread out evenly on either side of its channel due to waves hitting it head on, spreading the deposited sediment out. E.g. Tiber River 3. Birds Foot: They extend reasonably far into a body of water and form when the river’s current is stronger than the sea’s waves. Bird’s foot deltas are uncommon because there are very few areas where a sea’s waves are weaker than a river’s current. As the same suggests it looks like a birds foot. E.g. Mississippi river
  50. 50. Meanders  Meandering channels are produced when the thalweg follows a sinuous path through pool and riffles to cause erosion on the outer bank and deposition on the inside bank. This imparts a secondary flow called helical (Helicoidal) flow which is a spiral flow elevating the water on the outside of the meander with a return current at the inside of the meander. This produced the river cliff and point bar. A cross section of a meander would show that on the outside bend, the channel is very deep and concave. This is because the outside bend is where the river flows fastest and is most energetic, so lots of erosion by hydraulic action and abrasion takes place. River cliffs form on the outside bend as the river erodes laterally. The inside bend is shallower with a gentle slip-off slope made of sand or shingle that is brought across from the outside bend by the helicoidal flow and centripetal force of the river. The river flows much slower on the inside bend so some deposition takes place, contribution to the slip-off slope.
  51. 51. River cliff and Slip off slope formation  River cliff  Water flows fastest on the outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff.  Slip off slope In contrast, on the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel. This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition. Over time a small beach of material builds up on the inner bend; this is called a slip-off slope
  52. 52. Oxbow Lakes
  53. 53. Oxbow Lakes  As the outer banks of a meander continue to be eroded through processes such as hydraulic action the neck of the meander becomes narrow and narrower. Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander usually during a flood event when the energy in the river is at its highest. The water now takes its shortest route rather than flowing around the bend. Deposition gradually seals off the old meander bend forming a new straighter river channel. Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake. Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up, the feature left behind is known as a meander scar.
  54. 54. The 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.
  55. 55. Alluvial Fans  Alluvium (material in a river) is dropped by the river when it loses momentum as it enters a wide, flat valley known as a piedmont, after leaving a narrow mountain channel. This happens as water velocity, gradient and speed reduces as the water enters a wide unconfined channel, so it is deposited at the junction. It is the terrestrial (land) equivalent of a delta
  56. 56. Definitions  Meander - a bend in a river  River Cliff - a small cliff formed on the outside of a meander bend due to erosion in this high energy zone.  Slip off Slope - a small beach found on the inside of a meander bend where deposition has occurred in the low energy zone.  Ox-bow lake - a lake formed when the continued narrowing of a meander neck results in the eventual cut through of the neck as two outer bends join. This result in the straightening of the river channel and the old meander bend becomes cut off forming an ox-bow lake.  Meander scar - feature left behind when the water in an ox-bow lake dries up.
  58. 58. The influence of human activity on the hydrological cycle  Precipitation: - Cloud seeding introduces silver iodide, solid carbon dioxide (dry ice) or ammonium nitrate into the air to encourage water droplets to form.  -Mixed success but in Australia and the USA it has increased precipitation by 10- 30%  -In Urban areas precipitation can be increased by 10% due to extra pollutants in the air
  59. 59. The influence of human activity on the hydrological cycle  The human impact on evaporation and evapotranspiration is relatively small in relation to the rest of the hydrological cycle but nevertheless important. There are a number of impacts:  Dams: the construction of large dams have increased evaporation. For example: Lake Nasser behind the Aswan Dam loses up to a third of water due to evaporation. Water loss can be reduced by using chemical sprays or covering the dam in a form of plastic.  Urbanisation: Leads to a huge reduction in evapotranspiration due to the lack of vegetation. There may also be a slight increase in evaporation because of higher temperatures and increased surface storage.
  60. 60. The influence of human activity on the hydrological cycle  If a river’s drainage basin or floodplain has been heavily urbanised, a river becomes much more prone to flooding. Urbanisation (generally) involves the laying down of tarmac and concrete, impermeable substances that will increase surface runoff into the river and therefore increase the river’s discharge.  Urbanisation often involves deforestation. This (obviously) reduces vegetation cover, reducing infiltration and increasing surface runoff into a river.  To stop roads and streets from flooding, humans will often build storm drains that collect rainwater and channel it into a river or stream. Humans will often send this water to the local river or stream so, although roads and streets won’t be flooded by rainwater the entire town will be as the rainwater enters the river much faster than it would without the storm drains.
  61. 61. Urbanisation - flooding
  62. 62. Deforestation - flooding
  63. 63. Flooding: (Physical Factors)  Flooding occurs when a river’s discharge exceeds its channel’s volume causing the river to overflow onto the area surrounding the channel known as the floodplain. The increase in discharge can be triggered by several events. The most common cause of flooding is prolonged rainfall. If it rains for a long time, the ground will become saturated and the soil will no longer be able to store water leading to increased surface runoff. Rainwater will enter the river much faster than it would if the ground wasn’t saturated leading to higher discharge levels and floods.  As well as prolonged rainfall, brief periods of heavy rain can also lead to floods. If there’s a sudden “burst” of heavy rain, the rainwater won’t be able to infiltrate fast enough and the water will instead enter the river via surface runoff. This leads to a sudden and large increase in the river’s discharge which can result in a flash flood.  Although many floods are triggered directly by precipitation just a few hours after it falls some floods can be triggered by precipitation that fell many months ago. Precipitation that falls as snow can remain as snow on the ground until it melts. This mightn’t be until the end of winter, so potentially several months. When the snow does melt, large volumes of meltwater will enter the river increasing its discharge and triggering floods. These floods are often annual, occurring every year when snow melts in the spring. In Bangladesh, for example, melting snow in the Himalayas triggers annual floods in the summer.  Flash floods can also be triggered by slightly more catastrophic events. Erupting volcanoes can trigger very large flash floods called jökulhlaups when glaciers are partially or even fully melted by an erupting volcano or some other form of geothermal activity. The meltwater can enter rivers and greatly increase the river’s discharge leading to a flood. The eruption of Eyjafjallajökull1 in 2010 triggered jökulhlaups as the volcano had been capped by a glacier that melted when it erupted. Similarly earthquakes can bring about landslides – loosened soil may be deposited in rivers causing overflowing.
  64. 64. Effects of flooding  Flooding can have numerous social, economic and environmental effects that can vary depending on the demographics of a population and the economic development of an area.  Social Effects  The biggest, most obvious effect is death. Floods, especially flash floods, will kill people. Flood water can travel surprisingly quickly and weighs3 a lot, so people can easily get swept away by floods. Large chunks of debris and objects like cars can easily get picked up by floodwater and can easily kill a person should they get hit by the debris. In a LEDC, you’re generally going to get much more deaths than you would in a MEDC. In a MEDC, people and governments are better prepared for floods. Rescue services can be dispatched to a flood quickly in a MEDC whereas in a LEDC, rescue teams mightn’t arrive until several hours after the flood started.  During a flood, sewage pipes are often broken and raw sewage leaks into the floodwater. This has two effects. First, it contaminates not just floodwater but drinking water too which leads to a spread of waterborne diseases such as cholera especially in LEDCs where emergency drinking water mightn’t be available. Second, the sewage gets into people’s homes.  In LEDCs, famines can follow floods which can lead to even more deaths. Floods will commonly inundate farmland because farmland normally develops on floodplains. If the floodwater is polluted by sewage, it will contaminate the farmland and make any food grown on it dangerous to eat. Furthermore, cattle are often killed by floods which can lead to people starving because they either don’t have a source of food or don’t have a source of income to buy food with.
  65. 65. Effects of Flooding  Economic Effects  The big economic effect of a flood is property damage. Water can cause a lot of damage to property and when it picks up large chunks of debris such as cars, it can act like a wrecking ball, taking out chunks of buildings when cars crash into them. Very large and powerful floods can even dislodge buildings from their foundations and move them. In a MEDC, property damage is often extensive as people have lots of expensive possessions. This isn’t the case in LEDCs but that’s only because people don’t have a lot to lose in the first place. This means that the overall cost of a flood is generally substantially higher in a MEDC than in a LEDC.  Floods can cause extensive damage to infrastructure such as power lines, roads, water pipes etc. Bridges frequently collapse during a flood as they aren’t designed to withstand the high discharge of the river. The Northside Bridge in Workington, Cumbria collapsed when there were large floods in 2009. Repairing bridges and other types of infrastructure is very costly. Not only this, it can lead to a decline in the local economy as businesses are unable to operate without power or road connections. Unemployment can even increase if businesses are unable to fully recover from a flood. The economic impact of infrastructure damage and unemployment is larger in MEDCs since these countries have modern and expensive infrastructure in place. In LEDCs, this infrastructure is lacking, so there isn’t much economic damage. In fact, in a LEDC, floods can lead to positive economic effects in the long term. An influx of funding to a less developed area from charities and NGOs after a flood can result in new infrastructure being constructed that is substantially better than the previously existing infrastructure. This, in turn, creates new economic opportunities in an area by, for example, creating new trade routes.  Another economic benefit comes from when a river floods and deposits sediment across the floodplain. This improves the fertility of the floodplain and can improve agricultural yield in an area (assuming the floodwater wasn’t polluted).
  66. 66. Effects of flooding  Environmental Effects  Floodwater that is contaminated with sewage will pollute rivers and land when it drains back into the river. Similarly, if the river floods onto farmland, the water can be polluted by pesticides and other chemicals sprayed onto the farmland that, when drained back into the river, can pollute it and kill off wildlife that inhabits the river. If the floodwater isn’t polluted though, flooding can create wetlands that can help introduce new habitats for many species of animals.  Vegetation may be destroyed, along with natural habitats and animal species.
  67. 67. Reducing impacts of floods  1) Prediction  Using weather satellites to predict high rainfall amounts  Estimating rainfall and snow pack amounts  Using river gauges to study river levels over time and map flood recurrence  Create computer flooding models including information on human infrastructure and what would be most at risk.   2) Preparing people for floods  Loss sharing adjustments (e.g. disaster aid and insurance)  Removal of settlements from flood plains  Education on what to do in a flood.   3) Prevention and amelioration of floods  There are two types of flood protection methods which act to prevent or ameliorate flooding.  Hard engineering= Defence schemes that halt a rivers natural processes.  Soft engineering = involves the use of the natural environment surrounding a river, and the schemes often work with the river’s natural processes. Examples on next slides…
  68. 68. Soft/Hard Engineering to prevent floods
  69. 69. Soft/Hard Engineering to prevent floods
  70. 70. Advantages Disadvantages  Flood and drought control  Irrigation – 60% water from Aswan Dam is used for irrigation and up to 4000km of the desert is irrigated.  Hydro-electricity – accounts of 7000million kW hours each year.  Improved navigation  Recreation and tourism – Aswan Dam contributes 500million to the Egyptian economy each year.  Water losses – provide less than half the amount of water expected due to evaporation.  Salinisation  Displacement of population – up to 100000 Nubian people have been removed from their ancestral homes  Seismic stress – the earthquake of November 1981 is believed to have been caused by the Aswan Dam; as water levels in the dam decrease so too does seismic activity increase.  Loss of nutrients – it is estimated that it costs 100million to buy commercial fertilisers to make up for the lack of nutrients each year.  Diseases have spread – such as bilharzia Dams
  71. 71. Droughts Is an extended period of dry weather leading to conditions of extreme dryness.  Absolute drought is a period of at least 15 days with less than 0.2mm of rainfall  Natural causes of droughts: Insufficient rainfall can be caused by several factors: 1. Global atmospheric circulation leads to descending air over sub- tropical areas and therefore a lack of rain ( no clouds are formed) 2. An area’s distance from the sea can limit the amount of water carried by the wind 3. Some places are affected by rain shadow effects. This is where air passes over mountains, and rain is released, but the air has therefore lost all its moisture as it reaches the far side of the mountains
  72. 72. Human causes of droughts  Deforestation: Reduced vegetation cover results in lower rates of transpiration. Less water vapour in the atmosphere leads to fewer clouds formed. Soil exposed to direct sunlight dries up quickly.  Enhanced greenhouse effect: Global warming can cause droughts in places with drier climates. High temperatures increase the rate of evaporation, drying up land, rivers and lakes.  Over use of water: places with rapid population growth require more water for hoes, industry and agriculture. Water sources such as rivers and ground water may not be able to sustain an increase in water usuage.
  73. 73. Impacts of droughts  Economic impacts  Economic impacts range from direct losses in the broad agricultural and agriculturally related sectors (including forestry and fishing), to losses in recreation, transportation, banking, and energy sectors. Other economic impacts would include added unemployment and loss of revenue to local, state, and federal government.  Environmental Impacts.  Environmental losses include damages to plant and animal species, wildlife habitat, and air and water quality; forest and range fires; degradation of landscape quality; and soil erosion. These losses are difficult to quantify, but growing public awareness and concern for environmental quality has forced public officials to focus greater attention on them.  Social impacts  Social impacts mainly involve public safety, health, conflicts between water users, and inequities in the distribution of impacts and disaster relief programs. As with all natural hazards, the economic impacts of drought are highly variable within and between economic sectors and geographic regions, producing a complex assortment of winners and losers with the occurrence of each disaster.
  74. 74. How to reduce impacts of droughts:  Management of watershed and agricultural practices: reistance crops prevent desertification.  Using proper irrigation techniques – helps conserve water.  Cloud seeding – enables water droplets to form easily.
  75. 75. Model Answer
  76. 76. Hard engineering techniques  Dams – these are built across the river channel to stop the flow of the water. A lake or reservoir will form behind the dam and water can be let out in a controlled manner to prevent flooding. One of the disadvantages of a dam is that a large area of land has to be flooded and this can destroy natural habitat or even mean humans have to be relocated. Dams are expensive to build and maintain although they can be used to produce HEP. Dams do provide a high level of control to reduce the chances of flooding and are very effective in reducing the risk in this way however, they stop sediment from flowing downstream and this can lead to greater erosion in a similar way that holding back material on the coast by using groynes reduces protection further along the cliff. A good example of a dual purpose dam is the Karibaon the Zambezi in Mozambique .  Levees (reinforcing or man-made) – a levee is a naturally occurring feature on the bank of a river in the middle and sometimes lower stages of a river, on the flood plain. When the river floods, any load it is carrying in suspension is dropped and the heavier material is dropped first, just on the river bank. Over time this will build up and has the effect of increasing the capacity of the river as the banks are higher. These levees may be enforced in some way by humans. Planting vegetation on them helps to protect them. Adding even more height to them is also effective. An artificial levee can be built from scratch and this has the same effect as a natural one. Levees are very common in Holland where much of the country (more than 25%) is below sea level and is at risk from flooding. The Dutch have a complex network ofwing dykes (see below) and levees to protect the land.
  77. 77. Hard engineering  Channel straightening – getting the water out of an area at risk of flooding as quickly as possible is a way to reduce likelihood of problems during times of peak discharge. Meanders may be cut through and the channel is literally straightened so that water can move very quickly. This takes the water away from built up area for example where water can cause havoc to houses and businesses. Straightening has been one of many management techniques used along the Mississippi in the USA. This of course means that the water reaches further downstream more quickly too. Straightening often just diverts the problem elsewhere rather than providing a solution.  Wing Dyke – these work in a similar way to groynes on a beach in that they trap sediment moving through the river channel. They are usually placed in pairs either side of the channel and once sediment has built up behind them water is forced between them more quickly. Some good examples are on theMissouri river in the USA. As with channel straightening, they mean that water reaches downstream more quickly so careful planning is needed when they are installed to lessen the impact of increased discharge further along the river’s path.
  78. 78. Soft engineering  Afforestation – This may be part of wetland and river restoration when vegetation may be planted to return an area to its original form. Large scale afforestation can not only lower flood risk by intercepting and storing water but it can reduce the erosion of soil which ends up in the river channel. Material in the river channel effectively decreases its depth and the river level is higher, increasing flood risk during times of high discharge. Afforestation is widely used in Australia to achieve several things which includes helping to manage water flow in a catchment area (drainage basin).
  79. 79.  Meteorology= The study of the Atmosphere  Weather= Short term atmospheric conditions of a particular place.  Climate= Long term atmospheric conditions of a particular place.  Atmosphere=is an area of transparent gases surrounding the earth  The gases stretch to 500-1000km above the earth’s surface  There are several layers to the atmosphere  The area between layers is called a pause  Weather occurs only in the lowest part of the earth’s atmosphere called the troposphere. Introduction
  80. 80. Layers of the Atmosphere
  81. 81.  Troposphere - layer characteristics:  Decrease of temperature with height (6.4 degrees per 1000m).  Increase in wind speeds with height.  Fall in pressure with height.  An unstable layer due to the presence of cloud, pollution water vapour and dust.  The tropopause marks the outer edge of the troposphere and the limit to the earth's weather and climate.  Stratosphere - layer characteristics:  Temperatures increase with height in this layer, and it is here that ozone is concentrated, which absorbs UV radiation from the sun.  Winds increase with height but pressure falls.  The boundary is marked by the stratopause.  Mesosphere - layer characteristics:  A rapid fall in temperature with height, caused by a lack of water vapour, cloud and dust).  Temperatures are extremely low and winds high.  Its boundary is marked by the mesopause.  Thermosphere - layer characteristics:  The outer layer of the atmosphere.  A rapid increase in temperature with height, exceeding 1000 degrees Layer Characteristics
  82. 82.  Is the amount of energy entering, leaving and transferring within the system.  Some parts of the earth receive lots of solar energy (surplus), whilst others receive little (deficit).  In order to transfer this energy around, to create some sort of balance, the Earth uses pressure belts, winds and ocean currents.  The energy budget has a huge effect on weather and climate .  Energy budgets are usually considered at a global scale (macro scale) and can be at a local scale (micro scale). Energy Budgets
  83. 83. Global energy budget
  84. 84.  Incoming radiation (short wave radiation)  Incoming solar radiation (insolation) is short wave radiation that comes directly from the sun (100%)  19% is reflected off clouds. And 6 % is lost to scattering ( radiation diverted by gas molecules)  17% of this is absorbed by the gases in the atmosphere such as carbon dioxide and ozone.  4% of this is absorbed by clouds  7% is reflected by the earth’s surface (called albedo)  So only 47% actually reaches the earth’s surface to be absorbed  Outgoing Radiation (Long wave radiation)  Energy received by the earth is converted into heat energy when it reaches the surface. As the ground warms, some is re-radiated as long wave radiation.  8% of this re-radiated energy is lost to space  Evaporation and condensation account for a loss of 25 % of the heat energy from the earth as heat energy is used up when liquid is turned into vapour (this is called latent heat transfer)  7% of this re-radiated energy is absorbed by clouds, water vapour and CO2 Global Energy Budget
  85. 85.  The daytime energy budget consists of Six processes: I = Insolation R = Reflected Solar Radiation S = Surface Absorption L = Latent Heat (Evaporation) S = Sensible Heat Transfer L = Long wave radiation Daytime energy budget
  86. 86. Insolation:  Atmosphere’s main energy input which is strongly influenced by cloud cover and latitude. At the equator, the sun’s rays are more concentrated than at the poles. (75% of insolation reaches equator, 5% reaches Poles) Reflected Solar Radiation-  The proportion of reflected solar radiation varies greatly with the nature of the surface.  The reflectivity of a surface is known as the albedo.  Fresh snow & ice have the highest albedos, reflecting up to 95% of sunlight.  Ocean surfaces absorb most sunlight, and so have low albedos. Surface Absorption-  Energy arriving at the surface has the potential to heat that surface, as heat is absorbed by it.  The nature of the surface has an effect, e.g. If the surface can conduct heat rapidly into the lower layers of the soil its temperature will be low. If the heat is not carried away quickly it will be concentrated at the surface & result in high temperatures there. Latent Heat (evaporation)  The turning of liquid water into vapour (evaporation) it consumes a considerable amount of energy.  When water is present at the surface, a proportion of the incoming solar radiation will be used to evaporate it.  Consequently, that energy will not be available to raise local energy levels and temperatures. Sensible Heat Transfer  This term is used to describe the transfer of parcels of air to or from the point at which the energy budget is being assessed. If relatively cold air moves in, energy may be taken from the surface, creating an energy loss. If warm air rises from the surface to be replaced by cooler air, a loss will also occur. This process is best described as convective transfer, and during the day it is responsible for removing energy from the surface and passing it to the air. Long wave Radiation  This is emitted by the surface, and passes into the atmosphere, and eventually into space.  There is also a downward-directed stream of long-wave radiation from particles in the atmosphere  The difference between the 2 streams is known as the net radiation balance.  During the day, since the outgoing stream is greater than the incoming one, there is a net loss of energy from the surface. Daytime energy budget
  87. 87.  The night time energy budget consists of Four processes L= Long wave radiation L= Latent Heat (condensation) S= Sensible Heat Transfer S=Subsurface Supply Night time Energy Budget
  88. 88.  Long Wave Radiation: During a cloudless night, little long wave radiation arrives back at the surface of the ground from the atmosphere and consequently the outgoing stream is greater than incoming stream leading to a net loss of energy. Under cloudy conditions this loss is reduced because long wave radiation can reflect off clouds back to the surface; they act like a blanket around the earth.  Latent Heat (Condensation): At night water vapour in the air, close to the ground can condense and form dew as the air is cooled by the cold surface. This releases stored energy, resulting in a net gain of energy.  Subsurface supply: Heat transferred by the sun to the surface during the day, may be released back to the surface at night which can off set the night time cooling at the surface  Sensible heat transfer still occurs and cold air moving into an area may reduce temperatures whereas warm air moving in will raise temperatures Night Time Energy Budget
  89. 89.  Absolute Humidity: The amount of water in the atmosphere  Relative Humidity: Ratio of the amount of water vapour currently in the air compared to how much the air can hold at that temperature (usually expressed as a percentage).  Saturated Air: Air with a relative humidity of 100%  Dew point: The temperature at which condensation occurs, allowing the formation of dew, mist or fog. WARMER AIR CAN TYPICALLY HOLD MORE WATER VAPOUR THAN COOLER AIR CAN Humidity
  90. 90.  Mist and Fog: are cloud at ground level. A cloud is a collection of water droplets. Mist occurs when visibility is between 1000m and 5000m. Whereas Fog occurs where visibility is below 1000m. So Fog is thicker cloud cover than mist.  These clouds form at ground level because, air can only hold a certain amount of moisture. Colder air can hold less moisture than warmer air. Once this maximum amount of moisture is reached, air is saturated and the water vapour in the air turns to liquid (dew point). This is when clouds form as condensation of water vapour to water droplets occur.  For these clouds of fog/mist to form close to the ground level , one of two things must have occurred: 1. Air must have been cooled close to the ground  e.g. Advection Fog: As warm, moist air passes horizontally over a cold surface, it is chilled, and condensation takes place as the temperature of the air is reduced and therefore it reaches dew (saturation) point  e.g. Radiation Fog: Occurs in low lying areas when the ground surface loses heat at night by long wave radiation and therefore the air immediately above it is cooled causing condensation and fog. 2. More water vapour must have been added to the atmosphere close to the ground.  This can occur over warm, wet surfaces like large lakes, where water is evaporated from the warm surface of the lake and condenses in the cold air above to form fog)  For mist or fog to form, condensation nuclei are needed (e.g. dust or salt particles in the air). These are more common in urban or coastal areas, so mist of fog are more common here. Mist and Fog
  91. 91. Temperature Inversions During the day the ground is heated by the sun’s short wave radiation, and then after a short time, it heats the air above it when it emits long wave radiation. At night the ground surface and the air, lose the heat energy they have absorbed during the day. However, the ground loses heat energy faster than the air as it is a more efficient conductor of heat. By the end of the night the ground surface is therefore very cold, and the air directly above it will be cooled too due to close proximity to the surface. However, the air layer above this, will be warmer as it has cooled at a slower rate than the ground surface, causing a temperature inversion. Temperature inversions usually occur during anti- cyclones when there is little air turbulence to allow these layers to mix. Temperature inversions act like a lid, causing pollutants to remain in the lowest atmosphere. A relative increase in temperature with height in the lower part of the atmosphere
  92. 92. The Global Energy Budget Atmospheric Energy • The atmosphere is an open system • 47% of insolation reaches the earths surface • The atmosphere receives 39% of heat back from the earths surface. • Most incoming short-wave radiation is let through, but some outgoing long wave radiation is trapped by green house gases, known as the green house effect. • There are variations in the amount of solar radiation due to two factors. Latitude and Season, resulting in an unbalance • (+) positive budget in the Tropics (more energy received) • (-) negative budget in the Poles (more energy lost) • At the equator there is little seasonal different • At higher latitudes, there are large seasonal differences due to decreased insolation and changes in day length.
  93. 93. Temperature decreases with height above sea level:  The atmosphere is heated from ground level upwards via long- wave radiation.  The higher up a mountain, the smaller the ground surface area available to heat the atmosphere above.  This, in combination with a decrease in the ability of the air to retain heat results in lower temperatures. NOTE*
  94. 94.  The Coriolis Effect is the deflection of moving objects caused by the easterly rotation of the Earth. In the northern hemisphere, air moving from high to low is deflected to the right of its path and to the left in the southern hemisphere. Coriolis Effect
  95. 95.  Wind: The horizontal movement of air on the Earth’s surface. It is a result from the difference in air pressure and always moves an area of high to low pressure. When the air temperature of an area increase the air expands and rises reducing the air pressure. When the temperature of the air decrease the air contracts and becomes denser and sinks increasing the air pressure. The temperature of the wind is influenced by the origin of where the wind has come from. Planetary surface winds
  96. 96.  Pressure is measure in Malabar's (mb) and is represented by isobars (lines of equal pressure).  Poor weather = low pressure  Fine Weather = high pressure  The North Hemisphere has much more land so there is a lot of seasonal change, whereas the South Hemisphere has a lot of water, so little seasonal change occurs.  Air pressure: The gases in the atmosphere press down on the Earth’s surface, exerting a force called air pressure.  It is differences in air pressure that cause different weather in our atmosphere. You don’t feel it because you have equal pressure pushing out from inside your body  Winds and air pressure: Changes in air pressure make winds blow. They are due to seasonal differences in the overhead sun.  Air moves from areas of high pressure to areas of low pressure, and this produces winds. “Winds blow from high to low !“ Pressure Variations
  97. 97. Pressure Variations • Doldrums (ITCZ): Areas of pressure in which sailing ships have a hard time moving due to lack of wind. • Trade winds (30>equator) Where lots of ships travel die to strong easterlies. • Coriolis effect: Due to the tilt of the earth and its movement on its axis, winds and ocean currents curve instead of traveling straight. This curving is known as Coriolis effect. • Hoarse Latitude: (30-60) little wind, so in the past, horses were thrown over board to remove some weight. • Summer in Southern Hemisphere, means winter in Northern Hemisphere… this increases differences in polar and equatorial air. • High level of Westerly's are stronger in NH in Winter
  98. 98.  Angle of the overhead sun, latitude and thickness of atmosphere: Lower latitudes (equatorial regions) have higher temperatures than higher latitudes (Poles) this is as a result of the amount of heating that each area receives. Places near the equator receive direct heat on a small surface area, and experience little energy loss via absorption, scattering and reflection, as there is a relatively small amount of atmosphere to pass through. Towards the Poles, the surface area to be heated increases, as does the amount of atmosphere to pass through, increasing losses via, absorption, scattering, and reflection.  Height above sea level: It is important to remember that the atmosphere is heated from ground level upwards via long-wave radiation. The higher up a mountain you go, the smaller the surface area available to heat the atmosphere above. This, in combination with a decrease in the ability of the air to retain heat results in lower temperatures.  Distance from land and sea: Land and sea have vastly different specific heat capacities (the amount of energy needed to raise 1kg of a substance by 1 degree). They have different abilities to absorb, transfer and radiate heat energy. Generally, land surfaces respond to heating on a daily basis (diurnal) meaning that differences between day and night temperatures can be into double figures, but sea surfaces respond over a period of months and retain heat for longer. The sea heats up and cools down more slowly than the land, acting to moderate temperatures for coastal locations. Exploring variations in Temperature and winds:
  99. 99. Surface pressure belts There are many pressure belts existing on Earth, due to the rotation and tilt of the Earth on its axis, these vary.  In the equator region, warm air rises causing a low pressure belt. Whereas at the polar regions cold air sinks, thus creating a high pressure belt. The sub polar regions, around latitudes 60-65 degrees North and South of the equator, the rotation of the Earth flings the bulk of the air towards the equator,, creating a low pressure belt. These four main pressure belts however are not continuous because the surface of the earth is composed of both land and water, which are heated in different ways. 1. The first main pressure belt is the equatorial low pressure belt, which extends 5 degrees north and south. Being at the equator is receives direct sunlight and thus the air here is warm, this air expands and rises, creating low pressure in the process. This is a region of calm air known as the Doldrums, due to its very little winds. 2. The second pressure belt, is the subtropical high pressure belt, that coincides with latitudes of 30- 33degrees north and south. The air that rises eventually meets the tropopause where it can rise no further, it cools while rising and spreads outwards towards the poles, gradually cooling back down to the surface of 30degrees, which causes an increase in air pressure. The air flings off the polar region due to the rotation of the earth and also descends in this region thus adding to the already high pressure existing in this region. The subtropical high pressure belt is an area of low winds and so it is also known as the horse latitudes (an area where ship crew would throw horses overboard to lighten the load and spare food after being caught in these areas). 3. The third major belt is the sub polar low pressure belt at latitudes 60-65degres. It is created mainly to the rotation of the Earth, which swings the bulk of air towards the equator, these are areas of storminess, especially in winter. 4. The forth and final pressure belt is the polar high pressure belt, located in the polar region. This belt is created because in this region the air is extremely cold and heavy, leading to a high pressure. 5. Pressure belts are caused mainly due to the temperature differences on the Earths surface and therefore move in response to the migration of the sun. The sun shines vertically oer the Tropical of Cancer on June 21st. At this time all the pressure belts move about 5degrees North. On the 21st September, the sunshine's vertically over the equator and on December 22nd the sun shines vertically over the tropic of Capricorn, thus all belts move 5degrees south (winter). The shifting of pressure belts affects the direction of wind flow, causing wind belts during the year and as these wind belts shift with the season, belts of precipitation change also.
  100. 100. Ocean currents Ocean currents can be either warm or cold and they act to either raise or lower temperatures of the coastal areas. Warm currents transfer heat away from the Equator and towards the poles whilst cold currents carry water towards the Equator. Major ocean currents circulate clockwise in the NH and anticlockwise in the SH. They are caused by the influence of prevailing winds blowing across the oceans, and mean in the same motion.  The dominant pattern is roughly circular and known as a GYRE  Gyres move clockwise in NH and Anti clockwise in SH, due to CORIOLIS EFFECT  Like atmospheric circulation, ocean currents help to redistribute energy across the earth. Because they cover 67% of the earth's surface, the oceans receive 67% of the sun's energy that reaches earth. The ocean holds on to this heat for longer than the land does and the ocean currents move this heat around, from the tropics to higher latitudes. In total, ocean currents transfer about 25% of the global heat budget. Ocean Currents
  101. 101. Ocean Currents
  102. 102.  Ocean currents flowing away from the equator are called warm currents. The water in these currents is not necessarily warm, but it's warm compared to what you would expect for that latitude. The Gulf Stream is a good example of a warm current. If a current flows towards the equator it is a cold current, for example the Canaries current.  Water always flows down toward the lowest point.  Water’s density is determined by the water’s temperature and salinity (amount of salt).  Cold water is denser than warm water.  Water with high salinity is denser than water with low salinity.  Ocean water always moves toward an equilibrium, or balance. For example, if surface water cools and becomes denser, it will sink. The warmer water below will rise to balance out the missing surface water Ocean Currents
  103. 103. Ocean Conveyor Belt (global)
  104. 104. 1. Cold, salty water from polar regions sink into the depths and move towards the equator 2. The densest water is found around Antarctica, due to the amount of salt left in the water after ice is formed. 3. Surface currents bring warm water to North Atlantic from the Indian and Pacific Oceans, loosing heat in the northern areas. Water sinks, reversing convection current. 4. North Atlantic Is warmer than North Pacific – this leads to more evaporation in the Atlantic. This evaporation leaves more salt behind, making water denser, causing it to sink. Water will then travel back to the North Pacific, picking up more water which reduces its density. Ocean Conveyor belt
  105. 105.  The ocean is not a still body of water. There is constant motion in the ocean in the form of a global ocean conveyor belt. This motion is caused by a combination of thermohaline currents (thermo = temperature; haline = salinity) in the deep ocean and wind-driven currents on the surface. Cold, salty water is dense and sinks to the bottom of the ocean while warm water is less dense and remains on the surface.  The ocean conveyor gets its “start” in the Norwegian Sea, where warm water from the Gulf Stream heats the atmosphere in the cold northern latitudes. This loss of heat to the atmosphere makes the water cooler and denser, causing it to sink to the bottom of the ocean. As more warm water is transported north, the cooler water sinks and moves south to make room for the incoming warm water. This cold bottom water flows south of the equator all the way down to Antarctica. Eventually, the cold bottom waters return to the surface through mixing and wind-driven upwelling, continuing the conveyor belt that encircles the globe. Ocean conveyor belt
  106. 106. Land-sea breezes These are Meso (small) scale / local winds caused a pressure gradient between land and sea.  Created on a daily basis, as a result of the differences in heating and cooling of the land and sea (i.e. specific heat capacities)  During the day, onshore winds are created, as land temperatures are higher than sea temperatures; thus low pressure is formed over the land, air rises and cools  Cool air then drifts out over the sea, increasing in density and starts to sink. Thus creating high pressure over the sea.  The sea breeze is caused by air flowing from high to low pressure (sea to land)  The situation is reversed at night, leading to high pressure over the land and thus an off shore breeze. Pressure is lower over the sea as it is warmer than the land, and air above it rises Local pressure gradients
  107. 107. The two winds that exist in mountain and valley locations are uphill, anabatic winds and downhill, katabatic winds.  Anabatic: An uphill wind develops under sunny morning conditions when slopes receive sunlight, become warm and then heat the atmosphere above them. Air above these slopes expands and rises. A pressure gradient results accompanied by a strong uphill wind  Katabatic: Downhill winds form, as heat is lost from a valley during the evening. Colder, denser air from higher areas drains into the valley Mountain and Valley winds
  108. 108. General circulation model • Warm air is transferred pole wards and is replaced by cold air moving towards the equator. • Air that rises is associated with low pressure, whereas air that sinks is associated with high pressure. • Low pressure produces rain, while high pressure produces dry conditions. • There are three major cells present: Hadley, Ferrell and Polar. • They shift northwards and southwards throughout the year due the shift in location of the sun’s rays focusing most intensely on the Earth’s surface.
  109. 109.  At the equator, trade winds meet and form the Inter-tropical Convergence Zone (ITCZ). Winds are light and known as the doldrums. Air is warm and unstable, having crossed warm oceans, and rises due to convection currents. As the air rises it cools and large cumulonimbus clouds develop. The pressure at the equator is low. Eventually, the rising air diverges 30 degrees north and south of the equator,where it cools via radiation and therefore falls. As the air contracts, more air can move in, increasing the air pressure at the subtropical high pressure zone. The dense air will then sink, causing stability. Air is then either returned to the equator at ground level, or travels to the Poles as warm south-westerly winds. Hadley Cell
  110. 110.  The Ferrel cell circulation is not as easily explained as the Hadley and Polar cells. Unlike the other two cells, where the upper and low-level flows are reversed, a generally westerly flow dominates the Ferrell cell at the surface and aloft. It is believed the cell is a forced phenomena, induced by interaction between the other two cells whereby it acts like a gear. The stronger downward vertical motion and surface convergence at 30°N coupled with surface convergence and net upward vertical motion at 60°N induces the circulation of the Ferrel cell. This net circulation pattern is greatly upset by the exchange of polar air moving southward and tropical air moving northward. This best explains why the mid-latitudes experience the widest range of weather types. Ferrell Cell
  111. 111.  This is the northernmost cell of circulation and its mean position is between 60°N and the North Pole. At the pole, cold, dense air descends, causing an area of subsidence and high pressure. As the air sinks, it begins spreading southward. Since the coriolis force is strongest at the poles, the southward moving air deflects sharply to the right. This wind regime is called the surface polar easterlies, although the upper winds are still predominantly from the southwest. Near 60ºN, the southeasterly moving air moving along the surface collides with the weak, northwesterly surface flow that resulted from spreading air at 30°N. This colliding air rises, creating a belt of low pressure near 60°N. Polar Cell
  112. 112.  At the Equator, the sun warms the Earth and this transfers heat to the air above, which causes it to rise. The rising air creates an area of low pressure with clouds and rain - this is called the Inter Tropical Convergence Zone (ITCZ) and is where the trade winds meet in the equatorial zone.  As the rising air cools, it begins to move away from the Equator and then further cooling, increasing density and diversion by the Coriolis force cause it to slow down and descend, forming the descending limb of the Hadley Cell.  The cool air sinks at 30o north and south of the Equator, creating an area of high pressure with clear skies and stable conditions - this is where sub-tropical jet streams are found  The cool air reaches the ground surface - some is returned to the Equator as surface winds (trade winds) whilst the remaining air is diverted polewards.  60o north and south of the Equator, warm south-westerlies/ north- westerlies which have collected moisture from the sea meet the cold air from the Poles - the warmer air is less dense and this causes it to rise, creating an area of low pressure  Some of the air joins the Ferrell Cell and moves back towards the Equator and the rest joins the polar cell and moves towards the Poles  At the poles, the cool air sinks to create a high-pressure zone - the high- pressure is then drawn back to the Equator as part of the surface winds Combined effect of the three cells
  113. 113.  Between the different atmospheric cells high up in the tropopause at a height of about 10km are the jet streams, named the polar jet stream (40-60°N+S) and the subtropical jet stream (25-30°N+S). These jet streams move air at a high speed (up to 300km/h) around the Earth horizontally and give rise to Rossby waves. The jet streams were first discovered when Zeppelins were blown off course in WW1. The greater the temperature difference – the greater the jetstream.  Rossby waves were discovered by Carl-Gustaf Rossby, a Swedish meteorologist, in the 1930’s. They are waves or zigzags in the jet streams as the travel around the Earth. The number of waves varies throughout the year but usually in summer it’s between four and six while in winter it’s three. Rossby waves are formed by major releif barriers (like mountains), thermal differences and uneven land-sea interfaces.  Jet streams have also been known to influence flights, for example it’s quicker to travel by aeroplane from London to New York then it is the other way around because the altitude planes travel at is similar to these high speed winds. Jet Streams and Rossby Waves
  114. 114. Jet stream and Rossby Wave The wave like meandering of air is described as a rossby wave, which are affected by major topographic barriers. As the pattern becomes more exaggerated it leads to blocking anticyclones.
  115. 115. A weather front is a term used in meteorology to describe the boundary where two air masses converge, sparking weather events. There is a cold front and a warm front. Fronts • Cold fronts often come with thunderstorms or other types of extreme weather. They usually move from west to east. Cold fronts move faster than warm fronts because cold air is denser, meaning there are more molecules of material in cold air than in warm air. • Strong, powerful cold fronts often take over warm air that might be nearly motionless in the atmosphere. Cold, dense air squeezes its way through the warmer, less-dense air, and lifts the warm air. Because air is lifted instead of being pressed down, the movement of a cold front through a warm front is usually called a low- pressure system. Low-pressure systems often cause severe rainfall or thunderstorms. • Warm fronts usually show up on the tail end of precipitation and fog. As they overtake cold air masses, warm fronts move slowly, usually from north to south. Because warm fronts aren't as dense or powerful as cold fronts, they bring more moderate and long-lasting weather patterns. Warm fronts are often associated with high-pressure systems, where warm air is pressed close to the ground. High- pressure systems usually indicate calm, clearweather.
  116. 116. {Section 2.3 Weather processes and phenomena
  117. 117.  Atmospheric moisture exists in all three states – vapour, liquid and solid.  Energy is used in the change from one phase to another.  When evaporation occurs, it takes 600 calories of heat to change 1 gramme of water from liquid to vapour, thus a heat loss occurs.  Condensation however released locked latent heat, causing a rise in temperature.  Changes between vapour and ice releases heat when vapour is converted to ice.  By contrast, heat is absorbed in the process of sublimation (snow patches that disappear without melting. Moisture in the atmosphere
  118. 118. { {Evaporation  Initial humidity of the air- if air is very dry then strong evaporation occurs; if it is saturated then very little occurs.  Supply of heat – the hotter the air, the more evaporation that takes place.  Wind Strength – under calm conditions air becomes saturated rapidly and therefore little evaporation occurs. Condensation  Condensation occurs when either enough water vapour is evaporated into an air mass for it to become saturated or when the temperature drops so that dew point is reached. The Cooling occurs in three main ways… 1. Radiation cooling of the air 2. Contact cooling of the air when It rests over a cold surface. 3. Adiabatic cooling of air when it rises. Factors affecting…
  119. 119.  Precipitation refers to all forms of deposition of moisture from the atmosphere – including rain, hail , snow and dew. Because rain is the most common form of precipitation in many areas, the term is sometimes applied for rainfall alone. For any type of precipitation to form, clouds must first be produced.  When minute droplets of water are condensed from water vapour, they float in the atmosphere as clouds. If droplets coalesce they form large droplets which, when heavy enough to overcome gravity, fall as rain.  The BERGERON THEORY suggests that for rain to form, water and ice must exist in clouds at temperatures below 0 degrees C. At such temperatures water droplets and ice droplets form and grow by condensation until big enough to overcome turbulence and cloud updrafts, so they fall. As they fall, crystals coalesce to form larger snowflakes, which generally melt and become rain as they pass into the warm air layers near the ground. Thus according to Bergeron, rain comes from clouds that are well below freezing at high altitudes, where the coexistence of water and ice is possible.  Other mechanisms must also exist as rain as rain also comes from clouds that are not so cold. These include….  Condensation on extra-large hydroscopic nuclei  Coalescence by sweeping, whereby a falling droplet sweets up others in its path  The growth of droplets by electrical attraction. Humidity and Precipitation
  120. 120.  Relates to the rising and sinking of air. This means that the temperature of the air is changed internally, without any other influence. It is the rising (expanding and cooling) and sinking (contracting and warming) of air that causes its temperature change.  Air moves for four reasons… 1. Convection: The most powerful lifting mechanism initiated by the heat of the Sun warming the ground, causing air to warm, expand and rise. 2. Orographic barriers: When air is forced to rise over a hill, mountain etc. 3. Turbulence: in air flow 4. Frontal Systems  When air rises from one elevation to another, the temperature changes. The decrease of pressure with height allows the rising parcel of air to expand. As it expands it uses up energy from within the parcel. Likewise when air is sinking it gains heat from contraction. Therefore adibiatic heating is an internal mechanism without any heat exchange. Adiabatic processes (lapse rates)
  121. 121.  The Environmental lapse rate (ELR) is the actual temperature decline with height – on average this is 6degrees per 1000 metres.  Adiabatic cooling and warming in dry air occurs at a rate of 10 degrees/ 1000m. This is known as the dry adiabatic lapse rate (DALR)  Air in which condensation is occurring cools at the lower saturated adiabatic lapse rates (SALR) between 4-9 degrees/1000m. This is because latent heat released in the condensation process partly offsets the temperature loss from cooling. The rate varies according to the amount of latent heat released. 4degrees being in warm saturated air, 9/1000 being in cold saturated air.  Lapse rates can be shown on a temperature height diagram. Adiabatic processes
  122. 122. Rocks and Weathering
  123. 123. Elementary Plate Tectonics • The theory of plate tectonics states that the Earth is made of a number of layers 1. The Crust: Thin outer layer which holds tectonic plates. 2. The Mantle: Thickest layer making up 82% of Earths volume. Made up of magma, with diameter of approximately 2900KM 3. The Outer Core: Hot layer surrounding inner core. Liquid layer made up of iron and nickel, with temperatures of approximately 5000degrees. 4. The Inner Core: Dense, solid core made of iron and nickel with temperatures exceeding 5500degrees Celsius. • There are two types of crust: 1. Oceanic: Dense, thinner plate made up of Basaltic rock, approximately 16km thick. 2. Continental: Much thicker plate made up of granite, silica and aluminium, less dense than oceanic plate. • The upper mantle and crust make up a layer called the Lithosphere which is broken into a number of plates. These move over the Asthenosphere, which is a plastic layer in the mantle, which drives plate movement.
  124. 124. Earths Structure
  125. 125. Alfred Wegener's theory for continental drift • Wegener in 1912 proposed his hypothesis on continental drift, using several lines of evidence to support his ideas that the continents were once joined together in one super continent called Pangaea (which means “entire earth” in Greek) These included… 1. The apparent fit of the continents like a jigsaw puzzle 2. The correlation of multiple fossils such as the mesosaurus, found in only South America and Africa. As a creation who could only live in shallow water, couldn’t swim well or fly, how could it have travelled over an entire ocean? Solution: The continents were once connected. 3. Matching rock formations and mountain chains found in South America and Africa, consisting of the same rock and same age. 4. Glacial striations found in tropical rainforests suggests that countries were always in their current climatic regions. • However whilst Wegener had some very valid points and a good argument, he had no driving mechanism to make this happen. He believes that somehow continents were pushing ocean plates along – however critics commented that continental plates lacked momentum to achieve this and thus his theory fell through.
  126. 126. Harry Hess’s hypothesis of Sea floor spreading 1960s • Harry Hess in the 1960s suggested that convection currents within the mantle could be forcing magma to rise and crack the crust above it forcing it apart. • He believes that as it welled up and cooled on the ocean floor at divergent zones, new oceanic crust was forming at mid ocean ridges, pushing older, colder and more dense crust towards deep sea trenches, where it is subducted, recycled back into the mantle or creates volcanism. • However when there is no trench for old crust to subject under (such as the coast of Africa), then in pushes the continent along with it as crust accumulates. • As there are few trenches in the Atlantic Ocean it is expanding • As there are many trenches in the Pacific Ocean it is shrinking • How did Hess support his theory? Rock Magnetism. Hess looked at the polarity on either sides of the ridge, a correlation of identical bonds between the two sides supported his theory. • Magnetic grains in the rock align with the Earth’s magnetic field at the time of cooling (known as paleomagnetism)
  127. 127. Sea floor spreading
  128. 128. J Wilson 1965 • In 1965 J Wilson linked together ideas of continental drift and sea floor spreading, developing the concept of plate tectonics. • Wilson said that Earth’s crust, or lithosphere, was divided into large, rigid pieces called plates. These plates “float” atop an underlying rock layer called the asthenosphere. In the asthenosphere, rocks are under such tremendous heat and pressure that they behave like a viscous liquid (like very thick honey). The term “continental drift” was no longer fully accurate, because the plates are made up of continental and oceanic crust, which both “drift” over Earth’s face. Tuzo Wilson predicted three types of boundaries between plates: mid-ocean ridges (where ocean crust is created), trenches (where the ocean plates are subducted) and large fractures in the seafloor called transform faults, where the plates slip by each other.
  129. 129. Types of Plate boundaries • Divergent/Constructive: These plates are moving away from each other. They are usually found in the middle of the oceans and mid ocean ridges are found here. • Convergent/Destructive: These plates are moving towards each other causing earthquakes, volcanoes, deep ocean trenches and fold mountains. • Transform/conservative: These plates are sliding past each other. At these zones land is not being created nor destroyed , however frequent earthquakes are common. An example is San Andrea Fault in California
  130. 130. How do the plates move? • There are three main theories explaining plate movement… 1. Convection currents: This states that huge convection currents occur in the earths interior causing hot magma to rise to the surface and then spread out at mid ocean ridges, whilst the cooler magma gets denser and sinks back deep into the mantle where it is reheated. 2. Dragging Theory: Plates are dragged or subducted by their oldest edge when they become cold and dense. Plates are hot at mid ocean ridges, but cool as they are pushed further away. As the cold plates descend at the trenches, pressure causes the rocks to become heavier and therefore they are subducted. 3. Hotspot: Hotspots are plumes of molten rock which rise underneath a plate penetrating weaknesses in the crust and resulting in volcanic activity. As plates are moving and hotspots stay still, these therefore led to chains of land creation such as Hawaii.
  131. 131. Convection Currents Hot Spots Dragging Theory
  132. 132. Subduction Zones • Subduction occurs when an oceanic lithospheric plate collides with another plate. As the density of the ocean plate Is similar to that of the asthenosphere it can easily be subducted. Subduction zones dip mainly at 30-70 degree angles. • If a continental and oceanic plate meet, the oceanic plate will be subducted beneath the continental as it is more dense. • Evidence of subduction: • The existence of certain landforms such as deep sea trenches and folded sediments (usually arc shaped and containing volcanoes) • Benioff zone – a deep active seismic area dipping away from the deep sea trench. • Earthquake focal mechanisms (ring of fire)
  133. 133. Island Arcs • Island arcs are features of oceanic/continental convergence. They are chains of volcanoes which are aligned in an arc shape and sit close to the boundary where two plates meet. During subduction hot re- melted material from the subducting slab rises and leaks into the crust forming a series of volcanoes. These volcanoes can make a chain of islands called Island arcs. Many are found in the Pacific and Western Atlantic.
  134. 134. Mountain Building • Plate tectonics are associated with mountain building. Where oceanic plates meet continental, the light less dense plate may be bulked and folded up creating fold mountains, such as the Andes. • Where two continental plates meet, both may be folded and buckled, forming mountains such as the Himalayas formed by the collision of the Eurasian and Indian plates. • The Indian subcontinent moved rapidly north during the last 70 million years, eventually colliding with the main body of Asia. The huge ocean Tethys has been entirely lost between these masses in the collision zone and the crust has thickened because Asia overrides India, resulting in crust thickening causing the uplift of the Himalayas.
  135. 135. Plate landforms
  136. 136. WEATHERING Decomposition and Disintegration
  137. 137. Weathering • Describes the processes that break up rocks. There are three types of weathering… 1. Chemical Weathering (Decomposition): Processes that break down rocks atom by atom through chemical reactions. Water plays a key role here. 2. Mechanical Weathering (Disintegration): The tearing apart and breaking of rocks through physically destroying them 3. Biological Weathering: When animals and vegetation (root wedging) break up rocks. • Hot wet climates enhance chemical weathering • Cold wet climates enhance mechanical weathering.