External geological processes and landscapes


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Introduction to external geological processes, agents and main landforms planned for 14 year-old students

External geological processes and landscapes

  2. 2. INDEX:I. BASIC CONCEPTS (section 1) Exogenous geological processes: weathering, erosion,transportation and sedimentation. Landscape shaping.II. TYPES OF LANDSCAPES (sections 2-5) Aeolian landscapes Glacial landscapes Torrential landscapes Fluvial landscapes Karst landscapes Coastal landscapes
  3. 3. UNIT 8 – I. BASIC CONCEPTSWeathering is the process of alteration and fragmentation of rocks bythe joined action of the atmosphere, water and living organisms.What is wheathering?It is a static process: it does not involve movement.Where does it take place?
  4. 4. There are two main classes of weathering processes:a) Physical weathering: It’s the breakdown of rocks and mineralsinto small-sized particles through physical forces.b) Chemical weathering: It’s the breakdown of rocks throughchemical reactions causing changes in their mineral composition.Living things can affect the rocks both physically (e.g. roots) andchemically (lichens) through their activity. We can consider them as athird type of weathering, called biological weathering.e.g. Freeze-thaw weathering or frost wedging in high mountain areas.e.g.: Hydrolysis, Oxidation, Carbonation and Dissolution.e.g.: Thermal stress weathering in deserts.
  5. 5. Water flows into cracks.The freeze-thaw cycle is repeated, andeventually the rock cracks.When the water freezes, it expands.The ice acts like a wedge.MechanicalweatheringFreeze-thaw weathering istypical of mountainous regionswhere temperature fluctuatesbelow and above the freezingpoint. Water in the rock cracks,freezes and melts repeatedlyovertime, acting as a wedge andcausing them to break.Freeze – thaw weathering:As a result of this process, great piles ofrocky fragments called scree areformed.
  6. 6. Hydrolysis of graniteGraniteGravelHydrolysis processes take place when slightly acid rainwaterdissolves some minerals like feldspars and transforms them intoclay minerals. Granite is a hard and resistant rock, but becomesweakened and disintegrated when feldspars are broken down byhydrolysis and quartz crystals remain as loose debris,unaffected.
  7. 7. There are some factors which can affect the processes of weathering.In general, weathering is more efficient when climatic conditions areextreme.•Hot and wet climate conditions enhance chemicalweathering. Why is this?•Low or changing temperatures and wet conditionspromote physical weathering.•Weathering processes are typically slower in dryregions, whether cold or hot.
  8. 8. What is erosion?What is transportation?What is sedimentation?It is the removal of weathered materials due to the actionof water, wind, ice and gravity. It is a dynamic process thatinvolves movement and wears rocks away.Eroded materials are carried away by gravity, wind,water or ice.Deposition and accumulation of transported materials insedimentary basins (depressions in the surface of the land or in theocean floor). This happens when the agent loses energy.
  9. 9. Landscape shapingSome landscapes are fascinating: river canyons, coastal cliffs, huge sanddunes, waterfalls… The responsible agents for these breathtaking reliefsare called external geological agents.The geological agents are liquid water, ice, and air when they move:•Wind•Glaciers•Surface runoff•Rivers•Groundwater•OceansAll these agents are powered by the Sun or the Earth’s gravity. The sunheats the air and water and causes the wind and oceanic currents. Gravityforces water and ice to move towards lower areas.
  10. 10. What geological agent has shaped these landscapes?
  11. 11. Landscape shaping takes place through three different processes:a) Erosion: it is the removal of the weathered rocks away from the siteof weathering.b) Transportation: eroded materials are carried away to anotherplace.c) Deposition: it’s the laying down of the transported rock materials.The resulting materials are called sediment.
  12. 12. Erosion, transportation, and deposition shape landscape creatinglandforms (ex. valleys, cliffs, dunes, etc.). The relief of Earth’s surface isthe result of destructive processes tearing down the land, andconstructive processes building it up.• If erosion exceeds deposition, part of the surface relief isdestroyed. Destructive processes are dominant and createerosional landforms.• If deposition exceeds erosion, new relieves are built up.Constructive processes are dominant and createdepositional landforms.
  13. 13. Are these erosional or depositional landforms?
  14. 14. UNIT 8 – II. TYPES OF LANDSCAPESHow does wind shape landscape? Aeolian landscapesWind is not a very powerful agent. Aeolian processes are onlyimportant in arid environments with sparse vegetation and a largesupply of fine and unconsolidated (loose) sediment.Wind removes and transports only fine particles from the soil surface,leaving the larger ones. When the wind stops or reduces its speed,suspended particles are deposited.
  15. 15. Aeolian erosional processes and landforms:Process LandformDeflation(the wind removes fine, looseparticles from the ground)Rocky deserts orpavements (reg).Abrasion(suspended particles erode therocks close to the surface)Mushroom-shaped rocks
  16. 16. Aeolian depositional processes and landforms:Process LandformWind reduces its speed anddeposits sandy particlesDunes (ex. Barchans) anddune fields. Sandy desertsor ergs.The finest particles of dusttravel very long distances.Loess
  17. 17. Gravel desert ordesert pavement(Reg)Deflation and sandcolumnMushroom-shapedrocks created byabrasionErg BarchanLoess deposits are formed far away from deserts.
  18. 18. Arches National Park (Utah)
  19. 19. How do glaciers shape landscape? Glacial landscapesGlaciers are moving ice masses. They form where there is more snowfall in winterthan melts during summer. There, snow accumulates over time, and turns slowlyinto ice which starts moving downhill due to gravity.They are only in polar regions or in high mountains.Continental glaciers Alpine glaciersCirqueTongue
  20. 20. Glacial erosional processes and landforms:Near the mountain tops, ice causes intense mechanical weathering anderosion, forming very steep relieves with vertical walls and sharp peakscalled horns.
  21. 21. CIRQUETarnGlacial erosional processes and landforms:Cirques, where snowfall accumulates and turns into ice, are round bowl-shaped depressions. When glaciers disappear, they are often occupied byround lakes called tarns.
  22. 22. Glacial erosional processes and landforms:Glacial tongues erode the bedrock forming characteristic U-shapedvalleys. Ice pulls chunks of rock up into the ice and scrape the underlyingbedrock due to abrasion.Striations arecommon inrocks of glacialareas.
  23. 23. Glacial depositional processes and landforms:Glacial erosion produces much rock debris which is dragged and depositedby glacial tongues forming moraines.
  24. 24. CirqueTongueTarn Steep relief HornHanging valley Melting lakeMoraines
  25. 25. When glaciers disappear they leave very characteristiclandscapes in mountainous regions.
  26. 26. How do rainfall, surface runoff and temporary watercoursesshape landscape? Torrential landscapesHow do rivers shape landscape? Fluvial landscapesThey are the result of liquid water running off the surface!!Liquid water is probably the most important geological agent in the world.
  27. 27. How do rainfall, surface runoff and temporary watercoursesshape landscape? Torrential landscapes.In arid regions with scarce vegetation and sporadic but torrential showers, rainfallintensity quickly exceeds soil infiltration capacity and important surface runoff isproduced, thus eroding and shaping landscape.Raindrops cause mechanicalweathering and disintegratessoil materials.Torrential erosional processes and landforms:
  28. 28. Torrential erosional processes and landforms:Surface runoff can erode and transportbig amounts or fertile soil: this is calledsoil loss.
  29. 29. Sometimes runoff concentrates on certain flow paths and cuts a well defined channel on thehillside, that grows from small gullies to ravines or deep gorges.Torrential erosional processes and landforms:GULLIES
  30. 30. GULLIESRAVINESTorrential erosional processes and landforms:
  31. 31. Torrential erosional processes and landforms:GORGES
  32. 32. Between gullies or ravines, there may betall and thin spires left, called fairychimneys.Torrential erosional processes and landforms:
  33. 33. Torrential depositional processes and landforms:Intense runoff transports a lot of soil particles which are deposited at theend of ravines forming alluvial fans or alluvial cones. Alluvial fans arehigh flood risk areas.
  35. 35. GulliesRavineAlluvial fan Fairy chimneysAlluvial cone
  36. 36. How do rivers shape landscape? Fluvial landscapesUPPER COURSE- Steep slope-Fast flow- Erosion is dominant- V-shaped valleys- Waterfalls and rapids- Gorges and canyonsMIDDLE COURSE- Gentler slope- Lower velocity- Erosion and deposition areequally important.-Transportation is dominant.- Wide flat-floored valleys.- Floodplains and terraces- Meanders and oxbow lakesLOWER COURSE- Very gentle slope- Very slow flow- Deposition is dominant- Very Wide floodplains.- DeltasIn wetter regions, where rains are not torrential, but maintained and regular, waterforms rivers. Rivers are permanent watercourses, usually freshwater, flowingtowards another river, an ocean, or a lake.
  37. 37. The upper course:The river flows fast down a steep slope. Erosion is dominant. Rivers cut a narrowV-shaped valley. Waterfalls, rapids, gorges and canyons are common too.
  38. 38. WATERFALLS
  39. 39. The middle course:The velocity of the water decreases as the slope gets gentler. Erosion anddeposition are equally important. Transportation is dominat. The river forms awider flat-floored valley, meanders, and a well-developed floodplain.
  40. 40. The middle course:When the river erodes its bed deeper, it creates a new floodplain. The“hanging” old floodplain is called a fluvial terrace.
  42. 42. The lower course:The slope flattens out and the water flow gets slower. Deposition is dominant.Valleys are very wide, with large floodplains. Most materials are deposited at themouth where a delta may appear.
  44. 44. How does groundwater shape landscape? Karst landscapesDepending on soil characteristics and on rainfall intensity, a certain amount ofrainwater infiltrates the ground, thus becoming groundwater. Groundwaterfills the pores and grooves in the rocks forming aquifers.
  45. 45. When acidic water containing carbon dioxide gets into rock cracks, some minerals,such as calcium carbonate or calcite (found in limestone) and gypsum can bedissolved. The rock is chemically weathered.Then many erosional landforms, such as caves and sinkholes, are formed.Acidic water gets into the rock joints anddissolves calcium carbonate, creating caves.When the roof of a cave has been intenselyweathered, it collapses and a sinkholeappears.
  47. 47. Sinkholes
  48. 48. If the water discharges some of its dissolved carbon dioxide, then calciumcarbonate dissolved into water precipitates forming depositional features.In caves, dripping water forms:stalactites (conic deposits which grow downward from the cave ceiling)stalagmites (which grow upward from the cave floor).In karst landscapes,disappearing streams andreappearing springs are alsocommon.
  49. 49. Stalactites and Stalagmites
  50. 50. Above-ground depositional features
  51. 51. Disappearing Streams and Springs
  52. 52. SinkholeCaves StalactitesStalagmitesDisappearingstreamReappearing spring
  53. 53. How do oceans shape landscape? Coastal landscapesCoastlines are very dynamic features, changing shape continually. In coastallandscape, moving ocean water is the geological agent involved.There are three types of ocean water motions, which affect coastal landscapes:a) Waves: They are created by wind blowing the surface of the ocean. Wavesstrike the shore repeatedly and disintegrate the rocks mechanically. They are alsovery effective agents of erosion, transportation and deposition.Destructive waves: If the backwash isstronger than the swash, the dislodgedparticles are removed (erosion).Constructive waves: If the swash isstronger than the backwash, depositionoccurs.So they also transport materials along the shore.
  54. 54. b) Tides: They are the cyclic rise and fall of seawater due to the Moon’s andSun’s gravitational attraction. They intensify the action of waves: theirerosive power increases at high tide and deposition mainly takes place at lowtide.c) Longshore currents: They are gradual movements of water parallel to theshore. They appear when prevailing wind blows regularly at the same angleto the coastline. They can transport many materials along the shore (beachdrift).
  55. 55. Coastal erosional processes and landforms:Wave action over the most exposed parts of the shore shapes vertical rockywalls called cliffs. Cliffs retreat progressively over time, giving rise to agrowing flat rocky surface called wave- cut platform.This is the process:
  56. 56. Wave erosive action can also create otheramazing landforms such as arches,caves and stacks.Coastal erosional processes and landforms:
  57. 57. Depositional landforms:Constructive waves normally deposit these sandy particles in sheltered areas, likebays. The accumulation of sand from erosion forms beaches.
  58. 58. Depositional landforms:Longshore currents also transport and deposit big amounts of sand. They can formspits: extended stretches of beach material that project out to sea and are joinedto the mainland at one end.Currents can also form tombolos: sand bars that connect a rocky island to themainland.
  59. 59. Spit TomboloBeachCliffsArchStack