Mass Movement


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Leaving Certificate Geography slides on Mass Movement (Landslides, rockfalls, avalanches, soil creep etc)

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Mass Movement

  1. 1. Mass Movement Rock Falls, Avalanches, Soil Creep, Landslides, Mudslides….
  2. 2. Mass Movement Definition – The movement of material downhill under the influence of gravity Loose weathered material on the surface of the Earth is known as Regolith When you have regolith on a slope (even a gentle slope) the possibility exists for Mass Movement Movement can be very fast (Km/hr) or very slow (mm/year) – but can dramatically change the Earth’s surface over time Mass Movement can also be triggered by natural or man- made factors – anything that makes regolith unstable
  3. 3. Triggers of Mass Movement
  4. 4. Factors affecting mass movement Effect of Gravity – the bigger they are, the harder (and faster) they fall Slope – the steeper the slope, the faster the mass movement Water – helps to lubricate the regolith, more water = more sliding potential Vegetation – roots help keep the regolith together, lack of vegetation means more potential for movement Human activities – anything that disturbs the regolith (e.g. road building, farming on steep slopes) Type of regolith – loose snow is more likely to move quickly than Glacial Ice
  5. 5. Slow Mass Movement Soil Creep – this is the slowest type of MM. It is probably the most common type of MM in Ireland and despite it’s slowness it can have significant impacts over time Main Processes Involved – Freeze Thaw, wetting and drying These processes cause the soil to expand (freezing, wetting) and shrink (thawing, drying) – eventually the soil moves downhill As little as 1mm of movement per year movement Soil Creep is a relatively dry type of movement – the soil “rolls” rather than “flows” downhill
  6. 6. Mechanism of Soil Creep
  7. 7. Soil Creep - effects
  8. 8. Soil Creep - effects
  9. 9. Soil Creep - effects Trees and Poles bend, ripples appear on hillsides
  10. 10. Solifluction A type of “Waterlogged Soil Creep” that mainly occurs where Permafrost is present Upper layers of the soil melt in summer, however the lower layers remain frozen This results in the upper layers “sliding” over the lower layers Regolith in this case has the consistency of treacle or honey Leaves behind curved bulges called “lobes”
  11. 11. Solifluction
  12. 12. Solifluction lobes Lobes
  13. 13. Fast and Wet Mass Movements When the regolith is very wet the mass movement can flow downhill quite rapidly – however when the movement stops the regolith can become dry and hard like concrete, which makes removal very difficult Digger trapped in a Lahar
  14. 14. Mudflows The main process at work here is heavy rain and flooding Because of the mixture of water, mud and loose rock they are very dangerous – think of a tsunami with rocks embedded in it – the greater mass leads to greater damage Can travel a long distance depending on the gradient - gradients over 25 degrees are needed
  15. 15. Lahar These occur when volcanic ash, not mud, mixes with water and flows downstream On tall volcanoes the snow can melt and mix with the ash to form a thick substance with the consistency of wet concrete – when this substance flows downhill a Lahar is formed Can cause massive damage and are very difficult to remove – think of Pompeii in Italy
  16. 16. Lahar Deposits, Guatemala
  17. 17. Landslides or Slumps These are very fast forms of mass movement (at least compared to soil creep) Need a “trigger” to take place – can be a storm, earthquake, shockwave from explosion etc Human activity (e.g. road building) can increase the likelihood of a landslide – DART between Dalkey often disrupted (see P189) A Rotational Slump is a landslide where regolith slips along a curved surface and falls back on itself
  18. 18. Landslides ure=related ture=related
  19. 19. Rotational Slip
  20. 20. Avalanches A sudden rapid movement of a mass of snow down a slope Often confused with landslides – landslides have little or no snow Major issue in Alpine regions – a single skier can act as a “trigger” and cause unstable snow to fall Because snow in high areas is frozen, it acts like dry particles of material – like dry sand ted
  21. 21. Rockfall
  22. 22. Rockfall (on Mars) Talus/Scree Freshly exposed cliff Dust Cloud Shows this is dry mass movement
  23. 23. Bog Burst A bog burst is a type of landslide where the regolith is peat bog Are often caused by inappropriate/poorly managed development in blanket bogs (bogs on mountains) Can cause severe damage to water supplies if the peat enters a river Most (in)famous example in Ireland was the Derrybrien bog burst in Co. Galway.
  24. 24. Derrybrien wind farm
  25. 25. Derrybrien wind farm
  26. 26. Wind Turbine Foundation
  27. 27. Damage after bog burst
  28. 28. Damage after bog burst
  29. 29. Classifying Mass Movement
  30. 30. Mechanisms of Mass Movement
  31. 31. Mass Movement caused by...
  32. 32. ...intensive agriculture (overcropping) Gullies
  33. 33. How to prevent mass movement?
  34. 34. How to prevent mass movement? 5 well developed points on how to stop or limit undesirable mass movement. 2/3 of a foolscap page