Physical Causes And Consequences Of Mass Movement

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Physical Causes And Consequences Of Mass Movement

  1. 1. 3.4.2.7 Mass movement Physical causes and consequences of mass movement 1. Understand the concept of slope as an open system with inputs and outputs. 2. Identify different types of movement: –fast movements (mudflows, slumping, landslides, avalanches) –slow movements (solifluction and soil creep). 3. Know the processes responsible for these movements, together with the hazards that can arise from them.
  2. 2. references • Waugh Geography an Integrated Approach pg 46- 49 • Bishop –g 123 - 127
  3. 3. Mass movement • Mass movement is the movement downhill of weathered rock material (regolith soil, loose stones and rocks) under the influence of gravity. It differs from mass transport, which is movement caused by wind water or ice. • In mass movement, soil, loose stone and rock material falls, rolls, slides or flows downhill. Although water or ice may be involved, gravity is the driving force. Landslides and mud flows involve the downslope movement of huge quantities of material and can completely alter a slope’s shape.
  4. 4. Mass movement - Describes all downhill movements of weathered material (regolith) – soil, loose stones and rocks, in response to gravity - When gravitational forces exceed forces of resistance slope failure occurs and material moves downhill
  5. 5. Slopes Slope movement is common particularly where there are: • steep gradients • weak rocks • heavy rainfall • basal undercutting ( river or marine erosion or human actions) WHY?
  6. 6. The Slope System
  7. 7. A slope is an open system affected by • Biotic • Climatic • Gravitational • Groundwater • Tectonic inputs. Inputs may vary in time and scale. The amount, rate and type of movement depends upon the degree of slope failure
  8. 8. Classification of Mass Movements Carson and Kirkby (1972) Based on the speed of movement an the amount of water present Waugh Fig 2.12 pg 46
  9. 9. A slope is an open system affected by biotic, climatic, gravitational, groundwater and tectonic inputs which vary in time and scale. The amount, rate and type of movement depends upon the degree of slope failure
  10. 10. • Mass movement may occur on slopes or it may involve subsidence or sinking of the ground. • Two forms of mass movement on slopes are landslides and creep.
  11. 11. Slow movements a. Soil creep b. Solifluction
  12. 12. Soil Creep (Waugh pg 46) • Creep is slow mass movement less than 1cm per year. • It is a continual process • Occurs mainly in humid climates where there is vegetation cover • Rock creep refers to the movement of individual boulders on a rock slope or the movement of boulders on a talus slope. • Soil creep is the slow downslope movement of soil. • The rates of down slope movement of individual rocks on talus slopes can vary from 40m per year to as little as a few centimetres per year. The faster movements may be caused by factors such as the impact of boulders onto the talus surface. The slower movements are more commonly caused by freezing of water in the spaces between boulders,
  13. 13. Freeze thaw • Wet – dry periods When the regolith • During times of heavy freezes the ice rainfall moisture increases the volume and crystals increase the weight of the soil causing volume of the soil by expansion and allowing the regolith to move 9%. . As the soil downhill under gravity. In expands particles are the dry period the soil will dry out and then contract. lifted at right angles to the slope = heave. When the ground thaws, the regolith contracts and
  14. 14. Soil creep usually occurs on slopes of about 5o and produces terracettes.
  15. 15. Terracettes • Terracettes are evidence of soil creep. • Terracettes are step like features – often 20 – 50 cm in height which develop as the vegetation is stretched and torn. • Terracettes appear as a ribbed pattern on the surface of steep slopes indicating that the surface layer is breaking up into small steps. The details of terracette formation are not known except that they tend to form on steep slopes with or without vegetation.
  16. 16. Evidence of creep: The weight of tombstones, the response of trees on slopes to sunlight and the activities of burrowing animals, insects and tree roots. Where soil creep does occur it is probably the result of the frost heave whereby individual particles rise and fall in response to expansion and contraction due to
  17. 17. Solifluction = ‘soil flow’ (Waugh page 47) • Solifluction is the slow downhill flow of saturated soil. • Movement averages between 5cm and 1m per year. • It is a common process in periglacial environments. • During winter season both the bedrock and regolith are frozen. In summer the surface layer thaws but the underlying layer remains frozen and acts like impermeable rock. • Because surface meltwater cannot infiltrate downwards and temperatures are too low for effective evaporation any topsoil will soon become saturated and will flow as an active layer over the frozen subsoil and rock. This process produces solifluction sheets or lobes – rounded tongue – like features reaching up to 50m in width and head – a micure of sand and clay formed in valleys and at the foot of sea cliffs. • The water reduces the effects of cohesion and friction, thus promoting movement. • The term gelifluction refers to solifluction that takes place on top of frozen ground.
  18. 18. Flow movements c. Earth flows d. mudflows
  19. 19. Mudflow • When soil or weak rock such as clay or volcanic ash becomes saturated it may actually begin to flow downhill. Such flows can be slow but most commonly they are very rapid exceeding 1km/hr. • Occur after periods of intensive rainfall when both volume and weight are added to the soil giving it a higher water content than an earth flow. • Mudflows often form a serious volcanic hazard as melted snow and ice from volcanic summits can combine with ash to form deadly flows called lahars. (Armero 1985).
  20. 20. Waugh fig 2.16
  21. 21. Chittagong mudslide 2007 • Bangladesh’s 2007 monsoon started with unusually heavy rain, intensified by a storm from Bay of bengal 9 – 10th June. • Heavy rain caused mudslides which engulfed slums in foothill areas of Chittagong on the coast in SE Bangladesh on 11th June • Death toll reported to be at least 128 of whom 59 were children • More than 150 injured • Experts had warned of increasing likelihood of landslides due to the Bangladesh governments failure to curb illegal hill cutting in Chittagong.
  22. 22. • Hill cutting creates flat sites for the construction of new houses. • Trees have also been cleared to build houses on hill tops. This can block the natural rills or gullies that drain the landscape. Water is then forced to enter the ground through cracks, which weakens the soil structure and promotes landslides.
  23. 23. Rapid movements Slides Rock falls
  24. 24. What is the difference between slides and flows? Flows undergo internal derangement Slides move ‘en masse’ and are not affected by internal derangement.
  25. 25. Landslides
  26. 26. 10 deadly landslides
  27. 27. There are 4 basic types of landslide categorised by the main method of movement downhill. A landslide may – fall – Slide – rotate – flow. • Falls usually occur from cliff faces. • The falling mass or rock usually breaks up along joints and or bedding planes.
  28. 28. Landslides • Occur when the line of failure occurs along a shear plane which is roughly parallel with the ground surface. • They can occur in both soil and rock material. • If the hill slope is not too steep, vegetation may prevent the moving mass from breaking up. • Almost all the landslides that occur on natural hill slopes are slides. • The shear plane often forms within a soil horizon or along the contact between weathered material and the underlying rock.
  29. 29. Landslides • These are sudden movements of material downslope under the influence of gravity. Landslides vary enormously in the amount of material moved and the velocities. • Some landslides are fluid like a river of mud. • Some have hardly any water in them at all and are almost entirely angular blocks of rock. • Most landslides are in contact with the ground. Movement takes place along a flat or planar slide plane. Landslides commonly occur along a bedding plane particularly when the underlying bed is impermeable leading to a high moisture content which will lubricate the slide surface. Landslides are very rapid and can cause huge damage and loss of life.
  30. 30. Las Colinas Landslide, El Salvador, Feb 2003
  31. 31. Often occurs where softer rocks like clay or sands overlie more resistant or impermeable rocks like limestone and granite
  32. 32. Rock falls (Waugh pg 48) • These are spontaneous debris movements on slopes that exceed 40o. • Rockfalls may result from extreme physical or chemical weathering in mountains, pressure release, storm wave action on sea cliffs or earthqyakes • Rockfalls are often caused by frost wedging in cracks or by excess water pressure developing in cracks. A crack forms at the top of a cliff and over a period of months or years it widens at an increasing rate until failure occurs and the cliff face comes crashing down. • Rockfalls begin by travelling through the air. The rapid movement usually occurs on the steepest slopes. Individual rock fragments or whole slabs of rock suddenly become detached and fall to the base of the slope. They may be detached by gradual process such as freeze thaw or by sudden and dramatic events such as earthquakes. • The angular debris collects at the base of the slopes to form scree or talus.
  33. 33. Slumps or rotational slides • Rotate around a shear (=slide) plane that resembles an arc of a circle rather than being parallel with the ground surface or are concave in shape. • Slumps often occur in excavations or roadside cuttings. • They are probably the most common form of visible mass movement in the UK. They can be seen along the coast in Norfolk and Lulworth. • They usually occur in weak rock (e.g. clay or sands that overlie more resistant or impermeable rock) or in soil that has become saturated and, in response to gravity, simply collapses
  34. 34. What factors affect slope processes • Ross fig 2.49 The slope system is an open system becaue there are inputs from outside (e.g. heat and precipitation) and outputs (e.g. water and weathered rock) into other systems. • Passive causes are those features of an area that make frequent landslides there likely e.g. rock type, bedding, landforms, water, climate, human influence
  35. 35. Factors that promote landslides • Natural Hazards and Disasters fig 64 pg 55 Holmes and Warn
  36. 36. Localised physical factors that determine landslides: Gradient Topography. Landuse Geology Permeablility Climate
  37. 37. Factors that influence soil development: • Rock structure • Lithology • soil • Climate • Vegetation • Human activity
  38. 38. Rock type • Igneous and metamorphic rocks are strong and capable of supporting near – vertical slopes, whereas sands and gravels can only support very gentle slopes. Landslides are more likely to occur where rocks or clays are weak. Bedding • Rock slabs may become detached along bedding planes or joints leading to rockfalls and landslides. Landslides are more likely where the bedding of the rock tilts towards the slope. Slopes with reverse dipping beds are more likely to be stable. Landforms • Landslides are more likely on steep hillslopes. The steeper the slope the more unstable it is likely to be. Water • Slope stability may be reduced where ground water seeps to the surface
  39. 39. Climate • The climate of an area will affect the type of weathering that operates on a slope and will govern the nature and presence or absence of water and vegetation. • Heavy rain and meltwater add volume and weight to the soil • Heavy rain increases the erosive power of a river at the base of a slope which might make the slope less stable • Frost may also be important in causing rockfalls
  40. 40. Tectonic activity • Some of the steepest slopes are found in tectonically active areas which result from the gradual uplift following plate collision. Earthquakes may also trigger slope failure Weathering • Weathering affects the upper slopes particularly any bare rock outcrops. Mechanical weathering particularly frost shattering will lead to a more jagged , angular, bare rock surface whereas chemical weathering will produce more rounded slopes
  41. 41. Vegetation • If a slope is forested or covered in bushes and grass it is less likely to be active. This is because it will protect a slope from the direct effects of rainfall and help bind together particles of rock and soil. Basal excavation • Basal excavation can take the form of a river undercutting a slope or the sea cutting a notch in a cliffline. Human activity such as road construction can have the same effect. Basal excavation can lead to a steepening of a slope so making it unstable. Time • The length of time that a slope has been exposed to weathering. Newly formed landscapes that are steep and unvegetated are actively weathered and eroded until they assume a shape that is in balance with their environment
  42. 42. terms Talus - an accumulation of fallen material found at the base of cliffs
  43. 43. Ross pg 56-62 Waugh 46 – 55 Bishop 123- 135
  44. 44. May 2005“Slope instability is entirely due to human activity.” Using examples, assess this statement. [20 marks]
  45. 45. May 06 • Diagrams A and B show two types of mass movement. • (i) Define the term mass movement. [2 marks] • (ii) Identify the type of mass movement taking place in each of the diagrams • A and B. [2 marks] • (iii) Explain the processes of mass movement taking place in diagram A and describe their effects on the natural and human landscape. [4+2 marks] • (iv) Discuss the relative contribution of natural and human factors to mass movements of the type shown in diagram B. [10 marks
  46. 46. May 07 Explain how the stability of slopes can be affected by natural and human factors. [20 marks]
  47. 47. May 08

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