# Mass Movement. ‘Mass movements’ is the term used to define the different ways that weathered material can move downslope. The two forces acting on the.

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Mass Movement

‘Mass movements’ is the term used to define the different ways that weathered material can move downslope. The two forces acting on the material are;- 1.Gravity, which pulls the particles down. 2. Friction, which stops them sliding downhill too fast.

Four main factors influence mass movements;- 1.How much water is involved in the action. 2.How much of an angle is needed on the slope. 3.How quickly the movement happens. 4.The amount of vegetation cover is also important in some types of movement. 2

1. What angles are rockfalls most likely to happen at? 2. Which of the two forces is the dominant one? 3. Which secondary processes are very important too? 4. So what condition ought the rock face to be in if a rockfall is to happen? Four more questions after the answers to these!

1. The best angle is between 70 and 90 degrees from horizontal. 2. Gravity is more effective here than friction. 3. Freeze-thaw and frost-shattering are important processes. 4. The face should be well- jointed or cracked for rockfalls to happen.

5. What angle does scree usually initially settle at? 6. Explain what is meant by ‘size- sorting’ and why it happens. 7. Why will weathering of the rock face cease in time? 8. What is the usual angle of an inactive scree slope?

5. Scree settles between 34 and 40 degrees, usually. 6. Particles travel down the slope depending on their weight and density. Lighter ones finish nearer to the face, heavier ones travel further, often moved by rolling over smaller ones. 7. If scree covers the rock face, freeze-thaw cannot act and weathering stops. 8. The inactive slope consolidates at 37 degrees where it usually grasses over.

Climbers call these types of screes ‘stone chutes’. Their technical name is talus slope.

A rockfall and talus slope in Arizona

2. Where can the slip plane/ glide plane be found? 1.What two roles does heavy rainfall play? 3.How does the result of this movement look different to a rockfall? 4.Slumps- special kinds of slides- have a different kind of slip plane- in what way? 5. What is the difference in the motion of the material in slumps?

2. The slip plane is below the surface. 1. It lubricates the slip plane and makes the material heavier and likely to move. 3. There are ‘bald’ spots visible on the slip plane and there is no sorting. Material moves as a ‘unit’. (see p88 fig 4.14.B) 4. A shear is curved, steeper at the top than the bottom. 5. Slumped ground has rotated whereas slides travel down in a straight line.

The Quirang in Skye shows spectacular rotational slumping.(see p89 fig 4.15) Spot the tourists?

2.What angles are more likely? 1. How gentle a slope can mudflows start on? 3. What shape does the material stop in? 4. Describe the two factors that help create mudflows. 5.How do you describe the speed of a mudflow? 6. What condition does the surface below the flow have to have?

2. Angles over 10 degrees are more common. 1. They can start on a slope as gentle as 5 or 6 degrees. 3. The material stops in a lobe/fan shape. 4. They need high rainfall and little vegetation cover. 5. Mudflows happen very quickly indeed! 6. The lower layers need to be impermeable to water.

This mudflow happened several years before the photo was taken- the scars are still very visible!

A mudflow in Central America, October 2005

2. Define the term ‘heave’. 1.What angle of slope does this require? 3.Why do particles not travel back at the same angle as they heave? 4.Describe how biological activity may enhance the activity. 5. Describe what terracettes look like. 6.Describe four results of soil creep.

2. Heave is the upward motion of particles. 1. Angles over 6 degrees are susceptible. 3. Gravity (and the effects of contraction) are the dominant forces here. 4. Plant roots and burrowing animals help the soil to move. 5. Terracettes are small-scale ridges parallel to the slope. 6. See the next slide for the answer.

6. a). Fence and telegraph posts lean down-slope as their supports are undermined by the creep. ( note these are the same effect) b). Soil piles up behind walls, creating deeper pockets, pushing walls over in time. ( note there are two effects here!) c). Roads may crack open parallel to the slope angle, and will need frequent repair. d). Terracettes spoil the smooth surface of fields, and affect land use.

Terracettes can be seen to the right of the river valley.

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