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

1. Mass Movement
Rock Falls, Avalanches, Soil Creep, Landslides, Mudslides….

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. Triggers of Mass Movement

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. 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. Mechanism of Soil Creep

7. Soil Creep - effects

8. Soil Creep - effects

9. Soil Creep - effects
Trees and Poles bend, ripples appear on
hillsides

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. Solifluction

12. Solifluction lobes
Lobes

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. 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
http://www.youtube.com/watch?v=KP_Zug2D-tY

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. Lahar Deposits, Guatemala

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. Landslides
http://www.youtube.com/watch?v=F31ywRvtNkY&feat
ure=related
http://www.youtube.com/watch?v=AiczROnfGbc&fea
ture=related

19. Rotational Slip

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
http://www.youtube.com/watch?v=PyzKtNZBZjc&feature=rela
ted

21. Rockfall

22. Rockfall (on Mars)
Talus/Scree
Freshly exposed cliff
Dust Cloud Shows this
is dry mass movement

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. Derrybrien wind farm

25. Derrybrien wind farm

26. Wind Turbine Foundation

27. Damage after bog burst

28. Damage after bog burst

29. Classifying Mass Movement

30. Mechanisms of Mass Movement

31. Mass Movement caused by...

32. ...intensive agriculture
(overcropping)
Gullies

33. How to prevent mass movement?

34. How to prevent mass movement?
5 well developed points on how to stop or limit undesirable
mass movement.
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