This documents covers mass wasting from the CSEC Geography syllabus. It looks at both soil creep and landslides.

1. CSEC GEOGRAPHY
MASS WASTING
O.JOHNSON
Mass wasting (also known as mass movement) refers to the movement of weathered material
downslope because of the influence of gravity. Two of the major form of mass wasting includes
soil creep and landslide
Landslide
A landslide is a sudden movement of rock or soil downslope under the influence of gravity. They
can have devastating consequences on the environment and all who inhabit it. Landslides can
block roads and railways, destroy buildings or even bury entire villages. All slopes are affected
by gravity, so loose material will eventually slip or fall. On gentler slopes, around 5 degrees, soil
creep operates which is so slow that is has relatively little effect on human life, property or
activity. Landslides are natural events and would occur without people, but human activities do
increase the risks, scale and frequency of these hazards.
Physical causes of landslides
Unconsolidated material on the slope
In 1998 the rains from Hurricane Mitch caused devastating mudflows from the unconsolidated
ash slopes of the unconsolidated ash slopes of the Caista volcano which killed 1900 people and
destroyed the town of Posoltega in Nicaragua
On steep slopes
All slopes are affected by gravity, so loose material will eventually slip or fall. The steeper the
slope the likelihood of experiencing a landslide increases. Landslides usually occur on steeper
slopes while soil creep occur on gentler slopes.
Type of rocks
Shake and clay are both slippery, especially when wet. Wet clay acts as a lubricant so rock beds
above it slip down more easily. Shale is made of several very thin layers, so it slides easily,
taking any rocks above with it

2. The block of limestone slides on clay as it becomes slippery when wet
Angle of bedding planes
Bedding planes being roughly parallel to the slope surface makes it easy for material to slip
downward along the bedding planes. Gravity can exert its force easily on the rock beds. If the
underlying rock layers are impermeable a rock is more susceptible to slide. This keeps all the
moisture in the top layer of rock so it becomes saturated quickly.
Erosion of the base of a slope by rivers or ocean waves (Basal cutting)
Slopes on a coast are subjected to coastal erosion. Erosion takes place at the base of the slope.
The base provides stability or support. If the slope above is made of unconsolidated or very wet
material and this becomes unsupported it is much more likely to collapse. When a river valley
erodes it produce a similar effect.

3. Precipitation
Heavy rains can cause slopes to become saturated which further weakens the slope. When the
material becomes saturated it becomes heavier and friction is reduce. When this happens a
landslide can take place. This is why so many slides occur after heavy and short intense period of
rainfalls especially in the tropics from tropical storms or hurricanes. Water does two things:
 It adds weight to the material, making the slope less stable
 It decreases friction which helps movement downslope
The torrential rain from hurricane Mitch in October 1998 caused flooding and landslides which
affected 3 million people in the Caribbean side of Nicaragua and Honduras. .
Volcanic eruptions
When a volcano erupts it ejects several types of materials. These include materials such as ash,
tephra and bombs. When the ejected material especially ash is mixed with water/ice slurry like
liquid is formed. These are term lahars. Perhaps the best known example is the one that occurred
in Colombia in 1985, when 21 000 of the
22000 population of Armero died following the eruption of the volcano Nevado Del Ruiz.
Earthquakes

4. The vibrations from an earthquake can destabilize slopes. The vibrations and shaking can cause
the soil to lose strength and may cause an unstable slope to collapse. Earthquakes of magnitude
4.0 and greater have been known to trigger landslides
In the 1970s a powerful earthquake caused the partial collapse of Huascaran mountain in Peru.
The avalanche of rock and ice, travelling at speeds in excess of 300km/h destroyed the town of
Yungay in the Rio Santa Valley and killed some 20, 000 persons.
Landslides are also influenced by human factors
Building on unstable slopes
It is normal for a human being to build his house on a terrain without knowing the geology. At
times they may build their houses on unstable slopes. The weight of the building adds pressure to
already unstable slope. Over time the increase pressure my result in a landslide.
In December 1999 there were hundreds of landslides in and around Caracas the capital of
Venezuela. These major causes were heavy rain and by the activity of humans. The other two
causes involved human factors: steep sided valleys with unstable slopes had been used for high
rise buildings and vegetation had been removed to make way for these. This disaster left 30000
dead and 200 000 homeless.
Removal of Trees/ Deforestation
Human activities, such as deforestation, can make landslides more likely. In a forest, tree roots
help to bind the soil together. When trees are removed the soil become exposed and there are no
forest trees to hold the soil together. Any intense rainfall can easily saturate the soil making it
heavier and unstable. Deforested slopes are therefore prone to landslides.
Undercutting the base of a slope by road building
When humans undercut the base of a road to facilitate road building the slope is left unsupported,
the materials above it will collapse. It has the same effect as physical erosion.
Dumping waste material
Waste from activities such as coal mining can be difficult to dispose of. In the village of Aberfan
in South Wales, UK, the material was piled up on the slopes of 25 degrees above the village. A

5. wet autumn in 1966 saturated the coal tip. The tip collapsed on the morning of 21 October,
engulfing the local junior school, killing 116 children and 5 teachers. This was one of the worst
disasters in the UK in the 20th
century.
Building Dams
One of the worst disasters of the 20th
century in Europe was in the north-east Italy in 1963. The
Vaoiant dam had been built across the narrow, steep-sided Piave valley. The rock beds of
alternate limestone and clay sloped towards the reservoir. The pressure of the weight of water in
the reservoir caused a small earthquake. A block of limestone slipped on the clay beneath, falling
into the water and forcing a 100 metre wave over the dam. Within seven minutes it hit three
small towns including Longarone. The speed of the disaster and the fact it happened at night
caused the high death toll of 3000. This could have been avoided through better planning and
quicker reaction to earlier ground tremors.
Soil Creep

6. Soil creep is the slow, gradual movement of soil and rock particles down a slope under the
influence of gravity. This is the slowest of downhill movements and is difficult to measure as it
takes places at a rate of less than 1cm a year. However unlike faster movements, it is an almost
continuous process. Soil creep occurs mainly in humid climates where there is a vegetation
cover. More materials end up at the bottom of a slope by this process than in any other way. It
can take place on gradients as slight as 2 degrees but more usually on those of over 5 degrees.
There are two major causes of creep both resulted from repeated expansion and contraction.
Freeze-thaw when regolith (weathered material) freezes, the presence of ice crystals increases
the volume of the soil by 9 percent. As the soil expands, particles are lifted at right angles to the
slope in a process called heave. Sediments can expand when they freeze get wet or heated up in
the sun. When the ground later thaws and the regolith contracts (shrinks), the particles fall back
vertically under the influence of gravity and so move down slope. Creep takes a long time
because of each particle only move a millimeter to a few centimeters at a time
Wetting and Drying -Wet dry periods during times of heavy rainfall, moisture increases the
volume and weight of the soil, causing expansion and allowing the regolith to move downhill
under gravity. In a subsequent dry period, the soil will dry out and then contract.

7. Evidences of soil creep
 There are many indicators that show a soil creep is happening. The clearest one is the
formation of terracettes. These are step like features, often 20-50 cm in height which
develops as the vegetation is stretched and torn.
 Trees also clearly show the effects of soil creep. As they slip gradually down a slope
they try to grow vertically as before, resulting in the bending of the trunk.
 Tilting of utility poles along the slope
 Build up accumulation of soil behind walls built along the slope.
 Walls bulge or break

8. Conditions influencing soil creep
1. Climatic environment
In temperate environments soil can creep downhill at between 1 and 2 mm per year, but in
tropical regions it is quicker , perhaps 3-6 mm . In cold semi – arid areas it is even faster.