What are the factors that influence mass movement?

Once rock material has been broken down into smaller, unstable pieces by weathering, the material has the potential to move down slope called mass wasting (also called a mass movement or a landslide) because of gravity.  Before looking into the various types of landslides, the factors that influence them must be examined. Ultimately, this comes down to the dynamics of two factors: gravity and steepness of slope. The steeper the slope, the greater potential for gravity to pull objects down. The shallower the slope, the less effect gravity has on pulling objects down.

There are several factors that influence mass wasting, but ultimately it is a battle between friction and gravity. If the friction on a rock is stronger than gravity for a particular slope, the rock material will likely stay. But if gravity is stronger, the slope will fail.The steeper the slope, the stronger the friction or rock strength must be to resist down slope motion. The steepest angle a slope can be before the ground will slide is about 35 degrees, called the angle of repose. Many times we will cut through a slope to make room for a road or other forms of development. So to help prevent the slope from sliding along these cut areas, retaining walls must be build. More on this later.

Composition of Slope Material

Another factor that determines mass wasting is the slope’s material. Mass wasting is more prone on slopes that contain clay and shale. Without going into great detail here, the shape and composition of individual clay particles can absorb water and prevent water from peculating through the ground. A layer of clay on a slope can prevent water from filtering through the slope. Instead, the water stays near the surface and saturates the ground. This can cause the surface layers to lose friction and slide.

Weight and Friction of Slope

A third factor that influences whether a slope will fail is the load or weight of that slope. Adding weight to a weakened slope can obviously cause it to slide easier, especially on steep slopes. This added weight tends to occur by building on top of weak slopes, increasing the steepness of the slope, or over-saturating the slope.Friction has been mentioned as a factor several times already, but there are a few more things must be said here. As already noted, as long as the friction along the slope is stronger than gravity, the ground is unlikely to slide. But if that friction is weakened, slope fail becomes more likely. There are several other ways friction can be reduced along a slope: wildfires, removal of vegetation, or adding too much water.Gravity is probably the ultimate driving force of mass wasting. The force of gravity pulls all things on the planet toward the center of the Earth. Without gravity, mass wasting would not occur. But unlike many of the other factors, humans have no influence or control on gravity.

Regional Climate Conditions

A region’s climate can also determine the likelihood of a landslide. Climate is based on temperature and precipitation. Mass wasting is prone in the spring-time when snow melt, water saturation, and runoff is greatest. Also the type of climate will help determine the type of mass wasting. Humid climates tend to have slides, where water-saturated slopes fail and fall. Drier climates tend to have rocks that fall; especially early spring. Canyons and places prone to wildfires tend to have debris flows.

Water Content within Slopes

The amount of water in the soil is a major factor in the stability of a slope. When you build a sand castle, water is needed to build the walls and towers. That is because water has surface tension and is attracted to each other. This allows you to build towers greater than the angle of repose. So a little water can actually prevent slopes from sliding. But too much water lubricates the individual grains of sediment decreasing friction between each grain, so the possibility of mass wasting increases. The increase of water within the soils can come from over watering, pipe or swimming pool leaks, or prolonged stormy weather. In Utah and many mountainous regions, spring runoff of snow melt increases the water content within the soil.

Gravity

Finally, gravity is the driving force of mass wasting. The force of gravity pulls all things on the planet toward the center of the Earth. But unlike many of the other factors, humans have no influence or control on gravity. For more information on what causes landslides in Utah, click here.

meaning of mass wasting in geography

Mass wasting is the downslope movement of rock materials due to the influence of gravity.

When such movement occurs after the materials have been lubricated by rainwater from melting snow it is referred to as MASS MOVEMENT.

The force involved in this process is gravity but it is only able to exert its influence when the materials overcome their initial resistance to movement.

This initial resistance is caused by the friction between the materials and the underlying rock mass.

Water plays an important role in helping the materials overcome this resistance.

Most of the movements are slow amounting to a few centimeters per year but sometimes they can be sudden and rapid. 

7 factors that affect or influence nature and speed of movement in mass wasting;

The nature and weight of materials.

If the layer of the weathered rock is very deep or thinly bedded, it will result in rapid mass wasting.

Thin beds increase the tendency for movement as there are more bedding planes over which movement can occur.

Massive rock overlying weak rocks such as clay or shale can slide more easily than if they were overlying sand.

Large rocks are likely to be overcome by gravity more easily than fine weathered materials

Amount of water.

The more saturated the material is the more likely it is to move. A mass of materials saturated with water moves more easily than a dry mass.

This is because water increases the weight of the materials and at the same time reduces the cohesion between particles of materials between the mass.

Water also acts as a lubricant along the bedding plane thus facilitating movements.  

The angle of the slope.

The steeper the slope, the faster the movement. This is because gravity becomes stronger with increasing the angle of the slope.

On a gentle slope, the movement is slower but is the slowest on a plain which is almost flat.

The climate of the area.

The amount and nature of rainfall received in the area determine the amount of movement that will occur.

The area which receives heavy rainfall experiences massive landslides, especially where the slope is steep.

Light rain penetrates slowly into materials and may take a long to saturate it or may not saturate it at all.

The resultant movement of the materials will therefore be slow. Alternate freezing and thawing encourage mass wasting as well.

In a dry climate, materials may be loose but they lack the added advantage of the water. the resultant movement is therefore likely to be slow

Vegetation covers the area.

Plants such as grasses, shrubs, and large trees, help to hold rock materials together, thus reducing their movement on the earth’s surface.

Bare surfaces are more likely to experience mass wasting than surfaces that have vegetation cover.

The presence of dense vegetation cover in the wet regions increases the rate of water intake (infiltration) into the soil and to the rock beneath the surface.

This can speed up the saturation of the rock materials and trigger off mass wasting.

Human activities.

People affect the stability of the earth’s surface through various activities such as cultivation, building, grazing animals, mining, clearing vegetation, and road construction.

Some of these activities result in the direct movement of materials, while others create favorable conditions for other factors to exert their influence.

Vibration from moving trains and vehicles as well as tremors caused by explosions may shake the ground causing some materials to move downslope.

Tectonic movement.

Earthquakes and volcanic eruptions cause the vibration of the earth which often trigger off the widespread movement of the materials such as landslides.

  The factors discussed above do not work in isolation. One factor can facilitate the effect of another factor and movement accelerated when they operate in combination.

For example, heavy rainfall which is falling on deeply weathered rock materials on the steep slope may cause massive landslides even if the surface is covered in thick vegetation.

Here, vegetation increases the rate of water intake into the rock materials beneath, causing the soil to be saturated

types of mass wasting

Slow Mass Wasting

this is Slow but steady movement of soil or loose rock debris down slope

Cause

• Alternate heating and cooling causing expansion and contraction of particles causing them to change their positions.
• Alternate wetting and drying of soil whereby when it‟s wet its compact and when dry the particles are loosened and tend to move away from each other.
• Trampling and burrowing of animals.
• External forces e.g. shaking by earthquakes, explosives, heavy vehicles, etc.
• Ploughing down hill
• Freezing of soil water causing it to expand which lifts particles at right angles to the slope in a process called heaving.

solifluction

Movement of saturated soil, gravel and weathered rock down a moderate slope.Common in mountainous and very cold climates

process of solifluction

talus creep

Slow and gentle movement of the mass of broken rock particles which accumulate at the base of cliffs (scree) downhill.

rock creep

Slow movement of individual rocks which lie on clay at a very low speed down slope in the presence of moisture

rapid mass wasting

Type of mass wasting involving large amounts of weathered material moving suddenly and fast down the slope

Mud Flow

Movement of oversaturated weathered material informs of liquid downslope.It occurs mainly in dry areas after heavy rains.

Earth Flow

Slump

o Erosion occurs on the weak rocks at the base of a cliff undercutting the weak rock.

o The overlying rocks break off causing the overlying rocks to slide down hill rotating around a curved plane.

Debris Slide

Sudden downhill movement of accumulated rock debris and other loose material downhill as a whole

Debris fall

Sudden free fall of debris from a vertical or hanging cliff to the base of the slope.

Rock Slide

Sliding down of masses of rock a steep slope along a bending plane, joint of fault.

Rock fall

Falling or rolling of individual rocks or boulders down a steep slope or a cliff.Most rapid of all mass wasting.

Avalanche

Sudden slipping and falling of a large mass of snow, ice, and loose rock materials down a mountainside

minimizing mass wasting hazards

all forms of mass wasting can be very destructive

the hazard of mass wasting tend to be predictable and largely avoidable

evidence of past events help identify areas prone to slope failure

mass wasting and its effects can be minimized through engineering which involves:

• reducing slope
• increasing drainage
• constructing barriers

READ MORE