MASS WASTING / MASS MOVEMENT - ADVANCED PHYSICAL GEOGRAPHY

MASS WASTING / MASS MOVEMENT – ADVANCED PHYSICAL GEOGRAPHY

Refer to the down movement of rock, regolith, and soil under the direct influence of gravity.

Mass movement refers to the creeping, flowing, sliding and falling of regolith and weathered materials down a slope under direct influence of gravity.

Mass movement does not require transporting medium such as water glacier, etc. It greatly influenced by addition of water or snow melt which act as a lubricant agent and reduce resistance of materials. it also adds to the materials a significance weight which lead them to move down a slope.

In the evolution of most landforms, mass wasting is the step that follow after weathering process i.e it is secondary of denudation after rock weathering.

Mass wasting is a disastrous process leading death to people, destroying properties, etc. Example at Kikuyu Mbeya in Tanzania 1955 after heavy rains, in 1968 it occurred in North of Kumasi in Ghana.

CLASSIFICATION (TYPES) OF MASS WASTING

Classification of mass wasting is very complex, generally there are different types of mass wasting according to different criteria like as follow;

i. Types of mass wasting according to the kinds of materials involved.

ii. Types of mass wasting according to the types of motion (movement).

iii. Types of mass wasting according to the rate of movement.

I: TYPES OF MASS WASTING ACCORDING TO THE KINDS OF MATERIALS INVOLVED

It depends upon weather the descending materials are bed rock or unconsolidated in nature. If solid and regolith dominate the terms debris, mud, or earth can be used their description. Example: Mud flow, Earth flow, etc.

On the other hand, when a mass of bed rock breaks and become lose, the term rock may be used in their description. Example: rock, etc. In general, according this type mass wasting can be;

Mud flow.
Earth flow.
Rock fall, etc.

II: TYPES OF MASS WASTING ACCORDING TO THE TYPE OF MOTION

Generally, kind of motion can be described as either fall, slide, or flow etc based on the way the materials move down a slope.

In this case there are;

Falling movement.
Sliding movement.
Flowing movement, etc.

III: TYPES OF MASS WASTING ACCORDING TO THE RATE OF MOVEMENT

The velocity or state of movement of the materials down a slope can be either slower or faster depending on the slope and materials involved.

In reality, saturated materials on a steep slope move rapidly than less saturated materials on a very gently slope.

Therefore, based on this category mass wasting process can be divided into two (2) major types.

These are;

Slow mass wasting (Gradually) eg. soil creep, solifluction, etc.
Rapid mass wasting (Suddenly) eg. rock fall, avalanche, etc.

GENERAL TYPES OF MASS WASTING

In general, mass wasting can be categorised into two (2) major types depending on speed of movement, amount of moisture, angle of slope, and nature of materials. These are;

Slow mass wasting (Gradually).
Rapid mass wasting (Suddenly).

1: SLOW MOVEMENT OF MASS WASTING

These are the processes where the downhill movement of materials are at minimal speed. It involves gradual processes. This involves the following;

a. Soil creep:

This is very slow down slope movement of soil and fine materials on a very gently slope. The ideal condition of movement includes alternate drying and melting, or alternate heating and cooling of the land. In soil creep the hard pan layer beneath the soil may act as a plane of movement. It is indicated by bend of tree trunks, bend of fences, bend (bulges) of electricity poles, etc.

b. Talus creep:

Is the individual rock blocks may move slowly down a slope. Is occurs commonly where individual rock block are lying over clay materials. in the presence of moisture, the clay surface becomes slightly slippery. The rock blocks may creep slowly down slope under the influence of gravity.

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c. Rock creep:

This is a slow or moderate to high movement of angular wastes of rocks of all size on a moderate slope in the mountaineous areas where freeze-thaw process frequently. The talus creep (sheet) move as a mass down a slope.

d. Solifluction (slugging):

This is the movement of a mixture of soil, gravel and weathered rock down a moderate slope. It is limited surface layer to flow over underlying frozen ground. Because during the summer, the surface layer melt (thawing) faster compared to the underlying layer which remain frozen to be impermeable.

2: RAPID MOVEMENT OF MASS WASTING

These are the processes which involve downhill movement of materials at a greater speed. It occurs at a steeper slope of the ground. This involves the following;

a. Earth flow:

Is the flow movement of materials saturated with water down the hill slope or in terraces. This movement is common in the areas where there is high humidity. Shallow scar can be formed on the hill slide as a result of this movement.

b. Mud flow:

Is the movement of large volume of unconsolidated materials, which are supersaturated with water. It involves a higher slope especially after periods of intensive rainfall, when both volume and weight are added to the soil giving it a higher water content than in earth flow. In volcanic area there are special type of mud flow called lahar.

c. Rock slumping (slipping):

The sliding down of a large block of land without necessarily breaking up into many parts. It may occur where soft rock (sand\clay) overlying on a more resistant rock like limestone or granite. These occur along the curved slope or road cutting. This movement cause occurrence of crescent scar.

d. Rock slide:

Is the sliding movement of the slab of rock down steep slope, such as cliff face or road cutting. It can be triggered off by earth quake, or human activities like mining or cultivation.

e. Block slide:

This is the fast movement of large rock blocks down a moderate to steep slope. This occurs when water lubricates the land surface made by thick layer of the hard rocks, underlying weaker layers. The piece of rock slides down slope as individual block very fast.

f. Rock fall:

It is falling movement of individual rock particles down hill at a very high speed. It may occur along the coast(cliff) as a result of wave action. However, earth quake can also induce rock fall.

g. Avalanche:

It is a sudden down hill movement of snow, ice or rock. It occurs due to the increase in weight causing it to slide rapidly down the mountain slope, carrying with it tons of rock. It is very common in cold climates and mountains areas where snow occur.

FACTORS INFLUENCING THE NATURE AND SPEED (RATE) OF MASS WASTING

Generally, any factors or contributing factors that increase the gravity of materials decreasing stability of the slope or reducing friction of materials will promote movement. Therefore, there are many factors influencing the nature (type) and rate (speed) of mass wasting (movement). These are: –

1. The nature and weight of materials:

Generally, loose materials very faster than consolidated materials. in reality, when hard rock overlies like clay and shale materials. there will occur slumping. The less movement process such as creeping very often take place when materials are high consolidated or where the rocks are very high compacted.

2. The extent of saturation (amount of water in the materials)

Heavy rains or alternate freezing and thawing encourage rapid mass wasting such as mudflow, lahar, avalanche, etc. water is very important factor in mass wasting as it overcome resistance of materials and aids gravity. In general, where materials are fine and well saturated there occur flowing process eg mud flow.

3. Gradient (angle and slope):

The steeper gradient the faster the rate of mass movement and the lower the slope the slower the rate of mass movement. Generally, on very gentle slopes the process of movement is very slow eg. soil creep and solifluction, while on steep slope there are occurrence of rapid mass wasting process like rock fall and avalanche.

4. Climate:

Wet climate such as equatorial and monsoon climates where the rain is high are characterised by heaving rapid movement of mass wasting. Example mud flow, land slide, etc.

In cold climates and mountaineous climate where freeze and thaw processes are common there occur avalanche, talus creep and solifluction processes.

5. Vegetation:

When the area has little or no vegetation to hold the materials firmly, it could be affected by fast mass wasting processes like land slide than in areas covered by vegetation. Basically, vegetation cover inhibits any movement of materials.

6. Natural catastrophes:

Natural hazards like tectonic activities such as volcanoes, and earth quakes often trigger mass movement processes like avalanche, lahar, rock fall and land slide.

7. Human activities:

Activities like mining, cultivation, bombing, road construction, etc. can be cause mass movement processes of various velocity and types.

EFFECTS OF MASS WASTING

The following are the effects of mass wasting. These are;

1. It leads formation of fertile soil: The materials which are moved down slope tend to accumulate at the foot of the hills or mountains. Thereafter, they form fertile soils which did not exist before. The accumulated fertile soils can support agricultural activities.

2. Mass wasting can destroy human properties and killing large number of peoples. Mass wasting can destroy buildings, means of transport, and lines of communication and can even cause loss of life. Eg. in 1985 mass wasting disaster in Nyeri District in Kenya. iii. Mass wasting accelerate soil erosion and land degradation. Mass wasting leading to the total loss of cover soil hence encouraging soil infertility on upper slopes therefore decline agricultural activities.

3. Process of mass wasting such as slumping may lead to occurrence of permanent scar on the land surface.

4. In some places mass wasting processes such as rock fall, mudflow, and avalanche are source of income generation as they attract of tourism.

5. Source of tourist attraction. Features resulting from mass wasting such as rockfall and avalanches attract tourists. For example, the avalanches of Switzerland attract a large number of tourists which in turn boost the economy of the country. vii. It leads formation of lakes. Landslide materials often end up in valleys where they accumulate and form barriers to flowing rivers. Water of the blocked rivers accumulates on the upstream side of the barriers and eventually forms a lake.

MEASURES OF MASS WASTING

The following are the controlling measures can help to combat problems of mass movement.

These are;

1. Removal of fluid materials on the landscape by using subsurface drainage systems etc.

2. To avoid building houses etc in areas prone to mass wasting processes eg. damp areas on steep slopes at the foot of hills etc.

3. Reforestation and afforestation along the steep slopes so as tree roots may bind soil together to reduce possibilities of mass wasting in form of soil creep or mud flow caused by heavy rainfall.

4. Population control along steep sloped area which would have cut the slope in order to set up residential areas there by increasing the loose material along the slope.

5. Provision of environmental education about the causes, effects and controlling measures of mass wasting.

6. Adaptation of better and modern farming methods along the steep slopes like terracing, contour ploughing and strip cropping. These reduce surface run off therefore reduce the possibility for the materials to flow down slope.

REVISION QUESTIONS

1. (a) Determine the meaning of mass wasting.

(b) Describe the major types of mass wasting

2. What are the factors which influence the occurrence of mass wasting? Then show its four impacts

3. Examine the effects of mass wasting and remedial check against the occurrence of mass wasting.

4. What do you understand by the term mass wasting? Briefly examine the factors affecting the nature and speed of mass wasting (Necta 1986 and Necta 1994)

5. Discuss the conditions that determine the different types of mass wasting (Necta 1997)

6. Discuss the classification of mass movements, what are the controlling factors in mass wasting. (Necta 2000)

MASS WASTING / MASS MOVEMENT – ADVANCED PHYSICAL GEOGRAPHY