Types of Mass Movements

Types of Mass Movements

The facilitators of mass movements discussed above determine the types of movement. Another factor which is the size of the individual particles also determines the types of movement. There is also the factor of nature of slope, whether gentle or steep slope is another determining factor. There are two distinguish types of mass movements which are identified based on the rate of movement. These are:

Creep
This is a slow movement of soil and regolith downslope. This type of
movement takes place on gentle slopes. Material of very small size are involved in this movement. It is called creep because the spaces created by the moving particles are so minute that the grains are wedging one another, as they move. Such material is usually soil or talus. Other conditions that lead to the creep type of movement may be heating and cooling of soil, ground ice, trampling by bigger animals and burrowing by smaller animals. These lead to disturbance of material and the underlying bedrock. Gravity then exerts a downward pull urging the particles to move progressively along the slope. There may be absence of moisture in the ease of larger particles as rocks which can embark on mass movements.

Creep is such a slow movement that is hardly noticed. The rate of movement is usually less than a centimeter per year and can be detected only by careful observations which reveal tilted posts, and fenced as well as bent and broken walls. Creep also affects the rock fragments in a talus slope as it causes the angle of the talus surface to become flatter gradually.

Earth flow in humid regions, a mass of saturated soil, weak clay, overburden or shale layers may slide downslope as earthflow.

This may affect just a few square metres. It moves by plastic flowage of thick mud. Effects of earthflow are that it can cause blockage of highways and rail-lines during period of heavy rainfall, though it does not purse any threat to life.

Mudflow forms part of mass movement but is so slow in movement. It is usually quoted as a spectacular form of mass movement that is environmentally hazardous, particularly in desert areas where vegetation does not protect the soil and where convective storms produce rain much faster. This ensures more surface water than the soil can absorb. The water runs down the slope forming a thin mud which flows down into streams, and continues to flow until it becomes so thickened that it must stop by itself. The great boulders that are buoyed up in the mud are carried along.
Mudflow also occurs on slopes of erupting volcanoes because the freshly fallen volcanic ash and dust is turned into mud by heavy rains. This flows down the slopes of the volcano.

Landslides and Slump
This is the most rapid movement (sliding) of large masses of rock particles under the influence of gravity caused mostly by the presence of moisture in the regolith. The roles of moisture are that it increases the weight of the over burden, reduces cohesion of the particles and smoothing the surface upon which the regolith move down slope through reduction of friction because it serves as a lubricant. All these result in shearing along weak planes slope (relief). The water as well enhances the type and rate of the movement of overburden.

Causes of Landslides
Apart from the various roles of moisture in motivating landslide, there are other factors that cause landslide. These include the geologic structure. Where sandy particles are underlain by an impermeable layer, such as clay, the surface of this impermeable layer easily becomes slippery once lubricated by moisture. The geologic structure (arrangement) is responsible for landslide occurrence where such arrangement forms a slope.

Earth tremor and earthquakes occurring in areas where rock with sedding planes is steeply dipping also causes landslide. Due to the vigorous shaking of the bedrock, the overlying layers are forced to move down the steeply dipping slope as landslide.

Types of Landslides
There are two types of landslides. This classification is based on many factors such as the characteristic of the movement; the types of slope involved, that is, whether very steep, moderately steep or gentle; the rate i.e. the velocity of movement; the distance covered by the moving material in the course of movement and the nature of the mass shippage. The two types are rock (debris), slide and slump. A mass movement is described as rock (debris) slide if the bedrock mass slips on relatively flat inclined rock-plane such as a fall or bedding plane. The movement is termed “Slump” if there is backward rotation on a curved up-concave slip plane. Arthur, N.S and Alan, H.S (1985) and Faniran, A. and Ojo, O. (1980) both describe debris slide as large masses of rock that slide downhill along a sloping joints or bedding surface. On the 1st of September 1969, Nigeria had the experience of a rock (debris) slide, when a granite boulder of about 200-300 tones slides over a distance of 80m during a heavy rainfall. Similarly, in 1980 a granite boulder cut from the regolith and roted into a compound and caused damage to a dwelling man, the slump usually takes place as a small independent movement and the particles assemble to form features that are commonly referred to as step-like terracettes or catsteps.

Figure 3:3: Slide and slump

Where a slope is vertical, rock debris fall rather than the slides. The material may be large enough to block roads, cause darns across rivers and burry areas including settlements, thereby causing damage to property and
even loss of life.