MASS MOVEMENTS(MASS WASTING)

MASS WASTING:
Mass wasting (also called mass movement), is the process whereby weathered material is moved a relatively short distance downslope under the direct influence of gravity.
 It is a collective term for the downslope transport of surface materials in direct response to gravity.
Mass movements are aided by gravity and no geomorphic agent like running water, glaciers, wind, waves and currents participate in the process of mass movements. That means mass movements do not come under erosion though there is a shift of materials from one place to another.

Gravity is the principal force responsible for mass wasting, but water is often a contributing factor.
Water contributes to weathering,

•  which prepares rock material for mass movement,
• adds weight to porous materials on a slope, 
• decreases the strength of unconsolidated slope material
•  and can increase the slope angle. 

Water is involved in many weathering processes that break and weaken rocks, making them more susceptible to mass movement.
The movement may be gradual or sudden, depending on the gradient of the slope, the weight of the weathered debris and whether there is any lubricating moisture supplied by rain water.

Mass movement is dominant where there are weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rains and scarcity of vegetation etc., favors mass movements.

Factors Influencing Mass Wasting:

1. Removal of support from below to materials above through natural or artificial means; where material become loosen due to various natural or cultural factors they are easily subjected to down slope movement as the lack base to hold them. 
2. Increase in gradient and height of slopes; where the slope is steep or vertically like cliff slope mass movement is rapid and where materials are large in size such as rock blocks fast or rapid mass movement takes place. 
3. Overloading through addition of materials naturally or by artificial filling; If additional material accumulates on a debris pile lying on a slope that is near the angle of repose, the newly added material may upset the balance (because the added weight overcomes the friction force that is keeping the pile from sliding) and may cause all or part of the material to slide downward.
4. Overloading due to heavy rainfall, saturation and lubrication of slope materials (Water): water performs for main activities which engineer mass movement. First additional of water acceralate weathering of rocks which provide raw material for mass movement. Second water adds weight of weathered materials on a slope. Also water decreases the strength of unconsolidated slope material, and lastly water increase the slope angle. For these reasons, mass wasting is particularly likely to occur during and after heavy rainfall, snowmelt or subsurface flow.
5. Occurrence of earthquakes, explosions or machinery; earthquakes , explosions during mining activities or war, tend to shaken the land and up sate the balance which in return forcing unconsolidated material to be set in motion down the slope as mass movement.
6. Removal of natural vegetation. Where there is severe removal of vegetation mass movement is likely to occur. Where there is vegetation, plant roots holds material together, on other hand vegetation provides a protective cover. But severe removal of vegetation makes weathered material prone to mass movement.
7. Clay: is another facilitator of mass wasting. Clays readily absorb water; this absorbed water combined with the fine-grained texture of the material, makes clay a very slippery and mobile substance. Any material resting on clay can often be set in motion by rainfall or an earthquake shock, even on very gentle slopes. Indeed, some clay formations are called quick clays because they spontaneously change from a relatively solid mass to a near-liquid condition as the result of a sudden disturbance or shock.
8. Permafrost: In subarctic regions and at high latitudes, mass wasting is often initiated by the uprooting action of frozen groundwater. The presence of thawed, water-saturated ground in summer overlying permanently frozen subsoil (permafrost) also contributes to mass wasting in such regions. Some geomorphologists assert that, in the subarctic, mass wasting is the single most important means of transport of weathered material.

CLASSIFICATION OF MASS WASTING.

Physical geographers classify mass wasting events according to the kinds of Earth materials involved and the ways in which they move. Mass wasting according to the way they move is classified into slow mass movement and rapid mass movement.

 With regard to the earth materials moved there is fine materials such as mudflow, medium materials such as soil creep while large materials includes likes of avalanches and rock fall.

Generally mass movements are categorized into fast and slow mass movement as materials moved being large or medium can be moved fast or slow.

SLOW MASS MOVEMENTS.

Soil creep: is a slow and gradual continuous movement of soil downhill slopes. The movement is gradual to the extent it is not very noticeable especially where the slope is fairly gentle or where the soil is well covered with grass or other vegetation.
 It is common in damp soil where the water acts as lubricants such that individual soil particles move over each other and over underlying rock.
 This movement also occur where there is overgrazing in such a way that continuous trampling of by herds of cattle on the slopes sets up vibrations which loosen soil and cause it to move.  Where there is soil creep it is evidenced by tilts of trees, fences, posts. Soil is also seen to accumulate at the foot of slope or behind obstacles such as walls.

Talus creep, rock creep, rock-glacier creep: the slow migration of particles to successively lower elevations. The movement is similar in occurrence as soil creep and both a collectively referred as creep. The only difference is on type and size of particles moved. Creep is so gradual that it is visually imperceptible; the rate of movement is usually less than a few centimeters per year. Yet creep is the most widespread and persistent form of mass wasting because it affects nearly all slopes where there are weathered materials at the surface.

Solifluction (soil flow); refers to the relatively slow downslope movement of water-saturated soil and/or regolith. Solifluction is most common in high-latitude or high-elevation tundra regions that have permafrost, a subsurface layer of permanently frozen ground.
Above the permafrost layer lies the active layer, which freezes during winter and thaws during summer. During the summer thaw of the centimeters- to meters-thick active layer, the permanently frozen ground beneath it prevents downward percolation of melted soil water.
 As a result, the active layer becomes a heavy, water-saturated soil mass that, even on a gentle incline, sags slowly downslope by the force of gravity until the next surface freeze arrives. Evidence of Solifluction exists in many tundra landscapes. It consists of irregular lobes of soil that produce hummocky terrain or mounds.

RAPID MASS MOVEMENT.

1. Earthflow.it is mostly dominant movement in humid region where by presence of precipitation makes materials on the surface getting saturated with water causing them to flow downhill under the influence of gravity. It is dominant in hillsides and terraces. Occurrence of earth flow results into leaving behind a shallow scar at the place where materials have been removed. Where materials deposits they form bench like terraces.
2. Mudflow: Is faster downslope movement of fine saturated grains of soil, usually moving in a specified channel partly or whole of its movement. They can be moving within gullies or canyon stream channel. They are the outcome of torrential rainfall or snowmelt along the hill especially where there is little or absence of vegetation. The movement is triggered by water saturating the debris causing saturated debris to be set in motion. The movement is also dominant in volcanic slope of recently erupted volcano where volcanic ashes, dusts and other fragments turn into muds and start to move down the slope stream of muds or tongues creating greater impact. This volcanic mudflow is called LAHAR.
3. Land slide or land slip: is the rapid movement of large mass of earth’s and rocks down a hill or mountain side. Where it occurs it is accompanied by little or no flows of water. Rain water from heavy rain act as a lubricant that facilitate movement of materials. The common forms of land slides are slump, debris slide, rock slide, and rock fall.
4. Rock fall is rapid falling down of large blocks of rocks which occur in area of extensive outcrop rocks on steep hill slope or canyon walls. It involves individual rocks or boulders that fall or roll freely from a steep slope or vertical slope like hill. The size of rock fall varies depending on manner in which rocks disintegrate. In glaciated area thawing of ice makes the rocks to lose its compaction with the land hence provide a room for gravity to overcome rock resistance.
5. Debris fall: it is common form of rapid mass wasting occurring on a cliff where by loose materials on top of a cliff or any slope which is almost vertical fall freely to the base of the slope. 
6. Rock slide: is the downward movement of rocks in a sliding manner on a gentle sloping area. In this movement water act as a lubricant.
7. Slump: is a form of landslide which involves the tearing away of rock materials along a concave plane.it occurs in a steep slope where massive sedimentary strata usually sandstone or limestone overlie weak rocks like clay or shale.
8. Avalanche: is the suddenly sliding and falling of a large mass of snow, ice and loose rock materials down a mountain side. The movement is dominant in temperate regions during winter or spring season. Accumulated quantities of ice usually break away and slide over hardened snow surface. During spring the thawing of masses of snow may also result in ice sliding downhill. Sliding materials are roll as they move downhill.