In this article we will discuss about:- 1. Meaning and Concept of Mass Movement 2. Classification of Mass Movements 3. Factors.
Meaning and Concept of Mass Movement:
Disintegrated and fragmented rock materials due to mechanism of weathering processes (mechanical, chemical, biotic and biochemical) are called rockwastes. Generally, movement of rock waste enblock down the hillslope is called mass-movement of rockwaste or simply mass movement.
‘Mass movement is the detachment and downslope transport of soil and rock material under the influence of gravity. The sliding or flowing of these materials is due to their position and to gravitational forces, but mass movement is accelerated by the presence of water, ice and air. This definition of mass movement permits consideration of the movement of earth materials at ail scales and at all rates’.
It is evident from the above definition that mass movement includes both, detachment of rock materials and their downslope transport enblock. ‘The collective term for gravitational or downslope movements of weathered rock debris is mass-wasting.
The term implies that gravity is the sole important force and that no transporting medium such as wind, flowing water, ice or molten lava is involved. Although flowing water is excluded from the process by definition, water nevertheless plays an important role in mass-wasting by over steepening slopes through surface erosion at their bases and by generating seepage forces through groundwater flow’.
If we look at the aforesaid two definitions of R.J. Chorley and A.L. Bloom it appears that the term mass movement is more sound and appropriate than mass wasting to describe enblock downslope transport of weathered materials ranging from very fine (soils) to very coarse and large sized rock materials (boulders).
In fact, the definition by R.J. Chorley and others is comprehensive one because it includes both the aspects of mass movements, viz. detachment of rock materials and their downslope transport whereas Bloom’s mass wasting describes only the process of downslope transport of weathered rock debris.
Emphasising the significance of tectonics in mass wasting (may be rock disintegration) and mass movements R.J. Chorley and others (1985) have remarked, that, ‘the relation between mass wasting and tectonics is a relatively clear one. Where rocks are shattered and relief is high, this is where mass movement is common and, in fact, the denudation of high mountains may……….. be the result of mass wasting rather than fluvial or glacial process’.
It is, thus, evident that mass movement of rock wastes includes the mechanisms of detachment of rock materials through different weathering processes, and unblock downslope transport of weathered rock debris by gravity force without any medium of transport (e.g., running water, wind, sea waves, glacier etc.) except some lubricating role of water or ice.
The rocks debris coming through mass movement are deposited at the foot-hill zone as scree or talus. The deposit of large boulders in conical shape is called talus cone. It is, thus, apparent that the most significant stimulating factor of mass movements is gravity force.
Classification of Mass Movement:
A wide range of variations in terms of rate, direction and type of movements is noted in mass movements in different places having varying environmental conditions. It is generally believed that mass movement of rock wastes occurs suddenly and instantaneously and hence all mass movements cannot be witnessed by man.
But in reality mass movements have long preparatory period and there are certain precursor events which herald the occurrence of mass movements but these are generally unnoticed. It may be mentioned that most of mass movements occur in mountainous areas and hence it is not possible to notice the precursor events such as restlessness of animals, deserting of hives by bees etc.
Hence, if a landslide comes as a surprise to eyewitness, it would be more accurate to say that the observers failed to detect the phenomena which preceded the slide. Mass movements are generally classified on the basis of causative factors e.g., rate of movement, direction of movement, type of movement, lubricating substance e.g., water, ice etc.
The direction of mass movement of rock-waste down the slope may be:
(ii) Lateral, and
Based on direction mass movement may be divided into vertical movement, lateral movement and diagonal movement of rockwaste.
Vertical mass movement is further divided into:
(b) Collapse earthfall.
Lateral mass movement includes:
(a) Block slide,
(d) Sackung etc.
Diagonal mass movement is divided into:
(a) Soil creep,
(c) Talus creep,
(e) Debris slide,
(g) Debris flow,
(h) Mud flow,
(i) Solifluction, and
(j) Avalanche etc.
R.J. Chorley (1985) have presented exhaustive classification of mass movement and mass wasting phenomena on the basis of direction of movement, type of movement and presence of transporting agent as given below:
Based on the rate of movement and water content mass movements are classified in 3 types:
(1) Large-scale rapid slide of rock waste. Water is needed as lubricating agent for such type of mass movement. Landslide is the typical example of this type.
(2) Slow flowage of rock waste and weathered debris. Partial saturation of rock debris is required for such mass movement and hence moderate quantity of water is needed as lubricating and stimulating agent. Rock creep, soil creep, solifluction etc., are typical examples of this type.
(3) Rapid flowage of weathered debris. Sufficient quantity of water is needed as lubricant. Earth flow, mudflow etc. are representatives of this type of mass movement.
A generalized classification of mass movement of rock wastes is presented as follows:
Factors of Mass Movement:
Any sort of mass movement of weathered debris with any rate whether on hillslope or valley side slope depends on the ratio between shearing forces (simply known as stress) and resistance of materials to shearing forces (i.e. shearing resistance of materials).
FS = strength or shearing resistance of materials/magnitude of shearing forces
where Fs = factor of safety
When the quotient of shearing resistance of materials (simply strength of materials) and magnitude of shearing force i.e., safety factor (FS) is less than 1.0 (i.e., when magnitude of shearing forces of hillslope or valley side slope exceeds the shearing resistance of materials resting on slopes) materials begin to move downslope and thus mass movement of weathered debris occurs. It is apparent that mass movement may occur when either shearing forces increase or shearing resistance of materials decreases. It may be pointed out that either of the two processes (increase in stress and decrease in resistance of materials to stress) may operate or both the processes may operate together.
Based on this corollary D.J. Varnes (1978) classified the factors which control mass movement of rockwastes in two broad categories and many subcategories:
(1) Factors which increase shearing forces (shear stress), and
(2) Factors which reduce resistance of materials to shear stress.
Recently, man has emerged as a significant factor of mass wasting and mass movement in almost all of the environmental conditions. His activities (e.g., deforestation for commercial wood and increase in agricultural land; construction of roads, dams, reservoirs; urbanization on fragile hillslope; manipulation of rivers, coastal areas etc.) destabilize hillslopes as well as valley side slopes and accelerate the process of mass wasting and mass movement and increase frequency and magnitude of different mechanisms of mass movement.
Increased deforestation, cultivation on cleared hillslope, construction of roads and reservoirs in the Himalayas have made the mountain ecosystem more fragile and vulnerable to increase frequency and magnitude.
Factors which increase the shear strength:
1. Removal of lateral support (undercutting-steepening of slope)
(a) Stream erosion,
(b) glacial erosion,
(c) Marine erosion by sea waves,
(d) Weathering (these factors lead to removal of lateral support), and
(e) Previous rockfall or slide, subsidence or faulting (these factors steepen the slope).
(B) (Anthropogenic factors)
(a) Construction of quarries, pits, canals, roads, and
(b) Alteration of water levels in lakes and reservoirs.
2. Surcharge (loading of slope):
(a) (Natural) weight of rain, snow, (anthropogenic) water from pipelines, sewers, canals,
(b) Accumulation of talus,
(c) Vegetation, trees,
(d) Seepage pressure of percolating water, and
(e) (Anthropogenic) construction of fill, waste piles, buildings.
3. Transitory earth stress (endogenetic processes):
(b) Vibrations, blasting, traffic, and
(c) Swaying of trees in wind.
4. Removal of underlying support:
(a) Undercutting by rivers and waves,
(b) Solution at depth, mining (anthropogenic),
(c) Loss of strength of underlying sediments, and
(d) Squeezing out of underlying plastic sediments.
5. Lateral pressure:
(a) Water in cracks,
(b) Freezing of water in cracks,
(c) Swelling (hydration of clay), and
(d) Mobilization of residual stress (pressure release).
Factors which decrease (reduce) the shear strength of materials:
1. Weathering and other physico-chemical reactions:
(a) Softening of fissured clays,
(b) Physical disintegration of granular rocks (frost action, thermal expansion etc.),
(c) Hydration of clay mineral causing decrease in particles cohesion, swelling,
(d) Base exchange (changes in physical properties),
(e) Drying (desiccation) of clays and shales (racking, loss of cohesion), and
(f) Removal of cement by solution.
2. Changes in intergranular forces due to water content (porewater pressure)
(b) Softening of material.
3. Changes of Structure.
(a) Fissuring of shales and consolidated clays.
(b) Remoulding of loess, sand and sensitive clay.
(a) Burrowing animals.
(b) Decay of roots.
It may be mentioned that generally all types of mass movements of rock wastes including soils and ice are collectively called as landslides which are variously classified on different bases i.e., direction of movement, type and rate of movement, nature of materials, presence or absence of lubricants etc. (tables 15.1 to 15.4). On an average, landslides (downslope movement of different types of debris enblock) are divided into five major categories e.g., fall, slide, topple, flows and lateral spreads. On the basis of nature of materials these are further subdivided into several types (table 15.4).
Instantaneous fall of weathered rock materials including large blocks from steep hillslopes or earthen materials from steep and cliffed valleysides of streams under the influence of gravity is called fall. The size of rock fragments depends on the size and pattern of rock joints. This type of movement involves vertical displacement of materials without water.
The velocity of fall is greatest of all other types of mass movement. According to A.L. Bloom, ‘fall is a distinct landslide process, but it is rarely independent of subsequent events.’ On the basis of materials fall is subdivided into rock fall, debris fall and earthfall.
Rock falls are relatively small landslides confined to the removal of individual and superficial blocks from a cliff base. Rock fall (fig. 15.1 A) is facilitated by granular and block disintegration of rocks under the processes of mechanical weathering and limited action of oxidation in sandstones.
According to M.J. Selby (1982) ‘most rockfalls are promoted by hydrofacturing, stress release, the wedging action of tree roots, and other weathering processes a common cause of rock falls is undercutting of a face by streams or the more rapid weathering of an underlying weak rock such as shale or mudstone.’
The frequency of rock falls depends on certain environmental conditions such as aridity/humidity factor, lithological and structural characteristics of rocks, nature of slope and vegetation etc. In humid areas rock falls are very common features but in hot arid areas they are of very rare occurrence.
Debris fall involves rapid rate of fall of weathered rock materials (which are finer than the materials involved in rock fall) downslope (it may be hillslope or steep valley side slope of streams) from great height. The fallen materials collect at the foot-hill or cliff base and form small mounds and ridges. Earthfall involves downslope movement of finer materials than debris fall.
Slides, very often known as landslides among general public, are most significant of all types of mass movements. ‘Mass-wasting wherein a mass of rock or weathered debris moves downhill along discrete shear surfaces is defined as a slide’ (A.L. Bloom). It may be pointed out that slides involve downslope displacement of both types of materials-weathered rock materials and soils. Slides in rock or soil are characterized by movement above a sharply defined shear plane.
In rocks such as slate, schist, and many sedimentary formations the shear plane follows a structural plane within the rocks such as a plane of foliation or bedding- and it is often straight’. Slides are promoted by a host of controlling variables such as nature of slopes (vertical and cliff slope is essential for slides), moderate lubrication by water, earth tremors, gravity, vertical and steeply inclined rock beds, base removal etc.
Slides are more frequent in certain locations having favourable condition viz.:
(1) Steep hillslope or steep valleysides of streams,
(2) Fault scarps,
(3) Rejuvenated fluvially eroded valleys,
(4) Sea coasts,
(5) Alluvial river valleys,
(6) Degraded hills and mountains (due to deforestation, road construction, settlement expansion etc.).
On the basis of nature of materials, direction and rate of movement (intensity) slides are divided into:
(1) Slump (which is further divided into rock slump, debris slump and earth slump),
(2) Rock slides,
(3) Debris slide, and
(4) Earth slide.
(i) Slumping involves intermittent sliding of rock fragments, rock blocks or soils downslope along a curved plane caused by rotational movement (figs. 15.1 C and 15.3) and displaced blocks (whether rock blocks or soil blocks) cover very short distance. Slump is promoted by undercutting of slope base (with hillslope or valley side slope of streams) by streams, seawaves (in case of coast land) and by human activities (quarrying).
In fact, ‘slump is the form of slide most common in thick, homogeneous, cohesive materials such as clay. The surface of failure beneath a slump block is spoon- shaped, concave upward or outward’ (fig. 15.3).
Slumping of alluvial deposits of valley sides of alluvial rivers of north India through undercutting of valley sides by hydraulic action of the steams during wet monsoon period is of common occurrence. Slumping is consuming a large chunk of rich agricultural lands every year along the Ganga valley in U.P. and Bihar. Based on the nature of materials involved slump is subdivided into rock slump, debris slump and earth slump.
(ii) Rock slide (also known as rock glide or block glide) is most significant of all types of slides wherein large rock blocks slide down the hillslope. ‘Rock slides may be very large and catastrophic in mountain regions where the large available relief permits accelerations of rock debris to velocities as great as those of rock fails and rock avalanches,’. Rock slides involve rapid movement of materials downslope.
Sometimes, the velocity is so high and mass of materials is so enormous that ‘rock slides can be dramatic forms of sliding mass- wasting if large masses of un-weathered rock slide downhill along a sloping joint or a bedding surface’. The Cross Ventre Slide of 1925 in Wyoming, USA and Turtle Mountain Slide of 1903 in Alberta, Canada, are typical examples of devastating landslides. The very massive landslides (rock slide), which occurred in the north-western side of Naini Lake (Nainital, Uttaranchal) in 1884, was so enormous that the debris filled a sizeable portion of the lake.
(iii) Debris slide is more extensive and occurs at larger scale than slump but there is little amount of water.
Debris slide is promoted because of two basic factors:
(1) saturation of rocks due to water, and
(2) sudden downslope movement of unconsolidated mantle rock.
The materials involved in debris slide is a mixture of soils and rock fragments (boulders). The debris collects at the foot-hill or the base of the valleys and forms interesting morphological features.
Diagonal downslope movement of rock fragments and soils along sliding plane with enough water is called flow (which is further divided into solifluction, debris flow, mud flow, earthflow, rock avalanche etc.). Flow involves downslope rapid movement of rock debris or soils saturated with water like viscous fluid. ‘Dry flows in sand or silt are known, but most flows are saturated with water.
Rates of movement are greater than for creep but range from imperceptibly slow to tragically rapid mud flows and avalanches.
Flows typically move as lobes or tounges:
(i) Debris flow involves downslope movement of enormous amount of viscous soils and boulders either separately or mixed together, and occurs mostly along river valley sides. The difference between debris flow, earth flow and mud flow is related to size of particles and amount of water. The size of particle decreases from debris flow to mudflow. The three terms form a series of progressively higher water content (i.e. water content increases from debris flow through earth flow to mud flow) but are often used interchangeably.
Debris flows have 20-80 percent particles coarser than sand sizes, whereas earth flows and mud flows are 80 per cent or more mud and sand. Mud flow is the most liquid “end member” of the series. ‘Debris flow occurs mostly due to availability of water, presence of loosely deposited soils and fine rock materials, lack of vegetation cover, Clay minerals in the soils, unstable slope, undercutting of slope (valley sides) by streams, earth tremors etc. Debris flows range in size from a few meters to over 1000 meters in width and may be tens of meters thick in places; more commonly they are 1 to 5 m thick’ . Debris flow is most common on gully heads in the riverine tracts of major alluvial rivers.
(ii) Earth flow is promoted by excessive water received mostly through rainfall so that the materials are oversaturated. Earth flow is more common on planar hillsides or valley-sides having alluvium, rich in clay minerals.
Debris flow of volcanic materials saturated with water on volcanic cones is called lahar. Heavy downpour mixing with falling volcanic dusts causes enormous mud flow as lahar on the steep slopes of volcanic cones which inflicts great damage to human health and wealth. For example, great lahar created on the steep slopes of Kelut Volcano in Japan in 1919 killed 5500 persons.
(iii) Mud flow differs from earth flow in that former may be noticed by the observer while the latter cannot be noticed because earth flow is not very common. The water content is more in mud flow than in debris flow and earth flow. Mud flow is most common along valley-sides of alluvial rivers and the debris (mud) so produced is transported by the rivers.
The necessary conditions which promote mud flow include:
(1) Steep and vertical slope,
(2) Presence of unconsolidated materials on the upper surface so that these, when mixed with water, become viscous fluid and slippery,
(3) Intermittent supply of sufficient water as lubricant, and
(4) Absence of vegetation.
Based on these factors Elliot Blackwelder (1928) considered arid regions as most favourable for mud flow.
C.F.S. Sharpe (1938) has divided mud flow into three categories on the basis of spatial characteristics e.g.:
(1) Mud flow of arid regions,
(2) Alpine mud flow, and
(3) Volcanic mud flow.
Very slow and imperceptible downslope movement of materials (colluvium) is called creep.
On the basis of materials involved in such movement creep is divided into:
(1) Soil creep (fine weathered rock debris as well as soil), and
(2) Rock creep (unweathered joint blocks). It may be pointed out that the rate of movement of materials (colluvia) under creep is so slow (a few millimeters per year) that it becomes practically difficult for the observers to notice it.
(i) Soil creep is also called as solifluction which occurs in a variety of climatic conditions ranging from tropical humid to periglacial climates. The process of debris movement in periglacial regions has been variously defined and a number of terms have been suggested. First J.G. Anderson (1906) proposed the term solifluction (solum-soil, fluere-flow) for slow movement of debris, soaked with water, from higher to lower slopes.
Solifluction term was replaced by congelifluction of J.Daylik (1951) to incorporate only soil flow in the periglacial climate having permafrost below an active layer. K. Bryan (1946) used the term cryoturbation which included all types of mass movement of regoliths under periglacial environment. Recently, gelifluction is used in place of congelifluction.
(iii) Rock creep involves downhill movement of rock debris having relatively great depth (upto 300m) but the movements is very slow and ranges between one meter to ten meters per year. ‘It is distinguished from soil creep by its great depth and isolation from daily and seasonal climatic conditions, and from land sliding by the lack of a single clearly defined failure plane and slow rate of deformation’ . The following conditions promote rock creep- deformation of rocks through bending, folding, bulging, fracturing, spreading; distortion and buckling of inclined rock beds of varying resistance, mechanical disintegration of rocks etc.
Topographic Expressions of Mass Wasting and Mass Movement:
Different types of mass wasting and mass movement create distinctive morphological features on hillslope and river valley sides and coastal lands. It may be pointed out that on one hand there is wide range of variation in mass movement because of varying controlling factors and conditions, on the other hand, there is almost uniformity in the resultant morphological features. The topographic features produced by mass movement say landslides include scars, ripple marks, terraces, meander widening etc.