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Discipline course -1 Paper Lesson Developer: The movement of weathered material under the influence

Aug 26, 2018

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  • Institute of Lifelong Learning, University of Delhi

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    Discipline course -1 Semester -1

    Paper Geomorphology Lesson- Mass Wasting

    Lesson Developer:

    Dr.Prabuddh Kr. Mishra

    College /Department: Bhim Rao Ambedkar College University of Delhi

  • Institute of Lifelong Learning, University of Delhi

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    Table of Contents

    3. Geomorphic Process

    3.2 Mass Wasting

    1.1 Meaning and concept

    1.2 Factors affecting mass waiting

    1.3 Classification of Mass movement

    Summery

    Exercise

    References

  • Institute of Lifelong Learning, University of Delhi

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    Mass Wasting

    1.1 Meaning and concept

    The movement of weathered material under the influence of gravity and with or without

    the influence of rainwater is called as mass movement or wasting. In other words, mass

    wasting is the downslope movement of soil or rock material under the influence or gravity

    without the direct aid of other medium such as water, air or ice, water and ice, however, are

    frequently involved in mass wasting by reducing the strength of rock of soil and by

    contributing to plastic and fluid behaviour of soil. Mass movements include both

    detachment of rock materials and their down slope transport. The collective term for

    gravitation or downslope movements of weathered rock debris is mass wasting.

    R.J. Chorley has remarked that, "the relation between mass wasting and tectonics is a

    relatively clear one. Where rocks are shattered, reliefs are 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.

    Mass movement is a common phenomenon in all high and steep hill country and it can also

    occur on very low-angle slopes. Very large land slides are most common in the technically

    and seismically active belts of rising mountains chains where steep slopes, rapidly incising

    rivers and glaciers in valley floors, jointed and fractured rock masses on slopes, severe

    physical weathering, fluctuations in groundwater pressure, and many joints dipping steeply

    out of the slopes, all contribute to instability.

    1.2 Factors which results in mass wasting

    1. Weathering reduces the shearing strength of materials by physical and chemical

    changes through granular disintegration, hydration and expansion, saturation and

    loss of compactness, drying and cracking etc. Original rocks materials may be

    subject to mass wasting because of their composition. For example, rocks and

    materials like schist, sand, gravel shale, soil etc. are easily sheared permitting fall,

    slide, flow, creep or subsidence.

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    2. The texture and structure of materials may also be conducive to mass wasting

    because of their roundness or compaction.

    3. Movements may takes place along the places of structural weakness such as

    bedding planes, foliation, cleavage, fractures and Joints and faults.

    4. Changes in environmental conditions including climatic factors and vegetational

    cover may result in mass-wasting, (less vegetation cover, more lose will be the soil

    and hence mass wasting also will be more).

    5. Slope gradient is, always an important factor. Thus, a complex of factors in usually

    involved in mass wasting, resulting in a variety of mass movement.

    The role of water in the reduction of shearing strength is particularly important. This is often

    referred to as lubrication of rock on weathered rocks debris. This really achieved through

    the introduction of inter-granular forces by pore water pressure and the buoyant effect of

    water. However, rapid mass wasting frequently takes place after heavy rains or rapid

    melting of snow cover and does emphasize the effect of water on gravity movements (Fig.

    4).

    To watch video on mass wasting click on following web link: https://www.youtube.com/watch?v=yXaUbzVh4bI

    1.3 Classification of Mass movement

    Shapre (1938) made one of the first attempts at classification of the various types of mass

    wasting and though several others have been proposed since then, his classifications are still

    widely used. He recognized four major types of mass wasting which he called slow flowage,

    rapid flowage, landslides and subsidence.

    Hutchinson proposed a classification of mass movement on slopes based on mechanisms of

    movement, morphology of the mass and the rate of movement and has identified as four

    major types: creep, freeze, thaw movements, landslides mass movement involving sinking.

    https://www.youtube.com/watch?v=yXaUbzVh4bI

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    A wide range of variations in terms of rate, direction and type of movements in noted in

    mass movement in different places, having varying environmental conditions. In reality

    mass movement have long preparatory period and there are certain precursor events which

    herald the occurrence of mass movements but these are generally unnoticed. 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 eye witnesses, it would be more accurate to say that the observers

    failed to detect the phenomena which preceded the slide (R.J. Chorley).

    Mass movements are generally classified on the basis of causative factors.

    a) Rate of Movement;

    b) Direction of movement;

    c) Lubricating substance; eg. water, ice etc.

    The direction of mass movement of rock waste down slope may be (i) vertical (ii) Diagonal (iii) Lateral.

    Vertical mass movements are further divided into (a) rock fall, (b) collapse earth fall.

    Diagonal mass movements is divided into (a) soil creep (b) rock creep 9 (c) talus creep (d)

    rock slide (e) debris slide (f) slump (g) debris flow, (h) mud flow (i) solifluction (i) avalanches

    etc. Lateral mass movement includes (a) block slide (b) spread (c) cambering (d) sacking etc.

    Several types of mass wasting/movement are distinguished are as follow (Table 1 and Fig. 2).

    Table.1 Classification of Mass Wasting

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    (Source: http://itc.gsw.edu/faculty/bcarter/physgeol/mass/type1.htm)

    Landslide

    All types of mass movement of rock waste 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 lubrications.

    Mass movements wherein a mass of rock or weathered debris moves downhill along

    discrete shear surfaces in defined as slide. 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 included rock beds, base

    removal etc. Slides are more frequent in certain locations having favourable conditions viz.

    (1) steep hill slopes or steep valley sides of streams (2) fault scarps (3) rejuvenated fluvially

    eroded valleys (4) sea coasts (5) alluvial river valleys (6) degraded hills and mountains.

    On the basis of nature of materials direction and rate of movement slides are divided into:

    slumps, rock slides, debris slide and earth slide.

    i. Slump

    http://itc.gsw.edu/faculty/bcarter/physgeol/mass/type1.htm

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    Slump Rock Slump Debris Slump Earth Slum p

    Slumping involves intermittent sliding of rock fragments, rock blocks or soils down slope

    along a curved plane caused by rotational movement and displaced blocks (whether rock

    blocks or soil blocks) cover very short distances. Slump is promoted by undertaking of slope

    base (with hill slope or valley side slope of streams) by streams, sea waves and by human

    activities. 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.

    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 material involved slumps is subdivided into

    rock slump, debris slump and earth slump.

    ii. Rock slide

    Rock slide is also known as rock glide or block glide. It is most significant of all types of slides

    wherein lar