- These movements transfer the mass of rock debris down the slopes under the direct influence of gravity. That means, air, water or ice do not carry debris with them from place to place but on the other hand the debris may carry with it air, water or ice.
- Gravity exerts its force on all matter, both bedrock and the products of weathering. So, weathering is not a pre-requisite for mass movement though it aids mass movements. Mass movements are very active over weathered slopes rather than over unweathered materials.
- mass movements do not come under erosion though there is a shift (aided by gravity) of materials from one place to another.
- Several activating causes precede mass movements. They are : (i) removal of support from below to materials above through natural or artificial means; (ii) increase in gradient and height of slopes; (iii) overloading through addition of materials naturally or by artificial filling; (iv) overloading due to heavy rainfall, saturation and lubrication of slope materials; (v) removal of material or load from over the original slope surfaces; (vi) occurrence of earthquakes, explosions or machinery; (vii) excessive natural seepage; (viii) heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks; (ix) indis- criminate removal of natural vegetation.
- Heave (heaving up of soils due to frost growth and other causes), flow and slide are the three forms of movements.
Slow mass Movements
- Creep is one type under this category which can occur on moderately steep, soil covered slopes. Movement of materials is extremely slow and imperceptible except through extended observation.
- Materials involved can be soil or rock debris.
- fence posts, telephone poles lean downslope from their vertical position that is due to the creep effect.
- Depending upon the type of material involved, several types of creep viz., soil creep, talus creep, rock creep, rock-glacier creep etc., can be identified.
- Also included in this group is solifluction which involves slow downslope flowing soil mass or fine grained rock debris saturated or lubricated with water.
- This process is quite common in moist temperate areas where surface melting of deeply frozen ground and long continued rain respectively, occur requently. When the upper portions get saturated and when the lower parts are impervious to water percolation, flowing occurs in the upper parts.
- These movements are mostly prevalent in humid climatic regions and occur over gentle to steep slopes.
- Movement of water-saturated clayey or silty earth materials down low-angle terraces or hillsides is known as earthflow.
- Quite often, the materials slump making step- like terraces and leaving arcuate scarps at their heads and an accumulation bulge at the toe.
- When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope.
- mudflow.-In the absence of vegetation cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flow down along definite channels. It looks like a stream of mud within a valley. When the mudflows emerge out of channels onto the piedmont or plains, they can be very destructive engulfing roads, bridges and houses. Mudflows occur frequently on the slopes of erupting or recently erupted volcanoes. Volcanic ash, dust and other fragments turn into mud due to heavy rains and flow down as tongues or streams of mud causing great destruction to human habitations.
- debris avalanche, which is more characteristic of humid regions with or without vegetation cover and occurs in narrow tracks on steep slopes. This debris avalanche can be much faster than the mudflow. Debris avalanche is similar to snow avalanche.
- Landslides -These are relatively rapid and perceptible movements. The materials involved are relatively dry. The size and shape of the detached mass depends on the nature of discontinuities in the rock, the degree of weathering and the steepness of the slope.
- Depending upon the type of movement of materials several types are identified in this category.
- Slump is slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place
- Rapid rolling or sliding of earth debris without backward rotation of mass is known as debris slide. Debris fall is nearly a free fall of earth debris from a vertical or overhanging face.
- Sliding of individual rock masses down bedding, joint or fault surfaces is rockslide. Over steep slopes, rock sliding is very fast and destructive. Rock fall is free falling of rock blocks over any steep slope keeping itself away from the slope. Rock falls occur from the superficial layers of the rock face, an occurrence that distinguishes it from rockslide which affects materials up to a substantial depth.
- In our country, debris avalanches and landslides occur very frequently in the Himalayas. There are many reasons for this.
the Himalayas are tectonically active. They are mostly made up of sedimentary rocks and unconsolidated and semi-consolidated deposits. The slopes are very steep. Compared to the Himalayas, the Nilgiris bordering Tamilnadu, Karnataka, Kerala and the Western Ghats along the west coast are relatively tectonically stable and are are mostly made up of very hard rocks;
but still, debris avalanches and landslides occur though not as frequently as in the Himalayas, in these hills. Why? Many slopes are steeper with almost vertical cliffs and escarpments in the Western Ghats and Nilgiris. Mechanical weathering due to temperature changes and ranges is pronounced. They receive heavy amounts of rainfall over short periods.( 2013 mains question)
- can be defined as “application of the kinetic energy associated with the agent to the surface of the land along which it moves”.Kinetic energy is computed as KE = 1/2 mv2 where ‘m’ is the mass and ‘v’ is the velocity.
- The work of the other two agents of erosionwaves and ground water is not controlled by climate. In case of waves it is the location along the interface of litho and hydro sphere coastal region — that will determine the work of waves, whereas the work of ground water is determined more by the lithological character of the region.
- If the rocks are permeable and soluble and water is available only then karst topography develops.
- It is a consequence of erosion. The erosional agents loose their velocity and hence energy on gentler slopes and the materials carried by them start to settle themselves. In other words, deposition is not actually the work of any agent.
- The coarser materials get deposited first and finer ones later. By deposition depressions get filled up. The same erosional agents viz., running water, glaciers, wind, waves and groundwater act as aggradational or depositional agents also.
Soil and Soil Contents
- A pedologist who studies soils defines soil as a collection of natural bodies on the earth’s surface containing living and/or dead matter and supporting or capable of supporting plants.
- Soil is a dynamic medium in which many chemical, physical and biological activities go on constantly.
- Soil is a result of decay, it is also the medium for growth. It is a changing and developing body.
- It has many characteristics that fluctuate with the seasons. It may be alternatively cold and warm or dry and moist. Biological activity is slowed or stopped if the soil becomes too cold or too dry
Process of Soil Formation
- Soil formation or pedogenesis depends first on weathering. It is this weathering mantle (depth of the weathered material) which is the basic input for soil to form.
- the weathered material or transported deposits are colonised by bacteria and other inferior plant bodies like mosses and lichens. Also, several minor organisms may take shelter within the mantle and deposits. The dead remains of organisms and plants help in humus accumulation.
- Minor grasses and ferns may grow; later, bushes and trees will start growing through seeds brought in by birds and wind. Plant roots penetrate down, burrowing animals bring up particles, mass of material becomes porous and sponge- like with a capacity to retain water and to permit the passage of air and finally a mature soil, a complex mixture of mineral and organic products forms.
- Pedology is soil science. A pedologist is a soil-scientist.
Five basic factors control the formation of soils:
(i) parent material;
(iv) biological activity;
(v) time. In fact soil forming factors act in union and affect the action of one another.
- Parent material is a passive control factor in soil formation. Parent materials can be any in- situ or on-site weathered rock debris (residual soils) or transported deposits (transported soils).
- Soil formation depends upon the texture (sizes of debris) and structure (disposition of individual grains/particles of debris) as well as the mineral and chemical composition of the rock debris/deposits.
- Nature and rate of weathering and depth of weathering mantle are important considerations under parent materials. There may be differences in soil over similar bedrock and dissimilar bedrocks may have similar soils above them. But when soils are very young and have not matured these show strong links
- with the type of parent rock. Also, in case of some limestone areas, where the weathering processes are specific and peculiar, soils will show clear relation with the parent rock.
- Topography like parent materials is another passive control factor.
- The influence of topography is felt through the amount of exposure of a surface covered by parent materials to sunlight and the amount of surface and sub-surface drainage over and through the parent materials.
- Soils will be thin on steep slopes and thick over flat upland areas. Over gentle slopes where erosion is slow and percolation of water is good, soil formation is very favourable.
- Soils over flat areas may develop a thick layer of clay with good accumulation of organic matter giving the soil dark colour. In middle latitudes, the south facing slopes exposed to sunlight have different conditions of vegetation and soils and the north facing slopes with cool, moist conditions have some other soils and vegetation.
The climatic elements involved in soil development are :
- moisture in terms of its intensity, frequency and duration of precipitation - evaporation and humidity;
- temperature in terms of seasonal and diurnal variations.
- Precipitation gives soil its moisture content which makes the chemical and biological activities possible.
- Excess of water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (illuviation). In climates like wet equatorial rainy areas with high rainfall, not only calcium, sodium, magnesium, potassium etc. but also a major part of silica is removed from the soil.
- Removal of silica from the soil is known as desilication. In dry climates, because of high temperature, evaporation exceeds precipitation and hence ground water is brought up to the surface by capillary action and in the process the water evaporates leaving behind salts in the soil. Such salts form into a crust in the soil known as hardpans. In tropical climates and in areas with intermediate precipitation conditions, calcium carbonate nodules (kanker) are formed.
- Temperature acts in two ways — increasing or reducing chemical and biological activity. Chemical activity is increased in higher temperatures, reduced in cooler temperatures (with an exception of carbonation) and stops in freezing conditions. That is why, tropical soils with higher temperatures show deeper profiles and in the frozen tundra regions soils contain largely mechanically broken materials.
- The vegetative cover and organisms that occupy the parent materials from the beginning and also at later stages help in adding organic matter, moisture retention, nitrogen etc.
- Dead plants provide humus, the finely divided organic matter of the soil. Some organic acids which form during humification aid in decomposing the minerals of the soil parent materials.
- Intensity of bacterial activity shows up differences between soils of cold and warm climates. Humus accumulates in cold climates as bacterial growth is slow.
- With undecomposed organic matter because of low bacterial activity, layers of peat develop in sub-arctic and k6tundra climates.
- In humid tropical and equatorial climates, bacterial growth and action is intense and dead vegetation is rapidly oxidised leaving very low humus content in the soil.
- Further, bacteria and other soil organisms take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants. This process is known as nitrogen fixation. Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and fixes nitrogen beneficial to the host plant.
- The influence of large animals like ants, termites, earthworms, rodents etc., is mechanical, but, it is nevertheless important in soil formation as they rework the soil up and down. In case of earthworms, as they feed on soil, the texture and chemistry of the soil that comes out of their body changes.
- Time is the third important controlling factor in soil formation. The length of time the soil forming processes operate, determines maturation of soils and profile development.
- A soil becomes mature when all soil-forming processes act for a sufficiently long time developing a profile.
- Soils developing from recently deposited alluvium or glacial till are considered young and they exhibit no horizons or only poorly developed horizons.
- No specific length of time in absolute terms can be fixed for soils to develop and mature.