Mass Movements, Erosion, Soil Formation & Exercises

5.7 Mass Movements — Gravity Pulls Down the Mountain

In Part 1 we saw that weathering breaks rocks down where they sit, without moving them very far. The next step in the chain of denudation is to actually shift that broken material downhill — and the mover is gravity itself. Mass movements transfer the mass of rock debris down slopes under the direct influence of gravity. There is no separate transporting agent: air, water or ice do not carry the debris from place to place; instead, the moving debris may carry some air, water or ice with it.

Materials on a slope have their own resistance to disturbing forces, and they yield only when the disturbing force becomes greater than the shearing resistance of the materials. Several conditions favour mass movements: weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rain, scarcity of vegetation, and so on. Although weathering is not a pre-requisite for mass movement, it certainly aids it — mass movements are most active over weathered slopes rather than over fresh, unweathered rock.

Activating Causes of Mass Movements

NCERT lists nine activating causes that precede a mass movement. They are worth memorising:

1. Removal of support from below the materials above — by natural undercutting (river, sea, glacier) or by artificial means (road cuts, quarrying).
2. Increase in gradient and height of slopes (uplift, fault scarps).
3. Overloading through addition of materials, naturally or by artificial filling.
4. Overloading due to heavy rainfall, saturation and lubrication of slope materials.
5. Removal of material or load from over the original slope surfaces (deforestation, terrace cutting).
6. Occurrence of earthquakes, explosions or vibrations from machinery.
7. Excessive natural seepage of ground-water.
8. Heavy drawdown of water from lakes, reservoirs and rivers — leading to slow outflow of water from beneath the slopes.
9. Indiscriminate removal of natural vegetation — soils lose their root-binding network.

Heave (heaving up of soils due to frost growth and other causes), flow and slide are the three forms of movement. The full spectrum can be arranged on a chart of moisture content against rate of motion:

Slow Movements

Creep^?

Creep is the slowest mass movement of all — so slow that a person watching the slope sees nothing happening, yet over a decade the slope has clearly shifted. Soils, soft rocks, talus and scree all suffer their own kind of creep. The classic clues are tilted telephone poles, leaning trees with bent trunks, displaced fences and slowly bowed retaining walls. Creep can be active over wet, clayey soils, or as rock creep on steep talus slopes.

Solifluction^?

Solifluction is a special kind of slow flow common in cold and high-altitude regions. It is the slow downhill movement of waterlogged soil and other unconsolidated material over a frozen sub-surface (permafrost). The water cannot percolate downward because the layer below is frozen, so the saturated topsoil flows like a thick paste under gravity. NCERT asks the student whether solifluction can be classed under rapid flow movements — the answer is no, it is slow. But because it is a flow rather than a slip, it sits between creep and the rapid flows.

Rapid Movements

The rapid mass movements are mostly seen in humid climates and over moderate to gentle slopes. They are noticeably wet: the water is what gives the moving mass its fluidity.

Landslides — Rapid & Perceptible

Landslides are relatively rapid and perceptible movements. The materials involved are relatively dry compared with flows. The size and shape of the detached mass depend on the nature of discontinuities in the rock, the degree of weathering, and the steepness of the slope. Several types are recognised:

Slump is the slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place (NCERT Figure 5.4). Debris slide is the rapid rolling or sliding of earth debris without backward rotation of the mass. Debris fall is nearly a free fall of earth debris from a vertical or overhanging face. Rockslide is the sliding of individual rock masses down bedding, joint or fault surfaces — over steep slopes, rock sliding is very fast and destructive (NCERT Figure 5.5 shows landslide scars in the Shiwalik Himalayas near the river Sarada at the India–Nepal border). Slides occur as planar failures along discontinuities like steeply dipping bedding planes. Rock fall is the free fall of rock blocks over any steep slope, keeping itself away from the slope. Rock falls occur from the superficial layers of a rock face — and that depth is what distinguishes them from rockslide, which affects materials to a substantial depth.

5.8 Erosion and Deposition — The Real Surface Sculptors

Erosion involves the acquisition and transportation of rock debris. When massive rocks break into smaller fragments through weathering and any other process, erosional geomorphic agents — running water, ground-water, glaciers, wind and waves — pick up the debris and carry it to other places, depending on the dynamics of each agent. Abrasion by the rock debris that is being carried adds to the cutting power of these agents. By erosion, relief degrades — that is, the landscape is worn down.

The five erosional agents are not all alike. Running water, glaciers and wind are climatically controlled; they represent the three states of matter — gaseous (wind), liquid (running water) and solid (glacier). The work of waves is controlled not by climate but by location — they act only along the interface of the lithosphere and hydrosphere, that is, the coast. The work of ground-water is determined more by the lithological character of the region: only where rocks are permeable and soluble, and water is available, can karst topography^? develop.

Deposition — The Quiet Half of the Story

Deposition is a consequence of erosion. When the erosional agents lose their velocity — and therefore their kinetic energy — on gentler slopes, the materials they were carrying begin to settle. Strictly speaking, deposition is not the work of any agent; the agents simply stop carrying. Coarser materials get deposited first, finer ones later. Through deposition, depressions are filled up. The same erosional agents — running water, glaciers, wind, waves and ground-water — therefore also act as aggradational or depositional agents. What happens to the surface as a result of erosion and deposition is taken up in the next chapter (Landforms and their Evolution).

5.9 Soil Formation — From Regolith to Living Skin

You see plants growing in soils. You play in the ground and you soil your clothes. But what exactly is soil? Soil is a dynamic medium in which many chemical, physical and biological activities go on constantly. It is a result of decay, and at the same time the medium for growth. It is a changing and developing body, with characteristics that fluctuate with the seasons — alternately cold and warm, dry and moist. Biological activity slows or stops if the soil becomes too cold or too dry. Organic matter increases when leaves fall or grasses die.

Process of Soil Formation

Soil formation, or pedogenesis, depends first on weathering. The weathering mantle — the depth of weathered material — is the basic input for soil to form. The sequence of events runs roughly like this:

1. Weathered material or transported deposits are first colonised by bacteria and inferior plant bodies like mosses and lichens.
2. Several minor organisms take shelter inside the mantle.
3. Dead remains of organisms and plants accumulate as humus.
4. Minor grasses and ferns grow; later, bushes and trees follow.
5. Plant roots penetrate downward; burrowing animals bring up particles; the mass becomes porous and sponge-like.
6. Finally, a mature soil — a complex mixture of mineral and organic products — forms.

Soil-forming Factors — Five Controls

Five basic factors control the formation of soils. They never work alone — each affects the action of the others.

1. Parent Material — Passive Control

Parent material is a passive control factor. It can be any in-situ weathered rock debris (giving residual soils) or transported deposits (giving transported soils). Soil formation depends on the texture (sizes of debris) and structure (disposition of grains) of the parent material as well as on its mineral and chemical composition. Nature and rate of weathering and depth of weathering mantle are the important considerations under parent materials. Soils may differ over similar bedrock and dissimilar bedrocks may produce similar soils — but young, immature soils show strong links with their parent rock. In limestone areas, where weathering processes are peculiar, the soil shows a clear relationship with the parent rock.

2. Topography — Passive Control

Topography, like parent material, is a passive control. Its influence is felt through (a) the amount of exposure of a surface to sunlight, and (b) 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 most favourable. Soils on flat ground may develop a thick clay layer with good organic matter giving the soil a dark colour.

3. Climate — Active Control

Climate is the most important active factor in soil formation. Two climatic elements matter:

• Moisture — its intensity, frequency and duration of precipitation; evaporation; humidity.
• Temperature — seasonal and diurnal variations.

Precipitation provides the soil's moisture content, which makes chemical and biological activities possible. Excess water carries soil components downward — a process called eluviation — and deposits them lower down (illuviation). In wet equatorial rainy areas, calcium, sodium, magnesium and potassium, and even a major part of silica, are removed from the soil; the loss of silica is called desilication. In dry climates, high evaporation pulls ground-water up by capillary action, and salts are left behind as a crust called a hardpan. In tropical climates with intermediate precipitation, calcium-carbonate nodules called kanker are formed.

Temperature works in two ways — increasing or reducing chemical and biological activity. Higher temperatures speed up chemical activity (with the exception of carbonation, which is more effective in cold water), cooler temperatures slow it down, and freezing stops it altogether. That is why tropical soils develop deep profiles, while frozen tundra soils contain mostly mechanically broken material.

4. Biological Activity — Active Control

The vegetative cover and organisms occupying the parent material from the very beginning contribute organic matter, moisture retention and nitrogen. Dead plants give the soil humus, the finely divided organic matter; some organic acids formed during humification help decompose the minerals of the parent material. Bacterial activity is intense in warm climates and slow in cold ones — humus accumulates in cold climates because decomposition is slow, producing layers of peat in sub-arctic and tundra climates. In humid tropical and equatorial climates, dead vegetation is rapidly oxidised, leaving very low humus content. Bacteria and other soil organisms convert atmospheric nitrogen into a form usable by plants — a process called nitrogen fixation. Rhizobium, a bacterium living in root nodules of leguminous plants, fixes nitrogen for the host plant. Earthworms, termites, ants and rodents rework the soil up and down, mechanically and chemically.

5. Time — Passive Control

Time is the third important controlling factor. The length of time the soil-forming processes operate determines the maturation of soils and the development of horizons (the profile). A soil becomes mature when all soil-forming processes have acted for a sufficiently long time, developing a full profile. Soils on recently deposited alluvium or glacial till are considered young and exhibit no horizons or only poorly developed ones. No specific length of time can be fixed in absolute terms for soils to mature — it depends on the interplay of all five factors.