GC Leong Test: Weathering, Mass Movement And Ground Water - 2 - BPSC (Bihar) MCQ

• Repeated wetting and drying also occurs at the coast, where rocks may be rapidly dried by sun and wind between tides. 

• When rocks are wetted, the outer layers absorb a certain amount of moisture and expand. 

• When they dry this moisture evaporates, and they quickly shrink. When this happens repeatedly, the outer layers split off.

• It should also be stressed that the wetting and drying of the rocks in deserts is probably just as important as temperature changes in mechanical weathering. The rocks dry very quickly indeed after being wetted by brief desert rain-storms.

Frost action: 

• In temperate latitudes, frost is a potent rock breaker. All rocks contain cracks and joints, pore spaces, and shower water or snow collected in such places. 

• When the temperature drops at night or during the winter, this water freezes, when water freezes, it expands by one-tenth -its volume and exerts a bursting pressure of almost 140 kg per square cm (2,000 lb. to the square inch). 

• Repeated freezing of this kind will deepen and widen the original cracks and crevices and break the rock into angular fragments. On mountain peaks, this process creates sharp pinnacles and angular outlines. Such peaks are described as frost-shattered peaks

• Soil Creep: This is a slow, gradual, but more or less continuous soil down hillslope movement. The movement is not very noticeable, especially when the slope is fairly gentle or when the soil is well-covered with grass or other vegetation. 

• Soil creep most common in damp soils where the water acts as a lubricant so that individual soil particles move over each other and the underlying rock. 

• It is also found where continuous trampling by animals grazing on the slopes sets up vibrations which loosen the soil and cause it to move. 

• Though the movement is slow and cannot readily be seen in action, the gradual movement tilts trees, fences, posts and so on, rooted in the soil. The soil is also seen to accumulate at the foot of slope or behind obstacles such as walls, which may eventually be burst by the weight of soil above.

• A mantle of weathered debris may become saturated with rainwater in arid regions after a storm and flow downslope as a semi-liquid mass. 

• In temperate and tundra occur when the surface layers of a frozen ground thaw in spring. Soil and rock debris, lubricated by water, flow easily over the underlying frozen subsoil.

• In areas of peat soils, the peat absorbs much moisture. However, if the saturation point is reached the peaty soil may flow downslope. In Ireland, such flows are known as 'bog-bursts'.

• Landslides may be caused because a river or the sea undercuts a steep slope to fall by gravity. 

• Earthquakes or volcanic disturbances may loosen rocks and start a landslide. 

• Human-made steepening both undercuts the slope and sets up vibrations which may loosen rocks or soil. 

• But often landslides are caused by the lubricating action of rainwater. Water may collect in joints or bedding planes in rocks so that one layer slides over another, especially in areas of tilted strata. 

• Slumping is particularly common where permeable debris or rock layers overlie impermeable strata such as clay. The clay halts water sinking through the permeable material. 

• The damp clay provides a smooth, slippery surface over which the upper layers easily slide.

• Man often enhances the possibility of landslides by clearing natural vegetation for agriculture or housing. 

• Removal of the plant cover allows more water to penetrate the soil and rocks. 

• In areas such as the Cameron Highlands, where steep slopes have been cleared, there is much evidence of minor slumps and slides, the old scars showing up clearly in the tea gardens. 

• Extensive landslides, whether natural or man-induced, can have disastrous consequences, burying villages, railway lines or people

• Porous rocks are those, like sandstone, which have many pore-spaces between the grains. 

• Water is easily absorbed by such rocks and may be stored in the pore-spaces. Permeable or pervious rocks are those which allow water to pass through them easily. 

• Thus most porous rocks are also permeable. However, some rocks are porous but impermeable. For example, Clay is highly porous since it is made up of innumerable wonderful particles with pore-spaces between them. 

• It thus absorbs a great deal of water. However, the pore-spaces are so small that the water does not move easily through the rock, impermeable.

• Water that seeps through the ground moves downward under the force of gravity until it reaches an impermeable rock layer through which it cannot pass.

• If there is no ready outlet for the groundwater in the form of a spring, the water accumulates above the impermeable layer and saturates the rock. 

• The permeable rock in which the water is stored is known as the aquifer. The surface of the saturated area is called the water table. 

• The well must be sunk to the permanent water table's depth if a constant water supply is to be obtained. 

• If the well is only sunk to the water table's wet-season depth, water will be unobtainable when the level drops in the dry season. 

• When a well is bored, the water usually has to be raised by hand or by mechanical pumping. 

• Wells are essential in arid areas where there is little surface water but where the underlying rocks contain groundwater.