GC Leong: Summary of Weathering, Mass Movement and Ground Water Notes | EduRev

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Weathering
(i) The process of wearing away of earth’s surface is commonly known as denudation & is generally carried out in four phases Weathering, Erosion, Transportation & Deposition.
(ii) Warm wet climate promotes rapid chemical weathering while dry climate provide good conditions for physical weathering.
Chemical Weathering
(i) Extremely slow & gradual decomposition of rocks due to exposure to air & water
(ii) For example Granite when exposed to weather is found to be rough surfaced because it is mainly made up of Quartz, Feldspar & Mica; Feldspar is more quickly weathered than Quartz hence is worn away, eventually leaving loosened quartz crystals.
(iii) Regolith → Weathered material from the rock or mineral remains of decomposed rocks.
(iv) When a soil cover on the rock exists, chemical weathering of the rock enhances because the soil absorbs rain water & keeps the underlying rock in contact with this moisture.
(v) Rain water absorbs organic acids from the soil & thus become a stronger weathering agent than pure water acting on a bare rock.
Types of Chemical Weathering
1. Solution
(i) Many minerals are dissolved by water especially with rain water which contains enough carbon dioxide to make it a weak acid.
(ii) For ex. in case of limestone, rain water dissolves calcium carbonate, of which rock is chiefly formed & hence joints & cracks in rock are quickly widened, woming it out easily.
(iii) Rocks are more resistant if they have fewer joints or cracks to harbour Moisture.
(iv) All rocks are subjected to solution upto some extent though the process may be much slower depending not only on mineral composition of the rock but also on its structure, density & climatic conditions it faces.
2. Oxidation

(i) Weathering by reaction of oxygen in presence of air & water with minerals present in the rock.
(ii) For example most rocks contain certain amount of iron, which when comes in contact with air is changed in iron oxide & finally into rust, which crumbles easily, loosening the overall structure of the rock.

3. Decomposition by Organic Acids

(i) Within the soil which covers most rocks are bacteria which thrive on decaying plant or animal material.
(ii) These bacteria produce acids when dissolved in water, help to speed up weathering of underlying rocks.
(iii) In some cases, microorganisms & plants like mosses or lichens can live on bare rock damp surface, absorbing chemical elements from the rocks as food & producing organic acids. Hence, they become the agent of both Chemical & Mechanical weathering.

Physical Weathering

(i) Also known as Mechanical Weathering.
(ii) Disintegration by Mechanical Process.

Types of Physical Weathering

→ By insolation, by Frost

1. By Insolation
Block Disintegration

(i) Mainly in dry desert areas, hot at day and cold by night.
(ii) Leads to expansion & contraction of rock setting up stresses in the rock
(iii) Finally leading to its disintegration.

Granular Disintegration

(i) Different minerals in rock lead to different rate of expansion & contraction of rock.
(ii) Leads to Fragmentation of rock for ex. Granite.

Exfoliation

(i) Stresses are naturally greatest near the surface & where there are sharp angles in the rock
(ii) Rectangular blocks are thus gradually rounded by splitting away of sharp comers.
(iii) Finally it leads to peeling off of rock’s outer layer
(iv) Exfoliation also takes place by repeated wetting & drying of rocks surface as during wetting its outer layer absorbs moisture & expand; when they dry this moisture evaporates & they quickly shrinks, finally leading to peeling of outer layer of the rock.

2. By Frost

(i) Mainly at high altitudes & cold climates where during day cracks & joints inside rock fill with water & during night they get frozen.
(ii) This leads to increase in volume of water in rock approx, by 9 %.

Biological Weathering

(i) By Men, Animals, Insects & Vegetation
(ii) Vegetation grows into crevices of rock cracks or in courtyards or building walls.


Mass Movement


(i) Movement of weathered material down the slope due to gravitational force.
(ii) Movement may be gradual or sudden depending on the gradient of the slope, weight of the weathered debris & presence of a lubricating agent such as water.


Soil Creep


(i) Slow & gradual but more or less continuous movement of soil down the hill slopes.
(ii) Movement is not very noticeable, especially when slope is fairly gentle or when soil is well covered with grass or other vegetation.
(iii) Most common in damp soils where water act as a lubricant so that individual soil particles move over each other & over the underlying rock
(iv) Though the movement is slow, the gradual movement tilts trees, fences, posts & so on which are rooted in the soil
(v) Soil is also seen to accumulate at the foot of the slope or behind obstacles such as walls, which may burst by weight of the soil accumulated.


Soil Flow / Mud Flow (Solifluction)


(i) When the soil is completely saturated with water, soil particles easily move over each other & over the underlying rock.
(ii) Soil act as a liquid mixture & soil flow or mud flow occur.
(iii) In Ireland such flows are known as Bog-Burst.


Landslide (Slumping or Sliding)


(i) Very rapid movements resulting in large mass of soil & rock falling suddenly.
(ii) Landslide usually occurs on steep slopes & by earthquakes & volcanic activities
(iii) Landslides are often caused by the lubricating action of rain water
(iv) Slumping is usually common where permeable debris or rock layer overlie impermeable strata such as clay.
(v) Water sinking through the permeable layer is halted by the clay.
(vi) Damp clay provides a smooth slippery surface over which the upper layers slides easily.
(vii) Man often enhances the possibility of landslide by clearing natural vegetation for agriculture & housing which allows more water to penetrate through soil & rocks.


GROUNDWATER


(i) When rain falls on earth it is distributed in various ways.
(ii) Some is immediately evaporated & thus returned to atmosphere as water vapour.
(iii) Some is absorbed by plants & gradually returned to atmosphere by transpiration from the leaves of the plant.
(iv) Much of it flow into rivers & streams eventually reaching seas & oceans as run off.
(v) A considerable amount of water received from rain or snow, however, percolates downward into the soil & rocks known as groundwater.
(vi) Groundwater plays an important role in mass movement & weathering and is also important as a mean of natural water storage.
(vii) It re-enters the hydrological cycle by way of springs.
(viii) A spring is simply an outlet of stored groundwater, released at a point where water table reaches the surface (a man-made outlet for groundwater is known as well).
(ix) The amount of water available to form groundwater depends to some extent on climate, nature of the rocks (absorbing power) & seasons of the year.
(x) Absorbing power of the rock is determined mainly by its porosity, permeability & its structure.
(xi) For ex. Sandstone is both porous & permeable, Clay is highly porous but impermeable, Granite is crystalline but pervious.


Water Table


(i) Water which seeps through the ground moves downward until it reaches an impermeable layer of rock through which it can not pass.
(ii) If there is no ready outlet for the groundwater in form of spring, water accumulates above the impermeable layer & saturate the rock. The permeable rock in which the water is stored is known as aquifer & surface of saturated area is called water table.
(iii) Depth of water table varies with seasons, relief & type of rocks, as it is far below in hilltops but is close in flat surface areas.


Springs


(i) The ground water stored in the rock is released onto the surface at points where the water table reaches the surface.
(ii) A spring is simply an outlet for such water.


Types of Springs:


1. In areas of tilted strata:
(i) Permeable and impermeable rocks alternate, water emerges at the base of the permeable layers.
2. In Well Jointed Rocks:
(i) Water percolate downwards until it reaches joints
3. Where a dyke or sill or impermeable rock is intruded through permeable rocks
4. In limestone or chalk escarpments.
5. In karst regions rivers often disappear under ground. Sometimes called a vauclusian spring but is better referred to as a resurgence.


Wells


(i) Stored water below ground
(ii) Important type of well- Artesian well, which owing to the nature of its formation is quite distinctive.
(iii) Where rock layers have been down folded into a basin shape.
(iv) Permeable strata such as chalk or limestone may be sandwiched between impermeable layers such as clay.


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