DENUDATION

• Denudation is an erosive process of breaking and removing the rocks from the surface of the earth.
• It is the wearing away of the terrestrial land by weathering, erosion, moving water, ice waves.
• Denudation is the process by which the land areas are continually being reduced and their shape modified by weathering and erosion.
• Denudation is the processes of breaking up and removing the rocks exposed on the earth surface.
• Denudation is the process by which the removal of material, through means of erosion and weathering, leads to a reduction of elevation and relief in landforms and landscapes.
• Denudation is the process of lowering and leveling of the earth surface by gradual breaking and wearing away of such earth’s features.
• Denudation is the name for the processes of erosion, leaching, stripping, and reducing the mainland due to removal of material from higher to lower areas like valleys, river valleys, lakes and seas with a permanent filling of low lands.

Denudation is the result of two main processes, Endogenous and exogenous. Endogenous process uplifts the surface and exposes the continental crust to exogenous process.

However, external forces also shape the land surface and lower the level of land by wearing it away. The agents which promote these breaking and lowering of the earth’s surface are called the agents of denudation.  Examples of agents of denudation are wind, running water, ice and waves.

Denudation involves three major denudation processes:-

• Weathering
• Mass wasting
• Erosion

 

• Weathering: Weathering is the process of decomposition of rocks into smaller particles.

This decomposition is done via three weathering means, physical, chemical and biological weathering. Decomposition and disintegration of rocks into fragments is done by physical weathering, in chemical weathering this decomposition is due to the chemical change in the rock formation, and in biological weathering animals and plants are responsible for decomposition of rocks.

 

Mass wasting: It is the dynamic process of going sloping down the weathered or loosened materials due to gravity. For example, Landslide, mud flow.

• Slow movement, e.g. soil creep, rock creep and solifluction

1. Soil and rock move very slowly downslope.
2. The movement is too slow to be observed directly.
3. However, there is evidence of this slow movement on the hill-slopes.
4. Fences, posts and telephone poles are tilted.
5. Tree trunks may bend downhill.
6. There is a thicker soil layer accumulated on lower hill-slopes.
7. Irregular soil creep or rock creep produces a stepped surface called terracettes.
8. The creep is faster on steeper slopes, but may occur on slopes where the angle is as small as 5 degrees.

Conditions favouring rapid movements include:

1. alternate heating and cooling,
2. alternate wetting and drying,
3. alternate freezing and thawing,
4. burrowing of animals,
5. trampling by animals, and
6. earthquakes and other vibrations.

 

Soil creep

1. Soil creep is the slow downslope movement of soil particles due to gravity.
2. During wet weather, the soil on the slope is saturated with water. The soil becomes bulky.
3. The surface soil grains are forced to rise upward perpendicular to the slope.
4. During dry weather, the soil on the hill-slope dries up. The soil contracts.
5. The surface soil grains move vertically downward due the gravity.
6. The movement is vertical – perpendicular to the horizontal plane, NOT perpendicular to the slope.
7. Alternate wetting and drying causes the soil grains to move downslope.

 

Rock creep

1. Rock creep is the slow downslope movement of rock fragments due to gravity.
2. Rock creep is caused by alternate heating and cooling of rock fragments.

 

Solifluction

1. Solifluction is the slow downslope movement of soil and rock debris saturated with water due to gravity.
2. Solifluction is caused by alternate freezing and thawing of soil and rock debris in high latitudes and high altitudes.

 

• Rapid movement, e.g. mud flow, landslide, rock fall.

When soil and rock move rapidly downslope, the movement can be observed directly.

Conditions favouring rapid movements include:

1. steep slope,
2. lack of surface vegetation so that the bare rock is exposed to the air,
3. rock less resistant to weathering, and
4. climate with alternate cold-dry and hot-wet season which speed up weathering.

 

Mud-flow and earth-flow

1. Mud-flow or earth-flow is the rapid downslope movement of weathered materials saturated with water due to gravity.
2. It is common after volcanic eruption when volcanic ash saturated with melting snow moves downslope.

 

Rock-fall

1. Rock-fall is the sudden free falling of rocks from a cliff or a steep slope due to gravity.
2. Partially weathered blocks of rock stands up as a tor on top of a cliff or a steep slope.
3. Heavy rainfall or earthquake may remove the underlying supporting materials.
4. This provides a ‘trigger action’ for the block of rock or tor to fall vertically from the cliff.

 

Landslide

1. Landslide is the rapid downslope movement of weathered materials due to gravity.
2. A thick layer of deeply weathered materials lies on top of an unweathered layer of parent bedrock.
3. The surface between these two layers forms the slide plane.
4. Weathered layer moves downslope along the slide plane.
5. Slumping is a landslide on a curved slope.

 

• Erosion: Erosion is the movement of rocks and weathered materials due to natural agents like rivers and glaciers.

 

Denudation measured on the basis of stream load measurements using gauging stations. This stream load waste is converted into the volumetric units and then estimate the fraction of this volume with the area of the watershed. The final result shows the estimate of wearing down of Earth’s surface in inches per 1000 years.

The rock surface of the continents of the Earth, on which we are living, is undergoing constant and continuous destruction, a process called denudation.

Rocks exposed at Earth’s surface are constantly being altered by water, air, changing temperature and other environmental factors.  The term weathering refers to the group of destructive processes that change the physical and chemical character of rock at or near Earth’s surface.  The tightly bound crystals of any rock can be loosened and altered to new minerals by weathering.

It is important to distinguish between weathering and erosion and between erosion and transportation.  Weathering breaks down rocks that are either stationary or moving.  Erosion is the picking up or physical removal of rock particles by an agent such as streams or glaciers. Weathering helps break down a solid rock into loose particles that are easily eroded.

Most eroded rock particles are at least partially weathered, but rock can be eroded before it has weathered at all.  A stream can erode weathered or un-weathered rock fragments.

After a rock fragment is picked up (eroded), it is transported. Transportation is the movement of eroded particles by agents such as rivers, waves, glaciers, or wind. Weathering processes continue during transportation.

A boulder being transported by a stream can be physically worn down and chemically altered as it is carried along by the water.

 

Types of weathering.

These are three types of weathering namely:-

• Mechanical weathering or disintegration.
• Chemical weathering or decomposition
• Biological weathering.

 

• Mechanical weathering or disintegration

This is the breakdown of rocks into small particles by the action of temperature, by impact from rain drops and by the abrasion from mineral particles carried in the wind.

Products of mechanical weathering

The products of mechanical weathering include everything from huge boulders found beneath the cliffs to the smallest silt.

Processes most commonly involved in mechanical weathering.

1. Mechanical unloading: This is the vertical expansion due to the reduction of vertical load by erosion. This will open existing fractures and may permit the creation of new fractures.

2. Mechanical loading: This is impact on rock and abrasion, by sand and silt size.  Wind borne particles that occur in deserts, impact on soil and weak rocks, by raindrops during intense rainfall storms.

3. Thermal loading: This is expansion by freezing water in pores and fractures in cold regions or by the heating of rocks in hot regions.

4. Welting and drying: Expansion and contraction associated with the repeated absorption and loss of water molecules from mineral surfaces and structures.

5. Crystallization: This is expansion of pores and fissures by crystallization within them, of minerals that were originally in solution.

Note: Expansion is only severe when crystallization occurs within a confined place.

6. Pneumatic loading: The repeated loading by waving of air trapped at the head of fractures exposed in the wave zones of a sea cliff.

 

Some terms used

Frost-heaving- gentle rising and falling in regular alternation: This occurs when the freezing of the soil results in the formation of layers of segregated ice at shallow depths. The frost heaving of foundations of buildings is also caused by forces originating in the active layer and is a common problem is the Arctic – extremely cold.

Buildings which are heated can be placed a little above ground- level with a large air space beneath them.  Cold air in winter then circulates under the building and contracts the heating effect from it. Piped services to the buildings are also placed above ground level to prevent their rupture by ground movement.

Insolation: In hot climates, when a rock surface is exposed to a considerable daily range of temperature as arid and semi-arid regions, the expansion that occurs during the day and the contraction at night, constantly repeated, weaken the structure of the rock.  The outer heated layers tend to pull away from the cooler rock underneath and flakes and slabs split off, a process known as exfoliation.  This weathering is called Insolation.

Un loading: One result of denudation is to reduce the load on an area as the removal of the rock cover proceeds, leading to relief of stress in the rocks.  The unloading allows a small vertical expansion which gives rise to the formation of “sheet” of rock by the opening of joints parallel to the ground surface. This is frequency seen in igneous rocks such as granite intrusion, where the sheet jointing is developed in the upper part of the mass; the ‘Sheets’ or slabs of a rock are commonly up to a meter or so in thickness

 

• Chemical weathering

This is the breakdown of minerals into new compounds by the action of chemical agents; such as acid in the air, in rain and in river water; although they act slowly, produce noticeable effects especially in soluble rocks. The rate of chemical weathering depends on temperature, the surface area and the amount of water.  Chemical weathering causes the old minerals to disintegrate and to form new minerals. Minerals which are originally formed at lower temperatures in the original igneous rocks during the process of cooling of magma prove more resistant to chemical weathering compared to those which were formed at high temperatures during the cooling of Magma.

Processes of chemical weathering

Processes are most commonly involved in chemical weathering are listed below and their rate of operation depends upon the presence of water and is greater in wet climates than in dry climates. Some commonly occurring processes in chemical weathering are:

 

1. Solution.

This is dissociation of minerals into ions greatly aided by the presence of carbon dioxide (CO2) in the soil profile, which forms carbonic acid (H2Co3) with percolating rain water.

 

2. Oxidation

This is the combination of oxygen (O2) with a mineral to form oxides and hydroxides or any other reaction in which the oxidation number of the oxidized elements is increased.

 

3. Reduction.

The release of oxygen (O2) from a mineral to its surrounding environment; ions leave the mineral structure as the oxidation number of the reduced element is decreased.

 

4. Hydration.

This is the absorption of water molecules into the mineral structure.

Note: This normally results in expansion; some clay expands as much as 60% and by admitting water hastens the process of solution, oxidation reduction and hydrolysis.

 

5. Hydrolysis

Hydrolysis is the reaction with water. Hydrogen ions in percolating water replace mineral cations; no oxidation reduction occurs. In other words, hydrolysis is a chemical reaction in which a compound reacts with water causing decomposition and the production of two or more other compounds.

 

6. Leaching

This is the migration of ions produced by the above processes. Leach. (To drain away from soil when dissolved in rain water, lose a mineral or chemical dissolved in rain water.

Note:  The mobility of irons depends upon their ionic potential. Calcium (Ca), Magnesium (Mg), Sodium (Na), and Potassium (K) are easily leached by moving sodium water, Iron (Fe) is more resistant, Silicon (Si) is difficult to leach and Aluminum (Al) is almost immobile.

 

7. Cation exchange.

This is the absorption onto the surface of negatively clay of positively charged cations in solution especially Calcium (Ca), Hydrogen (H), Potassium (K), and Magnesium (Mg).

 

• Biological weathering

Biological weathering is the physical disintegration or chemical decomposition by the actions of animals and plants.

Biological weathering by physical processes

The growth of plant roots may enlarge and prise open the joints or cracks of rocks.

Worms and rabbits may help to loosen the soil and bring fresh soil to the surface from below.

Biological weathering by chemical processes

Plants may help to promote chemical weathering by releasing humic acid.

Bacteria, in the presence of water, may decompose certain minerals from the soil.

 

Factors Affecting Denudation

The rate of denudation depends on the following factors:

1. Nature of relief: The higher the relief of a particular region, the higher the rate of denudation and the lower the relief of that region, the lower the rate of denudation.
2. The structure of the earth: The softer the structure of the earth, the greater the rate of denudation and vice versa.
3. Local climate of an area: Wet climates tends of increase the rate of denudation more than dry climate.
4. Influence of man: The higher the rate of man’s activities on earth surface, the greater the rate of denudation and vice versa.

 

Denudation involves four stages which include:

• Weathering: This is the gradual disintegration of rocks by physical, biological and chemical processes.
• Erosion: This is the active wearing away of the earth’s surface by agents of denudation such as running water, wind, ice and waves.
• Transportation: This is the active removal of eroded materials to a new position.
• Deposition: This is the dumping of the debris or eroded elements in certain parts of the earth where it accumulates to form soils.