Geomorphic processes; Weathering, mass wasting, erosion and deposition,soil formation,Landscape cycles, ideas of Davis and Penck

 

Table of content

• Weathering
• Mass Wasting
• Erosion and Deposition
• Soil Formation
• Landscape (Geological) Cycles
• Davis Cycle
• Penck Cycle

Weathering

 

Weathering is the general term applied to the combined action of all processes that cause rock to disintegrate physically and decompose chemically because of ex- posure near the Earth’s surface through the elements of weather. Among these elements temperature, rainfall, frost, fog and ice are the important ones. Weathering begins as soon as rocks come in contact with one or more than one elements of weather on the surface of the earth. In nature, generally both the disintegration and decomposition act together at the sametime and assist each other. We must remember that the weathered material (i.e. disintegrated and decomposed) lie in situ (i.e. at its original position). In this process no transportation or movement of material is involved other than its falling down under the force of gravity.

Weathering is the response of rocks to a changing environment. For example, plutonic rocks form under conditions at high pressures and temperatures. At the Earth’s surface they are not as stable as the conditions under which they formed. In response to the environmental change, they gradually weather (transform to more stable minerals).

Different types of Weathering are:-

1. Physical Weathering :-The mechanical breakup or disintegration of rock doesn’t change mineral makeup. It creates broken fragments or “detritus.” which are classified by size:

• Coarse-grained – Boulders, Cobbles, and Pebbles.
• Medium-grained – Sand
• Fine-grained – Silt and clay (mud).

Various process of Physical weathering are:-

• Development of Joints – Joints are regularly spaced fractures or cracks in rocks that show no offset across the fracture (fractures that show an offset are called faults).
• Crystal Growth – As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals. As these crystals grow they may exert an outward force that can expand or weaken rocks.
• Thermal Expansion – Although daily heating and cooling of rocks do not seem to have an effect, sudden exposure to high temperature, such as in a forest or grass fire may cause expansion and eventual breakage of rock. Campfire example.
• Root Wedging – Plant roots can extend into fractures and grow, causing expansion of the fracture. Growth of plants can break rock – look at the sidewalks of New Orleans for example.
• Animal Activity – Animals burrowing or moving through cracks can break rock.
• Frost Wedging – Upon freezing, there is an increase in the volume of the water (that’s why we use antifreeze in auto engines or why the pipes break in New Orleans during the rare freeze). As the water freezes it expands and
  exerts a force on its surroundings. Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles.

2. Chemcial weathering :-involves a chemical transformation of rock into one or more new compounds.  A group of weathering processes viz; solution , carnonation, hydration , oxidation and reduction acts on the roks to decompose, dissolve orreduce them to a fine clastic state through chemical reactions by oxygen ,surface /soil water and other acids. Water and air along with heat must be present to speed up all chemical reactions. Over and above the carbon dioxide present in the air, decomposition of plants and animals increases the quanitity of carbon dioxide underground . Chamical weathering involves four major processes:

• Oxidation is the process in which atmospheric oxygen reacts with the rock to produce oxides. The process is called oxidation. Greatest impact of this process is observed on ferrous minerals. Oxygen present in humid air reacts with iron grains in the rocks to form a yellow or red oxide of iron. This is called rusting of the iron. Rust decomposes rocks completely with passage of time.
• Carbonation is the process by which various types of carbonates are formed. Some of these carbonates are soluble in water. For example, when rain water con- taining carbon dioxide passes through pervious limestone rocks, the rock joints enlarge due to the action of carbonic acid. The joints enlarge in size and lime is removed in solution. This type of breakdown of rocks is called carbonation.
• Hydration is the process by which water is absorbed by the minerals of the rock. Due to the absorption of water by the rock, its volume increases and the grains lose their shape. Feldspar, for example, is changed into kaolin through hydration. Kaolin on Vindhyan Hills near Jabalpur has been formed in this manner.
• Solution is the process in which some of the minerals get dissolved in water. They are therefore removed in solution. Rock salt and gypsum are removed by this process.

3. Biotic weathering :- is a type of weathering that is caused by living organisms. Most often the culprit ofbiotic weathering are plant roots. These roots can extend downward, deep into rock cracks in search of water, and nutrients. In the process they act as a wedge, widening and extending the cracks.

Mass Wasting

 

Mass wasting is defined as the down slope movement of rock and regolith near the Earth’s surface mainly due to the force of gravity.   Mass movements are an important part of the erosional process, as it moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can then pick up the material and move it to even lower elevations.   Mass movement processes are occurring continuously on all slopes; some act very slowly, others occur very suddenly, often with disastrous results.  Any perceptible down slope movement of rock or regolith is often referred to in general terms as a landslide.  Landslides, however, can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. Mass wasting can be classified as:-

 

• Slope Failures – a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.
• Sediment Flows – debris flows down hill mixed with water or air.

 

Erosion and Deposition

 

Soil erosion is the deterioration of soil by the physical movement of soil particles from a given site. Wind, water, ice, animals, and the use of tools by man are usually the main causes of soil erosion. It is a natural process which usually does not cause any major problems. It becomes a problem when human activity causes it to occur much faster than under normal conditions.The removal of soil at a greater rate than its replacement by natural agencies (water, wind etc.) is known as soil erosion.
Soil erosion is of four types which are as follows:-

• Wind Erosion :-Winds carry away vast quantity of fine soil particles and sand from deserts and spread it over adjoining cultivated land and thus destroy their fertility. This type of erosion is known as wind erosion. It takes place in and around all desert regions of the world. In India, over one lakh kilometers of land is under Thar Desert, spread over parts of Gujarat, Haryana, Punjab and Rajasthan states. These areas are subject to intense wind erosion.
• Sheet Erosion :-Water when moves as a sheet takes away thin layers of soil. This type of erosion is called sheet erosion. Such type of erosion is most common along the river beds and areas affected by floods. In the long run, the soil is com- pletely exhausted due to removal of top soil and becomes infertile.
• Rill Erosion :-The removal of surface material usually soil, by the action of running water. The processes create numerous tiny channels (rills) a few centimeters in depth, most of which carry water only during storms.
• Gully Erosion :-When water moves as a channel down the slope, it scoops out the soil and forms gullies which gradually multiply and in the long run spread over a wide area. This type of erosion is called gully erosion. The land thus dissected is called bad lands or ravines. In our country, the two rivers Chambal and Yamuna are famous for their ravines in U.P. and M.P. states.

Deposition / Sedimentation – occurs when sediment settles out as winds/water current die down, or as glaciers melt. When sediment is transported and deposited, it leaves clues to the mode of transport and deposition. For example, if the mode of transport is by sliding down a slope, the deposits that result are generally chaotic in nature, and show a wide variety of particle sizes. Grain size and the interrelationship between grains gives the resulting sediment texture. Thus, we can use the texture of the resulting deposits to give us clues to the mode of transport and deposition. Sorting – The degree of uniformity of grain size. Particles become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density.

Soil Formation

 

Soil consists of rock and sediment that has been modified by physical and chemical interaction with organic material and rainwater, over time, to produce a substrate that can support the growth of plants.Soil is the uppermost layer of the land surface that plants use and depend on for nutrients, water and physical support.

Factors of soil formation are:-

• Parent material: soil formation depends on the mineral material, or organic material from which the soil is formed. Soils will carry the characteristics of its parent material such as color, texture, structure, mineral composition and so on. For example, if soils are formed from an area with large rocks (parent rocks) of red sandstone, the soils will also be red in color and have the same feel as its parent material.
• Time: Soils can take many years to form. Younger soils have some characteristics from their parent material, but as they age, the addition of organic matter, exposure to moisture and other environmental factors may change its features. With time, they settle and are buried deeper below the surface, taking time to transform. Eventually they may change from one soil type to another.
• Climate:Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils. Moisture determines the chemical and biological reactions that will occur as the soils are formed. Warmer climate with more rainfall means more vegetative cover and more animal action. It also means more runoff, more percolation and more water erosion. They all help to determine the kind of soils in an area.
• Relief:i.e. the landscape position and the slopes it has. Steep, long slopes mean water will run down faster and potentially erode the surfaces of slopes. The effect will be poor soils on the slopes, and richer deposits at the foot of the slopes. Also, slopes may be exposed to more direct sunlight, which may dry out soil moisture and render it less fertile.
• Organisms:The source and richness of organic matter is down to the living things (plants and animals) that live on and in the soils. Plants in particular, provide lots of vegetative residue that are added to soils. Their roots also hold the soils and protect them from wind and water erosion. They shelter the soils from the sun and other environmental conditions, helping the soils to retain the needed moisture for chemical and biological reactions. Fungi, bacteria, insects, earthworms, and burrowing animals help with soil aeration. Worms help breakdown organic matter and aid decomposition. Animal droppings, dead insects and animals result in more decaying organic matter. Microorganisms also help with mineral and nutrient cycling and chemical reactions.

 

Davis Cycle

 

After the upliftment of landmass by the tectonic forces the process of denudation is started. The rivers, valleys and associated landforms passes through distinctive stages, provided that there has been no significant interference by earth movements or by changes of sea-level or climate. This idealized concept of landscape evolution was introduced to geomorphology more than sixty years ago by W.M. Davis, who referred to the whole sequence of stage as a Cycle of Erosion.

The basic goal of Davisian model of geographical cycle and general theory of landform development was to provide basis for a systematic descriptions and genetic classification of landforms. According to this concept a landscape has a definite life history, and as the processes of land structure operate on it the surface features are marked by several changes in its life time. Thus, the evolution of landscape passes through a cycle, and cycle follows a definite sequence of development.

The successive stage of developmental sequences can be divided into three parts and may be identified as youth, maturity and old age. Davis presentation of scheme was both vigorous and vivid and his colourful analogy of the human life and landscapes both passing through the stages of youth, maturity and old age caught the imagination of scientific world.

• Youth:The uplift is complete and has stopped. Immediately erosion of the uplifted block sets in. The streams follow initial irregularities available without adjusting to the structure. These are consequent streams. The floors of the valley suffer down cutting while the summits remain almost unaffected. Increased relief heralds the beginning of mature age
• Maturity:At this stage, the vertical erosion slows down and the horizontal action increases. A characteristic feature is the erosion of mountain tops at a faster rate than lowering of the valley floor. The coming closer of lines ‘A’ and ‘B’ indicates emergence of a gentle slope. The subsequent streams gain importance now.
• Old Age:A gentle gradient, accentuated by horizontal action and deposition, reduces the erosion intensity. A thick layer of sediment represents the earlier erosion activity. The landforms get mellowed—lines ‘A’ and ‘B’ run parallel to each other. Relicts of mountains or monad knocks are dotting the water divides and a featureless plain—peneplane is produced.

In order to understand the evolution of a particular landscape it is extremely important to know the stage of development. But the geographical structure and the nature of rocks also exert an important influence on the fashioning of landscapes is a function of structure, process and time (as called as stage by the followers of Davis). These three factors are called as ‘Trio of Davis’.

Structure :means lithological (rock types) and structural characteristics (folding, faulting, joints etc.) of rocks. Time was not only used in temporal context but it was also used as a process itself leading to an inevitable progression of change of landform. Process means the agent of denudation including both, weathering and erosion (running water in the case of geographical cycle).

Process:Implies the factors or agents responsible for weathering and erosion.

Time:Implies the stage at which the cycle is—youth, maturity or old age.

Penck Cycle 

 

According to German geomorphologist Walther Penck, the characteristics of landforms of a given region are related to the tectonic activity of that region. Contrary to the Davisian concept that “landscape is a function of structure, process and time (stage)”, Penck put forward his view that geomorphic forms are an expression of the phase and rate of uplift in relation to the rate of degradation, where it is assumed that interaction between the two factors, uplift and degradation, is continuous. According to Penck’s view the landforms observed at any given site give expression to the relation between the two factors of uplift and degradation that has been or is in effect, and not to a stage in a progressive sequence.

Penck proposed three types of valley slopes on the basis of erosional intensity acting on crustal movements.

1. Straight slope:Indicating uniform erosion intensity and a uniform development of landforms or ‘Gleichformige Entwickelung’ in German.
2. Convex slope:Indicating waxing erosion intensity and a waxing development of landforms or ‘Aufsteigende Entwickelung.
3. Concave slope:Indicating waning erosion intensity and a waning development of landforms or ‘Absteigende Entwickelung.’

Different Phases according to Penck are:-

(a) Phase of waxing rate of landform development (Aufsteigende Entwickelung)
Endogenetic forces cause the slow rise of the initial land surface (Primarumpf) but later on the upliftment is rapid.
In this phase, because of upliftment and the increase in the channel gradient and stream velocity rivers continue to degrade their valleys with accelerated rate of valley deepening.
The rate of upliftment is faster than the rate of down-cutting. It results in the formation of gorges and narrow V-shaped valleys. Since the upliftment of landmass far exceeds the valley deepening, the absolute height goes on increasing.
Altitude of the summit of interfluves and valley bottom continues to increase due to the faster rate of upliftment than that of the vertical erosion.
This phase is characterized by the maximum altitude and the maximum relief (relative heights of the valley floors).

(b) Phase of uniform development of land form (Gleichformige Entwickelung)
This phase may be divided into three sub-phases on the basis of upliftment and consequent degradation

(i) The first sub-phase is characterised by the continuance of accelerated rate of uplift. The absolute height continues to increase because the rate of upliftment is still greater than the rate of down-cutting.
The maximum altitude or the absolute relief is achieved, but relative relief remains unaffected because the rate of valley deepening is almost equal to the rate of lowering of the summits of stream interfluves.
The valley walls are steep. This is known as the phase of uniform development because of uniformity in the rate of valley deepening and lowering of divide summits.
(ii) In the second sub-phase the absolute relief neither increases nor decreases. This is due to the fact that rate of upliftment and the rate of erosion are the same. However, in this phase the absolute height and the relative relief’s are unchanged. So this may be called the phase of uniform development of landforms.
(iii) In this sub-phase there is no more upliftment of land.

(c) Phase of Wanning development of landscape (Absteigende Entwickelung)
The erosional processes dominate in this phase. The lateral erosion rather than vertical erosion is more important. There is progressive decrease in the height of the landforms. In other words, the absolute and the relative relief decline.
The valley side slope consists of two parts, the upper and the lower. The upper segment continues to have steep angle which is called as gravity slope.
The lower segment of the slope is called wash slope. The wash slope is composed of talus materials of lower inclination which is formed at the base of valley sides.
The later part of this phase is marked by the presence of inselbergs and a series of concave wash slopes.
This type of extensive surface produced at the fag end of absteigende entwickelung has been labelled is endrumpf which may be equivalent to peneplain as envisaged by Davis in his cycle concept. Thus, the cycle of landscape development as envisaged by Penck ends in endrumpf.