WEATHERING AND MASS WASTING

Weathering:

Weathering is the mechanical breakdown or chemical decay of rocks insitu as a result of their exposure at or near the earth's surface.

Qn. Explain the meaning of the word INSITU.

Agents of weathering are

• air, 
• water, 
• ice and 
• biological organisms.

Factors influencing Weathering

Climate:

Elements of climate which influence weathering include temperature, frost, rain humidity and sunshine. The rate rate of weathering differs from one climatic region to another. e.g. (hot equatorial regions and cold temperate regions) 

Topography:

Steep slopes are highly likely to encounter higher rates of erosion than gentle rolling slopes. Materials on steep slopes are washed away quickly exposing rocks to agents of erosion compared to low lying areas where it takes time before the top rock material is washed away.

Chemical composition:

Variation in color and mineral composition affects the rate of weathering. Rocks that are dark in color absorb more heat than shiny rocks which tend to reflect light more often. The darker rocks absorb heat more and may break up due to expansion and contraction. Dark coloured rocks break up more easily than shiny ones.

Rock structure:

This refers to the way a rock is arranged. Some are arrange in layers while some are arranged in blocks. 
This arrangement affects the weathering process in each type of rock. 
For example, where jointing is circular, rocks undergo spheroidal weathering. 
Rocks may also be permeable or impermeable.

  Rock texture      

•       Texture refers to the size of the particles of grains in a rock.

•       The size of particles determines the rate of weathering in that rock.

•       Fine textured rocks like limestone are more affected by weathering.

 Biological Organisms

•       These vary from small ones like bacteria to very large ones like plants, animals and people.

•       Bacteria facilitates the rotting of organisms leading to the production of organic acids, which react with rocks and break up.

•       Roots of big plants penetrate rocks causing them to crack.

•       Human activities like drilling, blasting and digging accelerate the rate of weathering because they expose rocks.

•  Types and Processes of Weathering

•       There are three types of weathering :

–      Mechanical

–      Chemical

–      Biological

•       Mechanical Weathering 

•       Is the break up of rock material without any alteration in its chemical composition. There are no changes in constituent minerals during the break up.

•       It involves the fracturing of rocks, into large blocks, which break up into smaller rocks. These disintegrate into smaller fragments.

•       The main causes include:

–      Temperature changes

–      Frost action

–      Pressure release (unloading)

–      Rain water

–      Crystal growth

•       Temperature changes

•       Changes in temperature lead to physical weathering processes such as

•       block disintegration

•       Exfoliation

•       Granular disintegration

•       Block disintegration: Also known as block separation. Rocks break away from the original rock mass along joints and bedding planes which are lines of weakness.

–      Joints are created by shrinkage as magma cools or by tectonic forces.

–      Its caused by large diurnal temperature changes e.g. in deserts, where very high temperature are experienced during the day and very low temperatures during the night.

–      High temperatures cause rocks to expand while cool temperatures lead to rapid cooling at night.

–      Repeated expansion and contraction causes stress on rocks leading to cracks that eventually cause the rocks to separate.

•       Exfoliation: is a process through which rocks peel resulting in the formation of curved rock shells.

–      This process takes place in hot climates.

–      Rocks are poor conductors of heat. When heated, the outer part expands faster than the inner part and when cooled, it contracts faster than the interior. This process when repeated leads to the breaking away of the outer part. The curved nature explains the differences in the cooling patterns of the rocks.

–      This may also occur because of the occurrence of different minerals within a rock.

–      This may also occur because of forest fires.

–      This process is also called spalling or onion weathering.

–      Exfoliation results in the formation of exfoliation domes. These are characterized by rock hills.

–      They have a smooth surface and may have accumulated rock masses at the foot of the mountain.

–      Granular Disintegration: This is the break up of rock into small particles or grains.

•       Its common in coarse grained intrusive igneous rocks such as granite and in coarse clastic sedimentary rocks (sandstone)

•       It is caused by alternate heating and cooling of rocks which are made of different mineral particles.

•       The difference in expansion and contraction rates leads to stress and strain within rocks. Rocks break off from the main rock mass.

•       A large smooth sided dome may be formed from the granular disintegration of homogeneous rock.

   Frost Action

•       This is the action of ice on rocks which causes them to shatter along new lines of weakness. Within the tropics, it occurs on high mountains such as Ruwenzori and Kilimanjaro.

•       This is mainly caused by the freeze thaw action.

•       When water enters cracks, joints and pore spaces of a rock, it freezes and grows crystals which exert pressure widening cracks.

•       Higher temperatures cause the ice to melt leaving wide cracks, which are filled with water

•        A fall in temperature causes the water to freeze again causing cracks to widen further causing the outer part of the rock to break away.

•       This may also occur along joints in rocks leading to block separation.

•       Frost action results in sharp pointed rugged rock masses. High mountains form pyramidal peaks.

•       At base of the main rock, mass angular pieces accumulate as mounds of scree or talus.

Pressure Release

•       This process is referred as unloading.

•       Deep seated igneous and metamorphic rocks are under intense pressure caused by mountain building processes.

•       This pressure is maintained by the weight the overlying rock materials such as sediments deposited over the years.

•       Denudation removes the overlying rocks exposing the underlying rocks.

•       The upper parts expand due to reduced pressure leading to shells of rock breaking away through a process called sheeting.

•       The sheets of rock that break off are parallel to the ground surface.

•       Unloading is common in quarries where unloading is very rapid because people quickly remove the overlying rock.

Rain Water

•       Rain water causes mechanical weathering of rocks through a process known as slaking.

•       Its common in shale which has a high amount of clay mineral.

•       Its common in areas with wet and dry seasons.

•       During wet seasons, clay absorbs moisture making them to swell, while during the dry season they lose much water causing them to shrink.

•       This alternate wetting and drying of the mineral is repeated over time causes particles to break off in pencil like fragments. Rock outcrops of shale are found around port Reitz.

Crystal Growth

•       This process is common in dry climatic regions.

•       It involves the growth of salt crystals within a rock.

•       During dry seasons, water is drawn to the surface by capillary force and once on the surface, it evaporates.

•       As it evaporates, tiny salt particles formerly dissolved in water are deposited in the pores and cracks, where they accumulate as evaporation continues through a process called crystallization.

•       Crystals exert force that pushes some rock particles off the main rock (especially coarse grained rocks) such as sand stone.

•       The outer parts of the rock undergo granular disintegration forming sand.

•       Crystals widen pores forming alveoli. A further widening of alveoli form pits calledtafoni.

•       It also leads to the formation of rock niches and shallow caves at the bases of cliffs as well as rock arches and tafoni.

Chemical weathering

This involves changes in the chemical composition of minerals making up rocks.

New Minerals may be formed through decomposition and decay of rocks.

Chemical decay is affected by the following:

·         Presence of moisture

·         Gases

·         High temperatures

Processes involved in chemical weathering include:

• ·         Solution
• ·         Carbonation
• ·         Hydrolysis
• ·         Hydration
• ·         Oxidation

Solution:

·         Is a process through which minerals in rocks directly dissolve in water without changing their chemical composition without being altered.

·         It involves a change of state of the minerals from solid to solution.

·         This happens in rocks which are soluble in water.

·         Minerals include:

·         Olivine, Orthoclase and hornblende (fairly soluble), rock salt and gypsum (very soluble in water)

·         Water must be present for this process to take place.

Qn. Name places in which solution takes place in Africa (Certificate Geo pg. 60)

Carbonation:

·         Involves the direct reaction of rain water with rocks.

·         It’s common in limestone and chalk rocks.

·         Rain water combines with carbon dioxide in the atmosphere to form carbonic acid which reacts with limestone rocks. Limestone rocks contain calcite mineral (calcium carbonate).

·         Carbonic acid may also be formed in swamps, ponds, rivers, lakes or seas which lead to the same reaction with limestone rocks.

·         The process of carbonation is faster in cool areas than warmer areas because the rate of carbon dioxide absorption is faster in low temperatures.

Qn. Give examples of features formed in Karst regions.

Hydrolysis:

·         This involves the reaction of water with mineral substances in rocks to form new compounds.

·         This is common in igneous rocks.

·         This process is common in humid regions where water breaks into ions and combines with minerals in rocks.

·         Orthoclase undergoes hydrolysis to form kaolin and colloidal silica.

·         Kaolin is a rock that absorbs water and swells leading to its split. This leads to granular disintegration.

·         Basic rocks (basalt and gabbro) break down to produce iron. The iron combines with water to form limonite (the red coloring in rocks).

·         Hydrolysis also leads to peeling off in curved shells in fine grained igneous rocks leading to spheroidal weathering.

Qn. Define the term Saprolite. (Certificate geo F3Pg 61)

Hydration:

·         This is the process in which certain minerals in a rock take up water causing them to swell or expand. These are called hygroscopic.

·         Expansion leads to stress within a rock leading to disintegration.

Oxidation:

·         This is a process in which the minerals in the rock combine with oxygen, forming new minerals.

·         Oxidation is important in the presence of moisture.

·         Iron containing rocks are mainly affected by this process.

·         Ferrous oxide is converted into ferric oxide.

Biological Weathering

Is the breakdown of rocks due to the action of biological organisms which include:

·         Plants

·         Animals

·         People

In biological weathering, rocks are broken down in mechanical and biological weathering processes.

Action of Plants:

·         Plant roots grow into the joints and cracks of rocks. As they grow bigger, they widen the joints and cause the rocks to separate (Wedging mechanism).

·         The widening of rocks exposes rocks to moisture and air which facilitates the process of hydrolysis and solution.

·         Rotting of plants may cause organic acids to be produced. This acid reacts with minerals in rocks causing them to decay leading to a biological chemical process.

Action of animals:

·         Rabbits, moles and earthworms cause weathering through burrowing.

·         They break off small bits of rock from the main rock and expose them to weathering processes.

·         Large herds of animals such as cattle, zebra and wildebeasts pound rocks breaking them down leading to mechanical weathering.

Action of People:

·         Activities such as lasting of rocks in quarries using explosives lead to the mechanical breakdown of rocks.

·         Rocks are scooped away during open-cast mining.

·         Building and construction activities as well lead to mechanical weathering.

·         Air pollution leads to chemical weathering when rain water combines with the gases to form rain water which readily reacts with chemicals in rocks causing them to breakdown.

Relationship between various types of weathering

Most processes occur simultaneously.

·         Mechanical weathering is dominant in deserts while chemical weathering is dominant in places with lots of moisture.

·         One type of weathering may facilitate another type. E.g. Blasting of rocks exposes rocks (Biological weathering/action of people). This exposes rocks which may be penetrated by roots, widening them. This exposes rocks to moisture and gases leading to chemical weathering.

·         Mechanical weathering by block disintegration also allows roots to penetrate rocks leading to chemical weathering when moisture and gases enter the open spaces.

·         Rocks containing feldspar may be acted upon by carbonation and hydrolysis.

·         Carbonation acts on calcereous rocks turning calcite into calcium.

Weathering in different climates

Mechanical and chemical weathering take place in all climates but vary in intensity.

Equatorial climate:

·         Has high temperature and high humidity due to high rainfall throughout the year.

·         These conditions support chemical weathering which largely depends on moisture and temperature.

·         Mechanical and biological weathering may take place where there are large trees with large roots that penetrate cracks in rocks causing them to wedge apart.

·         The decay of plant material causes organic acids which react with rocks breaking them down chemically.

·         High temperatures may lead to exfoliation and granular disintegration.

Tropical Climate:

·         This has alternating wet and dry seasons.

·         Chemical and mechanical weathering processes are experienced during this period.

·         During the dry season, bare rock undergoes exfoliation and block disintegration. Crystal growth may also take place.

·         During the wet season, slaking and oxidation occurs.

·         Mechanical weathering is dominant during the dry season while chemical weathering is dominant during the wet season.

Tropical Climates:

·         Mechanical weathering is dominant throughout the year because of very high temperatures.

·         Chemical weathering is less because of limited temperature.

Mid Latitude climates:

·         Mechanical and chemical weathering takes place in these climates.

·         Frost action is common because of the low temperatures which cause the water to freeze.

·         Chemical weathering occurs more when rainfall is higher, because CO2 dissolves in water at higher rate when temperatures are lower.

Cold –Climates and mountain tops:

·         The dominant cause of weathering is frost action resulting in a mass of scree at the base of cliffs.

·         This occurs within the tropics on high mountains which at latitudes beyond 4300M.

·         Freeze thaw has also been detected in the Tibesti Mountains in the Sahara.

·         The rate of physical weathering is faster than chemical weathering.

·         Chemical weathering is limited because of low temperatures and limited amounts of liquid water.

·         Carbonation takes place because carbon dioxide dissolves in water at very low temperatures leading to the formation of carbonic acid.

Significance of Weathering:

1.       Weathering breaks down rocks masses leading to the formation of soil which is important for agriculture.

2.       It produces natural resources such as clay which is used in brick making and pottery.

3.       Weathering weakens rocks making it easier for people to exploit them through mining and quarrying.

4.       It prepares land which agents of erosion acts on. This leads to the modification of the original landforms which influences human activities.

5.       Some weathered granitic rocks offer beautiful scenery which may be used as a tourist site.