Formation of Soil

      Hi, Welcome to "The Civil Engineer", About myself, I am a Civil Engineer with 9+ years of Experience in Civil Engineering and I write articles about Civil Engineering and General things on this website on a regular basis. In this article, I explained about "Formation of Soil".

    The term Soil is derived from the Latin word "solium" which means the upper layer of the earth which can be dug or ploughed this is the definition of soil by agricultural experts in Agronomy

    But if you take Geology, they will say soil is the unconsolidated sediments of the underlying rocksthey say soil as mantle or regolith

     Both of these terms are collectively known as the soil in Soil Mechanics.

    The Exact definition of soil in Soil Mechanics or Geo Tech Engineering is given by, the solid particles created by the disintegration of rocks in the voids between the solid particles are filled with water, air and in some cases, organic particles known as soil.

    The Rock have strong cohesive bonds between the particle when these bonds become weak due to natural or anthropogenic(man-made) process they become disintegrated into granular soil.

    From the above discussion, I think you will be clear with these two things, In Agriculture Agronomy they refer to soil only as the topsoil(Soft soil)  and in geology, they refer to regolith as soil which both the topsoil(Soft soil) and the big boulder(Hard Soil) below them up to the real rock strata.

    Hence what the Geology refers to as Regolith is known as Soil(both soft soil and hard soil) as Soil in Soil Mechanics or Geo Tech Engineering.

Soil Formation Cycle

The process of formation of the rocks and the soil is a cyclic process and it repeats continuously, but between these two there are various stages which the soil enters.

    So, Everything start's with the Parent Rocks. The Parent Rock is the Rock from which the soil is formed. It May be from a mountain(High ground) or on a plane etc.

    Due to the Weathering Process (Disintegration of rocks) such as external internal forces such as wind, rain, heat from the sun, frost and man-made drilling etc., the rocks get into small particles which we call soil is formed. 

    Then the formed particles may remain above the parent rock, such kind of soil is known as residual soil or sedentary soil. In the residual type of soil when we as Civil engineers test this soil we may find boulders below the soil. Hence Boulder is an indication of residual soil. The size of the soil particles is finer at the top and increases when going deeper into the residual soil and the engineering properties also vary from the top layer to the bottom one.

    Due to weathering, the weathered particles(soil) get transported due to transportation agents such as wind, water and landslides etc to new places and get deposited above another rock, this kind of soil is known as transported soil.

    Hence Overall, the Soil undergoes four-stage,

  1. Erosion or Weathering
  2. Transportation
  3. Deposition or Sedimentation
  4. Upheaval 

    After Upheaval then, that upheaved soil goes again for erosion and the cyclic process begin's once again.

Types of Soil Formation

Soil is the weathered or degraded material of the rocks. Here I explained the various process of disintegration of rocks into soil. If you want a better understanding of the concept check my articles on the Definition of Soil and the Soil formation Cycle.

    The weathering of rocks into soil takes place either physically or chemically.

Physical weathering of Rocks into Soil

    In Physical weathering, the chemical composition of the weathered or eroded soil remains the same as that of the parent rock. The Various types of Physical weathering are as follows,

  1. Thermal weathering - Due to the change in the temperature of the surroundings such as deserts, the temperature during the day will be too high and during the night, the temperature will be too low like very cold this kind of huge temperature variation make the particles in the rocks to expand and contract at different level's due to the amount of temperature they receive or the composition of the rock may have different elements which have different thermal expansion. Due to this the rocks get disintegrated or weathered into the soil.
  2. Wedging of Ice - If there is a crack in the rock after rain the crack got filled with water then during the night like in deserts, the water becomes ice due to low temperature. The volume of Ice is more than water hence the expansion of the volume of water due to ice formation exerts pressure on the rocks and makes them disintegrate or weather into the soil.
  3. Spreading of Roots of Plants - In cracks of the rocks the seeds of the plants may fall and they start growing these roots penetrated deep into the rocks and put pressure that makes the rock disintegrate or weather into rocks.
  4. Abrasion -  The flowing river over rocks or the moving wind around the rock continuously creates an erosion or disintegration or abrasion of the rock into smaller soil particles.
Chemical weathering of Rocks into Soil

    In Chemical weathering, erosion or weathering makes a change in the chemical composition of the weathered soil which is different from the chemical composition of the parent rock. Some of the chemical weathering are listed below, 
  1. Hydration - In this process, the parent rock minerals react with the water and erode or weather to form the soil particles.
  2. Carbonation - In this process, the water reacts with the carbon dioxide in the atmosphere which forms carbonic acid, this carbonic acid reacts with the rocks and erodes them to form soil.
  3. Oxidation - In Oxidation, the oxygen ions combine with the Minerals in the rocks and cause erosion to form soil. Like rusting of iron.
  4. Solution - The rock minerals form a solution with water and cause erosion in the rocks into the soil.
  5. Hydrolysis -  In this process, the water splits into H+ and OH- ions. the Hydrogen cations react with the minerals of the rock and erode to form soil with a new chemical composition which is different from that of the parent rock.
Note: Clayey soils are formed by Chemical weathering or erosion of rocks.

Transportation of Soil

 The soil formed from the erosion or wethering of the rocks gets transported to various places by agents of transportation like,

  1. Water
  2. Wind
  3. Glacier
  4. Gravity
  5. The combined action of agents
Transportation by Water
    The transportation of the soil particle by water is mostly tank care by the rivers. Rivers carry soil and boulder by eroding the rocks from the top of the mountains through suspension and rolling over. When the velocity or the force of the river is very high they even take on large boulders then when the speed starts decreasing they deposit the larger and heavier ones first.

    Hence you see in the course of a river the boulder are kept near the origin or in the middle of the course of the river, where the fine particles were at the valleys or near the place where the river merges with the sea.

    The deposits that were formed by the river are known as Alluvial deposits.  If the deposits were formed in a lake it is known as lacustrine deposits. If the soil is deposited by the river into an ocean or sea it is known as a Marine deposit.

Transportation by Wind 

   Wind transports particles from fine particles to small size boulders or even big ones everything is based on the intensity of the winds. For example, you would have seen the dust storm produced in the deserts in movies like "The Mummy". The deposits formed by the transportation of the soil by wind are known as aeolian deposits.

   Some examples of wind deposits are sand dunes, loess etc.

Transportation by Glacier 

   The Glacial movement over rocks makes the rocks erode and the eroded or weathered particles of soil are carried too far away distance as the glacier. The deposits made by the glacier are generally known as drift. There were some specific names for some specific deposits made by the Glaciers.

    Till - The Deposits directly made by or after the melting of the ice of the glacier are known as Till.

    Terminal moraine -  When the glacier moves, during the course of the movement the terminal i.e at the end of the glacier, the ice melts and leaves some deposits which are known as the terminal moraine.

    Ground Moraine - The land on which the Till has been deposited is known as the Ground moraine.

    Glaciofluvial deposits -  Instead of directly getting deposited in the place of melting the melted water transports the soil and deposit in another place is known as the glaciofluvial deposits.

    Glacial Till is well-graded and has high shear strength.   

Transportation by Gravity 

     Due to gravity, the rocks in the ridges, and peaks or cliffs may drop and break into pieces from which soil will be formed. Colluvial Soil such as talus is formed by the deposition of gravity. Talus are irregular and coarse and are useful for many engineering works.

Transportation by combined actions

    In some cases, two or more agents will be involved the transportation of soil. For example, the soil's formed by the fall of a rock from a cliff due to gravity into a river, the soil is then transported by the river to a far way place for deposition. This kind of combination of agents involved in the transportation of the soil is known as the Combined action.


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