Weathering and Soil Formations | Geology Optional Notes for UPSC PDF Download

Exogenic Processes

• Exogenic processes, also known as denudational or destructional processes, originate from atmospheric forces and continuously shape the Earth's surface through weathering, erosion, and deposition. These processes, collectively termed denudation, result in the gradual stripping away and leveling of the landforms created by endogenic forces.

Weathering and Erosion

• Denudation encompasses both weathering and erosion. Weathering involves the in situ disintegration and decomposition of rocks, while erosion involves the dynamic removal and transportation of these materials to depositional basins.

Weathering: Sediment Production

You have read that weathering is combined action of the processes that cause rock to disintegrate physically and decompose chemically and biologically because of its exposure near the Earth’s surface through the elements of “weather” such as temperature, rainfall, frost, ice, etc.

Mechanical or Physical Weathering

• Unloading/Pressure Expansion
  Unloading occurs when the confining pressure on rocks is released due to erosion, causing expansion and the development of cracks and joints. Exfoliation, or sheeting, can result from this pressure release.
• Frost Action
  Frost action, or frost wedging, involves the expansion of water as it freezes in cracks and crevices, exerting pressure and eventually breaking apart rocks. Processes like hydrofracturing and frost heaving also contribute to mechanical weathering in cold environments.
• Salt-Crystal Growth
  Salt-crystal growth occurs in arid and coastal areas when salt crystals crystallize within rock fissures, exerting stress and leading to granular disintegration. Salt expansion, through processes like hydration stress, contributes to weathering.
• Root Wedging
  Root wedging occurs when plant roots penetrate cracks in rocks, exerting pressure and causing mechanical weathering. Biological agents like plants, termites, and marine organisms can contribute to both mechanical and chemical weathering processes.

Chemical Weathering

• Solution
  Water acts as a solvent, dissolving minerals and salts in a process known as solution. Soluble minerals like halite and gypsum are particularly susceptible to dissolution.

• Hydration
  Hydration involves the absorption of water molecules by minerals without altering their chemical composition. This process can lead to the formation of new minerals like gypsum from anhydrite.
• Oxidation and Reduction
  Oxidation occurs when minerals react with oxygen, while reduction involves the gain of electrons. These processes, particularly affecting iron-containing minerals, can weaken rocks and make them more susceptible to chemical attack.
• Carbonation
  Carbonation involves the formation of carbonates through the reaction of carbon dioxide with water. This process, dominant in the weathering of calcareous rocks, leads to the dissolution of minerals like calcite.
• Hydrolysis
  Hydrolysis involves the reaction of water with minerals, resulting in the exchange of ions and the formation of new minerals. This process, particularly affecting silicate minerals, can completely decompose or modify vulnerable primary minerals.
• Chelation
  Chelation involves the removal of metal ions from solids, which then form soluble organic matter-metal complexes. Organic acids from plant decomposition contribute to this process, leading to the transfer of metals in soil or rock.

Biological Weathering

• Biological weathering involves the mechanical or chemical breakdown of rocks by organisms such as plants, bacteria, fungi, and marine organisms. Processes like root wedging and chemical alteration contribute to the weathering caused by biological agents.

Weathering Products

The products of weathering, including clays, soils, and dissolved substances, are carried by natural processes like rivers to depositional basins. Soil formation, influenced by factors like time, climate, topography, and parent material, is crucial for sustaining life on Earth.

• Weathered Mantle and Regolith
  The weathered mantle, or regolith, comprises the weathered material above unaltered bedrock. It may include distinct layers and horizons, with the weathering front marking the boundary between fresh and weathered rock. Duricrusts and laterites are examples of weathering deposits enriched in specific minerals.

Soil - Concept

• Soil is a complex, multi-layered medium that covers a significant portion of the Earth's land surface. It is composed of mineral particles, organic matter, water, and air, and serves as a natural medium for the growth of plants.
• Texture and Composition
  • Sand, silt, and clay are the three basic components that give soil its texture.
  • The mineral texture of the soil varies depending on these three elements.
  • The higher organic layer, known as humus, is formed as leaves and other elements decay.
  • The humus content of soils has a significant impact on their fertility.
• Role in Ecosystem
  • Soil not only supports plant life but also plays an essential role in various biogeochemical cycles, making it crucial for life on Earth.
• Soil Formation (Pedogenesis)
  • Soil formation is a complex process that transforms raw mineral material into the soil that covers much of the Earth's terrestrial surface.

Soil Genesis

• Soil is the top layer of the Earth's crust that has been worn by plants and animals.
• A soil profile is a vertical passage through this zone that contains multiple identifiable layers or horizons that allow different types of soil to be identified.
• Soil is a thin layer of mineral particles on the surface of the earth that generally contains a significant quantity of organic matter and is capable of supporting living plants.

• It covers the portion of the Earth's outer skin that stretches from the surface to the maximum depth to which living creatures can penetrate, which is essentially the area covered by plant roots.
• Soil is defined by its ability to create and store plant nutrients, which is facilitated by interactions between a variety of variables including water, air, sunshine, rocks, plants, and animals.
• Soil, despite its sparse distribution across the terrestrial surface, serves as a critical interface between the atmosphere, lithosphere, hydrosphere, and biosphere.

Genesis of Soil-Structure

The physical and chemical degradation of rock exposed to the atmosphere, as well as the action of water trickling down from the top, starts the process of soil production.

• Weathering Process
  • Weathering is the process of disintegration.
  • It causes the solid rock to weaken and break down, as well as the fragmentation of cohesive rock masses and the formation of small rocks from larger ones.
• Regolith Formation
  • The main product of weathering is regolith ("blanket rock"), a loose coating of inorganic material that acts as a blanket over the unfragmented rock beneath it.
  • The regolith, in most cases, is made up of material that has weathered from the underlying rock and has a rudimentary particle size gradation.
  • It can sometimes contain material that was brought from another location by wind, water, or ice, leading to compositional differences.
• Soil Formation
  • The upper section of the regolith is soil, the final product of weathering, primarily made up of finely fragmented mineral particles.
  • It usually contains living plant roots, dead and rotting plant parts, living and dead tiny plants and animals, and varying amounts of air and water.
  • Soil is a stage in a never-ending continuum of physical-chemical–biological processes, rather than the final outcome of a process.

Soil Profile and Layers

A soil profile is a vertical section of soil that displays all of its strata, running from the surface to the source rock material.

• Regolith Composition
  • All weathered material within the soil profile is part of the regolith, consisting of the solum and the saprolite.
  • The solum encompasses the top horizons of the profile, representing the most worn part.
  • The saprolite is the least weathered layer above the solid, consolidated bedrock but beneath the regolith.
• Soil Horizon
  • A soil horizon is a distinct stratum of soil that runs nearly parallel to the soil surface, exhibiting unique qualities and characteristics from the layers above and below.
• Utility in Agriculture
  • The soil profile serves as a valuable tool for fertilizer management, providing insights into soil fertility.
  • Observing changes in the soil profile over time can yield information about soil health and productivity.
• Variations in Soil Profile
  • The profile of the soil varies as it weathers and as organic matter decomposes, influencing its fertility and structure.
  • A heavily worn, infertile soil often exhibits a light-colored subsurface layer where nutrients have leached away.
  • In contrast, highly fertile soils may feature a deep surface layer rich in organic matter.

Layers of Soil

• O Horizon: The O horizon is a surface layer composed of organic material at various stages of decomposition, particularly noticeable in forested environments with an accumulation of tree debris.
• A Horizon: The A horizon is primarily composed of minerals (sand, silt, and clay) with trace amounts of organic matter, often representing the surface layer of grasslands and agricultural soils.
• E Horizon: The E horizon is a subsurface layer severely leached of soluble nutrients due to precipitation or irrigation, typically appearing bright in hue and commonly found beneath the O horizon.
• B Horizon:  The B horizon is a subsurface layer formed by the deposition of minerals leached from the layers above, serving as a depositional location for these materials.
• C Horizon: The C horizon is the least weathered subsurface layer, consisting of unconsolidated, loose parent material, often referred to as saprolite.

Processes of Soil Formation

• Soil Enrichment
  • Soil enrichment involves the addition of organic or inorganic substances to the soil.
  • Examples include mineral enrichment of silt from river floods or wind-blown dust, and the transport of humus from the O horizon to the A horizon by water, resulting in organic enrichment.
• Removal
  • Removal processes entail the loss of substances from the soil body.
  • Erosion carries soil particles into streams and rivers, while leaching removes soil compounds and minerals via water flowing to lower levels.
  • Cheluviation, akin to leaching, involves the downward movement of elements facilitated by organic molecules, or chelating agents, such as plant acids.
• Translocation
  • Translocation refers to the movement of materials upward or downward within the soil.
  • Downward Translocation
    • Eluviation sees fine particles like clays and colloids moving downward, forming the E horizon, while illuviation accumulates material in the B horizon.
    • Wind-blown silt and dune sand may add to the top of the soil profile, enriching the A horizon with humus from decaying organic materials.
  • Upward Translocation
    • Calcium carbonate translocation occurs as surplus soil water travels downward to the groundwater zone, leading to decalcification and, in arid climates, calcification.
    • Salinization, due to upward movement of groundwater in desert settings, can occur as dissolved salts are deposited upon evaporation, negatively impacting plant growth.
• Transformation
  • Transformation involves the alteration of materials within the soil body.
  • Examples include the conversion of minerals from primary to secondary types and the degradation of organic materials into humus through humification.
  • In warm, moist conditions, organic matter can be completely transformed into carbon dioxide and water by microbes, leaving minimal organic matter in the soil.

Factors that Influence Soil Formation

The following are some of the factors that influence soil formation:

• Parent Material
  • Parent materials are the rocks from which soils originate, influencing soil color, mineral composition, and texture.
  • Weathering processes acting on surface rocks transform them into fine grains, serving as the foundation for soil development.
  • In Indian conditions, parent materials include ancient crystalline and metamorphic rocks, Cuddapah and Vindhyan rocks, Gondwana rocks, Deccan basalts, and Tertiary and Mesozoic sedimentary rocks.
• Climate
  • Temperature and rainfall are crucial components influencing soil formation.
  • They affect the effectiveness of weathering, water movement through soil, and microbial activity.
  • Different climates can produce the same soil from different parent materials, and vice versa.
  • Examples include the formation of laterite soil in monsoonal regions with heavy rainfall and black soil in areas with hot summers and limited rainfall.
• Topography
  • Relief or topographic features impact soil formation, with dense soil horizons on moderate slopes and thin horizons on steep slopes due to erosion.
  • Topography also affects soil distribution and plant types by redistributing climate variables and water runoff.
• Organisms
  • Living organisms, including plants, animals, and microorganisms, influence soil formation through their activities and the organic material they contribute.
  • Plant roots disturb soil as they grow and contribute organic material to the topsoil, while burrowing animals mix and aerate the soil.
  • Earthworms, in particular, play a significant role in soil improvement through their burrowing and mixing actions.

• Time
  • Soil formation is a slow process, taking hundreds to thousands of years for significant changes to occur.
  • Factors such as a warm, moist atmosphere aid in soil formation, but the properties of the parent material are typically more critical.
  • According to soil scientists, it takes around 500 years to create 2.5 cm (1 in.) of topsoil, highlighting the slow pace of soil formation.
• Human Activity
  • Human activity significantly impacts the physical and chemical composition of soil.
  • Clearing native plants for agriculture can lead to erosion, as the removal of organic matter-rich layers exposes soil to the risk of degradation.
  • Extensive agricultural practices, including plowing and planting, have dramatically altered the structure and content of agricultural soils over time.
  • These modified soils often receive separate classifications due to their significant differences from natural soils.

Significance of Soil Formation Processes

1. Nutrient Dynamics
   • Soil formation processes play a crucial role in nutrient dynamics over time.
   • As soils form, nutrients are continuously extracted and added, influenced by the conditions present during soil formation.
   • This ultimately determines the type and quantity of nutrients available in the soil, crucial for plant growth, human nutrition, and water filtration.
2. Environmental Resilience
   • Healthy soil formation contributes to landscape resilience against environmental challenges such as droughts, floods, and fires.
   • Soil with adequate structure and nutrient content can better withstand extreme weather events and support vegetation recovery.
3. Climate Regulation
   • Soil plays a vital role in climate regulation, acting as a carbon sink and storing more carbon than all the world's forests combined.
   • Healthy soil formation processes contribute to carbon sequestration, helping mitigate climate change by reducing atmospheric carbon dioxide levels.