Chapter 6 Geomorphic Processes

The Earth's surface, where we live, is characterized by significant unevenness, featuring mountains, plains, valleys, and other landforms. This varied topography is a result of ongoing interactions between internal and external forces shaping the Earth's crust.

The Earth's crust is dynamic, constantly undergoing vertical and horizontal movements driven by forces originating from within the Earth (endogenic forces). These internal forces create variations in the crust's elevation, building up mountains and uplands.

Simultaneously, the Earth's surface is subjected to external forces (exogenic forces), primarily driven by solar energy and gravity. These forces act to wear down elevated areas (degradation) through processes like erosion and fill up low-lying areas and basins (aggradation) by depositing eroded material. The process of lowering relief through erosion is often referred to as gradation.

The Earth's surface remains uneven because the constructive action of endogenic forces (building up the land) is constantly countered by the destructive action of exogenic forces (wearing down the land). As long as these opposing forces continue to operate, variations in relief will persist.

Understanding these geomorphic processes is vital because humans rely heavily on the Earth's surface for resources and sustenance. Human activities, particularly the intensive and sometimes unsustainable use of resources, can significantly impact these processes and the environment. While most landforms are shaped over very long geological timescales, human actions can accelerate changes and diminish the land's future potential. Knowledge of how landforms are created and modified, and the materials involved, is necessary for responsible resource management and preserving the Earth's surface for future generations.

Geomorphic Processes

Geomorphic processes are defined as the natural processes that cause physical stresses and chemical actions on earth materials, leading to changes in the shape and configuration of the Earth's surface. These processes are driven by forces originating both from within the Earth and from external sources.

• Endogenic Geomorphic Processes: These originate from within the Earth's interior. Major endogenic processes include Diastrophism and Volcanism.
• Exogenic Geomorphic Processes: These originate from outside the Earth's interior, primarily driven by solar energy and gravity. Major exogenic processes include Weathering, Mass Wasting (or mass movements), Erosion, and Deposition.

An exogenic geomorphic agent is any natural element (like water, ice, wind) that is capable of acquiring, transporting, and depositing Earth materials. When these agents become mobile due to gradients (slopes), they erode material from higher elevations, transport it downslope, and deposit it in lower areas. While technically distinct (process is the action, agent is the medium), in the context of exogenic processes, the terms geomorphic process and geomorphic agent are often used interchangeably.

Gravity plays a fundamental role in activating many geomorphic processes, both directly and indirectly. It is the force that drives all downslope movement of matter, including the movement of rock and soil in mass wasting and the flow of water and ice which are agents of erosion. Gravity also indirectly influences wave and tide-induced currents and winds. Without gravity and the presence of gradients (slopes or pressure differences), there would be no movement of material, and thus no erosion, transportation, or deposition would occur.

Endogenic Processes

The primary driving force behind endogenic geomorphic processes is the energy emanating from within the Earth. This internal energy is mainly generated from sources such as the radioactive decay of elements, residual heat left over from the Earth's formation, and minor contributions from rotational and tidal friction.

This internal heat creates geothermal gradients (temperature increases with depth) and heat flow within the lithosphere. Variations in these geothermal gradients, along with differences in crustal thickness and strength, cause the endogenic forces to act unevenly across the Earth's surface. This differential action is responsible for the initial unevenness of the Earth's tectonically controlled surface.

Diastrophism

Diastrophism encompasses all processes that involve the movement, uplift, or building up of significant portions of the Earth's crust. These processes typically operate slowly over geological time scales.

Key processes included under diastrophism are:

• Orogenic Processes (Orogeny): These involve mountain building, often through intense folding and faulting of the Earth's crust. Orogeny affects long, relatively narrow belts.
• Epeirogenic Processes (Epeirogeny): These involve the uplift or warping (broad, gentle bending) of large, relatively flat areas of the Earth's crust, forming continents or plateaus. Unlike orogeny, epeirogeny involves simple deformation.
• Earthquakes: While often caused by the sudden release of stress built up by larger diastrophic movements, earthquakes are also considered a component of diastrophism as they involve relatively localized movements and adjustments of the crust.
• Plate Tectonics: This is the most significant diastrophic process, involving the large-scale horizontal movement of rigid lithospheric plates across the Earth's surface. Plate interactions at boundaries are responsible for most orogeny, epeirogeny, and earthquake activity.

Processes like orogeny and epeirogeny, along with earthquakes and plate tectonics, can cause faulting (fractures with displacement) and fracturing (cracking without displacement) of the Earth's crust. These immense pressures, changes in volume, and increased temperatures (PVT changes) associated with diastrophism can also lead to the metamorphism of rocks.

Differences between Epeirogeny and Orogeny:

• Orogeny: Primarily a mountain-building process; involves severe folding and faulting; affects narrow, elongated belts; creates significant vertical relief.
• Epeirogeny: Primarily a continent-building process; involves broad uplift or subsidence/warping; affects large, stable areas; creates relatively gentle, regional changes in elevation.

Volcanism

Volcanism refers to the processes by which molten rock material (magma) and associated gases and particles rise from the Earth's interior towards or onto the surface. It includes the movement of magma within the crust and mantle, as well as the eruption of lava and other materials onto the surface. Volcanism leads to the formation of various intrusive (solidified below surface) and extrusive (solidified on surface) volcanic landforms.

The terms volcanism and volcanoes are closely related. Volcanism is the general term for all phenomena associated with the origin and movement of molten rock and its eruption. A volcano is the specific vent or structure (typically conical) on the Earth's surface through which volcanic materials are erupted.

Exogenic Processes

Exogenic processes operate on the Earth's surface and derive their energy primarily from two main sources: solar energy and gradients created by tectonic factors and gravity.

Solar energy drives atmospheric processes like wind, temperature variations, and the water cycle (precipitation, evaporation), which power agents like wind and running water.

Tectonic forces (endogenic processes) create the initial slopes and variations in elevation on the Earth's surface. Gravity then acts upon these sloping surfaces, causing all Earth materials to tend to move downslope.

Stress is a key concept in understanding how exogenic processes affect materials. Stress is force applied per unit area. Gravitational forces and forces from temperature changes, crystallization, and melting can create stresses within Earth materials. Shear stress, acting parallel to a surface, is particularly important as it can cause materials to deform, break, or slip.

Chemical processes, often facilitated by water and heat, can weaken the bonds between mineral grains or dissolve cementing materials in rocks, making them more susceptible to breaking and movement. Ultimately, the development of stresses within Earth materials is the basic reason behind weathering, mass movements, and erosion.

The major climatic elements controlling exogenic processes are temperature and precipitation. Different climatic regions, determined by factors like latitude, altitude, and the distribution of land and water, experience different intensities and types of exogenic processes.

Vegetation cover, which is strongly influenced by climate, also indirectly affects exogenic processes by protecting the ground from erosion or contributing to weathering through root action and organic acids.

Even within a single climatic region, variations in wind patterns, precipitation amounts and intensity, temperature ranges, and the presence of frost can cause local differences in how exogenic processes operate. For example, slopes facing different directions (aspect) receive varying amounts of solar radiation, affecting temperature and moisture conditions.

Beyond climate, the type and structure of rocks significantly influence how susceptible they are to exogenic processes. Rock structure includes factors like folds, faults, the orientation of layers, the presence of joints or fractures, the hardness or softness of constituent minerals, chemical reactivity, and permeability. Different rocks offer varying resistance; a rock might be resistant to one process but weak against another. Also, the same rock can react differently to the same process under different climatic conditions.

Most exogenic processes act slowly, often making their effects unnoticeable over short periods. However, their continuous action over long durations can drastically alter landscapes.

All exogenic geomorphic processes are collectively covered under the term denudation, which means "to strip off" or "to uncover." Denudation includes the processes of:

• Weathering: Breaking down rocks in situ.
• Mass Wasting/Movements: Downslope movement of weathered material under gravity.
• Erosion: Acquisition and transportation of rock debris by mobile agents.
• Transportation: Moving eroded material from one place to another.

Flowchart illustrating the main denudational processes (Weathering, Mass Movements, Erosion, Transportation) and the forces that drive them (Gravitational Forces, Molecular Stresses, Kinetic Energy).

The fundamental reason for the Earth's surface variations is the ongoing interplay between endogenic forces (creating relief) and exogenic forces (wearing down relief). The specific landforms created depend on the type and structure of Earth materials, the specific geomorphic processes acting, and their rates of operation, which are influenced by climate and other factors.

Weathering

Weathering is a fundamental exogenic process involving the disintegration (breaking down) and decomposition (chemical alteration) of rocks and minerals at or near the Earth's surface. It is caused by the actions of weather and climate elements like temperature, precipitation, and atmospheric gases.

A key characteristic of weathering is that it is an in-situ or on-site process, meaning the broken-down or altered material generally remains in place, with very little or no significant movement or transportation involved. This distinguishes it from erosion, which involves movement.

While minor localized shifts might occur within the weathered material, this is not the same as transportation in the context of erosion, where material is moved over distances by external agents.

The type and rate of weathering are influenced by complex factors including the geological nature of the rocks (mineral composition, structure), climate (temperature, precipitation), topography (slope, aspect), and vegetation cover. Climate is particularly important as it dictates the dominant weathering processes and influences the depth of the weathered layer (regolith).

Generalized diagram indicating how the depth of the weathered layer (regolith) varies across different climatic regions.

Weathering processes are typically categorized into three main types, although they often occur together:

Chemical Weathering Processes

These processes involve chemical reactions that alter the composition of rocks and minerals. The main chemical weathering processes are solution, carbonation, hydration, oxidation, and reduction.

• These reactions are typically facilitated by water (surface water, groundwater, or even moisture in the air) and atmospheric gases (oxygen, carbon dioxide). Heat generally accelerates these reactions.
• Carbon dioxide in the atmosphere dissolves in rainwater to form weak carbonic acid. Organic matter decay in soils also produces carbonic acid and other organic acids, increasing the acidity of soil water, which enhances the chemical breakdown of minerals.
• Chemical weathering can dissolve minerals (solution), form new minerals by adding water to existing ones (hydration), or break down minerals by reacting with oxygen (oxidation) or in oxygen-poor environments (reduction). These processes effectively decompose rocks or reduce them into fine particles.

Physical Weathering Processes

Also known as mechanical weathering, these processes break rocks into smaller fragments without changing their chemical composition. Physical weathering is caused by applied forces, which can originate from:

• Gravity: Overburden pressure, load from overlying material, or shearing stress.
• Expansion: Forces generated by temperature changes (thermal expansion/contraction), the growth of ice crystals in cracks (frost wedging), the growth of salt crystals (salt weathering), or the activity of plants and animals (root wedging, burrowing).
• Water Pressure: Forces related to the wetting and drying cycles of porous materials.

Many physical weathering processes are related to thermal expansion and pressure release (unloading). For instance, repeated heating and cooling cause minerals to expand and contract, weakening the rock over time (thermal fatigue). When deeply buried rocks are exposed at the surface due to erosion of overlying material, the reduction in pressure causes them to expand, leading to fracturing (sheeting or exfoliation). These processes, though seemingly small and slow, cause significant damage to rocks over prolonged periods due to the repetitive nature of the stresses.

Biological Activity And Weathering

Living organisms, including plants, animals, and microbes, can contribute to both chemical and physical weathering processes.

• Physical Contributions: Plant roots grow into cracks and joints in rocks, exerting pressure that widens and deepens the fractures (root wedging). Burrowing animals like earthworms, termites, and rodents churn and move soil and rock fragments, exposing fresh surfaces to chemical attack and allowing air and moisture to penetrate deeper. Human activities like farming, construction, and deforestation also physically disturb earth materials, accelerating weathering.
• Chemical Contributions: Decaying plant and animal matter in the soil produces organic acids (like humic acid and carbonic acid). These acids can dissolve minerals or enhance other chemical weathering reactions. Microorganisms, particularly bacteria, can also directly or indirectly influence mineral breakdown through metabolic processes.

Special Effects Of Weathering

Certain landforms or textures can result from specific weathering processes:

• Exfoliation: While often discussed under physical weathering processes like unloading and thermal expansion, exfoliation itself is a *result* of these processes. It is the peeling or flaking off of curved sheets or layers from the outer surface of rocks or bedrock. This results in smooth, rounded rock surfaces or formations known as exfoliation domes or tors. Exfoliation can occur due to pressure release when overlying rock is removed or sometimes due to significant temperature fluctuations causing differential expansion and contraction.

Image depicting exfoliation, where layers of rock are peeling off the surface, often resulting in rounded forms.

Significance Of Weathering

Weathering is a crucial process with numerous significant impacts:

• Regolith and Soil Formation: Weathering breaks down solid rock into smaller fragments, creating regolith – the layer of unconsolidated rocky material covering bedrock. Regolith is the essential parent material for the formation of soil.
• Facilitating Erosion and Mass Movements: Weathering weakens rocks and provides loose material (regolith and soil) that can be easily picked up and transported by agents of erosion or moved downslope by gravity during mass movements. While erosion can occur without prior weathering on some solid rocks, it is vastly more efficient and widespread where weathering has produced loose material.
• Landform Development: By breaking down rocks and enabling mass movements and erosion, weathering indirectly contributes to the shaping and evolution of landforms.
• Economic Significance: Weathering can lead to the enrichment and concentration of certain valuable mineral ores. As rocks containing disseminated trace amounts of valuable metals undergo chemical weathering, less soluble components (including valuable metals like iron, manganese, aluminum, copper) may be left behind as more soluble components are leached away by water. This process increases the concentration of the desired metal to an economically viable level for mining.
• Soil Fertility: Weathering releases essential mineral nutrients from rocks into the soil, making them available for plant uptake, thus contributing to soil fertility.
• Biodiversity: Soil, formed through weathering and biological activity, is the medium that supports most terrestrial plant life. The depth and characteristics of the soil, which depend on weathering, influence the type of vegetation that can grow, which in turn supports diverse ecosystems and biodiversity. Therefore, weathering is indirectly responsible for much of Earth's terrestrial biodiversity.

Mass Movements

Mass movements, also known as mass wasting, involve the downslope transfer of masses of rock debris, soil, or regolith under the direct influence of gravity. A key distinction is that, unlike erosion, mass movements do not require a mobile medium like running water, wind, or ice to transport the material. While these agents might influence the conditions (e.g., water adding weight or lubricating material), gravity is the primary driving force causing the bulk movement.

Mass movements can vary greatly in speed, ranging from very slow, imperceptible creep to extremely rapid falls and flows. They can involve shallow layers of surface material or deep columns of rock.

Weathering is not strictly a prerequisite for mass movement (rock falls can occur from unweathered cliffs), but it significantly aids the process by providing loose, unconsolidated material (regolith and soil) that is more susceptible to gravitational pull than solid bedrock. Mass movements are generally much more active and widespread over weathered slopes.

Materials on a slope have a certain resistance to movement (shear strength), dependent on factors like internal friction, cohesion, and the presence of water. Mass movement occurs when the force of gravity pulling the material downslope (shear stress) exceeds the material's shear strength.

Factors that favour mass movements include:

• Steep slopes or vertical cliffs.
• Weak, unconsolidated materials or thinly bedded rocks.
• Presence of faults, joints, or steeply dipping layers that provide planes of weakness.
• Abundant precipitation or torrential rains that saturate and lubricate slope materials, increasing weight and decreasing strength.
• Scarcity or removal of vegetation, which helps bind soil and increases slope stability.

Several factors can trigger or activate mass movements, often by disturbing the balance between downslope force and shear strength:

• Removal of support at the base of a slope (e.g., by erosion or construction).
• Increasing the gradient or height of the slope.
• Adding extra weight to the slope (overloading) through natural accumulation or artificial filling.
• Saturation of slope materials by heavy rainfall or water seepage.
• Removal of load from the upper part of the slope.
• Shaking from earthquakes, explosions, or machinery.
• Excessive natural seepage of water.
• Rapid drawdown of water levels in lakes or reservoirs adjacent to slopes.
• Indiscriminate removal of natural vegetation.

Mass movements manifest in various forms, including creep (very slow movement), flow (movement as a viscous mass), slide (movement along a distinct surface), and fall (free-falling material). These types can be distinguished by the speed and how the material moves (Figure 6.5 is a conceptual diagram showing these relationships).

Conceptual diagram illustrating different types of mass movements classified by their rate of movement (slow vs. rapid) and the amount of water present (relatively dry vs. wet/viscous).

Regarding the terms Mass Wasting vs. Mass Movements: "Mass movement" is perhaps more intuitive as it directly describes the movement of mass. "Mass wasting" focuses on the process of material being removed from slopes (wasted), but both terms are widely used and acceptable.

Solifluction, a slow flow of water-saturated soil over permafrost or frozen ground, involves the movement of a viscous mass. While it is a flow movement, it is typically very slow and falls under the 'slow flow' category, not rapid flow movements like debris flows or mudflows.

Landslides

Landslides are a category of mass movements characterized by relatively rapid and perceptible downslope movement of a mass of rock or debris. The materials involved are typically relatively dry compared to flows.

The nature of the material involved and the type of movement lead to different classifications of landslides:

• Slump: Involves the sliding of a coherent block or several blocks of rock or soil along a curved (rotational) failure surface. The material moves with a backward tilt relative to the slope. (Figure 6.6 shows slumping).

Diagram illustrating the characteristic curved failure surface and backward rotation of material in a slump landslide.

• Debris Slide: A rapid downslope sliding or rolling of a mass of unconsolidated rock fragments, soil, and debris, but without the characteristic backward rotation seen in a slump.
• Debris Fall: The nearly free fall of unconsolidated debris from a vertical or very steep slope.
• Rockslide: The rapid sliding of individual rock blocks or layers along pre-existing planes of weakness, such as bedding planes, joints, or faults. On steep slopes, rockslides can be very fast and highly destructive. They occur as planar failures along discontinuities that dip steeply downslope.
• Rock Fall: The free falling of individual rock blocks or fragments from a cliff face or very steep slope. Rock falls typically involve materials from the outer layers of the rock face, distinguishing them from rockslides which involve deeper movement along planes.

Image depicting evidence of past landslide activity (scars) on a steep mountainous slope, such as in the Shiwalik range.

Debris avalanches are extremely rapid mass movements of a chaotic mixture of rock, soil, trees, and other debris, often flowing like a fluid. They can be considered a type of rapid flow or a very large, fast-moving landslide.

Landslides and debris avalanches are particularly common in areas like the Himalayas due to several factors: the Himalayas are tectonically active (prone to earthquakes), they are composed of relatively young and sometimes unconsolidated sedimentary rocks and deposits, and they have very steep slopes. While the Nilgiris and Western Ghats in peninsular India are tectonically more stable and composed of harder rocks, they still experience landslides and rock falls. This is often due to extremely steep slopes, significant mechanical weathering from temperature changes, and intense rainfall concentrated over short periods, which saturates materials and triggers failure.

Erosion And Deposition

Erosion is the process by which weathered rock fragments and other Earth materials are acquired (picked up) and transported by geomorphic agents. Once rocks are broken down by weathering or other processes, mobile agents like running water, groundwater, glaciers, wind, and waves remove this material and move it to other locations.

These agents also contribute to erosion through abrasion, where the transported rock debris grinds and wears away the solid rock surfaces they move across.

Erosion leads to the degradation of relief, meaning it wears down and lowers the Earth's surface. Weathering weakens rocks and produces material, thereby aiding erosion, but erosion itself is the process of removal and transport. Weathering, mass wasting, and erosion are all considered degradational processes that reduce the elevation and relief of the landscape.

Erosion and transportation are driven by the kinetic energy of the geomorphic agents. These agents include:

• Wind: A gaseous agent, prominent in arid and semi-arid regions.
• Running Water: A liquid agent, including rivers and streams, active in most climates.
• Glaciers: A solid agent (moving ice), significant in glacial and periglacial environments.
• Waves and Currents: Agents acting along coastlines, driven by wind and gravity.
• Groundwater: Water within the subsurface, causing erosion primarily through solution in soluble rocks.

The work of wind, running water, and glaciers is heavily influenced by climatic conditions (temperature, precipitation, seasonality). However, the work of waves and currents is determined by coastal location and oceanic dynamics, and the work of groundwater is strongly controlled by the type and permeability of subsurface rocks (e.g., formation of karst topography in soluble rocks like limestone requires groundwater action).

Deposition is the natural consequence of erosion. When erosional agents lose their energy (e.g., as slope decreases, flow velocity drops, or wind speed diminishes), they can no longer carry their load of sediment. The transported material settles out and accumulates, a process called deposition.

Deposition is essentially a passive process resulting from a loss of energy by the transporting agent, rather than an active "work" performed by the agent itself. Larger, heavier particles typically settle out first, while finer particles are carried further before being deposited.

Deposition leads to aggradation, the building up of landforms or filling in of depressions, thus increasing relief in depositional areas. The same agents that cause erosion (running water, glaciers, wind, waves, groundwater) are also responsible for deposition when their energy decreases.

While both mass movements and erosion involve the shift of materials, they are distinct processes. Mass movement is driven directly by gravity acting on a mass of material, without an intervening mobile agent carrying the debris from place to place. Erosion, on the other hand, is driven by the kinetic energy of agents (water, wind, ice) that acquire and transport individual particles or dissolved substances. For example, a rockfall is mass movement; a river carrying sediment is erosion and transportation.

Appreciable erosion, particularly by agents like running water and wind, is significantly enhanced if rocks have been weakened or broken down by weathering. Weathering creates the loose material that is easily picked up and transported. However, erosion can also occur on relatively unweathered rock surfaces through processes like abrasion and hydraulic action (pressure from moving water), but it is generally less efficient than eroding loose material.

Soil Formation

Soil is a vital, dynamic medium covering much of the Earth's surface. It is a complex mixture where continuous physical, chemical, and biological processes occur. Soil is both a product of the decay and alteration of rock and organic matter and the essential medium for plant growth.

Soil is a changing and developing entity whose characteristics can vary with seasons, becoming warm or cold, dry or moist. Biological activity within the soil flourishes in suitable conditions but slows down or stops when it is too cold or dry. Organic matter is added to the soil when plants and animals die and decompose.

Pedology is the scientific study of soil, and a soil scientist is called a pedologist.

Process Of Soil Formation

The process of soil formation, known as pedogenesis, begins with weathering. The weathered rock material and/or transported deposits (like alluvium or glacial till) provide the basic input, forming the weathering mantle or regolith.

This initial material is then colonized by pioneering organisms, such as bacteria, algae, mosses, and lichens. Small invertebrates also inhabit the material. The dead remains of these organisms and initial plant life contribute organic matter, which decomposes to form humus.

Over time, minor grasses, ferns, and eventually larger plants like bushes and trees become established, often from seeds dispersed by wind or animals. Plant roots penetrate the material, mechanically breaking it apart and creating pathways for water and air. Burrowing animals mix the material, further increasing porosity and aeration. This leads to the mass of material becoming more porous and sponge-like, capable of retaining water and allowing air circulation. These processes continue over long periods, transforming the simple weathered material into a mature soil, which is a complex mixture of mineral particles, organic matter, water, and air, organized into distinct layers called horizons.

Therefore, weathering is absolutely essential as a pre-requisite for soil formation. Without the initial breakdown of solid rock into smaller particles (weathering), the raw material for soil (regolith/parent material) would not be available.

Soil-Forming Factors

The development of soil is influenced by the interaction of five basic factors:

1. Parent Material
2. Topography (Relief)
3. Climate
4. Biological Activity
5. Time

These factors do not act in isolation but interact and influence each other's effects during pedogenesis.

Parent Material

The parent material is the source of the mineral particles in the soil. It is considered a passive factor in soil formation as it is acted upon, rather than actively causing the process. Parent material can be either residual (weathered bedrock that stays in place) or transported (sediments deposited by water, wind, or ice). The characteristics of the parent material, such as its texture (size of particles), structure (how grains are arranged), and mineral and chemical composition, influence the properties of the soil that develops from it. The rate and type of weathering of the parent material also play a role. While soils often show strong links to their parent rock, especially when young, over long periods under certain conditions, soils from different parent materials can become similar, and soils from similar parent materials can diverge. However, in certain cases like soils over limestone, the relationship with the parent rock remains pronounced due to specific weathering processes.

Topography

Topography, or the shape and slope of the land, is another passive control factor. Its influence is primarily on drainage, runoff, and erosion, and the amount of solar radiation received (aspect). On steep slopes, soil is often thin because material is constantly removed by erosion and runoff. On flat areas or gentle slopes, where erosion is minimal and water can infiltrate (percolate) into the ground, soils tend to be thicker and more developed. Topography also affects soil moisture and temperature regimes, further influencing the biological and chemical processes involved in soil formation. For example, soils in depressions may become waterlogged, affecting the type of vegetation and decomposition processes.

Climate

Climate is a major active factor in soil formation, exerting significant control over the rate and type of weathering, the amount of water available for chemical and biological reactions, and the type of vegetation that grows. The key climatic elements are:

• Moisture: The amount, intensity, frequency, and duration of precipitation, balanced against evaporation and humidity, determine how much water is available for chemical weathering and biological activity. Water facilitates chemical reactions and transports dissolved substances and fine particles within the soil profile (a process called eluviation - washing out from upper layers, and illuviation - accumulation in lower layers). In wet climates, significant leaching of soluble minerals occurs; in very wet tropical climates, even silica can be removed (desilication). In dry climates, capillary action brings water to the surface, where it evaporates, leaving behind salts that can form hardpans. In areas with moderate rainfall and high evaporation, calcium carbonate nodules (kankar) can form.
• Temperature: Influences the rate of chemical and biological processes. Higher temperatures generally increase reaction rates, while lower temperatures slow them down. Chemical activity virtually stops in freezing conditions, but physical weathering like frost wedging becomes dominant. Tropical soils, formed under high temperatures and moisture, tend to have deeper profiles than soils in colder regions. In cold climates (sub-arctic, tundra), low temperatures inhibit bacterial decomposition, leading to the accumulation of undecomposed organic matter and the formation of peat layers.

Biological Activity

Biological activity is another important active factor. Plants, animals, and microorganisms living in and on the parent material contribute organic matter and influence the physical and chemical environment. The type of vegetation cover affects the amount and type of organic matter input and can help stabilize the soil. Dead plant material decomposes to form humus, which improves soil structure, water retention, and provides nutrients. Organic acids produced during humification enhance mineral weathering. Microorganisms, especially bacteria and fungi, are crucial for decomposition and nutrient cycling. Some bacteria, like Rhizobium in legume root nodules, perform nitrogen fixation, converting atmospheric nitrogen into forms usable by plants. The mechanical action of larger animals like earthworms and rodents mixes and aerates the soil. Earthworms, by ingesting and processing soil, alter its texture and chemistry.

Time

Time is considered a passive factor, representing the duration over which the soil-forming processes have been acting. The length of time significantly influences how developed or mature a soil becomes and the extent to which distinct soil horizons (layers) form. Young soils, like those developing on recent river deposits (alluvium) or glacial till, have weak or no horizon development and closely resemble their parent material. Mature soils, which have undergone soil formation for a long time, display well-defined horizons. There isn't a fixed time scale for soil maturity, as it depends on the interaction of all other factors; favorable conditions can lead to faster development.

It is necessary to separate the process of soil formation (pedogenesis - the actual physical, chemical, and biological transformations) from the soil-forming factors (the environmental influences that control the *rate* and *nature* of those processes). The factors don't perform the chemical reactions or physical breakdown themselves, but they dictate how effectively and in what direction the processes occur.

Time, topography, and parent material are considered passive factors because they provide the stage or the raw material, but they do not actively drive the chemical and biological transformations that constitute pedogenesis. Climate and biological activity, conversely, are active factors because temperature, moisture, and organic/microbial activity directly fuel and control the rate and type of weathering, decomposition, and horizon development.

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