Geomorphic ProcessesClass 11 Fundamentals of Physical Geography Notes

Geomorphic Processes

The surface of our Earth is constantly changing. It's not flat or uniform because it's a dynamic stage for a tug-of-war between two powerful sets of forces. Understanding these forces helps us appreciate the mountains, valleys, and plains we see around us and use our planet's resources wisely.

The forces that cause physical and chemical changes to the Earth's materials, altering the shape of its surface, are known as geomorphic processes.

These processes are driven by two types of forces:

• Endogenic Forces: These are internal forces that originate from within the Earth. They are responsible for building up the landscape, creating mountains, plateaus, and other elevations.
• Exogenic Forces: These are external forces that originate from the Earth's atmosphere, powered mainly by energy from the sun. They work to wear down the landscape, a process called degradation, and fill in basins and depressions, a process called aggradation.

The overall wearing down of relief variations on the Earth's surface through erosion is known as gradation.

Geomorphic Agents vs. Geomorphic Processes

It's helpful to understand the difference between a process and an agent.

• A process is the force applied to Earth's materials, like the force of freezing water expanding in a rock crack.
• An agent is the mobile medium that removes, transports, and deposits materials.

Geomorphic agents include running water, moving ice (glaciers), wind, waves, and groundwater. When these natural elements become mobile due to gradients (like a slope), they can transport earth materials.

Gravity is a crucial force in all geomorphic processes. It not only pulls matter downslope but also causes stresses on materials. Without gravity and gradients (differences in elevation or pressure), there would be no movement, and therefore no erosion, transportation, or deposition.

Endogenic Processes

The energy for endogenic processes comes from within the Earth. This energy is generated by radioactivity, rotational and tidal friction, and primordial heat left over from the Earth's formation. This internal heat flow causes two major endogenic processes: diastrophism and volcanism.

Diastrophism

Diastrophism includes all processes that move, elevate, or build up portions of the Earth's crust. It is a broad term that covers several types of movements:

• Orogenic processes: This is mountain building, which involves severe folding and affects long, narrow belts of the crust.
• Epeirogenic processes: This is continent building, which involves the uplift or warping of large parts of the Earth's crust with simple deformation.
• Earthquakes: These involve relatively minor local movements.
• Plate tectonics: This involves the horizontal movement of crustal plates.

These processes can cause faulting and fracturing in the crust. The changes in pressure, volume, and temperature (PVT) they create can also lead to the metamorphism of rocks.

Volcanism

Volcanism involves the movement of molten rock, called magma, towards or onto the Earth's surface. It also includes the formation of many intrusive (below the surface) and extrusive (on the surface) volcanic forms.

Exogenic Processes

Exogenic processes get their energy from the atmosphere, which is ultimately driven by the sun's energy, and from gradients created by tectonic activity. These processes are all covered by the general term denudation, which literally means "to strip off" or "to uncover."

Denudation includes:

• Weathering
• Mass Wasting/Movements
• Erosion
• Transportation

The intensity of these processes varies globally due to different climatic conditions (temperature and precipitation), vegetation cover, and rock types.

Weathering

Weathering is the action of weather and climate elements on Earth's materials. It is the mechanical disintegration and chemical decomposition of rocks.

Weathering is influenced by geological, climatic, topographic, and vegetative factors, with climate being particularly important. There are three main types of weathering processes.

Chemical Weathering Processes

These processes decompose, dissolve, or reduce rocks to a fine state through chemical reactions. Water, air (oxygen and carbon dioxide), and heat are necessary to speed up these reactions.

• Solution: The dissolving of minerals in water.
• Carbonation: The reaction of carbonate and bicarbonate with minerals.
• Hydration: The chemical addition of water to minerals, causing them to expand.
• Oxidation and Reduction: The reaction of minerals with oxygen, which can weaken the rock (like rusting).

Physical Weathering Processes

Also known as mechanical weathering, these processes break rocks apart without changing their chemical composition. They rely on applied forces such as:

• Gravitational forces: Overburden pressure and shearing stress.
• Expansion forces: Caused by temperature changes, crystal growth (e.g., salt), or animal activity.
• Water pressures: Controlled by cycles of wetting and drying.

Most physical weathering is caused by thermal expansion and pressure release. Though these processes are slow, they cause great damage over time due to repeated contraction and expansion.

Biological Activity and Weathering

This involves the contribution of living organisms to weathering.

• Physical: Burrowing animals like earthworms and rodents expose new surfaces to chemical attack. Plant roots can grow into cracks and mechanically break rocks apart.
• Chemical: Decaying plant and animal matter can produce acids (humic, carbonic) that help decompose minerals.

Special Effects of Weathering

Exfoliation is a result of weathering, not a process itself. It is the flaking off of curved sheets of rock from a larger rock mass, resulting in smooth, rounded surfaces. Exfoliation can be caused by unloading (pressure release) and thermal expansion and contraction.

Significance of Weathering

Weathering is a fundamentally important process for several reasons:

• It breaks down rocks into smaller fragments, preparing the way for soil formation.
• It is crucial for biodiversity, as forests and vegetation depend on the depth of the weathered material (mantle).
• It aids mass wasting and erosion, which change landforms.
• It can lead to the concentration of valuable ores of iron, manganese, and aluminum through a process called enrichment, making them economically viable to mine.

Mass Movements

Mass movements (or mass wasting) transfer rock debris down slopes under the direct influence of gravity. Unlike erosion, no geomorphic agent like water, ice, or wind carries the debris. Instead, the debris itself moves, and it might carry air, water, or ice with it.

These movements can be slow (like creep) or rapid (like landslides). While weathering is not a prerequisite, mass movements are much more active on weathered slopes.

Factors that favor mass movements include:

• Weak, unconsolidated materials
• Steep slopes
• Heavy rainfall and saturation of materials
• Earthquakes or explosions
• Indiscriminate removal of natural vegetation

Landslides

Landslides are relatively rapid and perceptible movements of relatively dry material. The type of landslide depends on the rock, the degree of weathering, and the slope's steepness.

• Slump: The slipping of rock debris with a backward rotation.
• Debris slide: Rapid rolling or sliding of earth debris without backward rotation.
• Rockslide: Sliding of individual rock masses down a joint or fault surface.
• Debris fall / Rock fall: The free fall of earth debris or rock blocks from a vertical or overhanging face.

Erosion and Deposition

Erosion involves the acquisition and transportation of rock debris by geomorphic agents like running water, wind, glaciers, and waves. As these agents move, they pick up weathered fragments and use them to abrade (scrape and wear down) other surfaces, which further aids erosion. Erosion is a degradational process that wears down the landscape.

Deposition is the consequence of erosion. When erosional agents lose their velocity and energy, typically on gentler slopes, they can no longer carry their load. The materials they were transporting start to settle.

• Coarser materials are deposited first, followed by finer ones.
• Deposition is an aggradational process that fills up depressions.

Soil Formation

Soil is a dynamic medium where chemical, physical, and biological activities constantly occur. It is both a result of decay and a medium for growth. The process of soil formation is called pedogenesis.

Process of Soil Formation

1. Weathering: The process begins with the weathering of parent rock, creating a mantle of weathered material. This is the basic input for soil.
2. Colonization: Bacteria, mosses, and lichens colonize the weathered material. Their dead remains contribute to the accumulation of humus (decomposed organic matter).
3. Growth: Grasses, ferns, and eventually bushes and trees begin to grow. Plant roots penetrate the material, and burrowing animals bring particles to the surface.
4. Maturation: The material becomes porous and sponge-like, able to retain water and permit air passage. Over a long time, a mature soil with a complex mixture of mineral and organic products forms.

Soil-forming Factors

Five basic factors control how soils form. They work together, and each one affects the action of the others.

Parent Material

This is a passive control factor. It refers to the weathered rock debris (residual soils) or transported deposits (transported soils) from which soil develops. The texture, structure, and chemical composition of the parent material influence the final soil type.

Topography

This is another passive control factor. Topography influences soil formation through drainage and exposure to sunlight.

• Soils on steep slopes are often thin due to erosion.
• Soils in flat, upland areas are typically thick.
• Gentle slopes with good water percolation are very favorable for deep soil formation.

Climate

Climate is an important active factor. The key climatic elements are:

• Moisture: Precipitation provides water for chemical and biological activities. Excess water can lead to eluviation (leaching of components downwards) and illuviation (deposition of those components at a lower level).
• Temperature: Higher temperatures generally increase the rate of chemical and biological activity, leading to deeper soils in tropical regions. In freezing conditions, activity stops.

Biological Activity

This is another active factor.

• Vegetation: Dead plants provide humus. Acids formed during humification help decompose minerals.
• Organisms: Bacteria play a crucial role. In cold climates, bacterial growth is slow, so humus accumulates. In warm, humid climates, bacteria rapidly oxidize dead vegetation, leaving little humus. Some bacteria also perform nitrogen fixation, converting atmospheric nitrogen into a form plants can use.
• Animals: Earthworms, termites, and rodents rework the soil, mixing it and changing its texture and chemistry.

Time

Time is a passive control factor. The length of time that soil-forming processes operate determines the soil's maturity. A mature soil has a well-developed profile with distinct horizons. Young soils, like those on recently deposited alluvium, show little to no profile development.