Geomorphic Processes

The Earth’s surface is continuously reshaped by both internal and external forces.

The changes brought by these forces in the form and structure of the Earth’s surface are called geomorphic processes.

Geomorphic processes include natural mechanisms such as:

• Weathering
• Erosion
• Mass wasting
• Deposition

These processes modify landforms over time.

The internal forces are called endogenic forces, while the external forces are called exogenic forces.

Endogenic processes are mainly land-building forces, while exogenic processes are mainly land-wearing forces.

Geomorphic Agents

A geomorphic agent is a mobile medium that erodes, transports and deposits Earth materials.

Important geomorphic agents are:

• Running water
• Wind
• Waves
• Glaciers
• Ocean currents

Movement of materials on the Earth’s surface occurs due to gradients such as differences in pressure and elevation.

Materials generally move from higher levels to lower levels under the influence of gravity.

Role Of Gravity In Geomorphic Processes

Gravity is the fundamental force that keeps materials in contact with the Earth’s surface.

It drives all downslope movements.

Gravity is responsible for:

• Mass wasting
• Downslope movement of materials
• Influencing the action of geomorphic agents

Indirectly, gravitational forces also influence tides.

Wind generates waves and ocean currents.

Without gradients and gravity, processes such as erosion, transportation and deposition would not occur.

Classification Of Geomorphic Processes

Geomorphic processes are broadly divided into:

• Endogenic processes
• Exogenic processes

Endogenic Processes

Endogenic processes are internal geomorphic processes operating within the Earth.

They are powered by energy from the Earth’s interior.

Examples of endogenic processes include:

• Volcanism
• Diastrophism
• Metamorphism
• Earthquake
• Landslides
• Faulting and folding

The main sources of energy for endogenic processes are:

• Primordial heat – residual heat retained from the time Earth was formed.
• Radioactive decay – heat produced by disintegration of radioactive elements such as uranium, thorium and potassium.
• Rotational and tidal friction – minor sources of internal heat caused by gravitational interactions.

These energy sources drive processes such as diastrophism, volcanism and earthquakes.

Endogenic processes may bring changes in pressure, volume and temperature. These changes may lead to the metamorphism of rocks.

Exogenic Processes

Exogenic processes operate on the Earth’s surface.

They are mainly land-wearing processes.

Examples of exogenic processes include:

• Weathering
• Erosion
• Transportation
• Deposition

Exogenic processes vary from place to place because of differences in climate.

Important controlling factors are:

• Temperature
• Precipitation
• Seasonal changes
• Latitudinal differences
• Distribution of land and water
• Type and distribution of vegetation
• Type and structure of rocks

Different rocks offer different degrees of resistance to weathering and erosion.

Denudational Processes

The term denudation includes all exogenic geomorphic processes.

The word denude means to uncover or strip off.

Denudation includes:

• Weathering
• Erosion
• Mass wasting
• Transportation

Each process operates under specific driving forces.

Process	Driving Force / Energy
Weathering	Gravitational, molecular stresses and chemical actions
Mass movements	Gravitational force
Erosion / Transportation	Kinetic energy

Weathering

Weathering is the process of disintegration and decomposition of rocks under the influence of weather and climate.

It involves little or no movement of material.

That is why weathering is called an in-situ or on-site process.

There are three main types of weathering:

• Physical or mechanical weathering
• Chemical weathering
• Biological weathering

Physical Or Mechanical Weathering

Physical weathering means breakdown of rocks without any change in their chemical composition.

It is mainly caused by:

• Temperature changes
• Pressure changes
• Wind
• Water

Types Of Physical Weathering

Thermal Weathering

In arid and semi-arid regions, high daily temperature variation causes rocks to expand during the day and contract at night.

Repeated expansion and contraction create stress in rocks.

This leads to cracking and fragmentation.

Thermal weathering includes:

• Granular disintegration
• Block disintegration

Granular Disintegration

Granular disintegration occurs due to differential expansion and contraction of minerals within rocks.

The rock breaks down grain by grain into small particles.

Block Disintegration

Block disintegration occurs in regions with large temperature ranges.

Rocks break along joints and cracks into large rectangular blocks.

It is common in rocks such as granite.

Frost Wedging Or Freeze-Thaw Weathering

Water expands when it freezes.

When water enters rock cracks and freezes, it exerts pressure on rock walls.

Repeated freezing and thawing widens the cracks and finally breaks the rock.

This type of weathering is common in cold and mountainous regions.

Exfoliation Or Onion Weathering

Exfoliation is a type of physical weathering in which outer layers of rocks peel off in concentric sheets.

It occurs due to alternate heating and cooling or release of overlying pressure.

Repeated expansion and contraction separate surface layers from the main rock mass.

This forms dome-shaped structures.

It is common in granite rocks of arid and semi-arid regions.

It is also called onion weathering.

Chemical Weathering

Chemical weathering involves decomposition and disintegration of rocks through chemical reactions.

Important chemical weathering processes include:

• Solution
• Hydrolysis
• Carbonation
• Oxidation
• Hydration

Water, oxygen, carbon dioxide and suitable temperature speed up chemical reactions.

Solution

In solution, soluble minerals present in rocks dissolve in water.

Over a long period, these minerals are washed away.

This may lead to the formation of caves.

Hydrolysis

Hydrolysis is the chemical breakdown of rock material when it reacts with water.

It forms new minerals such as clay.

Example:

• Feldspar in granite changes into clay through hydrolysis.

Carbonation

Carbonation occurs when water reacts with carbon dioxide to form carbonic acid.

This weak acid reacts with minerals and helps in cave formation.

Oxidation

Oxidation occurs when oxygen combines with minerals, especially iron, in the presence of water.

This weakens and disintegrates the rock.

Example:

• Rusting of iron

Hydration

Hydration is the absorption of water by minerals.

It increases their volume and causes rock deformation.

Example:

• Anhydrite absorbs water and forms gypsum.

Biological Weathering

Biological weathering refers to breakdown and alteration of rocks by plants, animals and human activities.

Important examples include:

• Plant roots entering cracks and breaking rocks.
• Burrowing animals exposing rocks to air and moisture.
• Earthworms, termites and rodents disturbing rock material.
• Decomposition of plant and animal matter producing organic acids.
• Human activities such as ploughing, cultivation and removal of vegetation.

Root Wedging

Plant roots enter cracks in rocks.

As roots grow, they exert pressure on rocks and break them apart.

This process is called root wedging.

Organic acids such as humic acid and carbonic acid also increase chemical weathering.

Mass Movement

Mass movement is the transfer of a mass of rock and debris down slopes under the direct influence of gravity.

It may be slow or fast.

Unlike erosion by running water, wind, glaciers or waves, mass movement is not directly controlled by geomorphic agents.

Mass movement is common in areas with:

• Unconsolidated materials
• Thinly bedded rocks
• Faults
• Abundant precipitation
• Scarcity of vegetation

Activating Factors Of Mass Movement

Mass movement may be activated by:

• Removal of support from below by natural or artificial means
• Increase in slope gradient
• Heavy earthquakes
• Natural seepage
• Removal of natural vegetation

Types Of Mass Movement

Mass movements are classified on the basis of speed.

The main types are:

• Slow movement
• Rapid movement
• Landslides

Slow Movement

Slow movements take place gradually.

Solifluction

Solifluction is a slow process in which soil flows over the surface as a water-saturated mass.

The movement is completely dependent on water.

As water content increases, the soil becomes soggy and loses its cohesive strength.

High water content reduces friction, allowing gravity to move the soil downslope.

Solifluction often creates gentle, terrace-like features.

Soil Creep

Soil creep is a widespread mass movement found across the Earth’s surface.

It generally occurs on very steep slopes.

It can involve any kind of debris.

It is not dependent on moisture and can occur in any type of climate.

Soil creep can be identified by:

• Trees bent downward at their trunks
• Tilted exposed rock beds
• Heaps of weathered material at the base of mountain slopes

Rapid Movements

Rapid movements are mostly common in humid climatic regions.

They occur on gentle to steep slopes.

Earthflow

Earthflow is the movement of water-saturated clayey or silty Earth materials down low-angle terraces or hillsides.

It occurs in a localised manner.

It may take place on any type of slope surface.

Mudflow

Mudflow is a very rapid movement of water-saturated materials.

It is characteristic of valleys.

It is intensified by:

• Absence of vegetation
• Heavy rainfall

Mudflow usually occurs on a large scale.

Debris Avalanche

Debris avalanche is a very rapid movement of debris along steep slopes.

It commonly occurs in humid regions.

It moves along narrow tracks.

It is faster than mudflow.

Landslides

Landslides are sudden and devastating rapid slide movements.

They can transport and bury settlements and destroy ecosystems for a long time.

The materials involved in landslides are relatively dry.

Landslides are most common in areas with:

• Steep slopes
• Unconsolidated soil surfaces

Main triggering factors are:

• Heavy rainfall
• Earthquakes

Forms Of Landslides

Slump

A slump occurs when a great mass of bedrock moves downward by rotational slip from a high cliff.

The main cause of slumping is erosion at the base of the slope.

This erosion reduces support for overlying sediments.

Debris Slide

A debris slide is more extensive and occurs on a large scale.

The material involved is a mixture of soil and rock fragments.

Rock Slide

Rock slides usually follow a zone of weakness.

Water increases slippage.

During downward movement, collisions break the rock mass into rubble.

Rock slides may affect material up to considerable depth.

Rock Fall

Rock fall is the free-falling of rock blocks from steep slopes.

The accumulated debris at the base of the slope is called talus.

Erosion And Deposition

Erosion involves the acquisition and transportation of rock debris.

Like weathering and mass wasting, erosion is a degradational process.

It is caused by the dynamic activity of erosive agents such as:

• Wind
• Running water
• Glaciers
• Waves
• Groundwater

Wind, running water and glaciers are largely controlled by climatic conditions.

Waves and groundwater are less directly dependent on climate.

The work of waves depends on their coastal location where the lithosphere and hydrosphere meet.

The work of groundwater is controlled more by the lithological character, or nature of rocks, of the region.

Erosion degrades relief.

It wears down the landscape and continuously changes the Earth’s surface.

Denudational processes such as erosion and transportation are controlled by kinetic energy.

Deposition

Deposition occurs when erosional agents lose velocity and energy.

This usually happens on gentle slopes.

When this happens, the materials carried by agents begin to settle.

Coarser materials are deposited first.

Finer materials settle later.

The same geomorphic agents act both as erosional and depositional agents.

They are:

• Running water
• Wind
• Glaciers
• Waves
• Groundwater

Deposition fills up depressions and forms new depositional features.

Soil-Forming Factors

Pedology is the science of soil.

There are five basic factors that control soil formation:

• Parent material
• Topography
• Climate
• Biological activity
• Time

Parent material and topography are considered passive control factors.

Climate, biological activity and time are considered active control factors.

Parent Material

Parent material may be:

• In-situ weathered rock debris
• Transported deposits

Soil formation depends on the parent material’s:

• Texture
• Structure
• Mineral composition
• Chemical composition

The nature and rate of weathering and the depth of the weathering mantle are important.

Young and immature soils show a strong relationship with parent material.

Topography

Topography affects soil formation through:

• Exposure to sunlight
• Surface drainage
• Subsurface drainage

Soil is generally thin on steep slopes and thick over flat upland areas.

In gentle slope regions, erosion is slow and water percolation is adequate.

This makes soil formation favourable.

Flat areas may develop thick soil layers with good organic matter accumulation.

In middle latitudes, south-facing slopes receive more sunlight and develop different soils and vegetation compared to north-facing slopes, which are cooler and moister.

Climate

Climate is one of the most important soil-forming factors.

The main climatic elements are:

• Moisture
• Temperature

Moisture includes:

• Intensity of precipitation
• Frequency of precipitation
• Duration of precipitation
• Evaporation
• Humidity

Temperature includes:

• Seasonal variation
• Diurnal variation

Precipitation provides moisture to soil and makes chemical and biological activities possible.

Eluviation And Illuviation

Excess water causes downward movement of soil components.

This process is called eluviation.

The deposition of these materials in lower horizons is called illuviation.

Desilication

In high rainfall regions, calcium, sodium, magnesium and potassium are removed from soil.

A major part of silica is also removed.

This process is called desilication.

Hardpan And Kankar

In dry climates, evaporation exceeds precipitation.

Groundwater rises to the surface by capillary action.

When water evaporates, salts are left behind in the soil.

The accumulation of salts forms a crust called hardpan.

In areas with intermediate precipitation, calcium carbonate nodules called kankar are formed.

Temperature also affects soil formation.

High temperature increases chemical and biological activity.

Cool conditions reduce it.

Freezing conditions may stop chemical reactions.

Tropical soils develop deeper profiles, while tundra soils largely contain broken and undecomposed material.

Biological Activity

Vegetation and soil organisms contribute organic matter.

They also improve moisture retention and add nitrogen to soil.

Organic acids formed during humification help decompose minerals in the parent material.

Bacterial activity and organisms such as rats, rabbits, ants, termites and earthworms help soil formation.

Certain bacteria convert atmospheric nitrogen into a usable form for plants.

This process is called nitrogen fixation.

Rhizobium bacteria found in the root nodules of leguminous plants fix nitrogen beneficial to plants.

Time

Soil becomes mature when soil-forming processes operate for a sufficiently long period.

With time, a well-defined soil profile develops.

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FAQs On Geomorphic Processes