The surface of the Earth is not a smooth, permanent feature. It is constantly being shaped and reshaped by a variety of natural processes. These processes are called geomorphic processes, from the Greek words "geo" (earth), "morph" (form) and "logos" (study). Geomorphic processes are the physical and chemical actions that cause changes to the Earth's surface, wearing down the land, transporting material and building new landforms. They are responsible for the endless variety of relief features we see, from high mountains and deep valleys to vast plains and sand dunes.
Geomorphic processes can be divided into two broad categories based on the source of energy. Endogenic processes are powered by forces originating inside the Earth, such as heat from the interior, and include diastrophism (crustal deformation), volcanism and earthquakes. Exogenic processes are powered by external energy sources, mainly solar energy, and include weathering, mass wasting, erosion, transportation and deposition. The interaction of these two sets of processes creates and modifies the landforms of the Earth. This chapter focuses mainly on the exogenic processes, beginning with weathering, while landform evolution is discussed in the next chapter.
The relief of the Earth is the result of a balance, or contest, between the constructive endogenic forces that build up the land and the destructive exogenic forces that wear it down. The relief features of the Earth's surface are broadly classified as first order (continents and oceans), second order (mountains, plateaus and plains), and third order (valleys, dunes, deltas, etc.). The exogenic forces obtain their energy mainly from the Sun and from the gravitational force acting on materials. Endogenic forces obtain their energy from the Earth's internal heat and gravitational differentiation. The study of the interaction of these forces helps us understand the constant change occurring on the Earth's surface.
Exogenic processes operate on the Earth's surface and are driven by external energy, chiefly solar energy and gravity. They include weathering, mass wasting, erosion, transportation and deposition. These processes collectively work to reduce the relief of the land, a process called gradation. The gradational processes are divided into: 1. Degradation: The wearing down of the land surface, which includes weathering, mass wasting and erosion. 2. Aggradation: The building up of the land surface, which includes deposition of transported material.
The cycle of denudation (wearing away of land) begins with weathering, which prepares the material for erosion, after which the eroded material is transported and eventually deposited.
Weathering is the process of disintegration (physical breakdown) and decomposition (chemical breakdown) of rocks on or near the Earth's surface, caused by the action of weather, water, plants and animals. Weathering is the first and most important step in the process of denudation. It is of three main types: 1. Physical (Mechanical) weathering: Breakdown of rocks without any change in their chemical composition. 2. Chemical weathering: Decomposition of rocks through chemical reactions that change their chemical composition. 3. Biological weathering: Breakdown of rocks caused by the activity of living organisms such as plants, animals and microbes.
Physical weathering involves the mechanical disintegration of rocks. Important processes include: - Exfoliation (thermal expansion): In deserts, rocks expand on heating during the day and contract at night, causing the outer layers to peel off like the skins of an onion. - Frost weathering (freeze-thaw): Water in the cracks and joints of rocks freezes and expands by about 9%, exerting pressure that widens the cracks. This is common in high mountains and cold regions. - Salt weathering: In coastal and arid regions, the crystallisation of salts in the pores of rocks exerts pressure and causes disintegration. - Unloading and expansion: When the overlying rocks are removed by erosion, the pressure on the rocks below is reduced and they expand and crack, forming sheet joints.
Chemical weathering involves chemical changes in the rocks. Important processes include: - Solution: Minerals dissolve directly in water. For example, rock salt dissolves in water. - Carbonation: Carbon dioxide dissolved in rainwater forms weak carbonic acid, which reacts with minerals like limestone, dissolving them. This is important in karst landscapes. - Hydration: Minerals absorb water and expand, creating pressure on the rock. - Oxidation: Oxygen combines with minerals, especially iron, forming oxides. Rusting of iron-rich rocks is an example. - Hydrolysis: Water reacts with minerals to form new minerals, changing the rock's chemical structure.
The rate and type of weathering depend on several factors: climate (temperature and rainfall), the nature of the rock (hardness, jointing, mineral composition), the presence of vegetation, the slope and aspect of the land, and the length of time the rock has been exposed. Humid tropical regions experience intense chemical weathering because of high temperatures and rainfall, while cold and arid regions experience mainly physical weathering.
Mass wasting (or mass movement) is the downslope movement of rock, soil and regolith under the influence of gravity, without necessarily the involvement of a transporting agent like water or wind. Mass wasting occurs when the gravitational force acting on a slope exceeds the resisting forces that hold the material in place. It is classified on the basis of the speed and nature of the movement: 1. Slow movements: Creep (very slow, almost imperceptible movement of soil) and solifluction (slow flow of saturated soil in cold regions). 2. Rapid movements: Landslides, mudflows, earthflows, debris avalanches and rockfalls.
Mass wasting is common on steep slopes, in tectonically active regions and after heavy rains, and is an important process in shaping hill slopes and valleys.
Erosion is the removal and transportation of weathered material by natural agents such as running water, wind, glaciers, groundwater and sea waves. Erosion differs from weathering because it involves the transport of material. The main agents of erosion are: - Running water (fluvial): Rivers erode through hydraulic action, abrasion, solution and attrition, forming valleys, gorges and canyons. - Glaciers: Moving ice erodes through abrasion and plucking, forming U-shaped valleys, cirques and fjords. - Wind: Wind erodes in deserts through deflation and abrasion, forming mushroom rocks and yardangs. - Groundwater: Water in the ground dissolves soluble rocks, forming caves, sinkholes and karst topography. - Sea waves: Waves erode coastlines, forming cliffs, sea arches and stacks.
Transportation is the carrying of eroded material by these agents, and deposition (aggradation) occurs when the transporting agent loses energy and drops its load, forming landforms such as deltas, floodplains, dunes and moraines.
Weathering and the formation of soil are closely related processes. Soil is the uppermost layer of the Earth's surface composed of mineral particles, organic matter, water and air. Soil formation (pedogenesis) begins with the weathering of parent rock material and the accumulation of organic matter from the decay of plants and animals. The main factors controlling soil formation are: 1. Parent material (the rock from which soil is derived) 2. Climate (temperature and rainfall) 3. Relief (slope and altitude) 4. Organisms (plants, animals and microbes) 5. Time
Soil develops a characteristic profile of horizons (O, A, B, C) over time. The process of soil formation combines weathering with the biological activity of organisms, making soil a natural resource of vital importance.
| Type | Process | Example |
|---|---|---|
| Physical weathering | Mechanical breakdown without chemical change | Exfoliation, frost wedging, salt crystallisation |
| Chemical weathering | Chemical change of minerals | Carbonation, oxidation, hydrolysis, solution |
| Biological weathering | Action of living organisms | Root wedging, burrowing animals |
| Agent | Erosional Process | Resulting Landform |
|---|---|---|
| Running water | Hydraulic action, abrasion | V-shaped valleys, gorges, canyons |
| Glacier | Abrasion, plucking | U-shaped valleys, cirques, fjords |
| Wind | Deflation, abrasion | Mushroom rocks, yardangs |
| Groundwater | Solution | Caves, sinkholes |
| Sea waves | Hydraulic pressure, abrasion | Cliffs, sea arches, stacks |
| Type | Speed | Description |
|---|---|---|
| Creep | Very slow | Imperceptible movement of soil |
| Solifluction | Slow | Flow of saturated soil in cold areas |
| Landslide | Rapid | Sudden sliding of rock and soil |
| Mudflow | Rapid | Flow of water-saturated debris |
| Rockfall | Very rapid | Free fall of rock fragments |
Geomorphic processes are the forces that continuously shape and reshape the Earth's surface. Endogenic processes, powered by the Earth's internal heat, build up the land, while exogenic processes, powered by solar energy and gravity, wear it down. Weathering, the first step of denudation, breaks down rocks through physical, chemical and biological means. Mass wasting moves material downslope under gravity, and erosion, transportation and deposition by running water, glaciers, wind, groundwater and waves create the great variety of landforms on the planet. These processes also lead to soil formation, which sustains life. Understanding geomorphic processes is essential to explaining the landforms we study in the next chapter on landform evolution.
Keywords: Weathering, erosion, mass wasting, exogenic, endogenic, carbonation, exfoliation, frost wedging, gradation, pedogenesis.