Introduction to Erosion

Erosion is a natural geological process that involves the removal and transportation of disintegrated soil and rock particles, or sediment from one location to another by the action of various erosional agents. These agents include water, wind, ice (glaciers), or gravity. It plays a significant role in shaping and featuring the Earth’s surface over long periods of time. Erosion can occur in different environments and landscapes, including coastlines, and riverbeds. Like weathering erosion is a part of denudation. Both weathering and erosion run simultaneously. The actions of both processes are collectively known as denudation. Erosion and Different types of Erosion.

Cycle of Erosion

To understand the cycle of evolution of a landscape, let us start with a relatively flat upland area in a humid region.

  1. During the youth stage, the area remains more or less flat. The stream valleys are generally narrow.
  2. In the mature stage, the relief of the land increases, and the landscape is changed into one consisting of hills and valleys. The “relief” of an area may be defined as the maximum difference in the elevation.
  3. In the older stage, the streams will approach base level, and the land will be reduced to a peneplain. The “peneplain” is an undulating plain, which lies nearly at the base level. Some mounds or hillocks of hard rocks still persist. These hillocks are called “monadnocks”.

Types of Erosion

There are several types of erosion based on their association with their specific type of agents:

Water Erosion

 Water erosion is generally known as stream erosion. The term stream includes the channelized flow of any size, from the smallest brook to a very large river like the Amazon. Although the term “river” and “stream” are used synonymously, the term “river” is preferably used to denote the main stream into which several tributaries flow. The geological work of stram is to erode the valleys, transport the material that is eroded, and deposit the same in the lower reaches at favorable sites. The streams cause erosion in four ways:

    • Chemical Action: It includes the solvent and chemical action of water on country rocks. The chemical decay works along joints and cracks and thus helps in breaking the bedrock.
    • Hydraulic Actions: The swiftly flowing water hammers the uneven faces of jointed rocks exposed along its channel and removes the jointed blocks. This process of erosion is called ”Hydraulic Action”. At the bottom of a waterfall, the channels are eroded at an enormously rapid ratio by hydraulic action.
    • Abrasion: The flowing water uses rock fragments such as pebbles, gravel, and sand as a tool for scratching and grinding the sides and floor of the valley. This process of erosion is called “abrasion”.
    • Attrition: It is the breaking of the transported materials themselves due to mutual collisions. The attrition causes the rock fragments to become more rounded and smaller in size.

In addition to the above, the streams acquire their load by many other means. Much of the material carried by a stream is contributed by underground water, overland flow, and mass wasting.

Wind Erosion

    • Deflation: Wind removes loose, fine-grained particles from the surface, leading to the lowering of the land. The lifting and removal of loose material by wind is known as deflation. By this process, the land surface is gradually lowered. In many desert areas, deflation produces hollows or basins with their bottoms at the water table. Such basins containing some water are called “oasis”.
    • Abrasion: During dust storms the wind carries minute grains of sand in suspension. They dash and collide against the exposed rock masses and cause erosion. This process in which sand grains are used as tools for eroding rocks is called “abrasion”.
    • Attrition: The particles that travel with wind collide against one another. These mutual collisions lead to their further breakdown and the process is known as “attrition”.

Ice/Glacial Erosion

A “glacier” is a thick mass of ice, which moves over the ground under the influence of gravity. It originates on land from the compaction and recrystallization of snow. Glaciers form in places, where more snow accumulates each year than that melts away. They are found chiefly in high latitudes as in the Arctic region, or at high elevations as in the Himalayas, Rockies, Andes, and the Southern Alps of New Zealand above the snow line. The snowline is the lower limit of accumulating snow. Below the snowline, the snow melts in summer. The elevation of the snowline varies considerably. In polar regions, it may be at sea level, whereas in areas near the equator, the snowline may occur at 6000 meters. In the Himalayas, the snowline lies at altitudes varying between 4200 to 5700 meters. These ice bulks or glaciers are an active agent of erosion, and cause erosion in three ways; (i) by plucking or quarrying, (ii) by abrasion, and (iii) by frost wedging.

  • Plucking and Quarrying: While flowing over a jointed rock surface, the glacial ice adheres to blocks of jointed bedrock, pulls them out, and carries them along.
  • Abrasion: The moving ice grinds and polishes the rock surface with the help of the rock fragments, which are held firmly within the body of the glacier. The abrasion produces striations and grooves in the bedrock surface. A polished surface results when the glacier performs abrasion by fine silt-sized sediment. The ground-up rock produced by the grinding effect of the glacier is called “rock flour”.
  • Frost Wedging: Thawing and freezing of water in the cracks and joints of rocks break them by wedge action. In this manner, rock fragments of all sizes are added to the glacier.

Gravity Erosion

Gravity erosion is also known as mass wasting, mass movement, or slope movement. This refers to the downslope movement of rock and soil under the influence of gravity. It includes processes like landslides, rockfalls, and slumps, where high-lying materials move downhill under the influence of the force of gravity. Although “gravity erosion” is not a standard term (gravity is a fundamental force influencing various erosion processes), therefore a particular (standard) term; “mass wasting” is used for the erosion. It is caused by gravity without the involvement of water, ice, or wind. This process occurs when the force of gravity overcomes the resistance of the material holding it in place, leading to the movement of large volumes of material downslope. Mass wasting can occur in various forms, including landslides, rockfalls, slumps, and debris flows, and it can be triggered by factors such as heavy rainfall, earthquakes, volcanic activity, or human activities like deforestation and construction. It’s a significant geological process that can have severe impacts on landscapes, ecosystems, infrastructure, and human settlements.

The events of mass wasting can range from slow, gradual movements to rapid and destructive. Many factors can contribute to mass wasting, among which the following are the most important;

  • Gravity: The primary driving force behind mass wasting is gravity, which directly pulls materials downslope.
  • Slope Angle: The slope of the angle is a very important factor of mass wasting. Steeper slopes are more prone to mass movement because the gravitational force can act more effectively in overcoming resisting forces (friction, and cohesion).
  • Water Content: Water can significantly influence mass wasting by increasing the weight of the material and reducing internal cohesion, and friction. Rainfall, snowmelt, or other sources of water can trigger mass wasting events.
  • Vegetation: The presence or absence of vegetation can affect slope stability. Plant roots help bind soil and rock particles, thus providing stability and reducing the likelihood of mass wasting.
  • Geology and Rock Type: The type of rock and soil on a slope influences its susceptibility to mass wasting. Weak or fractured rocks are more prone to movement.
  • Human Activities: Activities such as construction, mining, and deforestation can alter the natural slope characteristics, making them more susceptible to mass wasting.

Common types of mass wasting include:

  • Creep: Creep is the imperceptibly slow, downslope movement of soil and earth materials. Rates of movement are often only a few centimeters per year, but the inevitability of creep can severely impact shallowly-placed structures.
  • Slides: A landslide is defined as the movement of a mass of rock, debris, or earth down a slope. Landslides are a type of “mass wasting,” which denotes any down-slope movement of soil and rock under the direct influence of gravity.
  • Flows: Flows are rapidly moving mass-wasting events. This movement happens when a loose material is mixed with abundant water, creating long runouts at the slope base. There are two types of flows; (i) the debris flow (coarse material) and (ii) earthflow (fine material). Both types are separated from each other depending on the type of material involved and the amount of water.

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