Erosion is a process in which rocks and soil are worn away and undergo transformations through the action of erosion agents such as water, wind, glacier movement, or temperature.
During erosion, particles of rock or soil are detached and displaced from their point of origin. It is a process of degradation and transformation of the relief which can either be slow and gradual, or completely drastic. It takes several million years to level (flatten, level out) a mountain or dig valleys, but it only takes a few minutes for an avalanche, a lahar (mud flow of volcanic origin), or a thunderstorm to transform the landscape.
Erosion and landscape modification can be caused by many factors known as erosion agents.
Example of the effect of erosion on landform: Bryce Canyon National Park in the United States
When raindrops hit the ground, the force of their impact breaks up the aggregates and disperses the particles that form the soil. This is called the splash effect. Thus, fine sands, silts, clays, and organic matter are easily washed away by water drops, as they are tiny particles.
Runoff occurs when water on a slope cannot seep into the ground quickly enough or cannot be intercepted by natural barriers. The greater the intensity of precipitation and runoff is, the larger the size and quantity of particles that are displaced. As water flows, it carries soil particles with it and causes erosion. It is mainly runoff that carves out the valleys.
Niagara Falls is a fine example of erosion. Niagara Falls is currently considered to be 11 kilometres from the site where it originally formed. Until the early 1950s, the erosion of Niagara Falls was one meter per year. Now, due to the different constructions, this rate is estimated at 36 cm per year.
Waves and currents cause erosion on the coastline. When the waves hit the shore at a certain angle, the result is coastal currents, parallel to the shore, which continually move the beach sands. All the waves produced by the wake of the boats, i.e. ship-generated waves, crash into the banks and cause erosion there as well.
Water can also seep through cracks in rocks and be absorbed by certain minerals (montmorillonite for example). These minerals expand and the increase in their volume can cause rocks (especially shales) to fragment. This is called hydroclasty.
Some rocks and minerals are soluble in water – some examples are limestone, carbonate, gypsum, sulphate, and sodium and potassium chlorides. Water dissolves these rocks and minerals and releases the chemical elements in the form of ions. Water can subsequently carry these elements over a great distance. This is another form of erosion.
Water in the form of ice can also be very erosive. For example, a moraine is a mass of mineral debris that has been carried by a glacier or an ice sheet. Rivers and glaciers shape valleys.
Wind acts like water. It removes soil particles and carries them further. Debris blown by the wind also causes friction erosion. It wears down and polishes surfaces. Soils become poorer. Moreover, wind erosion is the main physical factor responsible for the depletion of agricultural land.
Wind dries up the land and reduces its moisture content. Obviously, the stronger, more regular and dusty the wind is with fewer obstacles in its path, the greater its erosion power will be. Wind has a great power of erosion in dry regions where little vegetation is present, such as deserts.
Gravity allows several erosion processes to occur. Runoff, creep (slow movement of soil particles down slopes), landslides, avalanches, rock slides and the descent of muddy material on a slope are all erosion phenomena caused by gravity.
In places where temperature variations are significant (continental climate, polar climate, desert, high mountains, etc.), thermoclasticism (erosion due to temperature changes) occurs. For example, water that seeps through cracks in porous rocks takes on more volume as it changes from a liquid to a solid state. This water is able to burst rocks when it freezes and thaws. This is referred to as frost wedging.
Human beings, through some of their actions, increase erosion. For example, deforestation, agriculture, urbanization, and transport have the effect of accelerating erosion.
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When clearing land, the vegetation cover is removed and the soil is exposed to wind and landslides.
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When we build dams, the waterways are then diverted.
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By building roads, we increase runoff surfaces.
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When we use motor boats or personal watercrafts, we destroy habitat, increase water turbidity, and release nutrients that cause algae growth.
These activities have economic and environmental consequences. The best way to limit erosion is to preserve vegetation, as plant roots help hold the soil in place.
Certain factors accelerate the erosion process, including the following which should be taken into consideration.
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Some rocks are more sensitive than others to different types of erosion. For example, siliceous sandstone will not be easily dissolved; however, it will be very sensitive to the effects of frost. Conversely, a massive limestone rock will be easily eroded by water, but very unaffected by frost.
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The longer and steeper the slope, the more water is likely to erode the soil. And the greater the speed of water, the greater the leaching.
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Vegetation cover plays a major role in preventing erosion. In fact, the more residues and plants are present on the ground, the more they protect the ground from the impact of raindrops. Residues and plants slow down the speed of runoff and they promote improved infiltration of water into the soil.
Erosion does not only result in the loss of soil particles from the topsoil. For example, runoff water can carry away nutrients, fertilizers, and seeds from a field soil. Cultivating this field would not be as productive as it could be.
Sediments that are carried by water can silt up drainage ditches or cover spawning areas, for example. Water quality is thus reduced because pesticides and fertilizers carried with soil particles can contaminate water sources.