Erosion occurs when rocks break apart and move along their pathways, dispersing parts to other locations through natural processes like rivers or wind currents. Erosion may take place nearby – such as when stones wash into rivers – or can take place miles away, such as when one is carried off by gusts of wind to an entirely different destination.
Geologists study eroded sediments to understand how they formed. They look for grains with well-rounded surfaces formed over a longer time span by multiple forces acting upon them.
Weathering
Weathering is the process by which rocks break down into smaller pieces, while erosion refers to their movement from one place to another.
Water, wind and glacial ice all play their parts in erosion; its speed depends on what rocks and soils are being worn away – harder ones like granite wear away more slowly than softer rocks like limestone.
Water erosion can form canyons like the Grand Canyon and beaches with waves transporting sand into shallower waters, while chemical weathering breaks down rocks such as limestone to form sinkholes, caves and cliffs.
Abrasion
Rocks can wear away, or erode, by impact with other rocks and water or wind forces. Erosion occurs when bits of soil or rock move from one location to the next – usually caused by liquid water but wind and glaciers can also transport this process along.
Water erosion has created some truly extraordinary landscapes. Waterfalls sculpt cliffs into breathtaking shapes while wind erosion spreads sand like in China’s Badain Jaran section of Gobi Desert into towering sand dunes that eventually reach Badain Jaran section and then over time further smoothing rocks like those found at Arches National Park in Utah.
Hydraulic Action
Rivers, stream beds and coastlines all witness this type of erosion due to water’s forceful currents eroding rock particles away.
Water waves exert hydraulic action to gradually erode cliffs through hydraulic erosion. Air gets trapped between forward-moving water and rock surfaces, and when they retreat the explosion of compressed air weakens them further – known as pneumatic weathering.
Hydraulic action contributes to the creation of features like sea caves and blowholes, as well as to riverbank collapse and cliff collapse – shaping landscapes over time. Eroded material carried downstream then contributes to sediment deposition which leads to further erosion – including processes like abrasion, attrition and solution erosion.
Solution
Rain erosion can damage landscapes by washing away top soil, rocks, and sediment from their original locations, being transported by wind, ice, gravity or any combination thereof to new ones. Once enough kinetic energy has been dissipated from material being transported it becomes deposited adding further layers of sediment into an already growing pile of waste. Though erosion is a natural process it may be accelerated through human activities which in turn increases rates and frequency of potentially hazardous events.
Weathering and erosion are slow processes that we see all around us; potholes on roads result from water entering cracks in rocks that freeze over, plant roots clinging tightly into them, waves pounding rocks creating waves, waves creating beaches – these examples illustrate that erosion occurs either quickly like with mudslides or slowly like glaciers.
Deposition
Deposition refers to the process by which erosion wears away pieces of rock and soil from one location and deposits them elsewhere – either slowly, such as when wind blows sand onto beaches, or quickly in mudslides.
Chemical weathering is more prevalent in warm, humid environments due to water and reactants penetrating more easily through rock surfaces. Minerals with higher crystallization rankings in Bowen Reaction Series (see Chapter 4, Igneous Rock and Volcanic Processes ) typically experience rapid chemical weathering compared to those ranked lower on this series.
Mechanical weathering – which involves physical breakdown without altering their chemical composition – occurs across all types of rocks. When applied in fluid systems such as moving waters and glaciers (see Chapter 10, Mass Wasting ) sand and coarse sediment particles tend to move easily while fine silt and clay sediment remain in place.