External forces & their impact

Weathering is breaking down rocks, soil, and minerals as well as wood and artificial materials by contacting the atmosphere, water, and biological organisms of the Earth. Weathering takes place in situ, i.e. in the same place, with little or no movement. It should therefore not be confused with erosion involving the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity, and then transported and deposited elsewhere.

Erosion is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water.

Erosion is the opposite of deposition, the geological process in which earthen materials are deposited, or built up, on a landform.

Most erosion is performed by liquid water, wind, or ice (usually in the form of a glacier). If the wind is dusty, or water or glacial ice is muddy, erosion is taking place. The brown color indicates that bits of rock and soil are suspended in the fluid (air or water) and being transported from one place to another. This transported material is called sediment.

Physical erosion describes the process of rocks changing their physical properties without changing their basic chemical composition. Physical erosion often causes rocks to get smaller or smoother. Rocks eroded through physical erosion often form clastic sediments. Clastic sediments are composed of fragments of older rocks that have been transported from their place of origin.

Plant growth can also contribute to physical erosion in a process called bioerosion. Plants break up earthen materials as they take root, and can create cracks and crevices in rocks they encounter.

Ice and liquid water can also contribute to physical erosion as their movement forces rocks to crash together or crack apart. Some rocks shatter and crumble, while others are worn away. River rocks are often much smoother than rocks found elsewhere, for instance, because they have been eroded by constant contact with other river rocks.

Erosion is the physical removal and transport of material by mobile agents such as water, wind or ice.

The three common agents of erosion are:

  1. Water
  2. Wind
  3. Ice

These agents are mobile at the Earth’s surface and are responsible for the transport of sediment.

Erosion and mass wasting appear to be similar processes but have distinctly different causes. The movement of sediment by erosion requires mobile agents such a water, wind and ice. That is, the sediment is transported by the movement of the agents. Mass wasting (commonly referred to as landslides) involves the transfer of rock and soil downslope under the influence of gravity. Gravity is the key factor in mass wasting and the movement of material does not require a mobile agent.

Erosion (transport of sediment) usually ends with the deposition of sediments (and soil). Deposition occurs when the forces responsible for erosion are no longer sufficient to transport the sediment.

There are a wide variety of landscapes on the Earth’s surface where the deposition of sediments occur as the result of fluvial (rivers), aeolian (wind) and glacial (ice) erosion.