Whereas weathering refers to the breakdown of rock into smaller pieces through a variety of different processes, erosion refers to the assorted processes by which these pieces are removed and transported some distance from their origin. There are four main agents of erosion- gravity, water, wind, and glaciers- although there may be several different ways for each of these agents to erode a substance.
Erosion due to gravity is simply the movement of sediment, rock, etc. down-slope from an area of higher elevation to an area of lower elevation due to the force of gravity. Although gravity also plays a role in the other types of erosion, it is most pronounced as the primary agent of erosion in areas of higher topographic relief. A common example is the accumulation of talus, pieces of broken rock, at the bottom of a mountain or hillside (as seen in the picture below).
Glaciers are large masses of ice that develop as snow is successively compacted over time. Although there are several different types of glaciers, with complex interactions governing their movement, the erosion that results from the movement glaciers can be characterized as plucking, abrasion, and ice thrusting.
Most rock surfaces are not flat or regular, and they often include outcroppings that protrude from the surface. As a glacier moves over these areas where the rock is sticking up/out, differences in pressure and melting/refreezing ice within the glacier can act to break pieces off and incorporate these chunks of rock into the glacier itself. This process is known as plucking.
Similarly, ice thrusting occurs when rock or sediment is frozen to the bottom of the glacier. When the glacier begins to move after being stationary for some time, this rock or sediment is thrust forward with the glacier (and often up into the glacier). Eventually, this material will be deposited some distance away from where it was removed by the ice.
Lastly, the movement of glaciers can result in erosion through abrasion. Regardless of how the sediment and rock come to be carried along by the glacier, this material will be transported until it is finally deposited. During this time, some of these pieces will be carried along at the base of the glacier. As the glacier moves over the underlying rock, these pieces can gouge, carve, and grind up either the underlying rock or other pieces of loose rock.
The ability of wind to erode sediment depends upon the size of the material, the velocity of the wind, the duration of the wind, and the length of the area over which it can blow unobstructed. Smaller particles, such as grains of silt or clay, may be carried aloft in suspension and travel great distances before being deposited. However, larger-sized particles generally move by rolling or bouncing along the ground, a process known as saltation.
Water covers approximately 70% of the Earth’s surface, with over 200,000 miles of coastline and thousands of miles of rivers and stream systems. This, along with the large percentage of the areas which receive rainfall precipitation, make the importance of water’s role in erosion clear.
Along coasts, the primary mechanism of erosion due to water is wave action. Consider a sandy beach. As waves come toward the coast, sediment is disturbed and pushed toward the beach. As the waves break and flow back out, some of this sediment and sand from the beach flows back out as well with the water. The net result is that gentle wave is forever moving and rearranging sediment both on the sea bottom near the beach and the sediment on the beach itself.
However, waves can also erode material on rocky shores too. As waves crash into the rocks, pressure from this impact can work on preexisting fractures and joints, breaking off small pieces of rock that are carried back out into the water. These pieces can collide with each other, further breaking each other into smaller and smaller pieces, or they can be smashed into the rock along the shoreline, grinding down the rock and breaking off more pieces.
Something as simple as rainfall can also cause erosion, especially on farm and cropland. Raindrop erosion occurs from the impact of raindrops on bare soil. The energy of this impact can dislodge individual grains as well as break apart soil aggregates, clumps of soil particles which are being held together by some other medium (e.g. plant roots, clay, etc.).
If the rate of precipitation exceeds the rate of infiltration, the process by which water is absorbed into the ground, sheet erosion may result. As water collects on the surface, it will begin to flow as a thin sheet over the soil, taking loose soil with it. Over time, this flow will differentially erode the surface and form small channels or rills. Over successive precipitation events, these rills may become larger and larger, eventually earning the designation of gullies.
Lastly, rivers and streams have an incredible ability to erode and transport large volumes of sediment. A stream has two primary mechanisms by which it picks up material: entrainment and dissolution. Entrainment refers to the process by which the hydraulic action of the water, and eddies resulting from turbulence, lift loose particles and carry them along. Dissolution, as discussed in chemical weathering, refers to the process by which soluble materials are dissolved by the water (with the products of these dissolution reactions, ions, being carried along in solution).
The total sediment carried by a stream is referred to as load and there are three types: dissolved load, suspended load, and bed load. The dissolved load of a stream is the material which is transported in solution. The suspended load is the material which is carried along in the water without settling to the bottom. The bed load refers to material which is too large to be carried along in suspension and bounces along the bed, the process of saltation, or rolls along without being lifted. As these particles roll or bounce along the bottom, they can cause further erosion from abrasion.