Erosion is the natural process of wearing away and transporting soil, rock, or dissolved material from the Earth’s surface. This constant reshaping of the landscape is driven by dynamic external forces that move material from a source area to a depositional area. This process fundamentally alters topography over time. Based on the main agent responsible for the movement, erosion is categorized into four major types.
Erosion by Water
Liquid water is the most common agent of erosion, affecting landscapes from agricultural fields to river valleys. The process begins with rainfall, where the impact of raindrops dislodges soil particles, known as splash erosion. This detachment makes the particles available for transport by surface runoff.
As water flows over the ground in a thin sheet, it carries away loosened topsoil, a process called sheet erosion. This erosion is often gradual and may go unnoticed until significant loss of nutrient-rich material has occurred. When the flow concentrates into small, shallow channels, rill erosion begins, creating distinct grooves in the soil surface. Rills are typically less than 30 centimeters deep and can often be removed by farming practices.
Rills that deepen and widen due to concentrated water flow eventually form gully erosion. Gullies are large, deep channels that cannot be easily repaired by cultivation methods and severely limit land use. Rivers and streams also erode their channels through hydraulic action and abrasion. Hydraulic action involves the force of the water compressing air in cracks, while abrasion occurs when carried sediment grinds against the bed and banks. This action leads to the widening and deepening of valleys.
Erosion by Wind
Wind erosion, also known as Aeolian erosion, is effective in arid and semi-arid environments where vegetation cover is sparse and loose sediment is available. Wind removes and transports sediment through two primary actions: deflation and abrasion. Deflation is the lifting and removal of fine, loose particles like silt and clay by the wind, often leading to the lowering of the land surface.
Abrasion occurs when wind-transported particles, typically sand, strike and wear down exposed rocks and other surfaces. Sediment transport is categorized into three mechanisms: suspension, saltation, and creep. Suspension involves very fine particles (less than 0.1 millimeters) being lifted high into the air and carried over long distances, often resulting in dust storms.
Saltation is the primary mode of transport, accounting for an estimated 50 to 75 percent of total sediment flux. This process involves sand-sized particles (0.1 to 0.5 millimeters) bouncing or skipping along the ground surface. Creep involves larger particles (greater than 0.5 millimeters) that are too heavy to be lifted, instead rolling or sliding along the surface after being impacted by saltating grains.
Erosion by Ice
Glacial erosion occurs where large, moving masses of ice, known as glaciers, reshape the underlying landscape. Glaciers erode rock primarily through two mechanisms: plucking and abrasion. Plucking, also called quarrying, happens when glacial meltwater seeps into cracks in the bedrock. As the water refreezes, it expands and exerts pressure, breaking off large blocks of rock that the glacier then carries away.
Abrasion is the grinding action that occurs as the ice, laden with embedded rock fragments, scrapes across the bedrock beneath. This process smooths, scratches, and polishes the rock surface. The movement leaves behind linear grooves and scratches called glacial striations, which indicate the direction of the glacier’s flow. Glaciers create distinctive landforms, such as U-shaped valleys and bowl-shaped hollows known as cirques.
Erosion by Gravity
The force of gravity acts directly on surface materials, causing the downslope movement of soil and rock without a flowing agent like water, wind, or ice. This process is collectively known as mass wasting or mass movement. Mass wasting occurs when the slope is too steep or the material is too weak to resist the pull of gravity.
The speed and manner of movement vary dramatically, ranging from extremely slow to fast. Creep is the slowest form of mass wasting, involving the gradual downhill movement of soil, often indicated by curved tree trunks or tilted utility poles. This slow movement is facilitated by the repeated expansion and contraction of soil due to freezing and thawing or wetting and drying.
Faster movements include slides and flows. Slides, such as landslides or rockfalls, involve a block of material moving quickly along a distinct failure surface. Flows, like mudflows or debris flows, are rapid movements where the material behaves like a viscous fluid, typically involving loose sediment mixed with water. These events are often triggered by excessive rainfall or seismic activity.