Erosion is a fundamental process that relentlessly reshapes the surface of the Earth. It is defined as the movement of weathered material—rock fragments, soil, and sediment—from one location to another. After material is broken down by weathering, erosion serves as the transport mechanism to relocate the debris. This constant movement sculpts everything from smooth river valleys to sharp mountain peaks. This geological work is performed by four principal natural agents: liquid water, wind, solid ice, and the universal force of gravity.
Water Erosion
Liquid water is the most geographically widespread and effective agent of material movement on the planet. This erosion occurs across vast river systems, known as fluvial processes, where the volume and velocity of streams and rivers cut into banks and riverbeds. Water flowing over land as simple runoff also carries loose sediment downslope, especially in areas lacking dense vegetation cover. Coastal erosion occurs through the pounding of waves and the movement of tides, wearing away cliffs and transporting beach sand.
Water transports sediment through three primary mechanisms based on particle size and the flow’s turbulence. Dissolved load involves chemically weathered material carried within the water itself. Fine silt and clay particles are held up by the turbulent flow and constitute the suspended load, which can make the water appear muddy. Larger, heavier material, such as sand and gravel, is pushed, rolled, or bounced along the riverbed as bed load, constantly grinding against the bottom surface.
Wind Erosion
The movement of sediment by air, known as aeolian processes, is a significant force, particularly in arid and semi-arid environments where the ground is dry and vegetation cover is sparse. Wind lifts and carries particles in two distinct ways depending on their mass. The finest dust and silt, typically less than 0.1 millimeters in diameter, are carried high into the atmosphere as the suspension load. These particles can travel hundreds or even thousands of miles, sometimes resulting in massive dust storms.
The majority of sand transport occurs through a process called saltation. Saltation involves medium-sized particles (0.1 to 0.5 millimeters) that are too heavy for continuous suspension but light enough to be lifted into a series of short hops. When a saltating grain strikes the ground, it dislodges other particles, which then begin to hop themselves, creating a chain reaction of movement near the surface.
Glacial and Ice Erosion
Solid water, in the form of massive, moving glaciers and ice sheets, is a profoundly effective agent of landscape modification. Glaciers erode the land beneath them through two primary mechanical actions. Glacial plucking occurs when meltwater seeps into rock fractures, freezes, and expands, breaking off large blocks of rock. As the glacier moves, it lifts and carries these detached blocks away.
The second mechanism is glacial abrasion, which operates like geological sandpaper. Rock fragments and debris embedded in the base of the moving ice grind against the underlying bedrock, smoothing and polishing the surface. This abrasive action leaves behind fine rock flour and long, parallel scratches called striations. The constant freeze-thaw cycle of water in cracks also weakens rock, creating material ready to be scraped away by the ice.
Gravity and Mass Movement
Gravity acts as a direct, independent agent of material movement, pulling weathered rock and soil downslope in processes collectively termed mass wasting. Mass wasting occurs when the force of gravity pulling the material downward overcomes the slope’s shear strength, which is the internal resistance to movement. Water often acts as a lubricant to reduce friction, but gravity is the primary transport force.
The speed of mass movement varies widely, from rapid events like landslides and rockfalls to the almost imperceptible movement of soil. Slumping is a common type of slide where a mass of material moves as a coherent block along a curved surface. Creep represents the slowest form of mass wasting, involving the steady, gradual downslope movement of surface soil, often evidenced by tilted utility poles or bent tree trunks.