What Is Erosion?

Erosion is the detachment and transport of soil, sediment, and rock from one location to another by moving water, wind, ice, or gravity. Geologists distinguish it from weathering, which breaks material down in place without moving it, and from deposition, which is the end of the transport path. Together these processes form the denudation system that shapes landforms over timescales ranging from a single storm to tens of millions of years.

Erosion is quantified as a mass or volume removed per unit area per unit time, commonly in tonnes per hectare per year for soil or in millimeters of surface lowering per thousand years for landscapes. Rates vary over many orders of magnitude, from a fraction of a millimeter per thousand years on stable cratonic surfaces to centimeters per year on tilled slopes in intense rainfall. The gap between geologic background rates and rates on disturbed land is the central concern of soil conservation, because soil forms far more slowly than accelerated erosion removes it.

Water Erosion

Water is the dominant agent on most land surfaces. The sequence begins with raindrop impact, which detaches particles and seals the surface, reducing infiltration and generating overland flow. That flow removes a thin surface layer as sheet erosion, concentrates into small channels as rill erosion, and cuts deeper incisions as gully erosion where the flow accumulates. In stream channels, bank erosion and bed scour dominate, governed by boundary shear stress and the critical thresholds at which grains of a given size begin to move. Long-term average soil loss from agricultural hillslopes is commonly estimated with the Universal Soil Loss Equation and its successors, which multiply a rainfall erosivity factor by soil erodibility, slope length and steepness, cover management, and support practice terms. The USDA's Revised Universal Soil Loss Equation program maintains the current model versions, and the RUSLE2 user's reference guide prepared for the Agricultural Research Service documents how conservation planners apply the factors in practice.

Wind, Ice, and Coastal Processes

Wind erosion dominates in arid and semi-arid regions and on exposed, dry, loosely aggregated soils. Particles move by creep, saltation, and suspension, with saltating sand grains abrading the surface and ejecting further material in a self-reinforcing cascade that produces dust storms and dune fields. Glacial erosion works by abrasion, as debris frozen into basal ice grinds the bedrock, and by plucking, in which meltwater refreezing detaches jointed blocks, together carving the U-shaped valleys and cirques characteristic of glaciated terrain. Coastal erosion combines wave quarrying and abrasion at cliff bases with longshore sediment transport that redistributes beach material. The US Geological Survey national assessment of shoreline change applies consistent transect-based methods so that erosion rates can be compared between coastlines and across decades.

Control, Measurement, and Engineering Materials

Managing erosion means reducing the erosive force, increasing surface resistance, or both. Agricultural practice relies on residue cover, conservation tillage, contour farming, terracing, cover crops, and vegetated buffer strips; construction sites use silt fences, sediment basins, and erosion control blankets. Field measurement combines runoff plots, erosion pins, sediment traps, and stream sediment gauging with remote sensing and repeat lidar or photogrammetric surveys that resolve centimeter-scale surface change. The word also names a distinct engineering process: erosion of solid surfaces by particle impact, cavitation, or high-velocity droplets degrades pump impellers, turbine blades, pipeline elbows, and rocket nozzles, and is treated in tribology alongside abrasive and cavitation wear.

Applications

The study and control of erosion have applications in a wide range of fields, including:

• Agricultural engineering and soil conservation planning
• Watershed and reservoir management, including sedimentation forecasting
• Civil engineering for slope stability, embankments, and stormwater design
• Coastal engineering and shoreline protection
• Geomorphology and landscape evolution modeling
• Remote sensing and lidar-based topographic change detection
• Materials engineering, in the assessment of erosive wear on turbomachinery and piping