Erosion is the process involving the movement of soil, rock, and sediment from one location to another, constantly reshaping the Earth’s surface. This differs from weathering, which is the breakdown of rock into smaller particles without displacing them. Erosion transports this weathered material, fundamentally reshaping landscapes over time. Five primary natural forces act as the agents of erosion: running water, wind, glacial ice, gravity, and living organisms.
The Fluid Agents: Running Water and Wind
Running water, acting through rainfall, rivers, streams, and ocean waves, is the most powerful and common agent of erosion. The vast volume of material moved by water shapes landscapes ranging from massive canyons to tiny rill channels. Water transports sediment in three main ways: suspension, where fine particles are carried within the water column; dissolution, where soluble minerals are chemically broken down and dissolved; and abrasion, where carried sediment scrapes and grinds against the riverbed or banks.
The speed of the water directly controls its ability to erode, as faster flows carry significantly larger particles. This power leads to downcutting in river channels, forming V-shaped valleys, and lateral erosion that widens floodplains. Coastal erosion occurs when waves exert immense hydraulic action by compressing air into rock cracks, causing them to fracture.
Wind operates as a fluid agent, but its lower density limits it to moving much finer particles than water. Wind erosion is most pronounced in arid regions where sparse vegetation leaves the soil exposed. The primary mechanism is deflation, the lifting and removal of loose, fine-grained sediment like dust and silt, which sometimes creates large depressions.
Larger particles, typically sand, move through saltation, a bouncing motion where grains skip along the surface after being lifted by the wind. These airborne particles cause abrasion, sandblasting and smoothing exposed rock surfaces. Although less pervasive than water erosion, wind transports enormous quantities of fine material across continents, contributing to the formation of dunes and loess deposits.
The Mass Movement Agents: Glacial Ice and Gravity
Glacial ice is the most powerful erosional agent per unit area, capable of altering landscapes on a massive scale through slow, grinding motion. As ice masses move, they engage in plucking: meltwater seeps into bedrock cracks, freezes, and attaches rock fragments to the glacier’s base. The glacier’s forward movement then rips these fragments away, quarrying out large pieces of rock.
The fragments embedded in the ice mass act as tools for abrasion, scoring and polishing the underlying bedrock. This grinding leaves behind characteristic parallel scratches, known as striations, and carves out distinctive U-shaped valleys. The weight and viscosity of glacial ice allow it to move material that neither wind nor water could displace, reshaping mountainous and high-latitude terrains.
Gravity is the driving force behind all erosion, but it acts as a direct agent when causing mass wasting—the immediate, downslope movement of material without an intervening fluid. This process includes rapid events like landslides and rockfalls, where materials are pulled down steep slopes. Slower forms include slumps, where soil or rock slips along a curved surface, and creep, the gradual movement of soil down a slope.
Water often acts as a lubricant or adds weight, triggering mass wasting events, but the primary force remains the pull of gravity. These events are a significant component of landscape evolution, moving massive amounts of sediment and rock and creating landforms like talus slopes at the base of cliffs.
The Biological Agent
Living organisms, collectively considered a biological agent, contribute to erosion through natural activity and human intervention. Naturally, animals and plants influence soil and rock movement, primarily through bioturbation. Burrowing animals like earthworms and rodents constantly mix and loosen soil layers, bringing weathered material to the surface where it becomes vulnerable to transport.
Plant roots play a dual role; they often stabilize soil, but their growth into rock fissures exerts expansive pressure, physically breaking up the rock in a process called root wedging. The most significant impact, however, comes from human activity, known as anthropogenic erosion. Practices such as deforestation, poor farming techniques, and construction drastically accelerate the rate of soil movement beyond natural levels.