The Earth’s surface is constantly transformed by forces that break down and move material across the globe. This reshaping begins with the breakdown of solid rock into smaller fragments, a process that occurs continuously. When these broken-down pieces of rock, soil, and sediment are subsequently picked up and carried away from their original location, it represents the next major stage in the geological cycle. This movement of weathered material sculpts landscapes, from gentle slopes to deep canyons.
Erosion: Carrying Away the Weathered Rock
The process that describes weathered rock being carried away is called erosion. Erosion is the displacement of solids from the Earth’s crust, involving the movement of soil, rock, and other particles by natural forces. It is a mechanical process driven by the kinetic energy of a moving agent, which separates the material from its source and transports it to a new location. This movement is distinct from weathering, which involves the breakdown of rock while it remains in situ.
Weathering must occur first to create the fragments that erosion can transport. This breakdown happens in two primary ways. Mechanical weathering physically breaks the rock into smaller pieces without changing its chemical composition, such as when water freezes and expands in rock cracks (frost wedging). Chemical weathering involves a change in the rock’s mineral structure, often through reactions like oxidation or dissolution by acidic rainwater, creating new compounds like clay minerals or soluble salts.
Erosion acts upon these weathered products, whether they are physically broken fragments or chemically altered particles. The effect of erosion is the gradual wearing down of elevated landforms and the creation of sediment that will later form new geological layers. This continuous cycle of breakdown and movement is a fundamental part of the rock cycle, constantly renewing the materials on the planet’s surface.
The Agents of Movement
The transport of weathered rock is accomplished by four natural agents, each with its own mechanism of movement. Flowing water (rainfall runoff, rivers, or ocean currents) is the most widespread agent of erosion. Water picks up sediment through hydraulic action, where the force of the moving water dislodges particles. It carries the material downstream in suspension, rolls it along the bottom, or dissolves it completely. The speed and volume of the water determine the size and quantity of material it can transport, allowing fast-moving streams to move large boulders.
Wind acts as an erosional agent primarily in dry, sparsely vegetated environments like deserts or coastlines, a process known as aeolian transport. The wind lifts fine particles, such as silt and sand, carrying them over vast distances in the suspended load. Larger sand grains are moved closer to the ground by bouncing and skipping along the surface, a mechanism called saltation. This wind-driven movement can shape landforms like sand dunes and remove topsoil from agricultural areas.
Glacial ice represents a massive agent of erosion, moving material through glacial transport. As glaciers flow across the landscape, they pluck large blocks of rock from the bedrock and grind material beneath them in a process called abrasion. The resulting sediment, which can range from fine rock flour to house-sized boulders, is carried within or beneath the ice. This type of erosion creates unique features like U-shaped valleys and deep grooves in the underlying rock.
Gravity acts as an independent agent of transport, driving what is known as mass wasting. This process involves the downslope movement of rock and soil under the direct influence of gravity, without the aid of a flowing medium like water or wind. Examples include slow soil creep, rapid landslides, and rockfalls. These are often triggered when water saturates the material, increasing its weight and reducing internal friction. Mass wasting moves huge volumes of material quickly, especially on steep slopes.
Deposition: Where the Journey Ends
The final stage of this movement cycle is deposition, which occurs when the transporting agent loses the energy required to carry its load of sediment. As a river slows down upon entering a lake or the ocean, it loses its capacity to hold particles in suspension, causing them to settle out. The force of gravity and friction overcome the force of the moving fluid.
This process of settling and accumulation adds sediments, soil, and rocks to a new landform. The largest and heaviest particles, like gravel and pebbles, are deposited first. Finer silt and clay materials are carried further before settling. Over long periods, these accumulated layers can be compacted and cemented together, forming new sedimentary rocks such as sandstone and shale, completing the Earth’s surface cycle.