The movement of solid particles from their original location, known as sediment transport, is a fundamental force shaping the Earth’s surface. Sediments are loose materials, including fragments of broken down rock, minerals, and organic matter, ranging from microscopic clay to large boulders. This movement begins after the initial breakdown of rock (weathering) and the subsequent removal of the material (erosion). Sediment transport is the sustained journey of these particles, often over vast distances, from the source area to a final place of rest. Driven by natural forces, this continuous cycle constantly shifts material across continents and into the oceans.
The Agents of Sediment Movement
The energy required to lift and carry sediment particles is supplied by several natural agents. Water, primarily through rivers, ocean currents, and waves, is the most influential agent worldwide, capable of moving the largest volumes of material. The density and volume of flowing water allow it to suspend fine particles and roll much larger fragments along the bottom. Wind acts as a significant agent, particularly in arid or coastal environments, moving massive quantities of sand and fine dust across great distances in a process called eolian transport.
Glacial ice represents another powerful force, trapping sediment within its mass as it slowly flows across the landscape. Glaciers are unique because they transport sediments of all sizes, from fine clay to house-sized boulders, without sorting them. Gravity also serves as a direct agent of movement through mass wasting events, such as landslides, rockfalls, and mudslides. In these scenarios, gravity pulls material down a slope, often resulting in rapid sediment relocation. The competence of an agent, or the largest particle size it can move, is directly related to its velocity and energy level.
Mechanisms of Sediment Transportation
Sediment particles are moved by these agents through four distinct physical mechanisms, depending on particle size and the energy of the transporting fluid. The largest and heaviest particles, like pebbles and boulders, are moved by traction, which involves rolling or sliding along the riverbed or ground. This movement requires the highest energy and is often a grinding action that reduces particle size.
A slightly smaller size of sediment, typically sand, moves through a hopping or bouncing motion known as saltation. The fluid’s force temporarily lifts the particle off the bed, but gravity pulls it back down, causing it to strike the surface and dislodge other particles. Saltation is a significant process in both river systems and desert dune formation.
The finest particles, such as silt and clay, are carried in suspension, held aloft within the body of the moving water or air. These fine grains have a low settling velocity, meaning fluid turbulence is sufficient to keep them from sinking, often giving the water a muddy appearance. Suspended particles can travel for hundreds or thousands of miles, only settling out when the fluid flow slows dramatically.
The final mechanism is solution, which involves the invisible transport of dissolved ions and minerals. Chemical weathering breaks down soluble rocks like limestone, and the resulting dissolved material is carried entirely within the fluid. This dissolved load is transported until the water chemistry changes or evaporation occurs, leading to the precipitation of new minerals.
The Final Stage: Sediment Deposition
The journey of a sediment particle ends with deposition, the process of the material settling out of the transporting medium. This occurs when the velocity of the water, wind, or ice decreases to a point where it no longer has enough energy to sustain the movement of its sediment load. As the fluid slows, gravity overcomes the forces holding the particle in motion, and it drops to the surface.
The rate at which particles settle is governed by their settling velocity, which is faster for larger and denser grains. This difference in settling speed causes sorting, where sediments are separated by size. For example, when a river enters quiet water, the largest pebbles and sand settle out first, while the finest silts and clays are carried farther offshore. Over geologic time, accumulated layers of deposited sediment are compacted and cemented together (lithification), transforming the loose material into solid sedimentary rock.